647 research outputs found
First evidence for molecular interfaces between outflows and ambient clouds in high-mass star-forming regions?
We present new observations of the Cep A East region of massive star formation and describe an extended and dynamically distinct feature not previously recognized. This feature is present in emission from H2CS, OCS, CH3OH, and HDO at −5.5 km s−1 but is not traced by the conventional tracers of star-forming regions, H2S, SO2, SO, and CS. The feature is extended up to at least 0.1 pc. We show that the feature is neither a hot core nor a shocked outflow. However, the chemistry of the feature is consistent with predictions from a model of an eroding interface between a fast wind and a dense core; mixing between the two media occurs in the interface on a timescale of 10–50 yr. If these observations are confirmed by detailed maps and by detections in species also predicted to be abundant (e.g., HCO+, H2CO, and NH3), this feature would be the first detection of such an interface in regions of massive star formation. An important implication of the model is that a significant reservoir of sulfur in grain mantles is required to be in the form of OCS
Molecular ions in the protostellar shock L1157-B1
We perform a complete census of molecular ions with an abundance larger than
1e-10 in the protostellar shock L1157-B1 by means of an unbiased
high-sensitivity survey obtained with the IRAM-30m and Herschel/HIFI. By means
of an LVG radiative transfer code the gas physical conditions and fractional
abundances of molecular ions are derived. The latter are compared with
estimates of steady-state abundances in the cloud and their evolution in the
shock calculated with the chemical model Astrochem. We detect emission from
HCO+, H13CO+, N2H+, HCS+, and, for the first time in a shock, from HOCO+, and
SO+. The bulk of the emission peaks at blueshifted velocity, ~ 0.5-3 km/s with
respect to systemic, has a width of ~ 4-8 km/s, and is associated with the
outflow cavities (T_kin ~ 20-70 K, n(H2) ~ 1e5 cm-3). Observed HCO+ and N2H+
abundances are in agreement with steady-state abundances in the cloud and with
their evolution in the compressed and heated gas in the shock for cosmic rays
ionization rate Z = 3e-16 s-1. HOCO+, SO+, and HCS+ observed abundances,
instead, are 1-2 orders of magnitude larger than predicted in the cloud; on the
other hand they are strongly enhanced on timescales shorter than the shock age
(~2000 years) if CO2, S or H2S, and OCS are sputtered off the dust grains in
the shock. The performed analysis indicates that HCO+ and N2H+ are a fossil
record of pre-shock gas in the outflow cavity, while HOCO+, SO+, and HCS+ are
effective shock tracers and can be used to infer the amount of CO2 and
sulphur-bearing species released from dust mantles in the shock. The observed
HCS+ (and CS) abundance indicates that OCS should be one of the main sulphur
carrier on grain mantles. However, the OCS abundance required to fit the
observations is 1-2 orders of magnitude larger than observed. Further studies
are required to fully understand the chemistry of sulphur-bearing species.Comment: 12 pages, 5 figures, accepted by A&
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
Discovery Of A Molecular Outflow in the Haro 6-10 Star-Forming Region
We present high sensitivity 12CO and 13CO (1-0) molecular line maps covering
the full extent of the parsec scale Haro~6-10 Herbig-Haro (HH) flow. We report
the discovery of a molecular CO outflow along the axis of parsec-scale HH flow.
Previous molecular studies missed the identification of the outflow probably
due to their smaller mapping area and the confusing spectral features present
towards the object. Our detailed molecular line study of the full 1.6 pc extent
of the optical flow shows evidence for both blueshifted and redshifted gas set
in motion by Haro~6-10 activity. The molecular outflow is centered at
Haro~6-10, with redshifted gas being clumpy and directed towards the northeast,
while blueshifted gas is in the southwest direction. The molecular gas
terminates well within the cloud, short of the most distant HH objects of the
optical flow. Contamination from an unrelated cloud along the same line of
sight prevents a thorough study of the blueshifted outflow lobe and the mass
distribution at the lowest velocities in both lobes. The cloud core in which
Haro~6-10 is embedded is filamentary and flattened in the east-west direction.
The total cloud mass is calculated from 13CO(1-0) to be ~200Msun. The lower
limit of the mass associated with the outflow is ~0.25Msun.Comment: ApJ Accepted; 9 pages, 8 figures. For high resolution ps file use:
http://www.astro.umass.edu/~irena/haro.p
Candidate Rotating Toroids around High-Mass (Proto)Stars
Using the OVRO, Nobeyama, and IRAM mm-arrays, we searched for
``disk''-outflow systems in three high-mass (proto)star forming regions:
G16.59-0.05, G23.01-0.41, and G28.87+0.07. These were selected from a sample of
NH3 cores associated with OH and H2O maser emission and with no or very faint
continuum emission. Our imaging of molecular line (including rotational
transitions of CH3CN and 3mm dust continuum emission revealed that these are
compact, massive, and hot molecular cores (HMCs), that is likely sites of
high-mass star formation prior to the appearance of UCHII regions. All three
sources turn out to be associated with molecular outflows from CO and/or HCO+
J=1--0 line imaging. In addition, velocity gradients of 10 -- 100 km/s per pc
in the innermost densest regions of the G23.01 and G28.87 HMCs are identified
along directions roughly perpendicular to the axes of the corresponding
outflows. All the results suggest that these cores might be rotating about the
outflow axis, although the contribution of rotation to gravitational
equilibrium of the HMCs appears to be negligible. Our analysis indicates that
the 3 HMCs are close to virial equilibrium due to turbulent pressure support.
Comparison with other similar objects where rotating toroids have been
identified so far shows that in our case rotation appears to be much less
prominent; this can be explained by the combined effect of unfavorable
projection, large distance, and limited angular resolution with the current
interferometers.Comment: Accepted by ApJ main journal, the paper with the original quality
figures are available from
http://subarutelescope.org/staff/rsf/publication.htm
Water maser variability over 20 years in a large sample of star-forming regions: the complete database
Context. Water vapor emission at 22 GHz from masers associated with
star-forming regions is highly variable. Aims. We present a database of up to
20 years of monitoring of a sample of 43 masers within star-forming regions.
The sample covers a large range of luminosities of the associated IRAS source
and is representative of the entire population of H2O masers of this type. The
database forms a good starting point for any further study of H2O maser
variability. Methods. The observations were obtained with the Medicina 32-m
radiotelescope, at a rate of 4-5 observations per year. Results. To provide a
database that can be easily accessed through the web, we give for each source:
plots of the calibrated spectra, the velocity-time-flux density plot, the light
curve of the integrated flux, the lower and upper envelopes of the maser
emission, the mean spectrum, and the rate of the maser occurrence as a function
of velocity. Figures for just one source are given in the text for
representative purposes. Figures for all the sources are given in electronic
form in the on-line appendix. A discussion of the main properties of the H2O
variability in our sample will be presented in a forthcoming paper.Comment: 11 pages, 9 figures, to be published in Astronomy and Astrophysics;
all plots in appendix (not included) can be downloaded from
http://www.arcetri.astro.it/~starform/water_maser_v2.html or
http://www.ira.inaf.it/papers/masers/water_maser_v2.htm
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