348 research outputs found
A parsec-scale flow associated with the IRAS 16547-4247 radio jet
IRAS 16547-4247 is the most luminous (6.2 x 10^4 Lsun) embedded young stellar
object known to harbor a thermal radio jet. We report the discovery using
VLT-ISAAC of a chain of H_2 2.12 um emission knots that trace a collimated flow
extending over 1.5 pc. The alignment of the H_2 flow and the central location
of the radio jet implies that these phenomena are intimately linked. We have
also detected using TIMMI2 an isolated, unresolved 12 um infrared source
towards the radio jet . Our findings affirm that IRAS 16547-4247 is excited by
a single O-type star that is driving a collimated jet. We argue that the
accretion mechanism which produces jets in low-mass star formation also
operates in the higher mass regime.Comment: Accepted for publication in ApJL, 10 pages, 2 figure
High Angular Resolution Observations of the Collimated Jet Source Associated with a Massive Protostar in IRAS 16547-4247
A triple radio source recently detected in association with the luminous
infrared source IRAS 16547-4247 has been studied with high angular resolution
and high sensitivity with the Very Large Array at 3.6 and 2 cm. Our
observations confirm the interpretation that the central object is a thermal
radio jet, while the two outer lobes are most probably heavily obscured HH
objects. The thermal radio jet is resolved angularly for the first time and
found to align closely with the outer lobes. The opening angle of the thermal
jet is estimated to be , confirming that collimated outflows can
also be present in massive protostars. The proper motions of the outer lobes
should be measurable over timescales of a few years. Several fainter sources
detected in the region are most probably associated with other stars in a young
cluster.Comment: 9 pages, 2 figure
A multiwavelength study of young massive star forming regions: II. The dust environment
We present observations of 1.2-mm dust continuum emission, made with the
Swedish ESO Submillimeter Telescope, towards eighteen luminous IRAS point
sources, all with colors typical of compact HII regions and associated with
CS(2-1) emission, thought to be representative of young massive star forming
regions. Emission was detected toward all the IRAS objects. We find that the
1.2-mm sources associated with them have distinct physical parameters, namely
sizes of 0.4 pc, dust temperatures of 30 K, masses of 2x10^3 Msun, column
densities of 3x10^23 cm^-2, and densities of 4x10^5 cm^-3. We refer to these
dust structures as massive and dense cores. Most of the 1.2-mm sources show
single-peaked structures, several of which exhibit a bright compact peak
surrounded by a weaker extended envelope. The observed radial intensity
profiles of sources with this type of morphology are well fitted with power-law
intensity profiles with power-law indices in the range 1.0-1.7. This result
indicates that massive and dense cores are centrally condensed, having radial
density profiles with power-law indices in the range 1.5-2.2. We also find that
the UC HII regions detected with ATCA towards the IRAS sources investigated
here (Paper I) are usually projected at the peak position of the 1.2-mm dust
continuum emission, suggesting that massive stars are formed at the center of
the centrally condensed massive and dense cores.Comment: 6 figures, accepted by Ap
The open cluster NGC 6520 and the nearby dark molecular cloud Barnard 86
Wide field BVI photometry and CO(10) observations are presen ted
in the region of the open cluster NGC 6520 and the dark molecular cloud
Barnard~86. From the analysis of the optical data we find that the cluster is
rather compact, with a radius of 1.00.5 arcmin, smaller than previous
estimates. The cluster age is 15050 Myr and the reddening
E=0.420.10. The distance from the Sun is estimated to be
1900100 pc, and it is larger than previous estimates. We finally derive
basic properties of the dark nebula Barnard 86 on the assumption that it lies
at the same distance of the cluster.Comment: 21 pages, 8 eps figures (a few degraded in resolution), accepted for
publication in the Astronomical Journa
Infall, Outflow, Rotation, and Turbulent Motions of Dense Gas within NGC 1333 IRAS 4
Millimeter wavelength observations are presented of NGC 1333 IRAS 4, a group
of highly-embedded young stellar objects in Perseus, that reveal motions of
infall, outflow, rotation, and turbulence in the dense gas around its two
brightest continuum objects, 4A and 4B. These data have finest angular
resolution of approximately 2" (0.0034 pc) and finest velocity resolution of
0.13 km/s. Infall motions are seen from inverse P-Cygni profiles observed in
H2CO 3_12-2_11 toward both objects, but also in CS 3-2 and N2H+ 1-0 toward 4A,
providing the least ambiguous evidence for such motions toward low-mass
protostellar objects. Outflow motions are probed by bright line wings of H2CO
3_12-2_11 and CS 3-2 observed at positions offset from 4A and 4B, likely
tracing dense cavity walls. Rotational motions of dense gas are traced by a
systematic variation of the N2H+ line velocities, and such variations are found
around 4A but not around 4B. Turbulent motions appear reduced with scale, given
N2H+ line widths around both 4A and 4B that are narrower by factors of 2 or 3
than those seen from single-dish observations. Minimum observed line widths of
approximately 0.2 km/s provide a new low, upper bound to the velocity
dispersion of the parent core to IRAS 4, and demonstrate that turbulence within
regions of clustered star formation can be reduced significantly. A third
continuum object in the region, 4B', shows no detectable line emission in any
of the observed molecular species.Comment: LateX, 51 pages, 9 figures, accepted by Ap
Inner Structure of Protostellar Collapse Candidate B335 Derived from Millimeter-Wave Interferometry
We present a study of the density structure of the protostellar collapse
candidate B335 using continuum observations from the IRAM Plateau de Bure
Interferometer made at wavelengths of 1.2mm and 3.0mm. We analyze these data,
which probe spatial scales from 5000 AU to 500 AU, directly in the visibility
domain by comparison to synthetic observations constructed from models that
assume different physical conditions. This approach allows for much more
stringent constraints to be derived from the data than from analysis of images.
A single radial power law in density provides a good description of the data,
with best fit power law index p=1.65+/-0.05. Through simulations, we quantify
the sensitivity of this result to various model uncertainties, including
assumptions of temperature distribution, outer boundary, dust opacity spectral
index, and an unresolved central component. The largest uncertainty comes from
the unknown presence of a centralized point source. A point source with 1.2mm
flux of F=12+/-7 mJy reduces the density index to p=1.47+/-0.07. The remaining
sources of systematic uncertainty, the most important of which is the
temperature distribution, likely contribute a total uncertainty of < 0.2. We
therefore find strong evidence that the power law index of the density
distribution within 5000 AU is significantly less than the value at larger
radii, close to 2.0 from previous studies of dust emission and extinction.
These results conform well to the generic paradigm of isolated, low-mass star
formation which predicts a power law density index close to p=1.5 for an inner
region of gravitational free fall onto the protostar.Comment: Accepted to the Astrophysical Journal; 27 pages, 3 figure
Does Infall End Before the Class I Stage?
We have observed HCO+ J=3-2 toward 16 Class I sources and 18 Class 0 sources,
many of which were selected from Mardones et al. (1997). Eight sources have
profiles significantly skewed to the blue relative to optically thin lines. We
suggest six sources as new infall candidates. We find an equal "blue excess"
among Class 0 and Class I sources after combining this sample with that of
Gregersen et al. (1997). We used a Monte Carlo code to simulate the temporal
evolution of line profiles of optically thick lines of HCO+, CS and H2CO in a
collapsing cloud and found that HCO+ had the strongest asymmetry at late times.
If a blue-peaked line profile implies infall, then the dividing line between
the two classes does not trace the end of the infall stage.Comment: 21 pages, 8 figures, accepted by ApJ for April 20, 2000, added
acknowledgmen
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