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Issue
A&A
Volume 543, July 2012
Article Number L3
Number of page(s) 7
Section Letters
DOI https://doi.org/10.1051/0004-6361/201219429
Published online 27 June 2012

© ESO, 2012

thumbnail Fig. 1

Herschel maps of the DR21 environment showing a) 70 μm emission, b) column density, and c) dust temperature. The DR21 ridge is delimited roughly by the NH2 = 1023 cm-2 contour plotted in panels b) and c). The filaments selected using DisPerSE (see Sect. 3) are named and marked with dots along their crests in b) and c).

1. Introduction

The properties of dense structures in interstellar molecular clouds, e.g., the density and temperature distribution, chemical composition, and dynamics, are expected to determine the detailed outcome of the star formation process that varies between an isolated low-mass star and an OB star cluster. While previous observations have shown that clouds are filamentary (e.g., Bally et al. 1987), recent Herschel studies have revealed the ubiquity (e.g., Miville-Deschênes et al. 2010; Henning et al. 2010) and key importance of filaments for star formation. Notably, gravitationally unstable filaments are found to fragment into cores (André et al. 2010), and filaments generally show narrow central widths of about 0.1 pc (Arzoumanian et al. 2011). Resulting self-gravitating cores with sizes of 0.01 − 0.1 pc and masses up to 10   M⊙ are best candidates to form low- to possibly intermediate-mass stars (see reviews by Di Francesco et al. 2007; Ward-Thompson et al. 2007).

It is an important question which cloud structures lead to the formation of high-mass stars (≳10   M⊙) that are almost exclusively found in stellar clusters or associations (see review by Zinnecker & Yorke 2007). The Herschel imaging survey of OB young stellar objects (HOBYS, Motte et al. 2010) observes massive molecular cloud complexes within 3 kpc distance to probe the cloud environment of OB star-forming cores, their statistical evolution, and the effects of feedback on parental clouds. Studying the Vela C region, Hill et al. (2011) suggested that “ridges”, i.e., massive, gravitationally unstable filamentary structures of high column density (NH2 > 1023 cm-2) that dominate their environment could be preferential sites of high-mass star formation (cf. Nguyen Luong et al. 2011). The ridge in Vela C shows a complex substructure that could result from stellar feedback (Minier et al. 2012). Furthermore, intersecting filaments appear to mark sites of stellar cluster formation in the Rosette cloud (Schneider et al. 2012).

This study focuses on the DR21 ridge (also called DR21 filament), the densest and most massive cloud structure in the Cygnus X region (Schneider et al. 2006; Motte et al. 2007; Roy et al. 2011) at a distance of 1.4 kpc (Rygl et al. 2012). It hosts the embedded HII region DR21 (e.g., Roelfsema et al. 1989), the maser source DR21(OH), and massive protostars (Bontemps et al. 2010). Schneider et al. (2010) analysed molecular line emission and identified three “sub-filaments” F1 − F3 that connect to the ridge. Velocity gradients suggest that material is transported along them towards the ridge, and in the case of F3 a bend and possible direct connection to the DR21(OH) clump is traced (cf. Csengeri et al. 2011). This behaviour supports the scenario that sustained accretion of inflowing material plays a role in building up massive clumps and cores and that it sets the stage for high-mass star formation, as suggested by numerical simulations (e.g., Balsara et al. 2001; Banerjee et al. 2006; Smith et al. 2011). Here we exploit the unprecedented sensitivity of Herschel far-infrared and submillimeter continuum imaging to trace the detailed column density structure in this region.

2. Herschel observations and data reduction

Cygnus X North was observed in the parallel scan map mode with PACS (Poglitsch et al. 2010) and SPIRE (Griffin et al. 2010) on Dec. 18, 2010. To diminish scanning artefacts, two nearly perpendicular coverages of 2.8 × 2.8° were obtained in five photometric bands at 70, 160, 250, 350, and 500 μm with a scan speed of 20′′/s. The data were reduced using the Herschel interactive processing environment and the Scanamorphos software (Roussel 2012, for creating PACS maps).

The 70 μm map traces in particular heated dust towards star-forming sites (Fig. 1a). To quantitatively estimate the distribution of the cold dust, which generally represents most of the dust mass, we created maps of column density NH2 and dust temperature Td in the way described in Hill et al. (2011, 2012), including offsets to recover the absolute intensity level following Bernard et al. (2010). Computed from the 160, 250, and 350 μm bands, the maps provide an angular resolution of 25′′ (0.17 pc at 1.4 kpc). They have undefined values for pixel groups towards DR21, DR21(OH) (white pixels in Fig. 1b), and W75N (see Fig. B.1) due to saturation at 250 and 350 μm.

Table 1

Average properties of the DR21 filaments and ridge.

3. The DR21 filaments

We show in Fig. 1a compact 70 μm sources that are protostar candidates clustering along the DR21 ridge oriented north-south, with the most prominent peak being DR21 itself. Supposedly lying at the same distance, they show a strong northward decrease in luminosity. There are many filamentary streamers from the ridge to, e.g., the north-west and west, most of which were also detected in the mid-infrared with Spitzer (Marston et al. 2004; Hora et al. 2009). They mainly correspond to low column density structures (Fig. 1b), but several filaments are prominent. The extent of the DR21 ridge can be roughly defined by the NH2 = 1023 cm-2 contour enclosing an area of 2.3 pc2. Notably, the “sub-filaments” of Schneider et al. (2010), labelled F1 and F3, are recovered; both are resolved into nearly parallel northern and southern components that join close to the ridge. The northern part of the DR21 ridge shows two extensions in column density (“rabbit ears”) to the north and north-west (F1) that coincide with the coldest regions where the dust temperature drops as low as 14 K (Fig. 1c). The filaments are visible in the dust temperature map as structures of lower central temperature relative to the background of  ~19 K. The background column density level is  ~1022 cm-2 with a standard deviation of  ~1021 cm-2 mainly due to cirrus structure.

In the first step of characterising the DR21 filaments, we applied the DisPerSE software (Sousbie 2011) on the column density map to identify the filament crests, similar as Arzoumanian et al. (2011) and Hill et al. (2011), using a persistence threshold of 3 × 1021 cm-2. Unlike for these works, possible line-of-sight confusion needed to be addressed in our analysis because of the low-density foreground cloud Cygnus Rift and the close-by W75N cloud component (e.g., Schneider et al. 2006). We also avoided the strong DR21 outflow (e.g., Richardson et al. 1986; White et al. 2010). Subsequent studies will analyse the combined column density, temperature, and kinematical data for a more extended region, but here we conservatively limited this study to filaments that (1) have main velocities in 13\hbox{${}^{13}$}CO coherent with the ridge and no strong secondary velocity componentsa, and (2) have crest column densities above 1.5 × 1022 cm-2; the resulting selection is shown in Fig. 1. At positions along their crests spaced by half the beam FWHM we extracted perpendicular column density profiles extending over 2.5 pc in each direction, wide enough for the outer profiles to be sufficiently flat and trace the background. For background column densities, we adopted the profile minima that were median-smoothed over 2.5 beam FWHM to lower the influence of neighbouring structures.

thumbnail Fig. 2

DR21 filament properties over distance along their crest from the DR21 ridge. The distance scale is compressed beyond 3 pc. a) Crest column density (not background-subtracted), dashed curves show the background column density; b) crest dust temperature; c) mass per unit length, dashed lines gives the critical value.

Crest column density and dust temperature are anti-correlated as anticipated for all filaments with the possible exception of SW (Fig. 2a, b). Contrary to the others, the latter shows slightly decreasing central temperature outward from the DR21 ridge. This filament also shows the weakest outward column density decline. In general, lower temperatures are expected for structures of higher column density because of the increased shielding from the interstellar radiation field. However, the SW filament is running towards the warmer area around DR21 (Fig. 1a, c) and possibly into the outflow cavity. Its inverted density-temperature relation could thus be an effect of heating by DR21.

The perpendicular filament column density profiles generally show a flattened central part and non-Gaussian, roughly power-law wings. To examine the central width we performed a weighted fit of a Gaussian curve plus a constant level (cf. Arzoumanian et al. 2011). As weights a Gaussian curve with 0.17 pc FWHM was used to ensure the reproduction of the central flattening. The central width of each profile was derived as the deconvolved fitted Gaussian FWHM. The weighted mean values of the profile central widths lie between 0.26 and 0.34 pc (Table 1). These values are beyond the range of the typical  ~0.1 pc found by Arzoumanian et al. (2011), probably only partially caused by the lower spatial resolution. However, no correlation between central width and crest column density is present either for the profiles or for the mean values. Better statistics in the high-column density regime is needed to clarify a possible dependency.

To constrain the masses of the filaments we estimated an outer radius for each profile integration. A threshold per profile was derived by building the cumulative integral over radial distance from the crest and determining the first minimum of its derivative. For each filament we adopted its median threshold as integration radius, ranging from 0.24 to 0.58 pc (Table 1).

In theory, the thermal stability of filaments is to first order determined by the critical mass per unit lengthb (e.g., Inutsuka & Miyama 1992). Observations indicate that this is a proxy for the general stability (André et al. 2010). Most filament segments are thermally supercritical (Fig. 2c). The outer segments (beyond 1 pc) of the SW, F3N, and F3S filaments are only supercritical to within a factor of 3, and the longest, F1N filament shows a marginally subcritical segment. For thermally supercritical filaments, fragmentation and local spherical collapse occur faster than global collapse in the absence of other stabilising mechanisms (Pon et al. 2011; Toalá et al. 2012). In this case we expect the DR21 filaments to form cores and protostars. The S filament shows several local maxima of crest column density seperated by about 1 pc (Fig. 2a). Less regular and prominent, several peaks are also seen for the other filaments. At 70 μm there is one extended source towards the S filament (Figs. 1a, A.1a). Compact emission from protostar candidates is present towards the junction point of F3S and F3N, and towards the F1N and N filaments. The 250 μm map shows additional compact starless/prestellar core candidates towards all filaments, one of them identified as a dense core by Motte et al. (2007) (see Appendices A, B). This confirms that core and star formation is ongoing within the thermally supercritical filaments.

thumbnail Fig. 3

a) Column density map (in cm-2) of the DR21 ridge with the crest as black dots and the black NH2 = 1023 cm-2 contour. Note that white pixels are undefined because of saturation. Blue ellipses give the position and extent (FWHM = semi-axes) of the Motte et al. (2007) dense cores. b), c) Radial column density profiles towards the crest locations b and c marked in yellow/blue in a). The radial profiles are averaged over the marked crest dots. The yellow areas show the  ± 1σ ranges.

4. The DR21 ridge – an intersection of filaments

The DR21 ridge hosts 22 dense cores and  ~1/4 of the massive ones in the Motte et al. (2007) sample of the whole cloud complex, and thus represents the region that probably hosts the most massive forming star cluster in Cygnus X. We used the Motte et al. MAMBO 1.2 mm map to interpolate the missing pixels in the column density map and derived the mean ridge properties using a crest of 4 pc length following its main peaks (Fig. 3a, Table 1). If it were regarded as an individual filament, it would be highly unstable, in accord with the global collapse signature observed by Schneider et al. (2010) and the dense core detections (Fig. 3a).

The chain of cores dominates the structure of the ridge except between DR21 and DR21(OH), where a crest column density dip is present (location c in Fig. 3). Less clear than the northern branching, a possible additional filamentary extension coinciding with a core branches off to the south-east at the southern end of the ridge (at the very bottom of Fig. 3a). As illustrated in Fig. 3b for location b, corresponding to the dense core N40, the column density profile of the ridge generally shows narrow peaks and smoothly decreasing wings towards the cores. However, the profile towards location c shows at least one additional peak of 9 × 1022 cm-2 at 0.4 pc (Fig. 3c), indicating that there the ridge consists of more than one individual filament. Considered together with the branching of the northern ridge into the “rabbit ears” (the F1 and N filaments) and the possible southern branching, this finding suggests that the DR21 ridge is a complex intersection of several individual filaments. The dominating filament along the crest shows a central width of  ~0.34 pc, not significantly broader than the central widths of the filaments (Table 1).

5. Star formation in the DR21 ridge

Schneider et al. (2010) presented velocity gradients that suggest accretion of the filaments onto the DR21 ridge. Dust temperature dips are seen towards the filament-ridge connections (Fig. 2b), indicating that inflowing material has cooled at these locations. At present, the total mass in the selected filaments is about 1/3 of the ridge mass. It thus appears that the mass assembly process of the DR21 ridge is in a late stage, possibly driven by the gravitational potential of the ridge itself, and specifically the build-up of massive cores in the DR21/DR21(OH) clumps could have been supported by continuous accretion from the S/SW and F3S/F3N filaments, respectively. The filaments are for the most part gravitationally unstable and are forming cores and protostars, in contrast to the striations and “sub-filaments” in Taurus (Goldsmith et al. 2008; Palmeirim et al., in prep.) or Aquila (André et al. 2010). The high masses of the cores in the ridge could therefore not only be generated by the filament flows, but could also be caused by the merging with fragment cores of the filaments that form dense, small-scale convergent flows. For DR21(OH), Csengeri et al. (2011) showed possible fragmentation of the inflowing material. High-mass star formation involving a core merger process was also suggested by observations of NGC 2264-C (Peretto et al. 2006).

The Herschel observations emphasize the evolutionary gradient along the ridge: beyond DR21, the 70 μm luminosity of protostars strongly decreases northward, and the dust temperature shows lowest temperatures towards the northern part. The substructure of the DR21 ridge suggests that it was formed by the merging of individual narrow, intersecting filaments. The pronounced elongation and branching into filaments F1 and N with the highest mass per unit length indicate that the major components may be roughly parallel filaments oriented north-south. At present, the merging could have advanced farthest in the southern part and less in the north, where the two components appear to be separated. Extrapolating this scenario, we expect that the northern filaments will lead to the assembly of one or more additional massive clumps. This study suggests that high-mass star-forming ridges could be second-generation cloud structures formed via dynamical merging of gravitationally unstable filaments.

Online material

Appendix A: The S and F1N filaments in detail

Figure A.1 shows a zoom on the S and F1N filaments. Several compact, more or less elongated cores are seen in the 250 μm and column density map. The marked dense core N23 has an estimated mass of 13 M⊙.

thumbnail Fig. A.1

Detailed view of filament S (panels a)–c)) and F1N (d–f). The 70 and 250 μm units are MJy/sr, column density unit is cm-2. The black cross in panel a) indicates an extended 70 μm source. The blue ellipse in panels d)–f) gives the position and extent (FWHM  =  semi-axes) of the dense core N23 of Motte et al. (2007).

Appendix B: Herschel maps of DR21 and Cygnus X North

Figure B.1 shows the DR21 environment in the Herschel maps of the HOBYS Cygnus X North observations together with a RGB colour-composite image.

thumbnail Fig. B.1

Herschel maps of the DR21 environment in Cygnus X North obtained by HOBYS. Map units are MJy/sr. The last panel shows a RGB colour-composite image using the SPIRE 250 μm (red), PACS 160 μm (green), and PACS 70 μm (blue) maps.

a

We used the Schneider et al. (2010) FCRAO 13\hbox{${}^{13}$}CO (1 − 0) data and excluded filament segments showing additional velocity components above the 20% level of the primary peak.

b

Mlinecrit=2cs2/G\hbox{$M_{\rm line}^{\rm crit} = 2c_{\rm s}^2/G$} with the isothermal sound speed cs=RTd/μH2\hbox{$c_{\rm s}=\sqrt{R T_{\rm d}/\mu_{\rm H_2}}$} and Td at the crest position, resulting in Mlinecrit≈30 M⊙\hbox{$M_{\rm line}^{\rm crit} \approx 30~M_\odot$}/pc.

Acknowledgments

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, UKSA (UK); and NASA (USA). PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KU Leuven, CSL, IMEC (Belgium); CEA, LAM (France); MPIA (Germany); INAF-IFSI/OAA/OAP/OAT, 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/INAF (Italy), and CICYT/MCYT (Spain). This work was supported by the ANR (Agence Nationale pour la Recherche) project “PROBeS” (ANR-08-BLAN-0241).

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All Tables

Table 1

Average properties of the DR21 filaments and ridge.

In the text

All Figures

thumbnail Fig. 1

Herschel maps of the DR21 environment showing a) 70 μm emission, b) column density, and c) dust temperature. The DR21 ridge is delimited roughly by the NH2 = 1023 cm-2 contour plotted in panels b) and c). The filaments selected using DisPerSE (see Sect. 3) are named and marked with dots along their crests in b) and c).
In the text
thumbnail Fig. 2

DR21 filament properties over distance along their crest from the DR21 ridge. The distance scale is compressed beyond 3 pc. a) Crest column density (not background-subtracted), dashed curves show the background column density; b) crest dust temperature; c) mass per unit length, dashed lines gives the critical value.
In the text
thumbnail Fig. 3

a) Column density map (in cm-2) of the DR21 ridge with the crest as black dots and the black NH2 = 1023 cm-2 contour. Note that white pixels are undefined because of saturation. Blue ellipses give the position and extent (FWHM = semi-axes) of the Motte et al. (2007) dense cores. b), c) Radial column density profiles towards the crest locations b and c marked in yellow/blue in a). The radial profiles are averaged over the marked crest dots. The yellow areas show the  ± 1σ ranges.
In the text
thumbnail Fig. A.1

Detailed view of filament S (panels a)–c)) and F1N (d–f). The 70 and 250 μm units are MJy/sr, column density unit is cm-2. The black cross in panel a) indicates an extended 70 μm source. The blue ellipse in panels d)–f) gives the position and extent (FWHM  =  semi-axes) of the dense core N23 of Motte et al. (2007).
In the text
thumbnail Fig. B.1

Herschel maps of the DR21 environment in Cygnus X North obtained by HOBYS. Map units are MJy/sr. The last panel shows a RGB colour-composite image using the SPIRE 250 μm (red), PACS 160 μm (green), and PACS 70 μm (blue) maps.
In the text

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