1,293 research outputs found
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
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.
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
Chemical Diversity in High-Mass Star Formation
Massive star formation exhibits an extremely rich chemistry. However, not
much evolutionary details are known yet, especially at high spatial resolution.
Therefore, we synthesize previously published Submillimeter Array
high-spatial-resolution spectral line observations toward four regions of
high-mass star formation that are in various evolutionary stages with a range
of luminosities. Estimating column densities and comparing the spatially
resolved molecular emission allows us to characterize the chemical evolution in
more detail. Furthermore, we model the chemical evolution of massive warm
molecular cores to be directly compared with the data. The four regions reveal
many different characteristics. While some of them, e.g., the detection rate of
CH3OH, can be explained by variations of the average gas temperatures, other
features are attributed to chemical effects. For example, C34S is observed
mainly at the core-edges and not toward their centers because of
temperature-selective desorption and successive gas-phase chemistry reactions.
Most nitrogen-bearing molecules are only found toward the hot molecular cores
and not the earlier evolutionary stages, indicating that the formation and
excitation of such complex nitrogen-bearing molecules needs significant heating
and time to be fully developed. Furthermore, we discuss the observational
difficulties to study massive accretion disks in the young deeply embedded
phase of massive star formation. The general potential and limitations of such
kind of dataset are discussed, and future directions are outlined. The analysis
and modeling of this source sample reveals many interesting features toward a
chemical evolutionary sequence. However, it is only an early step, and many
observational and theoretical challenges in that field lie ahead.Comment: 14 pages, 9 figures, accepted for the Astronomical Journal, a high
resolution version can be found at
http://www.mpia.de/homes/beuther/papers.htm
Search for massive protostar candidates in the southern hemisphere: II. Dust continuum emission
In an ongoing effort to identify and study high-mass protostellar candidates
we have observed in various tracers a sample of 235 sources selected from the
IRAS Point Source Catalog, mostly with dec < -30 deg, with the SEST antenna at
millimeter wavelengths. The sample contains 142 Low sources and 93 High, which
are believed to be in different evolutionary stages. Both sub-samples have been
studied in detail by comparing their physical properties and morphologies.
Massive dust clumps have been detected in all but 8 regions, with usually more
than one clump per region. The dust emission shows a variety of complex
morphologies, sometimes with multiple clumps forming filaments or clusters. The
mean clump has a linear size of ~0.5 pc, a mass of ~320 Msolar for a dust
temperature Td=30 K, an H_2 density of 9.5E5 cm-3, and a surface density of 0.4
g cm-2. The median values are 0.4 pc, 102 Msolar, 4E4 cm-3, and 0.14 g cm-2,
respectively. The mean value of the luminosity-to-mass ratio, L/M ~99
Lsolar/Msolar, suggests that the sources are in a young, pre-ultracompact HII
phase. We have compared the millimeter continuum maps with images of the mid-IR
MSX emission, and have discovered 95 massive millimeter clumps non-MSX
emitters, either diffuse or point-like, that are potential prestellar or
precluster cores. The physical properties of these clumps are similar to those
of the others, apart from the mass that is ~3 times lower than for clumps with
MSX counterpart. Such a difference could be due to the potential prestellar
clumps having a lower dust temperature. The mass spectrum of the clumps with
masses above M ~100 Msolar is best fitted with a power-law dN/dM proportional
to M-alpha with alpha=2.1, consistent with the Salpeter (1955) stellar IMF,
with alpha=2.35.Comment: 83 pages, 10 figures, 3 tables. Accepted for publication by A&A. The
full paper, including Fig.2 with the maps of all the individual regions,
complete Tables 1 and 2 can be found at
http://www.arcetri.astro.it/~starform/publ2005.ht
First ALMA maps of HCO, an important precursor of complex organic molecules, towards IRAS 16293-2422
The formyl radical HCO has been proposed as the basic precursor of many
complex organic molecules such as methanol (CHOH) or glycolaldehyde
(CHOHCHO). Using ALMA, we have mapped, for the first time at high angular
resolution (1, 140 au), HCO towards the Solar-type
protostellar binary IRAS 162932422, where numerous complex organic molecules
have been previously detected. We also detected several lines of the chemically
related species HCO, CHOH and CHOHCHO. The observations revealed
compact HCO emission arising from the two protostars. The line profiles also
show redshifted absorption produced by foreground material of the circumbinary
envelope that is infalling towards the protostars. Additionally, IRAM 30m
single-dish data revealed a more extended HCO component arising from the common
circumbinary envelope. The comparison between the observed molecular abundances
and our chemical model suggests that whereas the extended HCO from the envelope
can be formed via gas-phase reactions during the cold collapse of the natal
core, the HCO in the hot corinos surrounding the protostars is predominantly
formed by the hydrogenation of CO on the surface of dust grains and subsequent
thermal desorption during the protostellar phase. The derived abundance of HCO
in the dust grains is high enough to produce efficiently more complex species
such as HCO, CHOH, and CHOHCHO by surface chemistry. We found that
the main formation route of CHOHCHO is the reaction between HCO and
CHOH.Comment: Accepted in Monthly Notices of the Royal Astronomical Society; 19
pages, 12 figures, 7 table
SiO collimated outflows driven by high-mass YSOs in G24.78+0.08
We imaged the molecular outflows towards the cluster of high-mass young
stellar objects G24.78+0.08 at high-angular resolution using SiO emission,
which is considered the classical tracer of protostellar jets. We performed SiO
observations with the VLA interferometer in the J = 1-0 v=0 transition and with
the SMA array in the 5-4 transition. A complementary IRAM 30-m single-dish
survey in the (2-1), (3-2), (5-4), and (6-5) SiO lines was also carried out.
Two collimated SiO high-velocity outflows driven by the A2 and C millimeter
continuum massive cores have been imaged. On the other hand, we detected no SiO
outflow driven by the young stellar objects in more evolved evolutionary phases
that are associated with ultracompact (B) or hypercompact (A1) HII regions. The
LVG analysis reveals high-density gas (10^3-10^4 cm-3), with well constrained
SiO column densities (0.5-1 10^15 cm-2). The driving source of the A2 outflow
is associated with typical hot core tracers such as methyl formate, vinyl
cyanide, cyanoacetilene, and acetone. The driving source of the main SiO
outflow in G24 has an estimated luminosity of a few 10^4 Lsun (typical of a
late O-type star) and is embedded in the 1.3 mm continuum core A2, which in
turn is located at the centre of a hot core that rotates on a plane
perpendicular to the outflow main axis. The present SiO images support a
scenario similar to the low-mass case for massive star formation, where jets
that are clearly traced by SiO emission, create outflows of swept-up ambient
gas usually traced by CO.Comment: Astronomy & Astrophysics, in pres
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