1,580 research outputs found
High-spatial-resolution CN and CS observation of two regions of massive star formation
Molecular line CN, CS and mm continuum observations of two intermediate- to
high-mass star-forming regions - IRAS20293+3952 and IRAS19410+2336 - obtained
with the Plateau de Bure Interferometer at high spatial resolution reveal
interesting characteristics of the gas and dust emission. In spite of the
expectation that the CN and CS morphology might closely follow the dense gas
traced by the dust continuum, both molecules avoid the most central cores.
Comparing the relative line strengths of various CN hyperfine components, this
appears not to be an opacity effect but to be due to chemical and physical
effects. The CN data also indicate enhanced emission toward the different
molecular outflows in the region. Regarding CS, avoiding the central cores can
be due to high optical depth, but the data also show that the CS emission is
nearly always associated with the outflows of the region. Therefore, neither CS
nor CN appear well suited for dense gas and disk studies in these two sources,
and we recommend the use of different molecules for future massive disk
studies. An analysis of the 1 and 3mm continuum fluxes toward IRAS20293+3952
reveals that the dust opacity index beta is lower than the canonical value of
2. Tentatively, we identify a decreasing gradient of beta from the edge of the
core to the core center. This could be due to increasing optical depth toward
the core center and/or grain growth within the densest cores and potential
central disks. We detect 3mm continuum emission toward the collimated outflow
emanating from IRAS20293+3952. The spectral index of alpha ~ 0.8 in this region
is consistent with standard models for collimated ionized winds.Comment: 5 pages, 2 tables, 9 figures, accepted for Ap
The kinematics of molecular clumps surrounding hot cores in G29.96-0.02 and G31.41+0.31
We present high angular resolution interferometric observations of the 3 and
1.3mm continuum emission, and HCO+(1-0) and SiO(2-1)v=0 lines, obtained with
the Owens Valley Radio Observatory millimeter-wave array, toward two hot cores
(HCs) associated with two well known ultracompact (UC) HII regions: G29.96-0.02
and G31.41+0.31. These HCs are believed to host young forming massive stars
which have been suggested to be surrounded by massive rotating accretion disks.
The aim of these new observations is to study the structure and kinematics of
the molecular clumps surrounding the HCs and nearby UCHII regions at moderately
high angular resolution. Our observations reveal that the clumps within which
the HCs and UCHII regions are embedded have a complex kinematical structure.
The total mass of the clumps is estimated to be in the range 1000-3000 Msun,
consistent with previous findings. Our observations also show compelling
evidence that the clump in G29.96-0.02 is contracting onto the HC position,
suggesting that the accretion process onto the massive young stellar object
embedded in the HC is still ongoing. In these objects the kinematical structure
that we observe is also compatible with the presence of a massive rotating disk
within the HC, even though we cannot prove this suggestion with our data. The
case of G31.41+0.31 is more complicated, and our data, although consistent with
the presence of an inner disk and an infalling envelope around it, do not have
the required spatial resolution to resolve the different structures.Comment: 13 pages, 15 figs, A&A in pres
Water masers in the massive protostar IRAS 20126+4104: ejection and deceleration
We report on the first multi-epoch, phase referenced VLBI observations of the
water maser emission in a high-mass protostar associated with a disk-jet
system. The source under study, IRAS 20126+4104, has been extensively
investigated in a large variety of tracers, including water maser VLBA data
acquired by us three years before the present observations. The new findings
fully confirm the interpretation proposed in our previous study, namely that
the maser spots are expanding from a common origin coincident with the
protostar. We also demonstrate that the observed 3-D velocities of the maser
spots can be fitted with a model assuming that the spots are moving along the
surface of a conical jet, with speed increasing for increasing distance from
the cone vertex. We also present the results of single-dish monitoring of the
water maser spectra in IRAS 20126+4104. These reveal that the peak velocity of
some maser lines decreases linearly with time. We speculate that such a
deceleration could be due to braking of the shocks from which the maser
emission originates, due to mass loading at the shock front or dissipation of
the shock energy.Comment: 11 pages, 8 figures. Accepted for publication in A&
Infall of gas as the formation mechanism of stars up to 20 times more massive than the Sun
Theory predicts and observations confirm that low-mass stars (like the Sun)
in their early life grow by accreting gas from the surrounding material. But
for stars ~ 10 times more massive than the Sun (~10 M_sun), the powerful
stellar radiation is expected to inhibit accretion and thus limit the growth of
their mass. Clearly, stars with masses >10 M_sun exist, so there must be a way
for them to form. The problem may be solved by non-spherical accretion, which
allows some of the stellar photons to escape along the symmetry axis where the
density is lower. The recent detection of rotating disks and toroids around
very young massive stars has lent support to the idea that high-mass (> 8
M_sun) stars could form in this way. Here we report observations of an ammonia
line towards a high-mass star forming region. We conclude from the data that
the gas is falling inwards towards a very young star of ~20 M_sun, in line with
theoretical predictions of non-spherical accretion.Comment: 11 pages, 2 figure
Dissecting a hot molecular core: The case of G31.41+0.31
We made a detailed observational analysis of a well known hot molecular core
lying in the high-mass star-forming region G31.41+0.31. This core is believed
to contain deeply embedded massive stars and presents a velocity gradient that
has been interpreted either as rotation or as expansion, depending on the
authors. Our aim was to shed light on this question and possibly prepare the
ground for higher resolution ALMA observations which could directly detect
circumstellar disks around the embedded massive stars. Observations at
sub-arcsecond resolution were performed with the Submillimeter Array in methyl
cyanide, a typical hot molecular core tracer, and 12CO and 13CO, well known
outflow tracers. We also obtained sensitive continuum maps at 1.3 mm. Our
findings confirm the existence of a sharp velocity gradient across the core,
but cannot confirm the existence of a bipolar outflow perpendicular to it. The
improved angular resolution and sampling of the uv plane allow us to attain
higher quality channel maps of the CH3CN lines with respect to previous studies
and thus significantly improve our knowledge of the structure and kinematics of
the hot molecular core. While no conclusive argument can rule out any of the
two interpretations (rotation or expansion) proposed to explain the velocity
gradient observed in the core, in our opinion the observational evidence
collected so far indicates the rotating toroid as the most likely scenario. The
outflow hypothesis appears less plausible, because the dynamical time scale is
too short compared to that needed to form species such as CH3CN, and the mass
loss and momentum rates estimated from our measurements appear too high.Comment: Astronomy and Astrophysics, in pres
Interferometric mapping of Magnetic fields: G30.79 FIR 10
We present polarization maps of G30.79 FIR 10 (in W43) from thermal dust
emission at 1.3 mm and from CO J= line emission. The observations were
obtained using the Berkeley-Illinois-Maryland Association array in the period
2002-2004. The G30.79 FIR 10 region shows an ordered polarization pattern in
dust emission, which suggests an hourglass shape for the magnetic field. Only
marginal detections for line polarization were made from this region.
Application of the Chandrashkar-Fermi method yielded mG
and a statistically corrected mass to magnetic flux ratio , or essentially critical.Comment: 11 pages, 2 Figures, Published in Ap
The structure of molecular clumps around high-mass young stellar objects
We have used the IRAM 30-m and FCRAO 14-m telescopes to observe the molecular
clumps associated with 12 ultracompact (UC) HII regions in the J=6-5, 8-7 and
13-12 rotational transitions of methyl-acetylene (CH3C2H). Under the assumption
of LTE and optically thin emission, we have derived temperature estimates
ranging from 30 to 56 K. We estimate that the clumps have diameters of 0.2-1.6
pc, H_2 densities of 10^5-10^6 {cm^{-3}}, and masses of 10^2-2 10^4 M_\odot. We
compare these values with those obtained by other authors from different
molecular tracers and find that the H_2 density and the temperature inside the
clumps vary respectively like n_{H_2} ~ R^{-2.6} and T ~ R^{-0.5}, with R
distance from the centre. We also find that the virial masses of the clumps are
~3 times less than those derived from the CH3C2H column densities: we show that
a plausible explanation is that magnetic fields play an important role to
stabilise the clumps, which are on the verge of gravitational collapse.
Finally, we show that the CH3C2H line width increases for decreasing distance
from the clump centre: this effect is consistent with infall in the inner
regions of the clumps. We conclude that the clumps around UC HII regions are
likely to be transient (~10^(5) yr) entities, remnants of isothermal spheres
currently undergoing gravitational collapse: the high mass accretion rates
(~10^{-2} M_\odot yr^{-1}) lead to massive star formation at the centre of such
clumps.Comment: 15 pages, 11 figures, A & A in pres
Submillimeter Observations of The Isolated Massive Dense Clump IRAS 20126+4104
We used the CSO 10.4 meter telescope to image the 350 micron and 450 micron
continuum and CO J=6-5 line emission of the IRAS 20126+4104 clump. The
continuum and line observations show that the clump is isolated over a 4 pc
region and has a radius of ~ 0.5 pc. Our analysis shows that the clump has a
radial density profile propto r ^{-1.2} for r <~ 0.1 pc and has propto r^{-2.3}
for r >~ 0.1 pc which suggests the inner region is infalling, while the infall
wave has not yet reached the outer region. Assuming temperature gradient of
r^{-0.35}, the power law indices become propto r ^{-0.9} for r < ~0.1 pc and
propto r^{-2.0} for r >~ 0.1 pc. Based on a map of the flux ratio of
350micron/450micron, we identify three distinct regions: a bipolar feature that
coincides with the large scale CO bipolar outflow; a cocoon-like region that
encases the bipolar feature and has a warm surface; and a cold layer outside of
the cocoon region. The complex patterns of the flux ratio map indicates that
the clump is no longer uniform in terms of temperature as well as dust
properties. The CO emission near the systemic velocity traces the dense clump
and the outer layer of the clump shows narrow line widths (< ~3 km/s). The
clump has a velocity gradient of ~ 2 km/s pc^{-1}, which we interpret as due to
rotation of the clump, as the equilibrium mass (~ 200 Msun) is comparable to
the LTE mass obtained from the CO line. Over a scale of ~ 1 pc, the clump
rotates in the opposite sense with respect to the >~ 0.03 pc disk associated
with the (proto)star. This is one of four objects in high-mass and low-mass
star forming regions for which a discrepancy between the rotation sense of the
envelope and the core has been found, suggesting that such a complex kinematics
may not be unusual in star forming regions.Comment: 34 pages, 13 figures, Accepted for publication in the Ap
Direct Detection of a (Proto)Binary-Disk System in IRAS 20126+4104
We report the direct detection of a binary/disk system towards the high-mass
(proto)stellar object IRAS20126+4104 at infrared wavengths. The presence of a
multiple system had been indicated by the precession of the outflow and the
double jet system detected earlier at cm-wavelengths. Our new K, L' & M' band
infrared images obtained with the UKIRT under exceptional seeing conditions on
Mauna Kea are able to resolve the central source for the first time, and we
identify two objects separated by ~ 0.5'' (850 AU). The K and L' images also
uncover features characteristic of a nearly edge-on disk, similar to many low
mass protostars with disks: two emission regions oriented along an outflow axis
and separated by a dark lane. The peaks of the L' & M' band and mm-wavelength
emission are on the dark lane, presumably locating the primary young star. The
thickness of the disk is measured to be ~ 850 AU for radii < 1000 AU.
Approximate limits on the NIR magnitudes of the two young stars indicate a
high-mass system, although with much uncertainty. These results are a
demonstration of the high-mass nature of the system, and the similarities of
the star-formation process in the low-mass and high-mass regimes viz. the
presence of a disk-accretion stage. The companion is located along the dark
lane, consistent with it being in the equatorial/disk plane, indicating a
disk-accretion setting for massive, multiple, star-formation.Comment: 12 pages, 3 figures (1 pseudo colour), 1 table; colour figure
replaced with jpg file; to be published in ApJL; (back after temoprary
withdrawal due to non-scientific reasons.
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