Issue |
A&A
Volume 520, September-October 2010
|
|
---|---|---|
Article Number | A49 | |
Number of page(s) | 21 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/201014481 | |
Published online | 30 September 2010 |
Dynamic star formation in the massive DR21 filament
1
Laboratoire AIM, CEA/DSM - INSU/CNRS - Université Paris
Diderot, IRFU/SAp CEA-Saclay, 91191 Gif-sur-Yvette, France e-mail: nschneid@cea.fr
2
OASU/LAB-UMR5804, CNRS, Université Bordeaux 1, 33270 Floirac,
France
3
I. Physikalisches Institut, Universität zu Köln,
Zülpicher Straße 77, 50937 Köln, Germany
4
Laboratoire de radioastronomie, UMR CNRS 8112, École normale supérieure et
Observatoire de Paris, 75231 Paris, France
5
Zentrum für Astronomie der Universität Heidelberg,
Inst. für Theor. Astrophysik, Albert-Ueberle Str. 2, 69120 Heidelberg, Germany
6
Kavli Institute for Particle Astrophysics and Cosmology, Stanford
University, Menlo Park, CA 94025, USA
Received:
22
March
2010
Accepted:
24
May
2010
Context. The formation of massive stars is a highly complex process in which it is unclear whether the star-forming gas is in global gravitational collapse or an equilibrium state supported by turbulence and/or magnetic fields. In addition, magnetic fields may play a decisive role in the star-formation process since they influence the efficiency of gas infall onto the protostar.
Aims. By studying one of the most massive and dense star-forming regions in the Galaxy at a distance of less than 3 kpc, i.e. the filament containing the well-known sources DR21 and DR21(OH), we attempt to obtain observational evidence to help us to discriminate between these two views.
Methods. We use molecular line data from our 13CO 1 0, CS 2
1,
and N2H+ 1
0 survey of the Cygnus X region obtained with the
FCRAO and high-angular resolution observations in isotopomeric lines
of CO, CS, HCO+, N2H+, and H2CO, obtained with the IRAM
30 m telescope, to investigate the distribution of the different phases
of molecular gas. Gravitational infall is identified by the presence
of inverse P Cygni profiles that are detected in optically thick lines,
while the optically thinner isotopomers are found to reach a peak
in the self-absorption gap.
Results. We observe a complex velocity field and velocity dispersion in the
DR21 filament in which regions of the highest column-density, i.e., dense
cores, have a lower velocity dispersion than the surrounding gas and
velocity gradients that are not (only) due to rotation. Infall
signatures in optically thick line profiles of HCO+ and 12CO
are observed along and across the whole DR21 filament. By modelling the
observed spectra, we obtain a typical infall speed of ~0.6 km s-1 and mass accretion rates of the order of a few 10-3 yr-1 for the two main clumps constituting the filament.
These massive clumps (4900 and 3300
at densities of around
105 cm-3 within 1 pc diameter) are both gravitationally
contracting (with free-fall times much shorter than sound crossing times
and low virial parameter α). The more massive of the clumps,
DR21(OH), is connected to a sub-filament, apparently “falling” onto
the clump. This filament runs parallel to the magnetic field.
Conclusions. All observed kinematic features in the DR21 filament (velocity field, velocity dispersion, and infall), its filamentary morphology, and the existence of (a) sub-filament(s) can be explained if the DR21 filament was formed by the convergence of flows on large scales and is now in a state of global gravitational collapse. Whether this convergence of flows originated from self-gravity on larger scales or from other processes cannot be determined by the present study. The observed velocity field and velocity dispersion are consistent with results from (magneto)-hydrodynamic simulations where the cores lie at the stagnation points of convergent turbulent flows.
Key words: ISM: clouds / ISM: individual objects: Cygnus X / ISM: molecules / ISM: kinematics and dynamics / radio lines: ISM / submillimeter: ISM
© ESO, 2010
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