326 research outputs found
ATCA and Spitzer Observations of the Binary Protostellar Systems CG30 and BHR71
We present interferometric observations with resolution of ~3 arcsecs of the
isolated, low-mass protostellar double cores CG30 and BHR71 in the N2M_\odotSpitzerSpitzer$ observations, we construct spectral
energy distributions (SEDs) and derive temperatures and luminosities for all
cores. Based on the morphology and velocity structure, we suggest that the
sub-cores in CG30 were formed by initial fragmentation of a filamentary
prestellar core, while those in BHR71 could originate from rotational
fragmentation of a single collapsing protostellar core.Comment: 31 pages, 10 figures, to be published by ApJ in Sep. 200
OVRO N2H+ Observations of Class 0 Protostars: Constraints on the Formation of Binary Stars
We present the results of an interferometric study of the N2H+(1--0) emission
from nine nearby, isolated, low-mass protostellar cores, using the OVRO
millimeter array. The main goal of this study is the kinematic characterization
of the cores in terms of rotation, turbulence, and fragmentation. Eight of the
nine objects have compact N2H+ cores with FWHM radii of 1200 -- 3500 AU,
spatially coinciding with the thermal dust continuum emission. The only more
evolved (Class I) object in the sample (CB 188) shows only faint and extended
N2H+ emission. The mean N2H+ line width was found to be 0.37 km/s. Estimated
virial masses range from 0.3 to 1.2 M_sun. We find that thermal and turbulent
energy support are about equally important in these cores, while rotational
support is negligible. The measured velocity gradients across the cores range
from 6 to 24 km/s/pc. Assuming these gradients are produced by bulk rotation,
we find that the specific angular momenta of the observed Class 0 protostellar
cores are intermediate between those of dense (prestellar) molecular cloud
cores and the orbital angular momenta of wide PMS binary systems. There appears
to be no evolution (decrease) of angular momentum from the smallest prestellar
cores via protostellar cores to wide PMS binary systems. In the context that
most protostellar cores are assumed to fragment and form binary stars, this
means that most of the angular momentum contained in the collapse region is
transformed into orbital angular momentum of the resulting stellar binary
systems.Comment: 35 pages, 9 figures (one in color), 6 tables. Accepted by ApJ (to
appear in Nov. 2007
Synthetic observations of first hydrostatic cores in collapsing low-mass dense cores II. Simulated ALMA dust emission maps
First hydrostatic cores are predicted by theories of star formation, but
their existence has never been demonstrated convincingly by (sub)millimeter
observations. Furthermore, the multiplicity at the early phases of the star
formation process is poorly constrained. The purpose of this paper is twofold.
First, we seek to provide predictions of ALMA dust continuum emission maps from
early Class 0 objects. Second, we show to what extent ALMA will be able to
probe the fragmentation scale in these objects. Following our previous paper
(Commer\c{c}on et al. 2012, hereafter paper I), we post-process three
state-of-the-art radiation-magneto-hydrodynamic 3D adaptive mesh refinement
calculations to compute the emanating dust emission maps. We then produce
synthetic ALMA observations of the dust thermal continuum from first
hydrostatic cores. We present the first synthetic ALMA observations of dust
continuum emission from first hydrostatic cores. We analyze the results given
by the different bands and configurations and we discuss for which combinations
of the two the first hydrostatic cores would most likely be observed. We also
show that observing dust continuum emission with ALMA will help in identifying
the physical processes occurring within collapsing dense cores. If the magnetic
field is playing a role, the emission pattern will show evidence of a
pseudo-disk and even of a magnetically driven outflow, which pure
hydrodynamical calculations cannot reproduce. The capabilities of ALMA will
enable us to make significant progress towards understanding fragmentation at
the early Class 0 stage and discovering first hydrostatic cores.Comment: 12 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Dust-temperature of an isolated star-forming cloud: Herschel observations of the Bok globule CB244
We present Herschel observations of the isolated, low-mass star-forming Bok
globule CB244. It contains two cold sources, a low-mass Class 0 protostar and a
starless core, which is likely to be prestellar in nature, separated by 90
arcsec (~ 18000 AU). The Herschel data sample the peak of the Planck spectrum
for these sources, and are therefore ideal for dust-temperature and column
density modeling. With these data and a near-IR extinction map, the MIPS 70
micron mosaic, the SCUBA 850 micron map, and the IRAM 1.3 mm map, we model the
dust-temperature and column density of CB244 and present the first measured
dust-temperature map of an entire star-forming molecular cloud. We find that
the column-averaged dust-temperature near the protostar is ~ 17.7 K, while for
the starless core it is ~ 10.6K, and that the effect of external heating causes
the cloud dust-temperature to rise to ~ 17 K where the hydrogen column density
drops below 10^21 cm^-2. The total hydrogen mass of CB244 (assuming a distance
of 200 pc) is 15 +/- 5 M_sun. The mass of the protostellar core is 1.6 +/- 0.1
M_sun and the mass of the starless core is 5 +/- 2 M_sun, indicating that ~ 45%
of the mass in the globule is participating in the star-formation process.Comment: Accepted for A&A Herschel Special Issue; 5 pages, 2 figure
Magnetic field evolution in Bok globules
Using the Submillimeter Common-User Bolometer Array (SCUBA) at the James
Clerk Maxwell Telescope (JCMT), we obtained submillimeter polarization maps of
the Bok globules B335, CB230, and CB244 at 850micron.
We find strongly aligned polarization vectors in the case of B335 and CB230,
indicating a strong coupling of the magnetic field to the dust grains. Based on
the distribution of the orientation and strength of the linear polarization we
derive the magnetic field strengths in the envelopes of the globules. In
agreement with previous submillimeter polarization measurements of Bok globules
we find polarization degrees of several percent decreasing towards the centers
of the cores.
Furthermore, we compare the magnetic field topology with the spatial
structure of the globules, in particular with the orientation of the outflows
and the orientation of the nonspherical globule cores. In case of the globules
B335 and CB230, the outflows are oriented almost perpendicular to the symmetry
axis of the globule cores. The magnetic field, however, is aligned with the
symmetry axis of the prolate cores in the case of the Bok globules B335 and
CB230, while it is slightly aligned with the outflow axis in the case of the
Bok globules CB26 and CB54. We discuss the possibility that the different
orientations of the magnetic field relative to the outflow directions reflect
different evolutionary stages of the single globules.
The complete version of this article (containing all figures) can be
downloaded from
http://spider.ipac.caltech.edu/staff/swolf/homepage/public/preprints/mfe.ps.gzComment: ApJ, in pres
Shapes of Molecular Cloud Cores and the Filamentary Mode of Star Formation
Using recent dust continuum data, we generate the intrinsic ellipticity
distribution of dense, starless molecular cloud cores. Under the hypothesis
that the cores are all either oblate or prolate randomly-oriented spheroids, we
show that a satisfactory fit to observations can be obtained with a gaussian
prolate distribution having a mean intrinsic axis ratio of 0.54. Further, we
show that correlations exist between the apparent axis ratio and both the peak
intensity and total flux density of emission from the cores, the sign of which
again favours the prolate hypothesis. The latter result shows that the mass of
a given core depends on its intrinsic ellipticity. Monte Carlo simulations are
performed to find the best-fit power law of this dependence. Finally, we show
how these results are consistent with an evolutionary scenario leading from
filamentary parent clouds to increasingly massive, condensed, and roughly
spherical embedded cores.Comment: 16 pages, incl. 11 Postscript figures. Accepted by Ap
Pebbles in an Embedded Protostellar Disk: The Case of CB26
Planetary cores are thought to form in proto-planetary disks via the growth
of dusty solid material. However, it is unclear how early this process begins.
We study the physical structure and grain growth in the edge-on disk that
surrounds the ~1 Myr old low-mass (~0.55 Msun) protostar embedded in the Bok
Globule CB26 to examine how much grain growth has already occurred in the
protostellar phase. We combine the SED between 0.9 m and 6.4 cm with
high angular resolution continuum maps at 1.3, 2.9, and 8.1 mm, and use the
radiative transfer code RADMC-3D to conduct a detailed modelling of the dust
emission from the disk and envelope of CB 26. We infer inner and outer disk
radii of around 16 au and 17222 au, respectively. The total gas mass in
the disk is ~0.076 Msun, which amounts to ~14% of the mass of the central star.
The inner disk contains a compact free-free emission region, which could be
related to either a jet or a photoevaporation region. The thermal dust emission
from the outer disk is optically thin at mm wavelengths, while the emission
from the inner disk midplane is moderately optically thick. Our best-fit
radiative transfer models indicate that the dust grains in the disk have
already grown to pebbles with diameters of the order of 10 cm in size. Residual
8.1 mm emission suggests the presence of even larger particles in the inner
disk. For the optically thin mm dust emission from the outer disk, we derive a
mean opacity slope of 0.60.4, which is consistent with the presence of
large dust grains. The presence of cm-sized bodies in the CB 26 disk indicates
that solids grow rapidly already during the first million years in a
protostellar disk. It is thus possible that Class II disks are already seeded
with large particles and may contain even planetesimals.Comment: Accepted for publication in A&A; 17 pages, 14 figure
Fragmentation and dynamical collapse of the starless high-mass star-forming region IRDC18310-4
Aims: We study the fragmentation and dynamical properties of a massive
starless gas clump at the onset of high-mass star formation. Methods: Based on
Herschel continuum data we identify a massive gas clump that remains
far-infrared dark up to 100mum wavelengths. The fragmentation and dynamical
properties are investigated by means of Plateau de Bure Interferometer and
Nobeyama 45m single-dish spectral line and continuum observations. Results: The
massive gas reservoir fragments at spatial scales of ~18000AU in four cores.
Comparing the spatial extent of this high-mass region with intermediate- to
low-mass starless cores from the literature, we find that linear sizes do not
vary significantly over the whole mass regime. However, the high-mass regions
squeeze much more gas into these similar volumes and hence have orders of
magnitude larger densities. The fragmentation properties of the presented
low-to high-mass regions are consistent with gravitational instable Jeans
fragmentation. Furthermore, we find multiple velocity components associated
with the resolved cores. Recent radiative transfer hydrodynamic simulations of
the dynamic collapse of massive gas clumps also result in multiple velocity
components along the line of sight because of the clumpy structure of the
regions. This result is supported by a ratio between viral and total gas mass
for the whole region <1. Conclusions: This apparently still starless high-mass
gas clump exhibits clear signatures of early fragmentation and dynamic collapse
prior to the formation of an embedded heating source. A comparison with regions
of lower mass reveals that the linear size of star-forming regions does not
necessarily have to vary much for different masses, however, the mass
reservoirs and gas densities are orders of magnitude enhanced for high-mass
regions compared to their lower-mass siblings.Comment: 11 pages, 10 figures, accepted to Astronomy and Astrophysics,
high-resolution version with all figures included can be found at
http://www.mpia.de/homes/beuther/papers.htm
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