75 research outputs found
SMA CO(2-1) Observations of CG30: A Protostellar Binary System with a High-Velocity Quadrupolar Molecular Outflow
We present interferometric observations in the 12CO (2-1) line and at 1.3 mm
dust continuum of the low-mass protostellar binary system in the cometary
globule CG30, using the Submillimeter Array. The dust continuum images resolve
two compact sources (CG30N and CG30S), with a linear separation of ~8700 AU and
total gas masses of ~1.4 and ~0.6 M_sun, respectively. With the CO images, we
discover two high-velocity bipolar molecular outflows, driven by the two
sources. The two outflows are nearly perpendicular to each other, showing a
quadrupolar morphology. The northern bipolar outflow extends along the
southeast (redshifted, with a velocity up to ~23 km/s) and northwest
(blueshifted, velocity up to ~30 km/s) directions, while the southern pair has
an orientation from southwest (blueshifted, velocity up to 13 km/s) to
northeast (redshifted, velocity up to ~41 km/s). The outflow mass of the
northern pair, driven by the higher mass source CG30N, is ~9 times larger than
that of the southern pair. The discovery of the quadrupolar molecular outflow
in the CG30 protobinary system, as well as the presence of other quadrupolar
outflows associated with binary systems, demonstrate that the disks in (wide)
binary systems are not necessarily co-aligned after fragmentation.Comment: 12 pages, 3 figures, to be published by ApJL in October 200
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
Evolution of Magnetic fields in Bok globules?
We study the influence and structure of the magnetic field in the early
phases of low-mass star formation using polarization maps of Bok globules at a
wavelength of 850micron, obtained with the Submillimeter Common-User Bolometer
Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT). We discuss
observations of the following sources: CB26 - a small globule with a nearly
dispersed dense core and a young and large circumstellar disk, CB54 - a large
globule with a massive dense core and a deeply embedded young stellar cluster,
and B335, CB230, and CB244 - three nearby, relatively isolated small globules
with low-mass protostellar cores.
We find strongly aligned polarization vectors in the case of CB26, B335, and
CB230, while the vector orientations in the case of CB54 and CB244 are more or
less randomly distributed. The degree of polarization, amounting to several
percent, was found to decrease toward the center in each source. Assuming
dichroic emission by aligned non-spherical grains as the polarization
mechanism, where the magnetic field plays a role in the alignment process, we
derive magnetic field strengths and structures from the observed polarization
patterns.
We compare the magnetic field topology with the morphology and outflow
directions of the globules. In the Class 0 sources B335, CB230, and CB244, the
magnetic field is oriented almost perpendicular to the ouflow direction. In
contrast, the inclination between outflow axis and magnetic field direction is
much more moderate (36deg) in the more evolved Class I source CB26.Comment: to be published in Conf. Proc. "Magnetic Fields and Star Formation
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
IRAM-PdBI Observations of Binary Protostars I: The Hierarchical System SVS13 in NGC1333
We present millimeter interferometric observations of the young stellar
object SVS13 in NCG1333 in the N2H+(1-0) line and at 1.4 and 3mm dust
continuum, using the IRAM Plateau de Bure interferometer. The results are
complemented by infrared data from the Spitzer Space Telescope. The millimeter
dust continuum images resolve four sources (A, B, C, and VLA3) in SVS13. With
the dust continuum images, we derive gas masses of 0.2-1.1 M_sun for the
sources. N2H+(1-0) line emission is detected and spatially associated with the
dust continuum sources B and VLA3. The observed mean line width is ~0.48 km/s
and the estimated virial mass is ~0.7 M_sun. By simultaneously fitting the
seven hyperfine line components of N2H+, we derive the velocity field and find
a symmetric velocity gradient of about 28 km/s/pc across sources B and VLA3,
which could be explained by core rotation. The velocity field suggests that
sources B and VLA3 are forming a physically bound protobinary system embedded
in a common N2H+ core. Spitzer images show mid-infrared emission from sources A
and C, which is spatially associated with the mm dust continuum emission. No
infrared emission is detected from source B, implying that the source is deeply
embedded. Based on the morphologies and velocity structure, we propose a
hierarchical fragmentation picture for SVS13 where the three sources (A, B, and
C) were formed by initial fragmentation of a filamentary prestellar core, while
the protobinary system (sources B and VLA3) was formed by rotational
fragmentation of a single collapsing sub-core.Comment: 26 pages, 9 figures, accepted by Ap
Dust SEDs in the era of Herschel and Planck: a Hierarchical Bayesian fitting technique
We present a hierarchical Bayesian method for fitting infrared spectral
energy distributions (SEDs) of dust emission to observed fluxes. Under the
standard assumption of optically thin single temperature (T) sources the dust
SED as represented by a power--law modified black body is subject to a strong
degeneracy between T and the spectral index beta. The traditional
non-hierarchical approaches, typically based on chi-square minimization, are
severely limited by this degeneracy, as it produces an artificial
anti-correlation between T and beta even with modest levels of observational
noise. The hierarchical Bayesian method rigorously and self-consistently treats
measurement uncertainties, including calibration and noise, resulting in more
precise SED fits. As a result, the Bayesian fits do not produce any spurious
anti-correlations between the SED parameters due to measurement uncertainty. We
demonstrate that the Bayesian method is substantially more accurate than the
chi-square fit in recovering the SED parameters, as well as the correlations
between them. As an illustration, we apply our method to Herschel and sub
millimeter ground-based observations of the star-forming Bok globule CB244.
This source is a small, nearby molecular cloud containing a single low-mass
protostar and a starless core. We find that T and beta are weakly positively
correlated -- in contradiction with the chi-square fits, which indicate a
T-beta anti-correlation from the same data-set. Additionally, in comparison to
the chi-square fits the Bayesian SED parameter estimates exhibit a reduced
range in values.Comment: 20 pages, 9 figures, ApJ format, revised version matches ApJ-accepted
versio
HD 135344B: a young star has reached its rotational limit
We search for periodic variations in the radial velocity of the young Herbig
star HD 135344B with the aim to determine a rotation period. We analyzed 44
high-resolution optical spectra taken over a time range of 151 days. The
spectra were acquired with FEROS at the 2.2m MPG/ESO telescope in La Silla. The
stellar parameters of HD 135344B are determined by fitting synthetic spectra to
the stellar spectrum. In order to obtain radial velocity measurements, the
stellar spectra have been cross-correlated with a theoretical template computed
from determined stellar parameters. We report the first direct measurement of
the rotation period of a Herbig star from radial-velocity measurements. The
rotation period is found to be 0.16 d (3.9 hr), which makes HD 135344B a rapid
rotator at or close to its break-up velocity. The rapid rotation could explain
some of the properties of the circumstellar environment of HD 135344B such as
the presence of an inner disk with properties (composition, inclination), that
are significantly different from the outer disk.Comment: Accepted for publication in Astronomy & Astrophysics, 9 pages, 12
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