A&A 453, 1003-1026 (2006)
M. A. Thompson1,2 - J. Hatchell3 - A. J. Walsh4 - G. H. Macdonald2 - T. J. Millar5,6
1 - Centre for Astrophysics Research, Science & Technology Research Institute, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
2 - Centre for Astrophysics & Planetary Science, School of Physical Sciences, University of Kent, Canterbury CT2 7NR, UK
3 - School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
4 - School of Physics, University of New South Wales, NSW, 2052, Australia
5 - Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, PO Box 88, Manchester, M60 1QD, UK
6 - School of Mathematics and Physics, Queen's University Belfast, Belfast BT7 1NN, UK
Received 20 October 2005 / Accepted 28 March 2006
We present a SCUBA submillimetre (450 and 850 m) survey of the environment of 105 IRAS point sources, selected from the Wood & Churchwell (1989a) and Kurtz et al. (1994) radio ultracompact (UC) H II region surveys. We detected a total of 155 sub-mm clumps associated with the IRAS point sources and identified three distinct types of object: ultracompact cm-wave sources that are not associated with any sub-mm emission (sub-mm quiet objects), sub-mm clumps that are associated with ultracompact cm-wave sources (radio-loud clumps); and sub-mm clumps that are not associated with any known ultracompact cm-wave sources (radio-quiet clumps). 90% of the sample of IRAS point sources were found to be associated with strong sub-mm emission. We consider the sub-mm colours, morphologies and distance-scaled fluxes of the sample of sub-mm clumps and show that the sub-mm quiet objects are unlikely to represent embedded UC H II regions unless they are located at large heliocentric distances. Many of the 2 5 SCUBA fields contain more than one sub-mm clump, with an average number of companions (the companion clump fraction) of 0.90. The clumps are more strongly clustered than other candidate HMPOs and the mean clump surface density exhibits a broken power-law distribution with a break at 3 pc. We demonstrate that the sub-mm and cm-wave fluxes of the majority of radio-loud clumps are in excellent agreement with the standard model of ultracompact H II regions. We speculate on the nature of the radio-quiet sub-mm clumps and, whilst we do not yet have sufficient data to conclude that they are in a pre-UC H II region phase, we argue that their characteristics are suggestive of such a stage.
Key words: stars: formation - ISM: HII regions - ISM: dust, extinction - submillimeter - radio continuum: ISM
Ultracompact (UC) H II regions are perhaps one of the most reliable tracers of recent massive star formation. UC H II regions are defined as dense, compact bubbles of photoionised gas less than 0.1 pc in diameter, surrounding and excited by massive young stellar objects (YSOs). The estimated age of UC H II regions is between 104-105 years, inferred from their spatial diameters and the typical expansion rates of H II regions. Ultracompact H II regions should thus trace the earliest embedded phases of massive star formation, wherein the YSOs have just begun to ionise their surrounding natal molecular clouds. For recent reviews of UC H II regions see Churchwell (2002), Kurtz et al. (2000) or Garay & Lizano (1999).
Most UC H II regions have to date been identified by their centimetre-wavelength free-free emission, due to the ability of radio-wavelength radiation to penetrate the dense molecular gas and dust cores within which the UC H II regions are embedded. There are two types of radio surveys that have been used to find UC H II regions, each relying on snapshot radio interferometry either targeted toward colour-selected IRAS point sources (Wood & Churchwell 1989a; Kurtz et al. 1994) or over unbiased regions of the Galactic Plane (Becker et al. 1990, 1994; Zoonematkermani et al. 1990; Giveon et al. 2005). The targeted surveys rely upon the Wood & Churchwell (1989b) IRAS colour selection criteria for UC H II regions, which propose that UC H II regions possess distinctive far-infared colours and can be identified by a discrete region in a 25/12 m versus 60/12 m colour-colour diagram.
Few UC H II regions have been mapped at a sufficient angular scale or resolution to investigate their dense molecular surroundings and this means that we are relatively ignorant of the location of the UC H IIs within their embedding cores, the global physical properties of the cores and the wider star-forming neighbourhood. Improving our understanding of the enviroments in which massive stars form is crucial to our comprehension of massive star formation and the impact that massive stars have upon their surroundings. In particular it is becoming apparent that a number of UC H II regions are in fact bright compact components of more extended emission (Kurtz et al. 1999; Kim & Koo 2001). One possible explanation for these extended regions is that the hierarchichal structure of molecular clouds can give rise to H II regions with both ultracompact and extended components if the O star exciting the H II region is displaced from the centre of its embedding core (Kim & Koo 2001, 2003).
Recently developed millimetre and sub-millimetre wavelength bolometers such as SCUBA (Holland et al. 1999), SHARC (Hunter et al. 1996), MAMBO (Kreysa et al. 1998) and SIMBA (Nyman et al. 2001) have made it possible to rapidly map dust emission from extended regions over arcminute scales at resolutions of 10-20 . A number of sub-millimetre and millimetre continuum mapping studies of massive star-forming regions have been carried out (e.g. Hill et al. 2005; Walsh et al. 2003; Mueller et al. 2002; Hatchell et al. 2000; Hunter et al. 2000) but the largest surveys have so far been aimed at H2O (Mueller et al. 2002) or CH3OH masers (Walsh et al. 2003; Hill et al. 2005) rather than UC H II regions. Although the individual populations of H2O masers, CH3OH masers and UC H II regions are known to overlap somewhat (e.g. Walsh et al. 1998; Palagi et al. 1993), without a dedicated large-scale UC H II region survey it is difficult to understand the potentially different physical characteristics of masing versus non-masing regions. Nevertheless, surveys of this type are beginning to reveal the density distributions and physical properties of massive star-forming cores. Additional dust clumps located near to those containing the active star formation traced by masers or UC H IIs are also being discovered (Hill et al. 2005; Walsh et al. 2003; Hatchell et al. 2000) and the implication is that these clumps may represent earlier evolutionary stages than the maser cores or UC H II regions.
In order to more fully understand the environments of UC H II regions, constrain the physical properties of their embedding dusty clumps and search for nearby associated clumps, we have carried out a sub-mm SCUBA imaging survey of 105 UC H II regions from the Wood & Churchwell (1989a) and Kurtz et al. (1994) radio catalogues. In this paper, the first of a series reporting our study, we present a description of the survey procedure, the SCUBA images of the UC H II regions and their sub-mm fluxes, plus a statistical analysis of their sub-mm colours, morphologies, clustering properties and likely physical natures. A detailed treatment of the physical properties (e.g. temperature, mass, density profile) of each of the sub-mm sources detected in the survey and their correlation with the presence of hot molecular cores will be presented in subsequent publications.
In Sect. 2 we outline the sample selection, observational and data reduction procedure. The 450 and 850 m images and the fluxes of the sources detected in the survey are presented in Sect. 3. We investigate the sub-mm detection statistics of the UC H II regions and their positional association with sub-mm cores, the morphology of the sub-mm clumps and their clustering properties in Sect. 4. Finally, in Sect. 5 we summarise the initial results of the survey and its implications for the future study of massive star-forming regions.
The sample of UC H II regions to be observed was drawn from the radio catalogues of Wood & Churchwell (1989a, hereafter WC89a), and Kurtz et al. (1994, hereafter KCW94). Each of these catalogues consists of a list of compact radio sources observed with the VLA towards IRAS FIR point sources. Although each radio survey consisted of single VLA pointings toward individual IRAS sources their initial survey selection criteria differ somewhat. The WC89a survey selected their target objects from known compact H II regions (Wink et al. 1982; Wood et al. 1988a,b) that were likely to contain ultracompact components as suggested by their long-wavelength spectra. The KCW94 survey on the other hand selected their targets from a sample of bright IRAS point sources ( Jy) that solely fulfilled the Wood & Churchwell (1989b) FIR colour criteria for UC H II regions. KCW94 made no attempt to screen their sample for well-known objects of other types (e.g. planetary nebulae, compact or extended H II regions) and as will be seen later certain of the KCW94 objects are not true UC H II regions.
The combined WC89a and KCW94 surveys consist of observations of 143 individual IRAS point sources, of which radio emission from UC H IIs was detected toward 101 IRAS point sources. Many of the IRAS sources display multiple radio components: out of 101 IRAS sources with associated radio emission a total of 150 discrete ultracompact radio components are identified in the WC89a and KCW94 surveys. Our initial SCUBA sample was selected from the 143 individual IRAS sources observed by WC89a and KCW94. As the SCUBA "instantaneous'' field of view (FOV) is very similar to the primary beam of the VLA at 2 cm and the IRAS 100 m beam FWHM we chose to observe each IRAS point source with a single SCUBA field.
There are 42 IRAS sources from the WC89a and KCW94 surveys towards which no radio emission was detected at the time of the WC89a and KCW94 surveys. It is possible that these objects may either represent H II regions extended over scales 10 , which would have been missed by the snapshot interferometric observations of the radio surveys, UC H II regions that were too faint to be detected in the original surveys, or potential massive protostellar objects in an evolutionary phase prior to that of UC H II regions. We searched the SIMBAD database of astronomical catalogues for more recent radio observations of these fields and discovered that ultracompact radio components had been found toward 21 of these IRAS point sources (predominantly from the 5 GHz galactic plane survey of Becker et al. 1994 and Giveon et al. 2005). In compiling this list of ultracompact radio components we matched each survey against the others to determine a unified source list. Where two (or more) surveys report a radio source of similar brightness within 5 of one another we consolidated these sources as a single detection. Positions and fluxes are used from the survey with the highest signal-to-noise ratio (generally the WC89a or KCW94 surveys). Thus our initial sample comprises 122 IRAS point sources with associated compact centimetre-wave emission ("UC H II regions'') and 21 IRAS point sources that are not associated with any detected compact centimetre-wave components. The latter 21 IRAS sources were included in our sample so that we could compare their sub-mm fluxes and morphologies with those dust cores known to contain UC H II regions.
Whilst the initial sample to be observed consisted of 143 IRAS point sources, due to the manner in which our observations were carried out (flexibly scheduled mode, which is described further in Sect. 2.2) it was not possible to complete observations of the entire sample. We were able to map 105 out of the total number of 143 IRAS point sources, of which 94/122 UC H II-associated IRAS point sources and 9/21 non-UC H II-associated point sources were mapped. The mapped fields and their coordinates are given in Table 1. In terms of the fields containing known ultracompact radio components our survey is thus 80% complete. As the selection of the mapped fields from within the larger sample was essentially random (depending upon hour angle, weather conditions, instrument availability etc.) we are confident that there is little selection bias within our final list of UC H II regions.
|UC H II||Field centre (J2000)||Dist.||Dist.||UC H II||Field centre (J2000)||Dist.||Dist.|
|field name||RA||Dec||(kpc)||Ref.||field name||RA||Dec||(kpc)||Ref.|
|Distance references are: (1) Mehringer et al. (1998); (2) Wood & Churchwell (1989a); (3) Acord et al. (1998); (4) Churchwell et al. (1990); (5) Fish et al. (2003); (6) Hofner et al. (1994); (7) Kurtz et al. (1994); (8) van der Tak et al. (2000); (9) kinematic distance evaluated from CH3OH maser velocity given by Szymczak et al. (2000); (10) Harju et al. (1998); (11) kinematic distance evaluated from CH3OH maser velocity given by Caswell et al. (1995); (12) kinematic distance evaluated from Hydrogen radio recombination line velocity given by Kuchar & Clarke (1997); (13) Blum et al. (2000); (14) Zavagno et al. (2002); (15) Motte et al. (2003); (16) Palagi et al. (1993); (17) Araya et al. (2002); (18) Watson et al. (2003); (19) Codella et al. (1994).|
Our observations were carried out using the SCUBA (Holland et al. 1999) common-user bolometer array in operation at the James Clerk Maxwell Telescope (JCMT). The observations were performed in a flexibly-scheduled mode whereby observations are not scheduled over a pre-defined period but are carried out over a 6-month observing semester according to the appropriate weather (atmospheric opacity) band, the visibility of the sources from the telescope and the scientific priority of the observations. The survey data were thus obtained by visiting observers over several separate periods across the 98B (Aug.-Jan.) and 99A (Feb.-July) observing semesters at the JCMT.
SCUBA is comprised of two bolometer arrays, a short-wave array of 91 pixels optimised for operation at 450 m and a long-wave array of 37 pixels optimised for operation at 850 m. Both arrays simultaneously sample a similar field of view (approx 2 square), although the spacing between individual bolometers on the array means that not all of the field of view is sampled instantaneously. To fill in the gaps in spatial coverage the telescope secondary mirror is moved in a 64-point pattern ("jiggling''), whilst also chopping at a frequency of 1 Hz to remove the sky emission. This procedure is commonly known as a "jiggle-map'' and provides maps with full spatial sampling at both wavelengths.
As the SCUBA FOV is approximately the same size as the VLA primary beam at 2 cm and the IRAS 100 m beam area we obtained a single jiggle-map centred at the coordinates of either the UC H II region radio emission or the position of the IRAS source (where no radio information was available). For the UC H II regions contained in the KCW94 survey we mistakenly used the IRAS positions rather than the radio positions, which resulted in the array being slightly offset from the main sub-mm emission. However, the SCUBA FOV was found to be larger than the positional offsets introduced and thus there is little effect upon the data in question. Additional maps with positional offsets were obtained for a number of sources where the mapped emission was affected by noisy bolometers or by being located at the edge of the FOV.
Each source was integrated on for three of the 64-point SCUBA jiggle cycles (also known as SCUBA "integrations'') for a total on-source integration time of 192 s. This typically resulted in 1-rms noises of 1.5 Jy per 8 FWHM beam at 450 m and 0.1 Jy per 14 FWHM beam at 850 m. All sources were observed with a chop throw of 120 in azimuth. As SCUBA is located at the Nasmyth focus of the JCMT and does not possess a beam rotator, this means that the chop positions are also a function of elevation.
Regular pointing checks were made and the average pointing offset was found to be 5 . Hourly skydips were carried out at approximately the same azimuth as the observations to estimate the atmospheric zenith optical depth at at 450 and 850 m. Values derived from the skydips were compared to and found consistent with those extrapolated from the fixed-azimuth measurements at 225 GHz made every 10 min by the CSO tipping radiometer. Absolute flux calibration and beam maps of the primary flux calibrator Uranus were also obtained at least once per observing session.
The data were reduced using a combination of the automated SCUBA reduction pipeline ORACDR (Economou et al. 2002), the SCUBA reduction package SURF (Jenness & Lightfoot 2000) and the Starlink image analysis package KAPPA (Currie & Bell 2002). The reduction procedure for 450 and 850 m data was essentially the same and proceeded along the following outline. Initially the chopping and nodding positions were subtracted from the on-source data to form a time-ordered series of sky-subtracted bolometer measurements. Given that the UC H IIs are likely to be embedded in larger giant molecular cloud complexes, it is likely that extended emission from these regions is present in the chopping positions and is subtracted from the on-source sub-mm fluxes by this procedure. The fluxes quoted in this paper should thus be strictly regarded as lower limits to the true sub-mm fluxes.
The time-ordered bolometer data were then corrected for atmospheric extinction using an optical depth value interpolated from skydips carried out before and after the jiggle-map. At this stage bolometers with a mean noise in excess of 100 nV were blanked and transient bolometer noise spikes were removed by applying a 5 clip to the data. As several of the sources were found to be strongly centrally-peaked, care was taken to avoid the removal of the central pixel by this despiking procedure. Residual sky variations between individual bolometers were removed by specifying emission-free bolometers and using the SURF task remsky. The time-ordered data were then regridded to J2000 sky coordinates with the SURF task rebin.
Absolute flux calibration was carried out using the calibration maps of Uranus. Predicted fluxes for Uranus were estimated using the values given by the Starlink package FLUXES (Privett et al. 1998) and on the JCMT calibrator webpage respectively. Flux correction factors (FCFs) for each wavelength were then determined by dividing the predicted flux by the measured peak value of the calibrator. By monitoring the variation in FCF over each observing period it was found that the absolute flux calibration was accurate to 30% at 450 m and 10% at 850 m. These errors in calibration are predominantly due to variations of the line-of-sight optical depth on timescales smaller than can be accounted for by skydips. Each jiggle-map was calibrated in units of Jy/beam by multiplying by the appropriate FCF. The FWHMs and peak values of the telescope main and error beams were determined by fitting two Gaussians to azimuthal averages of the maps of the primary calibrator (Uranus).
A number of images, mainly taken on a single night, were found to be essentially uncalibratable at 450 m due to either large values of or rapid variations in the 450 m atmospheric optical depth. Although there is a close linear relation between the 450 and 850 m zenith optical depth (Archibald et al. 2002), the atmosphere is typically more unstable and much more optically thick (by a factor of 4 or more) at 450 m. The result of this is that whilst the 450 m data was unusable for the affected periods, the 850 m data was relatively unaffected. We do not present the affected 450 m data from these periods, which is a total of 18 images.
The calibrated images were then converted into FITS format and deconvolved to remove the contribution from the error beam. The deconvolution was performed using the clean task in MIRIAD (Sault et al. 1995) with a circularly symmetric two-component Gaussian beam derived from azimuthal averages of the primary calibrator maps in a manner similar to that described in Hogerheijde & Sandell (2000). Each image was cleaned down to a cutoff level of twice the 1 rms noise and then restored back to a resolution appropriate for the wavelength (8 for 450 m and 14 for 850 m). The advantage of this technique is that the different error beam contributions from each wavelength are removed, facilitating comparison of 450 m and 850 m maps and allowing the integrated fluxes to be determined more accurately.
After deconvolution to remove the error beam the 450 m images were inspected and those displaying insufficient signal-to-noise to permit accurate source identification were smoothed from a native resolution of 8 to the same resolution as that in the 850 m images (14 ). Sacrificing the angular resolution of the 450 m images results in an improvement of the signal-to-noise ratio for extended emission by approximately a factor of 3, which considerably aided in the identification of faint 450 m sources.
We present the final deconvolved SCUBA images in Figs. 1 and 2, marked with the positions of the UC H II regions in each image. Figure 1 contains images of sources that were detected at 450 and 850 m, whereas Fig. 2 contains images of those sources with data only at 850 m, either because the 450 m emission was undetected or due to the 450 m calibration problems discussed in the previous section. Only those SCUBA fields with detected sub-mm sources are included in the figures. Sub-mm emission was detected toward 90% of the sample (92 out of 105 fields). Here we outline a few attributes of the SCUBA images in order to aid in their interpretation.
"Holes'' in the 850 m images result from the blanking of noisy bolometers in the data reduction process. Typically between 1 and 3 bolometers were blanked from each 850 m image and any flux from sources found near the blanked image regions may be underestimated. Individual sources whose fluxes may be affected are indicated in Table 2. Noisy bolometers were also blanked from the 450 m data, but as the short wavelength array in SCUBA has a higher spatial sampling, the blanking of bolometers from the short-wavelength array does not cause gaps in the spatial coverage of the array. Negative "bowls'' of emission are also present in some images (e.g. G13.19+0.04 in panel 73L, and G25.38-0.18 in panel 75L) and these bowls are caused by the presence of sub-mm emission in the chopping positions (i.e. approximately 120 in azimuth from the field centre). The morphology of a number of these bowls indicates that the emission is compact and resembles that of the positive emission features seen in the sub-mm image. These compact negative bowls may indicate the presence of other sub-mm cores located near the chopping positions, possibly similar to those associated with the UC H II regions in the main image.
The flux contour levels on each image were determined by a dynamic range power-law fitting scheme in order to emphasise both the low-level and bright emission. Logarithmic contour levels were fitted to the dynamic range of the image D (defined as the image peak divided by the 1rms noise) following the relation , where N is the number of contours (in this case 9) and i is the contour power-law index. The minimum power-law index used was one, which results in linear contours starting at a level of 3 and spaced by 3. This dynamic power-law contouring scheme was found to give excellent results in both high and low dynamic range images, concentrating the contours around low surface brightness features in the images to adequately represent low-level structure in the images.
The SCUBA images show that the sub-mm emission associated with UC H II regions follows a range of morphologies: from single, bright, centrally peaked compact cores (e.g. G20.08-0.14 in panel 22L, G35.02+0.35 in panel 46L); to more complex multiply-peaked regions with peaks both associated and unassociated with known UC H II regions (e.g. G28.29-0.36 in panel 36L); and ridge-shaped structures with multiple condensations strung along the ridge (e.g. G27.28+0.15 in panel 33L, G79.30+0.28 in panel 59L). Often the sub-mm cores that are not associated with the UC H II regions in an image are of comparable brightness or sometimes even brighter than the neighbouring cores that are (e.g. G8.14+0.23 in panel 4L, G25.72+0.05 in panel 31L). The SCUBA images of the non-UC H II associated IRAS point sources (those indicated by a in Table 1) also reveal the presence of bright radio-quiet sub-mm cores similar in characteristics and appearance to the cores that are known to be associated with UC H II regions (e.g. G13.19+0.04 in panel 73L). If these radio-quiet sub-mm cores are forming massive stars in a similar manner to their radio-loud UC H II-associated neighbours, it is possible that the radio-quiet cores represent an earlier evolutionary phase of star formation prior to the development of an UC H II region as suggested by Garay et al. (2004). We will dwell further on this hypothesis in Sect. 4.
Individual sources within the 450 and 850 m images were identified using the source extraction algorithm SEXTRACTOR (Bertin & Arnouts 1996). Whilst SEXTRACTOR was initially developed as a source extraction routine for visible and infrared-wavelength images, it nevertheless provides the means of separating the often closely blended sub-mm cores in the SCUBA images and measuring their individual fluxes in a consistent fashion. As SEXTRACTOR treats each pixel of the target image in a statistically independent way, whereas each of our sub-mm images is gridded in pixels smaller than the beam FWHM (and are thus not statistically independent), it was necessary to account for the beam area in the final measured fluxes and their uncertainties. We ran SEXTRACTOR on each of our images, setting the analysis threshold to be three times the rms noise in each image. The rms noise in each image was determined by hand from the standard deviation of source-free pixels, as it was found that SEXTRACTOR could not make an accurate automatic determination of this quantity in the relatively small and crowded SCUBA images (as compared to typical visible-wavelength data).
|Figure 1: SCUBA images from the survey with detections at both 450 and 850 m. The images displayed here are those discussed further in the text of the paper. Each UC H II region is represented by a pair of images at 450 m (left image) and 850 m (right image). Coordinates are Right Ascension and Declination in the J2000 system. Crosses indicate the positions of ultracompact H II regions from Wood & Churchwell (1989a), Kurtz et al. (1994), Becker et al. (1994) or Giveon et al. (2005). All images have been deconvolved with a model of the JCMT beam to remove the contribution from the error lobe and 450 m images with limited signal-to-noise have been smoothed to the same resolution as the 850 m images to improve the source detections. The full version of this figure is available in electronic form.|
A combined source catalogue was compiled from the individual SEXTRACTOR runs on each SCUBA image. Beam-corrected 450 and 850 m fluxes were determined for each source in the catalogue (using the FLUX_AUTO keyword) and were checked against a random sample of fluxes determined by manual aperture photometry. In all cases the fluxes determined by SEXTRACTOR were consistent with those measured by manual aperture photometry. We also compared the measured fluxes of those sources from our catalogue that have been published in the literature. We share 11 fields with the methanol maser survey of Walsh et al. (2003) and in all but two cases the integrated and peak fluxes are consistent to within the uncertainties. An inspection of the two images (G15.04-0.68 and G28.29-0.36) where the flux measurements are inconsistent reveals that the likely cause is due to the confused nature of these regions and the different chop positions used by ourselves and Walsh et al. (2003). Our measured fluxes of the sub-mm cores associated with G76.38-0.62 , G25.65+1.05 and G106.80+5.31 (also known as S 106, RAFGL 7009S and S 140 respectively) also agree very closely with the sub-mm measurements of Richer et al. (1993), McCutcheon et al. (1995) and Minchin et al. (1995), taking into account the slightly different filter bandwidths used in these observations.
The combined source catalogue with the centroid position of each sub-mm source, its peak flux and integrated flux at both 450 and 850 m are given in Table 2. The positions quoted for each source are usually taken from the 850 m images (unless otherwise stated) as the the 850 m images have a considerably higher signal-to-noise ratio. Where it was possible to deblend sources at 450 m but not at 850 m images the centroid positions are those measured from the 450 m images. The errors quoted in Table 2 include the absolute calibration errors of 30% at 450 m and 10% at 850 m combined in quadrature with the intrinsic measurement error derived from the image rms noise.
|UC H II||IRAS PSC||850 m flux upper limit|
Out of the total 105 SCUBA fields observed as part of the survey we have identified 155 sub-mm sources at either 450 or 850 m. Approximately 10% of the fields observed (13 in total) did not display any sub-mm emission down to a level of 3. Approximately half of the non-detected SCUBA fields (7/13) are associated with ultracompact centimetre-wave radio components. As these objects do not display significant sub-mm emission, which indicates a low column density of dust, it is unlikely that the centimetre-wave radio emission traces embedded UC H II regions unless the distance to these objects is large. We will discuss these objects in more detail in Sect. 4. The non-detected SCUBA fields and their upper 850 m flux limits are are indicated in Table 3.
In this section we describe selected fields from the survey in further detail to aid in the interpretation of their sub-mm emission. We briefly describe their morphology in the sub-mm, their associated UC H II regions (or lack of them) and whether the fields are better known by another name than their galactic coordinates.
During the survey we identified three distinct types of object: ultracompact cm-wave sources that are not associated with any sub-mm emission ("sub-mm quiet objects''), sub-mm clumps that are associated with ultracompact cm-wave sources ("radio-loud clumps''); and sub-mm clumps that are not associated with any known ultracompact cm-wave sources ("radio-quiet clumps''). In this section we investigate the likely nature of each kind of object. We also examine the broad properties of the sample as a whole, focusing upon their morphology and clustering properties. We postpone a detailed analysis of the physical properties of each sub-mm clump to a later paper in the series (Thompson et al. 2006), where we present an in-depth study of the temperature, mass and density of each sub-mm clump.
From an inspection of Figs. 1 and 2, it is clear that the vast majority of ultracompact radio sources investigated in this study are positionally associated with bright sub-mm emission and are thus likely to be young UC H II regions embedded within molecular cloud clumps. Nevertheless we have identified a substantial population of ultracompact radio sources in the fields surveyed that are not associated with any detected sub-mm emission. For brevity we hence refer to these objects as "sub-mm quiet objects''. What is the likely nature of these sub-mm quiet objects? The standard model of an UC H II region is a massive YSO embedded within a dusty molecular cocoon that is optically thick to the visible-UV radiation of the YSO. The compact cm-wave radiation results from free-free emission from the ionised gas of the UC H II region but the bulk of the radiation is emitted in the sub-mm, far- and mid-infrared regions through dust reprocessing of the shorter-wavelength radiation by the optically thick cocoon (e.g. Crowther & Conti 2003; Churchwell 2002). Following this picture sub-mm quiet objects are unlikely to be associated with massive dust cocoons and thus may not be true UC H II regions.
We investigate this possibility by constraining the upper limit to the mass of any molecular clump associated with the sub-mm quiet objects. The VLA surveys possess sufficient sensitivity to detect UC H II regions excited by B0 stars out to a distance of 20 kpc (Giveon et al. 2005) but the sensitivity of our SCUBA survey is such that we would not detect clumps of less than a few hundred solar masses at this distance. There is thus the potential that the sub-mm quiet objects may represent a population of distant UC H II regions embedded in relatively low mass clumps.
In the following discussion we refer only to the 850 m data, as the signal-to-noise ratio of these images is typically a factor or 10 higher that that at 450 m. A total of 80 radio sources identified from the literature search described in Sect. 2.1 were found to be unassociated with 850 m emission to a level of 3, or a typical flux limit of 0.4-0.6 Jy/beam. These sub-mm quiet objects are listed in Table 4 along with their corresponding SCUBA field, the 850 m flux upper limit, the distance to the radio component (if known) and an upper limit to the mass of any molecular clump that could be associated with the sub-mm quiet objects. Masses were calculated using the method outlined in Hildebrand (1983) assuming the upper flux limits and distances given in Table 4, a dust temperature of 30 K, dust emissivity and a mass coefficient of g cm-2 (from Kerton et al. 2001). For cases where a kinematic distance ambiguity exists we have calculated the mass upper limits at both the near and far distances. We have also assumed that each sub-mm quiet object in a single SCUBA field lies at the same distance.
|UC H II field||Known ultracompact radio components||850 m flux limit||Distance||Clump mass limit|
|G8.67-0.36||[GBH05] 8.66530-0.34198||[GBH05] 8.66324-0.34337||0.9||4.6||53|
|[GBH05] 8.66202-0.34038||[GBH05] 8.66730-0.34437|
|G10.30-0.15||[WC89] 010.30-0.15A||[GBH05] 10.30726-0.14585||0.6||6.0||65|
|[GBH05] 10.30485-0.14319||[GBH05] 10.32159-0.15542|
|[GBH05] 10.30605-0.14551||[GBH05] 10.30485-0.14319|
|G10.62-0.38||[GBH05] 10.59905-0.38336||[GBH05] 10.59905-0.38336||0.9||4.8||57|
|G13.19+0.04||[GBH05] 13.19014+0.04077||[GBH05] 13.18643+0.04999||0.4||4.4/12.1||21/161|
|G13.87+0.28||[GBH05] 13.87696+0.28238||[GBH05] 13.87595+0.28479||0.6||4.5||35|
|G18.15-0.28||[GBH05] 18.15158-0.28016||[GBH05] 18.14744-0.29764||0.7||4.2||34|
|G19.07-0.27||[WC89] 019.07-0.27||[GBH05] 19.06392-0.27384||0.5||5.4||40|
|[GBH05] 19.06256-0.27062||[GBH05] 19.06732-0.28637|
|G19.49+0.14||[GBH05] 19.48265+0.16098||[GBH05] 19.48660+0.15574||0.3||2.0/14.0||3/145|
|[GBH05] 19.48283+0.15668||[GBH05] 19.49724+0.13390|
|G23.46-0.20||[WC89] 023.46-0.20A||[GBH05] 23.43595-0.20366||0.4||9.0||82|
|[GBH05] 23.43821-0.20869||[GBH05] 23.43876-0.21157|
|[GBH05] 23.43876-0.21157||[GBH05] 23.44160-0.21151|
|G25.72+0.05||[GBH05] 25.69526+0.03343||[GBH05] 25.69562+0.03499||0.2||14.0||120|
|G27.28+0.15||[GBH05] 27.28030+0.16537||[GBH05] 27.27497+0.15800||0.5||15.2||338|
|[GBH05] 27.27070+0.14850||[GBH05] 27.27809+0.14873|
|[GBH05] 27.28271+0.16406||[GBH05] 27.27569+0.14696|
|G27.49+0.19||[GBH05] 27.49356+0.19212||[GBH05] 27.49617+0.19389||0.4||2.5/12.6||7/168|
|G28.60-0.36||[GBH05] 28.59890-0.36738||[GBH05] 28.60723-0.36382||0.3||5.2/9.7||22/75|
|[GBH05] 28.59173-0.36057||[GBH05] 28.60375-0.36693|
|G28.80+0.17||[GBH05] 28.79740+0.17314||[GBH05] 28.80450+0.16777||0.5||6.4/8.5||62/109|
|G28.83-0.23||[GBH05] 28.82159-0.22657||[GBH05] 28.81780-0.22737||0.4||5.1/9.8||30/111|
|G37.77-0.20||[GBH05] 37.76595-0.19983||[GBH05] 37.76913-0.20613||0.4||8.8||83|
|G39.25-0.06||[GBH05] 39.25972-0.05001||[GBH05] 39.24685-0.06521||0.4||11.4||160|
|G41.52-0.04||[GPSR5] 41.525+0.039||GPSR5 41.507+0.030||0.2||...||...|
|G42.90+0.57||[WC89] 042.90+0.57A||[WC89] 042.90+0.57B||0.3||5.2||21|
It can be seen from Table 4 that the 850 m flux upper limits preclude high mass clumps toward all but the furthest sub-mm quiet objects. Objects closer than a distance of 10 kpc cannot be embedded in clumps with masses in excess of a few tens of solar masses. The most extreme source in Table 4 is [KCW94] 110.209+2.630 which at an assumed distance of 0.7 kpc has an upper limit to the mass of an associated molecular clump of only 0.3 . Even at distances greater than 10 kpc the masses of any associated molecular clumps are only a few hundred solar masses (472 in the furthest object in the sample; [GBH05] 6.55647-0.09042). This mass limit is close to the corresponding median mass of the detected sub-mm clumps in the sample; the median flux and distance in our sample of detected sub-mm clumps is 7 Jy and 5.2 kpc, corresponding to a median mass of 530 . It is possible that those sub-mm quiet objects with distances in excess of 10 kpc may be part of a relatively low clump-mass population of embedded UC H II regions.
If the nearby sub-mm quiet objects with distances less than 10 kpc are embedded within molecular cloud clumps then these clumps must possess masses of less than a few tens of solar masses. There is considerable uncertainty regarding the minimum mass cloud clump required to form a high mass star, some studies suggest star formation efficiencies of 5-10% (Clark et al. 2005) or 10-30% (Lada & Lada 2003, and references therein). One thus might naively expect that a 10 star should only form in a clump of at least 30-200 . Observational values for the mass of high-mass star-forming clumps are at the higher end of the scale, ranging from 720 (Mueller et al. 2002) to 104 (Hatchell et al. 2000). The masses of the sub-mm quiet objects are thus below the lower mass end of the observational scale and close to the minimum clump mass suggested by a simple consideration of star formation efficiency. We thus conclude that it is unlikely that sub-mm quiet objects nearer than 10 kpc are true UC H II regions, unless they are embedded within extremely low-mass clumps.
It is possible that the sub-mm quiet objects represent planetary nebulae, which are known to emit cm-wave radiation at the mJy level and possess relatively low-mass dust envelopes that could fall below our SCUBA detection limits. An alternate explanation is suggested by the fact that in a significant number of our SCUBA survey fields (10/32 fields) the sub-mm quiet sources are strongly clustered, particularly objects from the Giveon et al. (2005) survey. Moreover, the majority of these "clusters'' are often found at the peripheries of radio-loud or radio-quiet sub-mm clumps, for example G19.07-0.27 in panel 19L, G27.49+0.19 in panel 34L and G28.60-0.36 in panel 76L of Fig. 1. The Giveon et al. survey, like all of the VLA surveys described in this paper, is a snapshot interferometer survey with limited uv coverage that significantly limits the largest angular scale visible in the survey data. The bright components of "resolved-out'' large angular scale structures often appear as clusters of apparent point sources and it is possible that the clustered Giveon et al. (2005) objects trace bright knots within extended structures rather than clusters of UC H II regions.
|Figure 3: 850 m SCUBA images of the UC H II field G19.07-0.27 overlaid with countours of MSX 8 m emission ( top) and NVSS 20 cm emission ( bottom). Crosses indicate the position of ultracompact radio components identified from the WC89a, Becker et al. (1994) and Giveon et al. (2005) surveys.|
This hypothesis is bolstered by the fact that a number of the clustered Giveon et al. objects are correlated with extended MSX 8 m emission signifying PAH emission from the fringes of an extended H II region. As an example we present MSX 8 m and NRAO VLA Sky Survey (NVSS) 20 cm images of the SCUBA field G19.07-0.27 in Fig. 3. NVSS data were used by Kurtz et al. (1999) to identify extended continuum emission toward a small sample of UC H II regions, as the longer wavelength and more compact VLA configuration of NVSS considerably improves its sensitivity to large-scale structure. Figure 3 reveals that the sub-mm clump G19.077-0.289SMM lies at the eastern edge of an extended region of 8 m and 20 cm emission, which probably marks the position of an extended H II region. The cluster of Giveon et al. (2005) objects lies within this extended H II region and are more than likely bright components of the extended free-free emission of the H II region rather than individual UC H II regions.
The overall appearance of the G19.07-0.27 region in the sub-mm, mid-IR and radio is consistent with the hierarchical structure hypothesis of Kim & Koo (2001, 2003) for H II regions with both ultracompact and extended components. As seen in Fig. 3 the "sub-mm loud'' ultracompact radio source associated with G19.077-0.289SMM lies to the south-west of the sub-mm peak, on the same face as the extended mid-IR and radio emission traced by MSX and NVSS. It is thus possible that the sub-mm loud ultracompact component corresponds to the position of an embedded O-star(s) whose H II region is generally constrained by the high ambient clump density within the clump (and hence has an ultracompact component) but has broken free from the clump into the lower density region to the west to form a champagne flow H II region. It is unclear what fraction of the ultracompact cm-wave sources contained in the WC89a, KCW94 or Giveon et al. (2005) catalogues are extended objects similar to G19.07-0.27. Nevertheless the Kim & Koo (2001) hierarchical structure hypothesis offers a plausible explanation for these clustered sub-mm quiet objects. Further investigation of the UC H II region catalogues is necessary to determine the fraction of extended interlopers in the present UC H II region catalogues and whether the extended H II regions are consistent with the Kim & Koo model.
The sub-mm clumps that were detected in the survey split into two types; those that are positionally associated with ultracompact radio sources and those that are not associated with any detected ultracompact radio sources. The former are referred to as "radio-loud'' clumps and the latter as "radio-quiet'' clumps. We define positionally associated as the position of the radio source lying within the lowest 850 m contour bounding the clump (the lowest contour corresponds to the 3 level in each image, see Sect. 3.1). It is important to note that all of our target fields have been observed with the VLA as part of the WC89a, KCW94, Becker et al. (1994) or Giveon et al. (2005) surveys and so the radio-quiet clumps are truly radio-quiet (to within the flux limits set by the VLA surveys) rather than simply radio-unobserved. We discuss the likely individual nature of the radio-loud and radio-quiet clumps in more detail in Sects. 4.3 and 4.5 but in this section we will briefly dwell upon the properties of the population of sub-mm clumps as a whole.
The sub-millimetre wavelength radiation detected from the clumps originates from thermal emission from dust grains contained within the molecular gas of the clumps and is an excellent optically thin tracer of the column density within the clumps (e.g. Hildebrand 1983). The wavelength dependence of the sub-mm emission depends upon both the temperature of the dust and its emissivity coefficient . The standard procedure in the determination of the column density (or mass) of a sub-mm clump, its temperature and grain emissivity is to fit a greybody model to the broad spectrum SED of the clump (e.g. Dent et al. 1998). Usually a combination of IRAS far-infrared and millimetre/sub-millimetre measurements provide the SED, but for many of the clumps detected in our survey (particularly those SCUBA fields with multiple sub-mm components) the IRAS data is hopelessly confused due to the large beam at 100 m (2 FWHM).
|Figure 4: Sub-mm "colour-luminosity'' diagram. Radio-loud clumps are indicated by filled circles and radio-quiet clumps are shown by open circles.|
As part of Paper II we present greybody and radiative transfer analyses for those clumps that are not confused over the large scale of the IRAS beam, but here we present an investigation of the sub-mm properties of the wider sample as a whole. One of the standard techniques used to investigate the properties of large stellar samples is the colour-magnitude diagram. This approach is not a standard tool in the study of star-forming cores, but was recently adapted by Crowther & Conti (2003) to investigate the mid-infrared properties of UC H II regions. We adapt their technique for the sub-millimetre by using the distance-scaled 850 m flux of the clumps as a surrogate for the clump luminosity and the 850 m/ 450 m flux ratio for the sub-mm colour, renaming the approach as a colour-luminosity diagram to avoid confusion.
Assuming that clumps are optically thin at both 450 and 850 m, their sub-mm colour should depend solely upon the source-averaged dust temperature and grain emissivity. Thus by plotting a sub-mm colour-luminosity diagram we are able to investigate different regions in the dust temperature/emissivity parameter space as a function of clump luminosity. The sub-mm colour-luminosity diagram of the sub-mm clumps detected in the survey is shown in Fig. 4. Only those clumps with unambiguous distances are included in Fig. 4. As many of the radio-quiet clumps lack known kinematic distances we assume that radio-quiet clumps located within the same SCUBA field as a radio-loud clump with a known distance lie at that known distance (i.e. the radio-loud and radio-quiet clumps are part of the same complex).
Two insights are immediately apparent from the colour-luminosity diagram shown in Fig. 4. The first is that, in general, the radio-loud clumps tend to have a higher distance-scaled 850 m flux than the radio-quiet clumps. The median distance-scaled 850 m flux for radio-loud clumps is 3.8 Jy, compared to 1.2 Jy for the radio-quiet clumps. A Kolmogorov-Smirnov test also indicates that the two distance-scaled flux distributions are unlikely to be drawn from the same population, with a probability of only 2.5% for a single flux distribution hypothesis. This may imply that the radio-quiet clumps form a separate lower-luminosity population to the radio-loud UC H II region-containing clumps (note that the typical UC H II detection limit from WC89a is of a B0.5 star at 5 kpc). The radio-quietness of these clumps may perhaps thus be due to the fact that the clumps are forming lower-mass stars rather than high-mass protostars in a pre-UC H II region phase. It must however be cautioned that, as the radio-quiet sample is almost certainly comprised of a mixed population of massive star-forming clusters in a pre-UC H II phase and low- to intermediate-mass star-forming clusters, it is impossible to make conclusions about the nature of the entire sample of radio-quiet clumps from this data.
The second insight from the sub-mm colour-luminosity diagram is that both radio-quiet and radio-loud
clumps possess a similar range of sub-mm colours. This implies that all the clumps in the diagram
possess a similar range of temperatures and dust emissivities. The sub-mm properties of the
radio-quiet clumps closely resemble those of the radio-loud clumps, suggesting that the clumps form a
fairly homogenous population. It is intriguing to speculate that the main difference between the two
is the presence or absence of an UC H II region as this naturally gives rise to the implication that
the clumps are in different evolutionary states. However, the tendency for the radio-quiet clumps to
exhibit generally lower distance-scaled 850 m fluxes implies that the radio-quiet clump
population may also contain clumps that are forming lower-mass stars rather than high mass protostars
in a pre-UC H II region phase. We will dwell further upon the nature of the radio-loud and radio-quiet clumps
in Sects. 4.3 and 4.5.
We examined the morphology of the sub-mm clumps detected in the survey by measuring their half-power diameters along both major and minor axes. The major and minor axes of the clumps were initially estimated by Gaussian fits. It was found during the fitting procedure that a significant fraction of the clumps could not be accurately fitted by Gaussians and so for consistency all major and minor axes were directly estimated from the 50% contours of each clump. Due to the better signal-to-noise of the 850 m images we restrict the following discussion solely to the morphology of the clumps at this wavelength. The half-power diameters of the clumps are more sensitive to the distribution of the central regions of the clumps rather than the low-lying flux in their outer regions. To investigate the general extent of the sub-mm emission and compare this to the recent survey of candidate high-mass protostellar objects (HMPOs) of Williams et al. (2004) we also measured the 850 m flux Y-factor (the ratio of integrated to peak flux) for each clump in the survey.. We plot histograms of the clump elongation (a/b), and Y-factor ( ) in Figs. 5 and 6 respectively.
The majority of the sub-mm clumps display small elongations, with a median elongation of 1.4, suggesting a predominantly spherical or low axis-ratio population. This of course assumes that the clumps observed at 850 m represent single objects. This assumption seems to generally hold for the clumps in this survey: elongated clumps with high signal-to-noise 450 m data remain as elongated single structures in the higher resolution 450 m images (e.g. G33.13-0.09 in panel 43S) and there is no correlation of elongation with distance, as would be expected if the more distant clumps are unresolved linear chains or ridges.
We do not see any differences between the morphology of radio-loud or radio-quiet clumps, histograms of the elongation of each population resemble each other closely and each sample possesses a similar median elongation (1.3 and 1.5 for radio-loud and radio-quiet respectively). The radio-quiet clumps are marginally larger than the radio-loud clumps, with median major and minor axes of 30 and 19 for radio-quiet clumps as compared to 24 and 19 for the radio-loud clumps respectively.
The Y-factors of the sample as a whole (plotted in Fig. 6) follow a distribution that is peaked at a value of 3. As in the histograms of elongation there is little difference between the histograms and median values of the Y-factor of radio-loud and radio-quiet clumps. The median value of the Y-factor for all sub-mm clumps detected in the survey is close to the value for candidate HMPOs observed by Williams et al. (2004). This may indicate that, as the clumps span a range in distance from 1-14 kpc, they could possess a scale-free envelope structure as suggested for candidate HMPOs by Williams et al. (2004). As mentioned previously we do not see any evidence that the objects in our study fragment at higher resolution, unlike the objects of Williams et al. (2004). However absence of evidence is not evidence of absence and caution must be applied to the hypothesis that the sub-mm clumps possess a scale-free envelope structure. Nevertheless we may at least draw the qualitative conclusion that, like the candidate HMPOs of Williams et al. (2004), a significant fraction of the mass of the clumps lies outside the central beam. If the radio-loud clumps in our survey represent embedded UC H II regions which are more evolved than the candidate HMPOs of Williams et al. (2004) then we may also conclude that the fraction of the mass in the outer region of the clump compared to that in the central beam does not evolve appreciably between the HMPO and UC H II stages.
|Figure 6: Histogram of 850 m clump Y factor (integrated flux divided by peak flux). For an unresolved source the Y-factor is obviously equal to one.|
A substantial number of UC H II regions from the survey display emission from more than one sub-mm clump in the SCUBA field of view (for example G11.94-0.62 in panel 13L, G29.96-0.02 in panel 37L and G79.30+0.28 in panel 59L). Almost half of the SCUBA fields that were imaged during the survey (44/105) are associated with two or more sub-mm clumps. Many of these "neighbouring'' clumps have not been observed in suitable kinematic distance tracers and so it is not possible to determine whether these clumps are located in the same molecular cloud complex, or are a chance alignment of objects at different distances. Nevertheless the data that are so far available for individual cases (e.g. G34.26+0.15, Hunter et al. 1998; G10.30-0.15 and G29.96-0.02, Thompson et al. in preparation and G30.78-0.02, Motte et al. 2003) suggests that a number of the multiple clumps are clusters of objects found within the same molecular cloud complex. Several of the neighbouring sub-mm clumps are associated with star formation tracers: either UC H II regions (e.g. G28.29-0.36 in panel 36L) or H2O masers (e.g. G11.94-0.62 in panel 13L; Hofner & Churchwell 1996), although a considerable number are radio-quiet. For an assumed distance of 5 kpc each 2 diameter SCUBA fields represents a 3 pc diameter region, similar to the parsec-scale condensations within Giant Molecular Clouds (Blitz 1993). There is thus the important implication that these "neighbouring'' sub-mm clumps may represent other sites of star formation within the same molecular cloud complex containing the recently-formed YSO(s) exciting the UC H II region.
We quantify the likelihood of finding "neighbouring'' clumps within a SCUBA field using the
companion clump fraction (CCF), a statistic more often used to investigate the multiplicity of
stars, but which has been used to investigate the multiplicity of candidate
HMPOs (Williams et al. 2004). The CCF gives a measure of the average number of companions per sub-mm
clump and is given by the formula:
We also determined the surface density of the sub-mm clumps using the mean surface density of companions (MSDC) technique outlined in Williams et al. (2004), so that we could compare the surface density of clumps found toward more evolved UC H II regions to that measured for the relatively more isolated and potentially less evolved candidate HMPO clumps. We determined the MSDC by measuring the linear distance between pairs of clumps in the SCUBA images and scaling the number of linear distance pairs in annuli of fixed radius by the area of the distance annulus to obtain a value for the surface density. Clumps located in the same SCUBA field were assumed to lie at the same heliocentric distance. We present a plot of surface density versus linear separation in Fig. 7.
|Figure 7: The mean surface density of companions for all the 850 m clumps detected in the survey. Error bars are determined from simple Poisson counting statistics. Solid lines indicate linear least-squared fits to the first 5 and last 3 data points respectively.|
Figure 7 shows that the MSDC of the sub-mm clumps follows two power laws, with a break at 3 pc. At small linear separations the index of the power law is -1.6, close to the value of -1.7 found for candidate HMPO clumps by Williams et al. (2004). But for linear separations over 3 pc the MSDC declines steeply with a power law index of -4.1. The underlying cause behind this break is unclear; it is possibly due to incomplete sampling at large linear separations, but as the numbers of distance pairs in each distance bin are relatively constant apart from the very largest value we do not feel that this is a likely explananation. The decline in surface density over a radius of 3 pc may reflect a natural size scale for the extent of massive star-forming clusters in molecular clouds. If this is the case, one would also expect to see a similar effect in the candidate HMPO sub-mm clumps, particularly as the selection criteria of the HMPO sample select against radio-loud companions within several arcminutes (or several pc at the typical heliocentric distance of the HMPOs). This effect is not seen within the Williams et al. (2004) sample, whose surface density is consistent with a single unbroken power law of index -1.7.
Our sample contains objects at a wide range of heliocentric distances (and thus a large range of linear distance pairs) and is limited at both small linear distances by the angular resolution of the JCMT and at large linear distances by the limited field of view of SCUBA. Our flux-limited, rather than mass-limited, sample may also cause a bias towards detecting bright, relatively nearby clumps at large linear separations or may underestimate the number of faint companion clumps. It is thus possible that the break in the power law is caused by a selection bias in our sample to particular ranges of linear distances. However, our sample of clumps observed towards UC H II regions is very similar to that observed by Williams et al. (2004) in terms of heliocentric distance and both surveys have a corresponding 850 m flux limit. We thus think it unlikely that the different MSDC distributions are caused by underlying selection biases in each sample. Further wide-field observations of a large sample of massive star-forming regions are required to more accurately the determine surface density distribution of sub-mm clumps and investigate whether the apparent steep decline in surface density at large separations is indicative of a natural size scale for regions of massive star formation.
Here we examine those sub-mm clumps associated with ultracompact radio sources (the radio-loud clumps) in order to determine whether their observed properties are consistent with massive clumps containing embedded UC H II regions. As we have already demonstrated, those ultracompact radio sources that are not associated with sub-mm emission (sub-mm quiet objects) are unlikely to be young embedded UC H II regions unless they lie at considerable heliocentric distance (10 kpc). So, do the radio-loud sub-mm clumps fit the standard picture of young UC H II regions that are still embedded within their natal molecular cloud cores?
|Figure 8: Comparison between the radio to sub-mm ratio ( left: 850 m vs. 2 cm; right: 850 m vs. 6 cm) and the Lyman photon flux inferred from the radio free-free emission of the UC H II regions. Solid lines represent linear least-squared fits to the data. Particular outliers to the fits are indicated and are discussed in the text.|
The standard model of UC H II regions as discussed by e.g. Crowther & Conti (2003) or Churchwell (2002) is of a small and young H II region photoionised by one or more high-mass stars and surrounded by a massive optically thick cocoon of dust and gas. Clearly the bright sub-mm clumps observed in our survey fit the picture of massive envelopes of dust and gas with sufficient dust to be optically thick to short-wavelength emission, but is the sub-mm emission consistent with being reprocessed radiation from the embedded star(s) responsible for exciting UC H II regions? If this were the case then a clear relation ought to exist between the sub-mm luminosity of the clumps (which arises from the reprocessed stellar luminosity) and the cm-wave radio emission of the associated UC H II regions (which depends upon the UV luminosity of the stars). This relation has been shown to exist for a small number of UC H II regions from the WC89a and KCW94 catalogues by Crowther & Conti (2003) using VLA cm-wave and IRAS 100 m flux data. Crowther & Conti's approach was unfortunately limited by confusion within the large 100 m beam of IRAS. Using our SCUBA images we can almost entirely remove these confusion issues and considerably enlarge the sample of UC H II regions over that of Crowther & Conti (2003).
In Fig. 8 we present plots of the 850 m/cm-wave flux ratio against the number of Lyman continuum photons ( ) for each radio-loud clump detected in our survey whose heliocentric distance is both known and unaffected by a near/far kinematic ambiguity. Lyman fluxes were calculated from the integrated radio fluxes of ultracompact radio sources associated with the sub-mm clumps as published in WC89a, KCW94, Becker et al. 1994 or Giveon et al. 2005 using the standard equation relating free-free continuum brightness to the Lyman continuum flux (e.g. Carpenter et al. 1990) and assuming that each radio source represents an UC H II region. Where multiple ultracompact radio sources were found to be associated with a single sub-mm clump, the flux of each was summed to provide a total radio flux before evaluating the Lyman continuum flux. Diagrams are plotted for both 2 and 6 cm data, as many of the objects observed in the WC89a survey do not possess 6 cm data.
Caution must however be applied to the comparison of the derived Lyman flux from each VLA survey. The VLA surveys of WC89a, KCW94, Becker et al. (1994) and Giveon et al. (2005) were taken at different wavelengths and so all possess different selection effects, flux limits, synthesised beam sizes and sensitivity to extended structures. The 6 cm fluxes are mostly drawn from the sample of WC89a which was based on the single-dish point sources from Wink et al. (1982) with a 6 cm sensitivity of 0.5 Jy and is therefore intrinsically biased against weak 6 cm emitters. The IRAS-selected sample of KCW94 which appears in the 2 cm plot (Fig. 8a) is however only limited by the sensitivity of their VLA observations. This explains the lack of low sources at 6 cm in Fig. 8.
It is also possible that as the shorter wavelength data are less sensitive to extended structure (for the same uv coverage and array configuration) the Lyman flux calculated from fluxes measured at this wavelength may be lower than that calculated from longer-wavelength data. We illustrate this effect in Fig. 9, where the 850 m/2 cm flux ratio is plotted against distance. As can be seen in Fig. 9 there is a trend towards higher flux ratio values at smaller distances. Partly this is due to selection against faint radio sources at large distances, but this effect may also be caused by selection against nearby (i.e. within 2 kpc) UC H II regions with low flux ratios (and high ) or that the 2 cm fluxes of nearby UC H II regions are underestimated due to the extended flux sensitivity issue described earlier.
We caution against overinterpretation of Fig. 8 but note that Fig. 9 reveals that the bias toward high values of / only becomes marked for UC H II regions with / or s-1. It can thus be seen that the correlation between the sub-mm/radio flux ratio seen in Fig. 8 holds for all except low values of or high values of the sub-mm/cm-wave flux ratio.
|Figure 9: The 2 cm to 850 m flux ratio of UC H II regions as a function of distance. Note the trend toward higher values of at small distances which may indicate that the 2 cm fluxes of nearby UC H II regions are underestimated due to their over-resolution by the VLA.|
Bearing in mind the caveats discussed in the previous paragraph, both plots in Fig. 8 show a clear linear relation between the 850 m/radio flux ratio and the Lyman continuum flux of the embedded UC H II regions. This linear relationship extends over more than four orders of magnitude in the number of Lyman photons ( ) and almost three orders of magnitude in the 850 m/radio flux ratio. There are fewer objects with low Lyman flux in the 6 cm plot and the scatter is larger at 6 cm as compared to the 2 cm plot, but in general both plots agree closely. Figure 8 also shows that we do not see objects with either low and low 850 m flux or high and high 850 m flux. This is unlikely to be caused by any of the selection effects discussed earlier, as both the VLA surveys and our own SCUBA survey have sufficient sensitivity to detect these types of objects (if they exist) in these regions of the plot, either at low or at low 850 m flux. A power-law fit to the data in both diagrams (following the form y=A 10bx) yields values for A of and at 2 and 6 cm respectively; and b of and , with straight-line fit coefficients of 0.9 and 0.7 for 2 and 6 cm respectively.
Figure 8 clearly shows that the radio-loud sub-mm clumps in our survey are in excellent agreement with the standard model of embedded UC H II regions and that the clumps are predominantly heated by their embedded massive stars. Hereafter in this paper we will refer to these ultracompact radio components as UC H II regions. The scatter in both diagrams is likely to be caused by the selection effects mentioned earlier, the absorption of Lyman continuum photons within the UC H II regions by dust (which would move points leftwards and upwards) or by the presence of stars of lower mass that contribute to the total IR flux but do not contribute to the Lyman flux (which would move points vertically upwards). We indicate two specific clumps in particular whose outlying positions in Fig. 8 may be due to these effects (G25.650+1.049SMM and G12.908-0.261SMM). We also indicate the object G10.100+0.738SMM in Fig. 8, whose outlying position at the bottom of the diagram is unlikely to be caused by either selection effects, Lyman photon absorption or the presence of lower-mass stars. This object is almost certainly a misclassified UC H II region. Walsh et al. (2003) classify this object as a planetary nebula, noting that the position of the sub-mm clump coincides with the position of the well-known planetary nebula NGC 6537. The position of G10.100+0.738SMM in Fig. 8 confirms this hypothesis.
The positional association of ultracompact radio components and sub-mm clumps has already been discussed in Sect. 4.2 where we identified both radio-loud and radio-quiet clumps; and in Sect. 4.3 where we showed that "sub-mm loud'' radio components are strongly consistent with being UC H II regions. The angular resolution of our SCUBA images not only identifies whether the UC H II regions in our survey are associated with sub-mm clumps, but also serves to study the (projected) position of the cm-wave radio emission with respect to the peak of their associated sub-mm clump. The location of UC H II regions within their embedding clumps potentially allows us to investigate the characteristics of the birthplaces of the massive OB stars that excite the UC H II regions. For example, are the massive stars born within dense central cores within the clumps or are their birthsites offset from the density peak of the clump as suggested by the hierarchical model of Kim & Koo (2001)?
In Fig. 10 we present a histogram of the angular distance between UC H II regions and the peak position of the closest sub-mm clump. For comparison we also plot a similar histogram of the angular distance between methanol masers and sub-mm clumps from Walsh et al. (2003), which is shown as a shaded histogram. As in previous sections we have restricted our analysis to the higher signal-to-noise 850 m data. As can be seen in Fig. 10 the two angular distance distributions are markedly different; methanol masers follow a very tightly peaked distribution whereas the UC H II regions show a dip at small distances with a much broader overall distribution.
The tight correlation of masers with sub-mm peaks is evidenced by Walsh et al's detection statistics; all but one of the 84 masers surveyed was associated with sub-mm emission and 83% were found within 5 of a sub-mm peak (Walsh et al. 2003). Figure 10 shows that the FWHM of the methanol maser angular distance distribution is 5 . Methanol masers are thus predominantly found toward the peak positions of sub-mm clumps, which suggests that the masers trace deeply embedded star formation at or very near (at a median distance of 5 kpc, 5 corresponds to a projected linear distance of 0.1 pc) to the centre of the dense dusty clumps traced by the sub-mm emission. The dip in the ultracompact radio component distribution shows that the majority of UC H II regions are found approximately twice as far from the sub-mm peak positions as methanol masers, i.e. UC H II regions are more likely to be offset from the centre of their dense dusty embedding clumps. The width of the ultracompact radio distribution is also twice as large as that of the methanol maser distribution.
The observed dip in the UC H II region distribution is small compared to the FWHM of the JCMT beam, but is twice as large as the typical pointing accuracy that was achieved during the observations (5 ). We examined our data carefully to exclude systematic errors in the pointing by comparing histograms for sources that were obtained on different nights. These histograms all displayed a similar distribution, indicating that systematic pointing effects are not the cause of the observed dip. We also note that the Walsh et al. (2003) data do not display a similar distribution, even though they were obtained under similar conditions and with the same instrument. Finally, exactly the same distribution was observed from independent data: the angular distance between the UC H II positions determined by Walsh et al. (1998) as part of their ATCA maser survey and the sub-mm peaks observed by Walsh et al. (2003). We thus conclude that the difference between the methanol maser and ultracompact radio component angular distance distributions is likely to be a real effect. A similar tight correlation between the positions of methanol masers and their associated sub-mm clumps is reported by Beuther et al. (2002), who found an average linear separation of just 0.03 pc between the masers and their associated millimetre continuum peaks
|Figure 10: Histogram of the angular distance of the UC H II regions to the nearest sub-mm peak (unfilled histogram) plotted over a histogram of the angular distance of the methanol masers from Walsh et al. 2003 to their nearest sub-mm peak (shaded histogram). Note the difference in the two populations in terms of the central peak and the width of the distribution.|
Why are methanol masers and ultracompact H II regions found at predominantly different distances from the peak of their associated sub-mm clumps? One clue may lie in the different evolutionary states that are traced by methanol masers and UC H II regions. Simple models of maser formation (Walsh et al. 1998; Codella & Moscadelli 2000) suggest that methanol masers are initially formed in the dense molecular environments of luminous pre-ZAMS high-mass stars, radiatively pumped by the strong mid-IR emission from the deeply embedded stars, then are destroyed as an UC H II region forms and expands around the young high-mass star. If this is the case then methanol masers should trace younger objects than UC H II regions.
Discounting projection effects, which should be negligible in our large sample, we identify three possibilities to explain the relative absence of UC H II regions in the centres of sub-mm clumps:
The clearing hypothesis may provide a better answer. As UC H II regions expand they begin to clear away their surrounding gas and dust, ultimately resulting in the eventual destruction of the molecular cloud in which the high mass star was born. As the UC H II expands in its early stages the first region to be cleared should be the dense dusty core within which the high mass star was born. These cores are typically 0.1 pc in diameter (Kurtz et al. 2000) and would hence be unresolved by SCUBA. As this core is destroyed the peak position of the larger clump observed by SCUBA (which does not resolve the individual cores comprising the clump) would shift to the next most dense core (i.e. next highest column density) within the clump, effectively changing the UC H II/sub-mm peak distance. This effect may be complicated by the fact that the UC H II region will still be surrounded by a shell of warm dust, but as long as the column density of the next most dense core is sufficiently higher than that of the shell surrounding the UC H II, then the observed sub-mm peak position will be offset from that of the UC H II. Hence, the methanol maser phase of high-mass star formation is tightly correlated with the peak of the SCUBA sub-mm clump, then the apparent peak of the clump shifts as the UC H II develops and destroys the dense core within which it is embedded. The width of the dip in the UC H II region distribution yields an estimate of the clearing radius, i.e. 5 or 0.1 pc at 5 kpc.
The difficulty with this hypothesis rests upon whether the relatively small diameter UC H II regions can clear dust on these scales. The diameters of UC H II regions measured by WC89 and KCW94 are typically 2-4 , but more recent observational results show that UC H II can in fact possess much larger ionised regions on arcminute scales (e.g. Kurtz et al. 1999; Kim & Koo 2001; Ellingsen et al. 2005). The dust is more difficult to clear than gas, but the dust sublimation boundary of UC H II regions can be as much as 0.3-0.4 times the ionised gas radius initially and throughout the expansion phase (Inoue 2002; Arthur et al. 2004). In addition, the analysis of Franco et al. (2000) suggests that in strongly-peaked density distributions ( ) the ionisation front of the UC H II region undergoes a runaway expansion that expands at 0.5 pc/Myr or more. From modelling of clumps associated with UC H II regions we know that their density distributions are steeper than r-3/2 (Hatchell & van der Tak 2003) and so it is likely that the UC H II regions in these steep density distributions rapidly destroy and clear their dense birthplaces, shifting the observed SCUBA sub-mm peak to the next highest column density peak within the clump.
Corroborating observational evidence to this hypothesis may be found in high-resolution interferometric studies of UC H II regions and their molecular environment. The edges of UC H II regions are often truncated by the presence of nearby dense hot molecular cores, e.g. the well-known UC H II regions G29.96-0.02 and G9.62+0.19 (Cesaroni et al. 1994). Confirmation of the small effect uncovered in our SCUBA imaging using higher resolution millimetre or sub-millimetre interferometry is a priority to determine whether clearing by the UC H II regions is indeed responsible.
We have already shown (in Sect. 4.3) that the radio-loud clumps are strongly consistent with the standard model of embedded UC H II regions, but what is the likely nature of the radio-quiet clumps? A cursory inspection of Figs. 1 and 2 and Table 2 does not reveal any immediate differences between the fluxes or morphology of radio-quiet and radio-loud clumps, with perhaps the only difference that the radio-quiet clumps possess on average lower peak and integrated sub-mm fluxes than the radio-louds. There are six main possibilities for the nature of the radio-quiet clumps:
With the current data it is not possible to completely rule out scenarios in which the radio-quiet clumps contain embedded UC H II regions that were not detected in the VLA surveys of WC89a and KCW94. From the radio flux upper limits of WC89a and KCW94 (typically 0.4-1 mJy/beam at 2 cm) we may evaluate a lower limit to the radius of a detectable optically thick UC H II region using Eqs. (4) and (5) from Molinari et al. (2000). Assuming an upper limit to the 2 cm flux of 1 mJy and a synthesised beam FWHM of 0 5, we derive a lower limit to the detectable radius of an UC H II of pc, where d is the distance to the UC H II region in pc.
Unfortunately, the distances to many of the radio-quiet clumps are highly uncertain, as there is often no kinematic tracer (radio-recombination line or maser) associated with the clump. If we assume that the radio-quiet clumps are at the same typical distance as those clumps containing UC H II regions then from Table 1, we expect the upper bound on the distance to be 14 kpc, at which distance the VLA surveys are only sensitive to UC H II regions pc. For a more fiducial distance of 5 kpc this upper limit falls to pc. In comparison, the initial Strömgren radius for an O6 star surrounded by a pure hydrogen nebula of density cm-3 is 10-3 pc (de Pree et al. 1995).
Assuming all the radio-quiet clumps are populated by UC H II regions at or above their dust-free Strömgen radius, the 2 cm upper limits rule out essentially all O stars and B0 stars in all but the furthest 20% of sources. Later-type B stars are not ruled out: B0.5 stars in all but the 20% closest sources, and later-type B stars at all survey distances, would fall below the 2 cm flux limit. Of course, the time to produce a Strömgren sphere is very short and UC H II regions subsequently expand, making them more observable, and although the expansion is inhibited over that expected by simple pressure arguments, WC89a see few sources at their initial Strömgren radii. On the other hand, both dust absorption and higher densities reduce the detectability by reducing the Strömgren radius. In the case of 90% UV absorption by dust, we would also fail to detect O9.5 stars in the more distant half of the sample. The closer the radio-quiet clump, the higher the probability of containing no UC H II or a very young and therefore compact UC H II. We thus conclude that it is possible for the radio-quiet clumps to contain late O or early B-type stars. A wider investigation of the kinematic distances of the radio-quiet clumps is required to reveal their potential to host massive stars or undetected UC H II regions.
Due to the fact that interferometers such as the VLA screen out extended radio emission, the converse that some H II regions are too extended to be detected is also true. The largest angular scales that the WC89a and KCW94 surveys were sensitive to is 10 at 2 cm. Because both of these surveys were carried out in snapshot mode, the sparse uv coverage of the resulting data means that the largest angular scale should be considered to be an upper limit. H II regions larger than 10 with a smooth surface brightness distribution would not have been detected, although bright components of clumpy H II regions would possibly have been detected as a "cluster'' of UC H II regions. This effect may be seen in some of the KCW94 UC H II regions, which merge to form a larger H II region in more compact VLA configuration images (Kurtz et al. 1999), or those clustered objects in the Giveon et al. (2005) catalogue which were discussed in more detail in Sect. 4.1. However, it is unlikely that all the radio-quiet clumps are associated with extended H II regions and in any case these objects inform us as to the prior massive star formation that has taken place in the region, not deeply embedded star-formation that may be currently taking place. These radio-quiet clumps fall into categories v) or vi) below.
The Jeans mass for a 50 K, molecular cloud is about 80 , which produces a 850 m flux of at 5 kpc. On this basis, a few weak, nearby, radio-quiet clumps could be unbound (about 15% of the sample), but the remaining 85% fall above this limit.
It is more difficult to rule out the presence of low-mass star forming clusters in our sample, particularly for the less luminous objects. Two examples of low-mass star forming clusters are NGC 1333 and L1448, which were recently observed at 850 m as part of a survey of the Perseus molecular cloud (Hatchell et al. 2005). Both NGC 1333 and L1448 contain several hundred of dust and gas within a few pc2, and their resulting 850 m fluxes are of the order 200-300 Jy. This implies that clusters of this type could have been detected out to a distance of 5.5 kpc in our survey (assuming a typical detection limit of <1 Jy at 850 m). We thus conclude that the weaker radio-quiet clumps detected in our survey may be forming clusters of low-mass stars. It is difficult to point to specific examples as many of the radio-quiet clumps do not possess measured kinematic distances. Follow-up observations to determine the distance (and hence luminosity) of these objects are a priority.
Of all of these possibilities the last two are by far the most intriguing, as they imply that the radio-quiet clumps may contain a population of still-accreting massive stars with newly-formed UC H II, and the long sought-after massive protostellar precursors to UC H II regions.
With the current body of observational data it is not possible to completely determine the nature of the radio-quiet clumps. A full investigation of the SCUBA-detected clumps near UC H II, using molecular line observations to probe the temperatures, chemical composition, structure and dynamics of the cores, will be reported at a later date.
We present the results of a sub-mm continuum imaging survey of UC H II regions, performed with the SCUBA bolometer array on the JCMT. A total of 105 IRAS sources from the UC H II region catalogues of Wood & Churchwell (1989a) and Kurtz et al. (1994) were mapped at 450 and 850 m using SCUBA's jiggle-mapping mode. We detected 155 sub-mm clumps within the SCUBA images and identify three kinds of object within our survey: "sub-mm-quiet objects'' which are ultracompact cm-wave radio sources that are not associated with sub-mm emission; "radio-loud'' sub-mm clumps which are associated with ultracompact cm-wave sources; and "radio-quiet'' sub-mm clumps which are not associated with detectable cm-wave emission. A number of IRAS point sources (14 in total) were found to exhibit no emission at 850 m greater than 0.1-0.3 Jy/beam. We draw the following conclusions from our survey:
The authors would like to thank all of the anonymous visiting observers and JCMT support staff who undertook the flexibly scheduled observations. We would also like to thank the referee for the many useful suggestions and careful reading of the paper. This research made use of data products from the Midcourse Space Experiment obtained from the NASA/ IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.. MSX data processing was funded by the Ballistic Missile Defense Organization with additional support from NASA Office of Space Science. This research would not have been possible without the SIMBAD astronomical database service operated at CCDS, Strasbourg, France and the NASA Astrophysics Data System Bibliographic Services. JH is supported at Exeter by a PPARC AF.
|UCH II field||Source||RA (J2000)||Dec (J2000)||Peak flux (Jy/beam)||Integrated flux density (Jy)|
|(h min s)||( )||450 m||850 m||450 m||850 m|
(a) A noisy bolometer clipped from the final rebinned image is located close to this source and thus the measured flux is a lower limit to the true
(b) These sources are separable in the unsmoothed 450 m image but blended in the 850 m image. The quoted 850 m flux is that of the blended single source. The coordinates quoted are measured from the 450 m image.
(c) These sources are separable in the 850 m images but blended into one source in the smoothed 450 m images. The quoted 450 m flux is the flux of the blended single source.
(d) A noisy bolometer clipped from the final rebinned 850 m image lies over this source and it was not possible to masure any sensible peak or integrated 850 m flux.
(e) This sources lies on the edge of the field of view and thus one or more of the quoted fluxes is a lower limit to the true flux.
|Figure 1: SCUBA images from the survey with detections at both 450 and 850 m. Each UC H II region is represented by a pair of images at 450 m (left image) and 850 m (right image). Coordinates are Right Ascension and Declination in the J2000 system. Crosses indicate the positions of ultracompact H II regions from Wood & Churchwell (1989a), Kurtz et al. (1994), Becker et al. (1994) or Giveon et al. (2005). All images have been deconvolved with a model of the JCMT beam to remove the contribution from the error lobe and 450 m images with limited signal-to-noise have been smoothed to the same resolution as the 850 m images to improve the source detections.|
|Figure 2: SCUBA images of the ultracompact H II regions in the survey with detections at 850 m only. Each UC H II region is represented by a pair of images at 450 m (left image) and 850 m (right image). Coordinates are Right Ascension and Declination in the J2000 system. Crosses indicate the positions of ultracompact H II regions from Wood & Churchwell (1989a), Kurtz et al. (1994), Becker et al. (1994) or Giveon et al. (2005). All images have been deconvolved with a model of the JCMT beam to remove the contribution from the error lobe.|