Issue
A&A
Volume 518, July-August 2010
Herschel: the first science highlights
Article Number L92
Number of page(s) 5
Section Letters
DOI https://doi.org/10.1051/0004-6361/201014618
Published online 16 July 2010
A&A 518, L92 (2010)

Herschel: the first science highlights

LETTER TO THE EDITOR

A Herschel[*] study of the properties of starless cores in the Polaris Flare dark cloud region using PACS and SPIRE

D. Ward-Thompson1 - J. M. Kirk1 - P. André2 - P. Saraceno3 - P. Didelon2 - V. Könyves2 - N. Schneider2 - A. Abergel4 - J.-P. Baluteau5 - J.-Ph. Bernard6 - S. Bontemps2 - L. Cambrésy7 - P. Cox8 - J. Di Francesco9 - A. M. Di Giorgio3 - M. Griffin1 - P. Hargrave1 - M. Huang10 - J. Z. Li10 - P. Martin11 - A. Men'shchikov2 - V. Minier2 - S. Molinari3 - F. Motte2 - G. Olofsson12 - S. Pezzuto11 - D. Russeil6 - M. Sauvage2 - B. Sibthorpe13 - L. Spinoglio3 - L. Testi14 - G. White15,16 - C. Wilson17 - A. Woodcraft13 - A. Zavagno5

1 - School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff, CF243AA, UK
2 - Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/ Service d'Astrophysique, C.E. Saclay, Orme des Merisiers, 91191 Gif-sur-Yvette, France
3 - INAF-IFSI, Fosso del Cavaliere 100, 00133 Roma, Italy
4 - IAS, Université Paris-Sud, Bât. 121, 91405 Orsay, France
5 - LAM/OAMP, Université de Provence, 13388 Marseille, France
6 - CESR, 9 Avenue du Colonel Roche, BP 4346, 31029 Toulouse, France
7 - CDS, Observatoire de Strasbourg, 11 rue de l'Université, 67000 Strasbourg, France
8 - IRAM, 300 rue de la Piscine, Domaine Universitaire, 38406 Saint Martin d'Héres, France
9 - Herzberg Institute of Astrophysics, Department of Physics and Astronomy, University of Victoria, Victoria, Canada
10 - NAOC, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100012, PR China
11 - CITA, University of Toronto, 60 St George Street, Toronto, Ontario, M5S 3H8, Canada
12 - Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
13 - UKATC, Royal Observatory, Blackford Hill, Edinburgh, EH93HJ, UK
14 - INAF, Largo Enrico Fermi 5, 50125 Firenze, Italy
15 - RAL, Chilton, Didcot, OX110NL, UK
16 - Dept. of Physics & Astronomy, Open University, Milton Keynes MK7 6AA, UK
17 - Dept. of Physics & Astronomy, McMaster University, Hamilton, Ontario, L8S 4M1, Canada

Received 31 March 2010 / Accepted 10 May 2010

Abstract
The Polaris Flare cloud region contains a great deal of extended emission. It is at high declination and high Galactic latitude. It was previously seen strongly in IRAS Cirrus emission at 100 microns. We have detected it with both PACS and SPIRE on Herschel. We see filamentary and low-level structure. We identify the five densest cores within this structure. We present the results of a temperature, mass and density analysis of these cores. We compare their observed masses to their virial masses, and see that in all cases the observed masses lie close to the lower end of the range of estimated virial masses. Therefore, we cannot say whether they are gravitationally bound prestellar cores. Nevertheless, these are the best candidates to be potential prestellar cores in the Polaris cloud region.

Key words: stars: formation - ISM: clouds - dust, extinction

1 Introduction

In this paper we present observations, performed with the ESA Herschel Space Observatory (Pilbratt et al. 2010), of the Polaris Flare region. In particular we use the large collecting area and powerful science payload of Herschel to perform imaging photometry using the PACS (Poglitsch et al. 2010) and SPIRE (Griffin et al. 2010) instruments. These observations were carried out as part of the guaranteed-time key programme to map most of the Gould Belt star-forming regions with Herschel (André et al. 2010). The Polaris Flare was first detected in HI as a spur of gas that appears to rise more than 30$^\circ$ out of the Galactic plane. This region is an area rich in IRAS cirrus emission (e.g. Low et al. 1984), and is sometimes known as the Polaris Cirrus cloud. It was mapped in CO by Heithausen & Thaddeus (1990). On the large scale this cloud appears to merge with the Cepheus Flare cloud (e.g. Kirk et al. 2009), and both clouds extend to high Galactic latitude.

\begin{figure}
\par\includegraphics[width=17.2cm,clip]{14618fg1.eps}\vspace*{-1mm}
\vspace{-1.5mm}\end{figure} Figure 1:

The densest part of the Polaris Flare region at some of the observed wavebands. Upper row: 160 $\mu $m from PACS, and 250 $\mu $m and 350 $\mu $m from SPIRE. Lower row: false-colour image (where 160 $\mu $m is shown in blue, 250 $\mu $m is shown in green, and 350 $\mu $m is shown in red), column density map (where red is <4, blue is 4-8, and yellow is > $8 \times 10^{21}$ cm-2), and colour temperature map (where blue is 10-11 K and yellow is 12-13 K). The contour levels on the column density map start at $4 \times 10^{21}$ cm-2, and the interval between successive contours is $1.5 \times 10^{21}$ cm-2. The same contours are repeated on the temperature map for ease of location. Five sources are seen above a column density of $4 \times 10^{21}$ cm-2. These are labelled cores 1-5 (in order of increasing RA) on the last two panels and are discussed in the text. The loop (loop 1) discussed in the text (containing cores 4 & 5) is clearly visible in all images. The reddest features on the false-colour image are the coldest, and the loop shows up clearly as redder than the surroundings. Likewise in the temperature map, the loop shows up as blue, indicating that it is the coldest feature on the map. The position of the IRAS source (IRAS 01432+8725) is marked with a star on the last two panels (adjacent to core 4).

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One of the denser regions in the cloud is known as molecular cloud 123.5+24.9, or MCLD 123.5+24.9 (e.g. Bensch et al. 2003) - hereafter MCLD 123 - at a distance of 150 pc (Bensch et al. 2003). It shows strong extended IRAS 100-$\mu $m emission and is generally believed to be gravitationally unbound with a mass of $\sim$18-32 $M_\odot$ (Grossmann et al. 1990; Bensch et al. 2003). A CO study by Falgarone et al. (1998) revealed a curved filament in MCLD 123 in 13CO and C18O - both in the J=2-1 transition. This filament is also apparent in some narrow velocity channels in the same transition of 12CO (Falgarone et al. 1998).

There is one IRAS source in the region, IRAS 01432+8725. This is listed in the IRAS catalogue as having a flux density at 100 $\mu $m of 2.88 Jy, but only upper limits at the other IRAS wavebands. There is also one Spitzer source that was only detected at a wavelength of 24 $\mu $m at coordinates RA (2000) = 01$^{\rm h}$58$^{\rm m}$27.5$^{\rm s}$, Dec (2000) = +87$^$40$^\prime$07 $^{\prime\prime}$. It has a peak flux density at 24 $\mu $m of 1.3 mJy/beam, where the Spitzer beam at this wavelength is 7 arcsec. This detection lies in a Spitzer calibration field in an unpublished archival dataset (AOR 33136386).

2 Observations

The SPIRE/PACS parallel-mode science demonstration observations of the Polaris cloud were performed on 2009 October 23 (Operation Day 162) at wavelengths of 70 $\mu $m and 160 $\mu $m with PACS, and at 250 $\mu $m, 350 $\mu $m and 500 $\mu $m with SPIRE. The 70- and 160-$\mu $m $\sim$6 deg2 scan map was taken with 60 arcsec/s scanning speed. The field was observed twice with both instruments by performing cross-linked scans in two nearly orthogonal scan directions. The combination of nominal and orthogonal coverages reduced the effects of 1/f noise and better preserved spatial resolution. The SPIRE data were reduced using HIPE version 2.0 and the pipeline scripts delivered with this version. These scripts were modified, e.g. observations that were taken during the turnaround of the satellite were included. A median baseline (HIPE default) was applied to the maps and the ``naive mapper'' was used for map making.

The PACS data were reduced with HIPE 3.0.455 provided by the Herschel Science Center (HSC). We used file version 1 flat-fielding and responsivity in the calibration tree, instead of the built-in version 3. Therefore the error in the final reduced flux scale was corrected manually with the corresponding correction values in the PACS wavelengths. Standard steps of the default pipeline were applied for data reduction starting from (level 0) raw data. Multi-resolution median transform (MMT) deglitching and second order deglitching were also applied. Baselines were subtracted from the level 1 data by high-pass filtering with a $\sim$1$^\circ$ filter width, avoiding obvious sources, whilst the full leg length was 2.5$^\circ$in the parallel mode.

Table 1:   The physical properties of the cores.

The PACS data of this field include transients of unknown origin after each calibration block, which seriously affected the ensuing frames. We processed these observations using data-masking and a narrower high-pass filter width than the image size in order to mitigate the calibration block artifacts. In this process, we may have removed spatial scales larger than the filter widths. The final PACS maps were created using the HIPE ``MADmap'' mapping method projected to the 3.2 and 6.4 arcsec/pixel size for 70 and 160 $\mu $m data, respectively.

3 Results

The Polaris Flare dark cloud region was observed at five wavelengths - 70, 160, 250, 350 and 500 $\mu $m. Figure 1 shows some of the main results. Only the densest part of the mapped region is shown. The area shown is just over half a degree square. The upper row of Fig. 1 shows the data from three of the wavebands: 160 $\mu $m from PACS; and 250 $\mu $m and 350 $\mu $m from SPIRE. The data have been smoothed to a common resolution of 24 arcsec, the approximate resolution of the 350-$\mu $m data. The images have also been re-gridded onto $10 \times 10$ arcsec pixels.

The lower row of Fig. 1 shows some images derived from the raw data: a false-colour image; a column density map; and a colour temperature map. The contours on the column density map are at 4, 5.5, and $7 \times 10^{21}$ cm-2. These are repeated on the temperature map to assist in source location. The Polaris cloud is clearly seen, and the raw data show a complex structure that is broadly similar at all wavebands. There are a number of filamentary structures seen in the data, with a few brighter cores embedded in the cloud.

There is a filamentary loop seen in all images that is centred roughly at Galactic coordinates l = 123.67, b = +24.89 - RA $(2000) = 01^{\rm h}$58$^{\rm m}$, Dec $(2000) = 87^\circ$40$^\prime$. We here label this feature loop 1. This is the same curved filament as was seen by Falgarone et al. (1998) in 13CO. They interpreted this as an edge of a cloud core. However, in the continuum we see it is clearly a loop with no filled centre. It was also detected in various transitions by Grossman & Heithausen (1992).

There is also a filament with an apparent bifurcation at roughly Galactic coordinates l = 123.48, b = +24.90 - RA $(2000) = 01^{\rm h}$42$^{\rm m}$, Dec $(2000) = 87^\circ$43$^\prime$. A bright core region is seen at the head of this bifurcation, which may be broken up into three components in the 160-$\mu $m data. The mean off-source pixel-by-pixel 1-$\sigma$ variation on the N(H2) map varies from 1.2 to $1.5 \times 10^{21}$ cm-2. Hence, we adopt a value of $4 \times 10^{21}$ cm-2 for the 3-$\sigma$ contour.

Five sources are seen in the column density map above a column density of $4 \times 10^{21}$ cm-2. We here label these cores 1-5 in order of increasing Galactic longitude - see lower right panels of Fig. 1. We list the core positions and their assumed distances in Table 1. The core mentioned above at the bifurcated filament is core 2, and loop 1 contains cores 4 and 5.

The IRAS source IRAS 01432+8725 lies an arcminute to the west of core 4. We believe this offset is sufficient that the two sources are different (the IRAS FWHM at 100 $\mu $m is 44 arcsec). Therefore, none of the cores is associated with an infrared source, and so these are all candidate starless cores (Myers et al. 1987). The IRAS source is coincident with the centre of the loop, and may in fact be loop 1 itself, as IRAS point sources that only show up at 100 $\mu $m have often in the past been shown to be simply bits of cirrus. The Spitzer source may be foreground, as it is only seen at the shortest wavelengths.

The reddest features on the false-colour image are the coldest, and loop 1 shows up clearly as redder than the surroundings. Likewise in the temperature map, the loop shows up as blue, indicating that it is the coldest feature on the map. Cores 4 & 5 appear to be the densest features on the map, with peak column densities in excess of 1022 cm-2. The column density contour of $4 \times 10^{21}$ was selected as the core boundary in each case. The radial sizes of the cores were estimated from the images as the equivalent radius of a circle with an area equal to that contained by the core boundary. The derived equivalent radii are listed in Table 1. Flux densities were measured within the core boundary contour in each case, and these are also listed in Table 1.

\begin{figure}
\par\includegraphics[width=17cm]{14618fg2.eps}
\end{figure} Figure 2:

Spectral energy distributions of cores 1 to 5. The peak flux density in a single $10 \times 10$ arcsec pixel was measured. This is shown on a log-log plot of $\lambda S_\lambda $ versus $\lambda $. The data are shown with 15% uncertainty error-bars. The upper limits at 70 $\mu $m are shown as arrows. The solid lines are grey-body fits of the form described in the text. The temperatures of the fits are listed in Table 1.

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4 Core properties

The core properties were estimated from the maps of column density and temperature. The flux densities of the pixels coincident with the column density peaks of each core are plotted on the spectral energy distributions (SEDs) shown in Fig. 2. Modified blackbody curves were fitted to the flux densities, and these are also shown in Fig. 2. These are the same fits that were used, pixel-by-pixel, to construct the column density and temperature maps shown in Fig. 1. The form of the fit (cf. Hildebrand 1983) that was used in each case is

\begin{displaymath}F_\nu = \Omega B_\nu(T) ( m_{\rm H} \mu N(H_2) \kappa_\nu ),
\end{displaymath} (1)

where $F_\nu$ is the flux density at frequency $\nu$, $\Omega$ is the solid angle of each pixel, $B_\nu(T)$ is the blackbody function at temperature T, $m_{\rm H}\mu$ is the mean particle mass ($m_{\rm H}$ is the mass of a hydrogen atom and $\mu $ was taken to be 2.86, assuming the gas is $\sim$70% H2 by mass), N(H2) is the column number density of molecular hydrogen, and $\kappa_\nu$ is the dust mass opacity.

We used the pixel-by-pixel SED fits to calculate the column densities, and hence the core masses. The value of $\kappa_\nu$ that should be used has been the subject of much controversy. Here we adopt the dust opacity recommended by Henning et al. (1995) and Preibisch et al. (1993) for clouds of intermediate density - n(H2$\leq$ 105 cm-3 - and we assume a standard gas to dust mass ratio of 100. This is a similar parameterization of the dust opacity to that used by Beckwith et al. (1990), namely that

\begin{displaymath}\kappa_\nu = 0.1 ~\textrm{cm}^2\textrm{~g}^{-1} \times (\nu/1000 ~{\rm GHz})^{\beta},
\end{displaymath} (2)

where we have set the dust opacity index $\beta$ to be equal to 2.

This is also consistent with the value used by André et al. (1993, 1996) and by Kirk et al. (2005) for prestellar and starless cores. The peak column densities and the temperature at the peak are listed in Table 1. The mass of each core was calculated by integrating the column density map within the selected core boundary. This is also listed in Table 1. From these, the volume densities were calculated, assuming that the cores are spherical. These, too, are listed in Table 1.

The temperatures of the SEDs are listed in Table 1. These are all quite low, with values of 10-12 K. This, and the lack of NIR emission from the cores, implies that the star formation process has yet to begin within these particular cores. Hence they are starless cores. This means that these cores should have no internal heating and should be heated solely by the external radiation field. This is similar to what is seen in other low-mass starless and prestellar cores (e.g. Ward-Thompson et al. 2002; or for a review see Ward-Thompson et al. 2007).

Core 5 was observed in the submillimetre by Bernard et al. (1999). Our results are consistent with their findings, allowing for the very different resolutions of the two sets of observations. Falgarone et al. (2009) measured the mean CO linewidths in MCLD 123. For so-called ``bright'' regions (i.e. high column densities) they found a mean linewidth of 0.4 kms-1 in this region. Heithausen et al. (2008) observed MCLD 123 in a number of transitions and found mean linewidths from 0.2 to 0.4 kms-1. Using this range of values we estimated a range of values for the virial masses of the five cores, and list these in Table 1.

We note that all of the cores have masses that are below the virial masses that we have estimated. However, given the uncertainties in the mass calculations, they could be consistent with the lower limit of the range of virial masses. Hence, we can only say that they may or may not be gravitationally bound, and these 5 cores may be on the edge of possibly becoming prestellar cores. Note that this is very different from the cores found in the Aquila region (André et al. 2010), a large fraction of which are clearly gravitationally bound - cf. Fig. 4 of André et al. (2010). Nevertheless, bound or unbound, the 5 cores we have selected are the closest to being gravitationally bound of any of the starless cores in Polaris.

5 Conclusions

We have presented Herschel data of the Polaris Flare dark cloud region, and in particular the region MCLD 123. We found a great deal of extended emission at wavelengths from 70 to 500 $\mu $m with both PACS and SPIRE. We noted some filamentary and low-level structure. We identified the five densest cores withinthis structure. We carried out a temperature, mass and density analysis of the cores. We compared their observed masses to their virial masses, and found that the observed masses are on the lower limit of the range of their estimated virial masses, and thus we cannot say for certain whether they are gravitationally bound.

Acknowledgements
J.M.K. acknowledges STFC for funding, while this work was carried out, under the auspices of the Cardiff Astronomy Rolling Grant. SPIRE was 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); 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); Stockholm Observatory (Sweden); STFC (UK); and NASA (USA). PACS was developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KUL, CSL, IMEC (Belgium); CEA, LAM (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).

References

Footnotes

...Herschel[*]
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

All Tables

Table 1:   The physical properties of the cores.

All Figures

  \begin{figure}
\par\includegraphics[width=17.2cm,clip]{14618fg1.eps}\vspace*{-1mm}
\vspace{-1.5mm}\end{figure} Figure 1:

The densest part of the Polaris Flare region at some of the observed wavebands. Upper row: 160 $\mu $m from PACS, and 250 $\mu $m and 350 $\mu $m from SPIRE. Lower row: false-colour image (where 160 $\mu $m is shown in blue, 250 $\mu $m is shown in green, and 350 $\mu $m is shown in red), column density map (where red is <4, blue is 4-8, and yellow is > $8 \times 10^{21}$ cm-2), and colour temperature map (where blue is 10-11 K and yellow is 12-13 K). The contour levels on the column density map start at $4 \times 10^{21}$ cm-2, and the interval between successive contours is $1.5 \times 10^{21}$ cm-2. The same contours are repeated on the temperature map for ease of location. Five sources are seen above a column density of $4 \times 10^{21}$ cm-2. These are labelled cores 1-5 (in order of increasing RA) on the last two panels and are discussed in the text. The loop (loop 1) discussed in the text (containing cores 4 & 5) is clearly visible in all images. The reddest features on the false-colour image are the coldest, and the loop shows up clearly as redder than the surroundings. Likewise in the temperature map, the loop shows up as blue, indicating that it is the coldest feature on the map. The position of the IRAS source (IRAS 01432+8725) is marked with a star on the last two panels (adjacent to core 4).

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In the text

  \begin{figure}
\par\includegraphics[width=17cm]{14618fg2.eps}
\end{figure} Figure 2:

Spectral energy distributions of cores 1 to 5. The peak flux density in a single $10 \times 10$ arcsec pixel was measured. This is shown on a log-log plot of $\lambda S_\lambda $ versus $\lambda $. The data are shown with 15% uncertainty error-bars. The upper limits at 70 $\mu $m are shown as arrows. The solid lines are grey-body fits of the form described in the text. The temperatures of the fits are listed in Table 1.

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In the text


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