Free Access
Issue
A&A
Volume 545, September 2012
Article Number A145
Number of page(s) 9
Section Galactic structure, stellar clusters and populations
DOI https://doi.org/10.1051/0004-6361/201118665
Published online 24 September 2012

© ESO, 2012

1. Introduction

Star formation occurs in the hearts of cold molecular clouds, where young stars begin their lives shrouded in the envelopes of cold dust and gas from which they condense. As the protostar evolves infalling material forms a circumstellar disc from which planetary systems may later form. During these earliest stages, most of the emission from these protostars is in the form of reprocessed stellar radiation emitted at longer, far-infrared (far-IR) wavelengths. These very earliest stages of star formation have remained hidden from us until the advent of modern infrared charge-coupled devices and bolometric arrays. The Herschel Space Observatory (Pilbratt et al. 2010) provides an unprecedented view of the earliest stages of star formation. The PACS (Poglitsch et al. 2010) and SPIRE (Griffin et al. 2010) instruments provide the capability for large scale mapping of star forming regions in the far-IR in relatively short amounts of time. One of the goals of the Herschel Gould Belt survey (André et al. 2010) is to obtain a census of prestellar cores and protostars in nearby clouds.

Chamaeleon I (hereafter Cha I) is a nearby region of low mass star formation, located at a distance of 160–180 pc (Whittet et al. 1997, and references therein), in a complex which also contains Chamaeleon II & III and Musca (Luhman 2008). The entire Cha I cloud covers an area of  ~5 deg2 and, at this distance, is one of the closest regions of star formation to the Earth. From spectroscopic surveys of confirmed young stellar objects (YSOs), the age of the cluster has been found to be  ~2 Myr (Luhman 2008, and references therein). Previously, the region has been observed from the optical to the radio by Spitzer, SEST, ISOCAM, 2MASS, and DENIS, among others (Luhman et al. 2008; Haikala et al. 2005; Persi et al. 2000; Gomez & Kenyon 2001; Cambresy et al. 1998). Extinction maps towards the cloud give values of AV in the range 5–20 mag, implying that even the protostars are not too deeply embedded, giving us an unimpeded view of the entire population of the region. The cloud is separated into two main parts: the northern cluster centred on the Herbig star HD 97300 and the reflection nebula Ced 112 (Cederblad 1946), and the southern clusters including two reflection nebulae Ced 110 and Ced 111 and the Infrared Nebula (Schwartz & Henize 1983, IRN). In his article in the Handbook of Star Forming Regions Luhman (2008) provides an in-depth and detailed review of the entire Chamaeleon complex and the population of YSOs within.

In this initial paper on the Chamaeleon complex, we present the first results of Herschel Gould Belt survey of Cha I, detailing the data reduction, map making process, and source extraction. We then present a comparison of the Herschel detected objects with those in previous catalogues, primarily focusing on the recent Spitzer surveys of the cloud (Luhman et al. 2008; Kim et al. 2009; Manoj et al. 2011), which provide reliable evolutionary classifications for the known members. Finally we discuss the mid- and far-IR properties of the young stars detected with Herschel by evolutionary class.

2. Observations and data reduction

The Cha I region was observed by Herschel as part of the Gould Belt survey (André et al. 2010). The observations (OBSIDS 1342213178, 1342213179) were performed using the parallel mode covering an area of  ~5.4 deg2 for both PACS (at 70 and 160 μm) and SPIRE (at 250, 350 and 500 μm) instruments, with a scanning speed of 60′′ s-1. The field was observed twice by performing cross-linked scans in two nearly orthogonal scan directions. The total observing time was  ~8 h, each observation being  ~4 h. The observing date was the 22nd of January 2011 for both observing directions. The full width at half maximum of the point-spread-functions (PSFs) for PACS in the parallel mode at 70 and 160 μm are 5.8′′  ×  12′′ and 11.6′′  ×  15.4′′, respectively. Beam sizes are 18.1′′, 25.2′′, and 36.9′′, for 250, 350, and 500 μm, respectively for SPIRE.

The data were reduced using the Herschel Interactive Processing Environment (HIPE, Ott 2010) version 7 using both the photProject and madMap algorithms for PACS observations, and naiveMap algorithm for SPIRE. However, the pipelines contained in this version produced some problems due to the characteristics of this kind of star forming regions: the baseline-removal part of the mapping algorithm produced negative flux values (overshooting) in some regions located next to the bright filamentary structures in the scan directions. For this reason we decided to use scanamorphos (Roussel et al. 2012) version 12, a map-making software designed to take advantage of the redundancy of the observations. This procedure allowed us to get rid of the negative flux values and the corresponding artifacts in the maps, making the overall photometric measurements more reliable when compared to the output from the HIPE maps (both for the PACS and SPIRE instruments). Scanamorphos maps suffer much less from areas of overshooting, in fact these features were no longer present. Checking for consistency between the photometry from HIPE and Scanamorphos maps for clearly detected sources unaffected by artifacts (>100 mJy) in both maps, we find no relevant differences in the photometry.

2.1. Source extraction

Source extraction was performed using getsources (Men’shchikov et al. 2010, 2012), which is designed specifically for use with Herschel data. The mosaics are prepared using prepareobs, part of the getsources package, which resamples the mosaics to a 3′′ pixel scale. Unlike other source extraction algorithms, the new method analyzes fine spatial decompositions of original images across a wide range of scales and across all wavelengths. As part of its multi-wavelength design, getsources removes the noise and background fluctuations from the decomposed images separately in each band, and constructs a set of wavelength-independent detection images that preserve information in both spatial and wavelength dimensions as well as possible. Sources are detected in the combined detection images by following the evolution of their segmentation masks across all spatial scales. Measurements of the source properties are performed in the original images at each wavelength after the background has been subtracted by interpolation under the sources’ “footprints” and after overlapping sources have been deblended. Based on the results of the initial extraction, detection images are “flattened” to produce much more uniform noise and background fluctuations in preparation for the second, final extraction. A more detailed description of the getsources software is provided in the forthcoming paper by Men’shchikov et al. (2012).

The final merged source catalogue provided 397 detections at the five available bandpasses of PACS and SPIRE over the whole Cha I field of the initial getsources extraction run with a combined significance across all detected bandpasses >7σ, where 347 have detections in more than one band. No constraints were placed a priori on the source detection, such as for source size, minimum flux level, or detection at multiple wavelengths, to ensure that no known source was missed. The fluxes returned by getsources include the peak flux of each detection and the total flux within an ellipse determining its extent on the sky at each specific wavelength. In this study, we use the measure of the total flux for each object detected. Of the 306 sources with detections at 250 μm, 49 have major axis  >30′′, greater than 1.65  ×  FWHM(PSF) at 250 μm. The uncertainties were also derived by the getsources routine. Typical uncertainties on the fluxes at each wavelength are less than 20%, with  ~75% having uncertainties less than 40% of the total measured flux. For this paper the lower limit for reporting fluxes was set at  ~0.1 Jy for each of the PACS 70, 160 μm, and SPIRE 250, 350, 500 μm bands. Twenty-six of the sources were detected in all five available bands, with forty-one detected at all four bands from 160 to 500 μm, and two hundred only detected in some bands longward of 160 μm. The remaining 130 sources are detected only in bands shortward of 250 μm.

thumbnail Fig. 1

Three-band false-colour image of the Chamaeleon I region, with PACS 160 μm in blue, SPIRE 250 μm in green and SPIRE 500 μm in red. The northern region, centred on Ced 112 and HD 97300, and two southern clusters, Ced 110 and Ced111 are visible. North is up and east is to the left.

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3. Results

3.1. Cross correlation with other catalogues

The Cha I region, given its close proximity to us, has been extensively studied at other wavelengths over the decades. To date, more than 200 young stars have been identified as members of the Cha I cloud. Previous studies using X-ray, optical, near-IR, mid-IR, and sub-mm instruments have provided a comprehensive view of young members from the young protostars (class I) to the evolved diskless members (class III). Herschel provides some of the highest resolution observations to date in the far-IR to submm; from these data we can identify the protostars (class 0 and class I), prestellar cores, and the emission from the remnant envelopes and cold outer disks of the more evolved class II young stars. Figure 1 shows the three-band false-colour image of a Herschel PACS and SPIRE view of Cha I, where the PACS 160 μm band is shown in blue, SPIRE 250 μm in green, and SPIRE 500 μm in red. The three Cederblad regions are marked on the image. While some of the emission at the shorter wavelengths arises from stellar sources, the cold dust emits strongly at 500 μm, where the underlying structure of the cloud is clearly observed.

To correctly identify the Herschel sources we mapped the detections and matched sources to those at shorter wavelengths where the positional offsets were less than 5′′. Given the lower positional accuracy in the Herschel astrometry due to the larger beam size of Herschel when compared to those at shorter wavelengths, we did not rely on blind matching only, but visually checked each object against the positions of objects in the Spitzer, ISOCAM, DENIS, and 2MASS catalogues and utilised the online Aladin software to compare the Herschel sources to the DSS optical images and the SIMBAD catalogue.

Luhman (2008) presents a review of previous surveys of the region, where they confirm a total of 237 YSOs associated with Cha I. Luhman et al. (2008) present a Spitzer IRAC and MIPS survey of Cha I where they report the detection of 204 YSOs in the mid-IR.These YSOs were classified using the slope of their spectral energy distributions (SEDs) and their positions on various colour–colour diagrams. Of these 204 YSOs, they found five class I (two in the northern, three in the southern groupings), ten flat spectrum (two in the north, eight in the south), ninety-four class II (43 north, 51 south), and ninety-five class III (44 north, 51 south). Of the 397 Herschel sources, a total of forty-nine, are matched to YSOs identified in the Luhman (2008) catalogue.Of these 49, four are class I (one in the north, three in the south) and one, Cha MMS1 is very young class 0 object (Reipurth et al. 1996), six are flat spectrum (one in the north, five in the south), thirty-four are in evolutionary class II, and three are considered to be transition disc objects: CS Cha, SZ Cha, and T54. The remaining source is Ced 110 IRS2, a class III, diskless member of Cha I, which was also identified in the Herschel data, indicating that it retains circumstellar material in an outer disk.

Recently, Belloche et al. (2011) presented a LABOCA 870 μm map of the Cha I cloud, and reported the detection of 59 starless cores and 21 sources associated with YSOs. When compared to the Herschel detections, we find 51 sources in common between the two catalogues, with an additional fourteen possible matches where the offset is greater than 25′′. Of the fifty-one common detections, all 21 of the Belloche detections associated with known YSOs are also detected with Herschel, two are associated with the ring of nebular emission surrounding HD 97300 (Kóspál et al. 2012) and the remaining twenty-eight are associated with candidate prestellar cores which lack near or mid-IR detections. We likely do not find counterparts for the other twenty-nine 870 μm detections due to different source extraction algorithms, the difference in the resolution between the catalogues, and the requirements for greater than 7-σ detections in the Herschel catalogue.

Haikala et al. (2005) used the SEST telescope to observe the Cha I region in C18O, 13CO, and C17O. They identified 71 clumps, to which we find 9 counterparts in the Herschel dataset with positional accuracy  <25′′. Four of these appear to correspond to Belloche et al. (2011) sources, adding five new candidates to the list of Herschel sources.

The region has been observed in X-rays with ROSAT by Lawson et al. (1996), with Chandra by Feigelson & Lawson (2004) and with XMM by Stelzer et al. (2004) to look for high energy emission from young stars with the purpose of studying their X-ray properties and to identify any class III diskless members of the cluster, which do not exhibit excess emission in the IR, but are bright in X-rays (Guenther et al. 2007) with emission 103–104 times their main sequence counterparts. Combining all three surveys, twenty-one of the 49 Herschel and Spitzer detected YSOs have counterparts in the X-ray catalogues.

Persi et al. (2000) presented a mid-IR study of Cha I cloud using ISOCAM on the Infrared Space Observatory (ISO). In the paper they reported the detection of 108 young stars, mainly class II, in two broad band filters at 6.7 μm and 14.3 μm covering a 0.5 square degree field of the cloud. Of the 108 young members detected with ISOCAM, 28 have counterparts detected in the Herschel field.

A survey of 82 young stars and brown dwarfs in Cha I was carried out by Manoj et al. (2011) using the Spitzer Infrared Spectrograph (IRS) mid-IR spectrograph. Their Table 3 shows that of the 49 known YSOs in our Herschel sample, sixteen are known to be in multiple systems. They also updated the evolutionary classification of the previously known YSOs and found that many of the disks show evidence of dust settling and evolution in the mid-IR. Kim et al. (2009) presented a Spitzer IRS survey of a number of transition disc candidates in Cha I, concluding SZ Cha, CS Cha, and T54 to be transition disks.

3.2. Herschel catalogue of known objects

The final getsources catalogue for the region included 397 sources with detections in one or more of the five available Herschel bandpasses. Having compared the Herschel catalogue to those at all other available wavelengths, we match 86 of the 397 Herschel detections to known sources. This includes 49 YSOs, two outflow features, two emission features near HD 97300, discussed in detail in Kóspál et al. (2012), 28 candidate prestellar/starless cores (Belloche et al. 2011), and five CO clumps (Haikala et al. 2005). Table 1 presents the identifications, evolutionary classification, the Herschel fluxes, and the spectral types for the identified YSOs in Cha I. The remaining 311 objects were not previously identified as young stars, emission features or cores in the cluster. The source extractions are preliminary at this point, hence the details of those sources not identified as a previously known YSO are not given here, but will be presented in a later publication, pending a more complete analysis of their SEDs. These may be deeply embedded protostars, prestellar cores, or condensations of dust in the filamentary structure of the cloud. These detections may also be background extragalactic contaminants. It is likely that the majority of the point-like sources that are located off-cloud in regions of lower AV are galaxies. Clements et al. (2010) estimate the number counts of galaxies in the Herschel SPIRE 250 μm field as 10–20 per sq. deg. at 100 mJy, rising to  ~100 galaxies per sq. deg. at 50 mJy; given the  ~5.4 sq. deg. coverage of the Cha I cloud and our sensitivity limits at  ~100 mJy, we could therefore expect a few hundred galaxies in the field. This number may be further reduced by the interstellar extinction over much of the observed field. In this paper, only the photometry of the previously known sources will be included in the discussion of the Herschel photometry, where a general comparison will be made between the known young stars and candidate cores.

While the northern region, Ced 112, contains a smaller population than the southern one, Ced 110 and 111, the two regions possess similar properties statistically, and this holds true with the inclusion of the Herschel data. Comparing to the Luhman et al. (2008) catalogue, we detect all but one of the known class I protostars with Herschel. The nondetected class I is seen only faintly, and it was not reliably separated from the cloud emission. The detection fraction with evolutionary class falls off rapidly; with 1 of 2 flat spectrum sources in the north, and 5 of 8 (63%) in the southern region detected. For the class II sources, which are distributed more evenly throughout the region, the overall detection fraction drops to 34%. We detect only one of the known class III sources over the whole cloud and only three transition disks. There is no difference in the fractions of previously known sources seen with Herschel in the northern Ced 112 region compared to the southern, Ced 110 and 111, region.

Table 1

Identifiers and fluxes of Herschel detected YSOs in Cha I.

4. Discussion

4.1. Spitzer colours of Herschel detected YSOs

Figure 2a shows the traditional Spitzer IRAC colour–colour figure (ccd) of the known YSOs in Chamaeleon I, while Fig. 2b shows the Spitzer IRAC-MIPS ccd of those YSOs. The grey dots are the members listed in Luhman et al. (2008), the black squares are those YSOs also detected with Herschel. The fluxes are plotted without dereddening of the IRAC data. There is little discernible difference in colour between those young stars detected by Herschel and those that are not, with no obvious trend towards redder IRAC colours discernible. This would imply that the extent and/or structure of the inner  ~1–5 AU disk, detected in the mid-IR with IRAC, does not have a strong bearing on whether or not an outer disc (10 to  > 100 AU) or remnant envelope will be detected at the longer Herschel wavelengths. Only one source is detected in the class III/transition disc region, the transition disc CS Cha. Similarly, only one of the class III sources is detected by Herschel.

Figure 3 shows the [3.6] vs. [3.6−4.5] colour–magnitude figure (cmd) of the Spitzer YSOs with the Herschel sources as above. Here it can clearly be seen that it is the brighter objects that have been detected with Herschel. From a total number of 150 YSOs on this cmd, 20/25 or 80% are detected with Herschel for m3.6 < 8.5, 26/61 or 43% are detected with Herschel for m3.6 < 10, and 4/89 or 4% are detected for m3.6 > 10. The Herschel detections are complete for objects with [3.6−4.5] colour  > 0.6 and m3.6 < 10.

In order to investigate further, the values of α2 − 8, the slope of the SED calculated between the KS band and the IRAC 8μm wavelength, of both the Herschel detected class II and the class II members not detected by Herschel were examined. The value of α2 − 8 is interpreted as a measure of the amount of infrared excess emission a star possesses; positive values indicating a protostar with emission from an envelope and negative values  > –2 indicating a class II young star. The α2−8 values utilised here were those presented in Table 8 of Luhman et al. (2008). The median α2−8 value of the class II sources detected with Herschel is –1.21  ±  0.77. The median α2−8 of the class II sources not detected with Herschel is –1.44  ±  0.55. The uncertainties presented are the standard deviation of the α2−8 values.These results are statistically similar; there is no suggestion that the sources with stronger emission from their inner disks are more likely to be detected than those with weaker emission.

thumbnail Fig. 2

Above: Spitzer IRAC [3.6–4.5] vs. [5.8–8.0] colour–colour figure derived from the Luhman et al. (2008) photometry. Below: Spitzer IRAC-MIPS [3.6–4.5] vs. [8.0–24] colour–colour figure of the Luhman et al. (2008) YSOs. Grey dots indicate the Luhman et al. (2008) identified YSOs. The black symbols indicate those YSOs detected with Herschel. The group of YSOs clustered around [0,0] in both diagrams are the class III sources.

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thumbnail Fig. 3

Spitzer IRAC [3.6] vs. [3.6–4.5] colour–magnitude figure derived from the Luhman et al. (2008) photometry, with grey dots indicating the Luhman et al. (2008) identified YSOs and black squares indicating those YSOs detected with Herschel. Only those YSOs with m3.6   μm brighter than 10 are likely to be detected with Herschel. The Herschel detected sample is complete for sources with [3.6–4.5] colour  > 0.6 and m3.6 < 10.

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thumbnail Fig. 4

Contours of the SPIRE 500 μm mosaic, showing the elongated structure of the dust, shown at levels of 30, 40, 45, 50, 55, 60, 65% of the peak flux 13.4 Jy/beam. The open circles indicate the positions of the Spitzer identified YSOs (Luhman et al. 2008), the filled circles are those YSOs also detected with Herschel. The “x” symbols mark the positions of the Herschel sources matched to Belloche et al. (2011) detections; excluding three which are off-field to the west. It may be noted that the Herschel detected YSOs are mainly located in and near the centres of the three clusters. North is up and east is to the left, coordinates are given in Equ J2000.

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4.2. Clustering of Herschel detected YSOs

Figure 4 shows the contours of the 500 μm SPIRE mosaic in grey, which traces the densities of cold dust in the cloud. Overlaid on the contours are the positions of the Luhman et al. (2008) YSOs detected in the Spitzer IRAC and MIPS observations, shown by the open circles. The filled circles are those of the YSOs that are also detected at Herschel wavelengths. The “x” symbols mark the locations of the twenty-nine Herschel detections with a counterpart in the Belloche et al. (2011) LABOCA catalogue. These objects include candidate class 0 protostars and prestellar or starless cores; they are located either in one of the three main Cederblad groups or are adjacent to other Spitzer identified YSOs. Further modelling of the SEDs of these sources will provide a better understanding of their nature, and will be presented in a later paper. The majority of the previously identified YSOs that are detected with Herschel are found to be clustered in the core of the northern region surrounding HD97300/Ced 112, and in the two clusters Ced 110 and Ced 111 to the south. The YSOs detected with Herschelare more likely to be contiguous with the denser regions of cold dust traced by the 500 μm emission than those that are not. The percentage of Herschel detected YSOs within the 50% contour in Fig. 4 is 60% (29/49), the percentage of YSOs not detected with Herschel within the 50% contour is 28% (52/183). In this regard, Cha I is similar to other clusters, such as Aquila (André et al. 2010) and IC 5146 (Arzoumanian et al. 2011) where the sources have been found to be contiguous with the filaments in the clouds; future work on the filamentary structure of the Chamaeleon clouds will indicate whether the young stars also trace the filamentary structure of Cha I (Alves de Oliveira et al., in prep.).

The YSOs detected with Herschel are also the brightest YSOs in the mid-IR as shown in Fig. 3. This may at first suggest that these are the more massive stars with correspondingly more massive outer disks and are therefore detected in Herschel due to their increased emission. However, from their spectral types, listed in Table 1, these are mostly M and K type stars. Specifically, all of the Herschel detected YSOs in the northern Ced 112 cluster surrounding HD 97300 have spectral type mid to late M. The southern population is more distributed but the majority are still M-K stars. Therefore, it is more likely that these stars represent the youngest segment of the Cha I population and still retain thick outer disks and/or envelopes.

4.3. Herschel fluxes and colours of detected sources

Figure 5 shows the Herschel log (F160) vs. log (F160/F250) flux-ratio diagram, where the black circles indicate the known class I objects, the asterisks identify the flat spectrum objects, the grey triangles mark the known class II sources, the inverted triangles indicate the transition disc members, and the open squares mark the positions of those Herschel sources also identified by Belloche et al. (2011) at 870 μm. While the difference in 160 μm flux between the three groups is not highly significant, the class II sources extend to fainter fluxes than the protostars or candidate cores. There is a distinct colour difference between the known YSOs and the Belloche identified Herschel objects. The YSOs have a bluer colour at log (F160/F250) than the cores, where the median colour of the class II sample was found to be and the median colour of the candidate cores was found to be . Though the difference in median colour is only significant to 1-σ, it is indicative of a trend towards bluer log (F160/F250) ratios for the more evolved young stars. The median fluxes for the protostars (class I and flat spectrum), class II, and candidate core detections at each of the five Herschel wavelengths are presented in Table 2. Table 3 lists the median colours for each class of object for the three colours shown in Figs. 5–7. In both tables the uncertainties presented give the interquartile range: [upper quartile – median] and [median – lower quartile], where the lower quartile is the value below which 25% of the sample fall, the upper quartile is the value below which 75% of the sample lie, and the interquartile range is the range of values which encompasses the central 50% of the sample.

Figures 6 and 7 show two Herschel colour–colour ratio diagrams: log (F160/F250) vs. log (F250/F350) and log (F160/F250) vs. log (F350/F500) where the symbols are the same as in Fig. 5. Again, the trend in median colour in log (F160/F250) can be clearly seen. There are no trends in the median colours between the known YSOs and the candidate cores at either the log (F250/F350) or log (F350/F500) colours. The range in colours is similar for the young stars and the candidate prestellar cores, though the candidate cores do extend towards redder log (F350/F500) colours, which is expected of more deeply embedded and younger objects, which are expected to be colder and to emit at longer wavelengths than the more evolved young stars.

Another comparison that can be made is that between the colours and fluxes of the identified protostellar class I/flat spectrum objects and the more evolved class II young stars. Our investigation indicates that there is some difference in the fluxes but not the colours between the protostellar and class II members of the cluster detected by Herschel. The protostars have brighter median fluxes at the four shorter wavelengths than the class II objects. Neither group have a significant peak in their median fluxes across the five Herschel bands: the median fluxes of the class II sources appear to be flat across the five wavelengths, while the protostars show some indication of a decrease in flux with increasing wavelength. There is no significant difference in colour between the protostellar and class II sources, though the class II show consistently redder median colours than the protostars.

Table 2

Median fluxes [Jy] of the Herschel detected sources in Cha I as a function of object type.

Table 3

Median colour of the Herschel detected sources in Cha I as a function of object type.

One Herschel detection is of particular interest: the variable star CS Cha, which exhibits IRAC colours similar to that of a field star, indicating that it has an optically thin inner disk, while exhibiting a large colour excess at [8–24], indicating that it is a transitional disc object. Kim et al. (2009) support its classification as a transition disc in their Spitzer IRS survey. Its Herschel colours and fluxes are similar to those of the other detected YSOs, its log (F160/F250) colour of 0.22 gives it a blue colour similar to those of the class II objects. Identified as a binary system by Guenther et al. (2007), Nagel et al. (2012) model CS Cha as possessing both a circumbinary and circumstellar disks, complicating the classification of CS Cha as a straightforward transition disk. We also detect two other class II sources which Kim et al. (2009) and Manoj et al. (2011) reclassify as likely transition disks. These two sources, SZ Cha and T54, do not exhibit obvious transition disc colours on the Spitzer ccds, being more similar to class II. SZ Cha exhibits a similarly blue Herschel colour as CS Cha, with log (F160/F250) colour of 0.19. T54 is only reliably detected at 70 μm.

thumbnail Fig. 5

Flux-colour ratio figure log (F160) vs. log (F160/F250) showing the Herschel photometry of all sources with required photometry detected in the field. The grey triangles represent the Luhman et al. (2008) class II objects, the class I objects are shown by black circles. The flat spectrum objects are marked by asterisks. The inverted triangles mark the transition disc objects. The Herschel detections with a counterpart in the Belloche et al. (2011) catalogue are marked with open squares. These objects may be class 0 sources or starless/prestellar cores. No trend in magnitude is present, however the known YSOs exhibit a bluer colour than the candidate cores. Note that redward is to the left in this diagram.

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thumbnail Fig. 6

Colour–colour figure log (F160/F250) vs. log (F250/F350) showing the Herschel photometry of all sources with required photometry detected in the field. The symbols are the same as in Fig. 5. The known YSOs appear bluer at log (F160/F250) than the candidate cores. However there is no difference in colour at log (F250/F350). Note that redward is towards the left in this diagram.

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thumbnail Fig. 7

Colour–colour figure log (F160/F250) vs. log (F350/F500) showing the Herschel photometry of all sources detected in the field. The symbols are the same as in Fig. 5. The known YSOs appear bluer at log (F160/F250) than the candidate cores. There is, however, no difference in colour at log (F350/F500). Note that redward is towards the left in this diagram.

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5. Conclusions

The Chamaeleon I star forming cloud has been observed as part of the Herschel Gould Belt survey of the entire Chamaeleon complex, utilising PACS and SPIRE parallel mode observations at 70, 160, 250, 350, and 500 μm to map the cloud with complete coverage at all five wavelengths. The main results from our initial study of Cha I are:

  • Using the recently developed getsources source extraction software we have detected 397 reliable sources in the observed Cha I field, 77 of which are previously known YSOs or cores. Of these 77, forty-nine are identified as YSOs. The other 28 detections are candidate class 0 members, or starless or prestellar cores associated with Belloche et al. (2011) detections. In addition to these, we also detect two outflow features, two emission features from the ring near HD 97300, and five CO clumps (Haikala et al. 2005). The remaining 311 detections may be associated with the cluster; however the majority lie in regions of low AV, and are likely to be background galaxies.

  • Of the 49 identified young stars, one is considered a class 0 protostar, four are class I protostars, six are flat spectrum protostars, thirty-four are class II pre-main sequence stars, three are considered to be transition disc objects: CS Cha, SZ Cha, and T54. Only one of the known class III young stars is detected in the Herschel images, Ced 110 IRS2.

  • By comparing the Spitzer colours and magnitudes of the detected and non-detected young members of the cloud we find no difference in colour between those young stars detected by Herschel and those that are not, with no obvious trend towards redder IRAC colours discernible. Those objects with the brightest 3.6 μm magnitudes are the most likely to be detected with 80% detected with m3.6 < 8.5, 43% are detected with Herschel for m3.6 < 10.5, and 4% are detected for m3.6 > 10.5.

  • The Herschel fluxes indicate that the protostellar and candidate cores have brighter fluxes at most wavelengths when compared to the class II sources. The candidate cores are more reddened than the known protostars and PMS members. There is no significant difference in colour between the protostellar and class II sources for any colour from Herschel wavelengths.

  • The three detected transition disc sources in the cluster exhibit bluer log (F160/F250) colours than the median value of the class II sources.

The Herschel observations provide unprecedented resolution in the far-IR, especially at 70 μm, enabling us to both extend the study of the known YSOs into the cold dust regime, and to observe embedded class 0 protostars and prestellar cores. Further work in the region is underway to understand the filamentary structure of the cloud and to model the SEDs of the Herschel sources to gain a greater understanding of the environment in which stars form and their evolution from embedded protostar to evolved stellar system.

Acknowledgments

We would like to thank the anonymous referee for their very helpful comments on the paper. P.R. and N.L.J.C. acknowledge support from the Belgian Federal Science Policy Office via the PRODEX Programme of ESA. The Herschel spacecraft was designed, built, tested, and launched under a contract to ESA managed by the Herschel/Planck project team by an industrial consortium under the overall responsibility of the prime contractor Thales Alenia Space (Cannes), and including Astrium (Friedrichshafen) responsible for the payload module and for system testing at spacecraft level, Thales Alenia Space (Turin) responsible for the service module, and Astrium (Toulouse) responsible for the telescope, with in excess of a hundred subcontractors. PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KUL, CSL, IMEC (Belgium); CEA, OAMP (France); MPIA (Germany); IFSI, OAP/AOT, OAA/CAISMI, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI (Italy), and CICT/MCT (Spain). SPIRE has been developed by a consortium of institutes led by Cardiff Univ. (UK) and including Univ. Lethbridge (Canada); NAOC (China); CEA, LAM (France); IFSI, Univ. Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCL-MSSL, UKATC, Univ. Sussex (UK); and Caltech, JPL, NHSC, Univ. Colorado (USA). This development has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); SNSB (Sweden); STFC (UK); and NASA (USA). This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This research has made use of 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. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. This research has made use of Aladin.

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All Tables

Table 1

Identifiers and fluxes of Herschel detected YSOs in Cha I.

Table 2

Median fluxes [Jy] of the Herschel detected sources in Cha I as a function of object type.

Table 3

Median colour of the Herschel detected sources in Cha I as a function of object type.

All Figures

thumbnail Fig. 1

Three-band false-colour image of the Chamaeleon I region, with PACS 160 μm in blue, SPIRE 250 μm in green and SPIRE 500 μm in red. The northern region, centred on Ced 112 and HD 97300, and two southern clusters, Ced 110 and Ced111 are visible. North is up and east is to the left.

Open with DEXTER
In the text
thumbnail Fig. 2

Above: Spitzer IRAC [3.6–4.5] vs. [5.8–8.0] colour–colour figure derived from the Luhman et al. (2008) photometry. Below: Spitzer IRAC-MIPS [3.6–4.5] vs. [8.0–24] colour–colour figure of the Luhman et al. (2008) YSOs. Grey dots indicate the Luhman et al. (2008) identified YSOs. The black symbols indicate those YSOs detected with Herschel. The group of YSOs clustered around [0,0] in both diagrams are the class III sources.

Open with DEXTER
In the text
thumbnail Fig. 3

Spitzer IRAC [3.6] vs. [3.6–4.5] colour–magnitude figure derived from the Luhman et al. (2008) photometry, with grey dots indicating the Luhman et al. (2008) identified YSOs and black squares indicating those YSOs detected with Herschel. Only those YSOs with m3.6   μm brighter than 10 are likely to be detected with Herschel. The Herschel detected sample is complete for sources with [3.6–4.5] colour  > 0.6 and m3.6 < 10.

Open with DEXTER
In the text
thumbnail Fig. 4

Contours of the SPIRE 500 μm mosaic, showing the elongated structure of the dust, shown at levels of 30, 40, 45, 50, 55, 60, 65% of the peak flux 13.4 Jy/beam. The open circles indicate the positions of the Spitzer identified YSOs (Luhman et al. 2008), the filled circles are those YSOs also detected with Herschel. The “x” symbols mark the positions of the Herschel sources matched to Belloche et al. (2011) detections; excluding three which are off-field to the west. It may be noted that the Herschel detected YSOs are mainly located in and near the centres of the three clusters. North is up and east is to the left, coordinates are given in Equ J2000.

Open with DEXTER
In the text
thumbnail Fig. 5

Flux-colour ratio figure log (F160) vs. log (F160/F250) showing the Herschel photometry of all sources with required photometry detected in the field. The grey triangles represent the Luhman et al. (2008) class II objects, the class I objects are shown by black circles. The flat spectrum objects are marked by asterisks. The inverted triangles mark the transition disc objects. The Herschel detections with a counterpart in the Belloche et al. (2011) catalogue are marked with open squares. These objects may be class 0 sources or starless/prestellar cores. No trend in magnitude is present, however the known YSOs exhibit a bluer colour than the candidate cores. Note that redward is to the left in this diagram.

Open with DEXTER
In the text
thumbnail Fig. 6

Colour–colour figure log (F160/F250) vs. log (F250/F350) showing the Herschel photometry of all sources with required photometry detected in the field. The symbols are the same as in Fig. 5. The known YSOs appear bluer at log (F160/F250) than the candidate cores. However there is no difference in colour at log (F250/F350). Note that redward is towards the left in this diagram.

Open with DEXTER
In the text
thumbnail Fig. 7

Colour–colour figure log (F160/F250) vs. log (F350/F500) showing the Herschel photometry of all sources detected in the field. The symbols are the same as in Fig. 5. The known YSOs appear bluer at log (F160/F250) than the candidate cores. There is, however, no difference in colour at log (F350/F500). Note that redward is towards the left in this diagram.

Open with DEXTER
In the text

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