198 research outputs found
CO in OH/IR stars close to the Galactic centre
Aims: A pilot project has been carried out to measure circumstellar CO
emission from three OH/IR stars close to the Galactic centre. The intention was
to find out whether it would be possible to conduct a large-scale survey for
mass-loss rates using, for example, the Atacama Large Millimeter Array (ALMA).
Such a survey would increase our understanding of the evolution of the Galactic
bulge.
Methods: Two millimetre-wave instruments were used: the Nobeyama Millimeter
Array at 115 GHz and the Submillimeter Array at 230 GHz. An interferometer is
necessary as a `spatial filter' in this region of space because of the
confusion with interstellar CO emission.
Results: Towards two of the stars, CO emission was detected with positions
and radial velocities coinciding within the statistical errors with the
corresponding data of the associated OH sources. However, for one of the stars
the line profile is not what one expects for an unresolved expanding
circumstellar envelope. We believe that this CO envelope is partially resolved
and that this star therefore is a foreground star not belonging to the bulge.
Conclusions: The results of the observations have shown that it is possible
to detect line profiles of circumstellar CO from late-type stars both within
and in the direction of the Galactic bulge. ALMA will be able to detect CO
emission in short integrations with sensitivity sufficient to estimate
mass-loss rates from a large number of such stars.Comment: 5 pages, 3 figure
Protostars and Outflows in the NGC7538 - IRS9 Cloud Core
New high resolution observations of HCO+ J=1-0, H13CN J=1-0, SO 2,2 - 1,1,
and continuum with BIMA at 3.4 mm show that the NGC7538 - IRS9 cloud core is a
site of active ongoing star formation. Our observations reveal at least three
young bipolar molecular outflows, all ~ 10,000 -- 20,000 years old. IRS9 drives
a bipolar, extreme high velocity outflow observed nearly pole on. South of IRS9
we find a cold, protostellar condensation with a size of ~ 14" x 6" with a mass
> 250 Msun. This is the center of one of the outflows and shows deep,
red-shifted self absorption in HCO+, suggesting that there is a protostar
embedded in the core, still in a phase of active accretion. This source is not
detected in the far infrared, suggesting that the luminosity < 10^4 Lsun; yet
the mass of the outflow is ~ 60 Msun. The red-shifted HCO+ self-absorption
profiles observed toward the southern protostar and IRS9 predict accretion
rates of a few times 10^-4 to 10^-3 Msun/yr. Deep VLA continuum observations at
3.6 cm show that IRS9 coincides with a faint thermal VLA source, but no other
young star in the IRS9 region has any detectable free-free emission at a level
of ~ 60 microJy at 3.6 cm. The HCO+ abundance is significantly enhanced in the
hot IRS9 outflow. A direct comparison of mass estimates from HCO+ and CO for
the well-characterized red-shifted IRS9 outflow predicts an HCO+ enhancement of
more than a factor of 30, or [HCO+/H2] >= 6 10^-8.Comment: 40 pages, 3 tables and 10 figures included; to appear in Ap
A new sample of OH/IR stars in the Galactic center
Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe
The detection of Class I methanol masers towards regions of low-mass star formation
Six young bipolar outflows in regions of low-to-intermediate-mass star
formation were observed in the 7_0-6_1A+, 8_0-7_1A+, and 5_{-1}-4_0E methanol
lines at 44, 95, and 84 GHz, respectively. Narrow features were detected
towards NGC 1333IRAS4A, HH 25MMS, and L1157 B1. Flux densities of the detected
lines are no higher than 11 Jy, which is much lower than the flux densities of
strong maser lines in regions of high-mass star formation. Analysis shows that
most likely the narrow features are masers.Comment: 12 pages, 6 figures, to be published in Astronomy Report
HI Narrow Line Absorption in Dark Clouds
We have used the Arecibo telescope to carry out an survey of 31 dark clouds
in the Taurus/Perseus region for narrow absorption features in HI (
21cm) and OH (1667 and 1665 MHz) emission. We detected HI narrow
self--absorption (HINSA) in 77% of the clouds that we observed. HINSA and OH
emission, observed simultaneously are remarkably well correlated. Spectrally,
they have the same nonthermal line width and the same line centroid velocity.
Spatially, they both peak at the optically--selected central position of each
cloud, and both fall off toward the cloud edges. Sources with clear HINSA
feature have also been observed in transitions of CO, \13co, \c18o, and CI.
HINSA exhibits better correlation with molecular tracers than with CI.
The line width of the absorption feature, together with analyses of the
relevant radiative transfer provide upper limits to the kinetic temperature of
the gas producing the HINSA. Some sources must have a temperature close to or
lower than 10 K. The correlation of column densities and line widths of HINSA
with those characteristics of molecular tracers suggest that a significant
fraction of the atomic hydrogen is located in the cold, well--shielded portions
of molecular clouds, and is mixed with the molecular gas.
The average number density ratio [HI]/[\h2] is . The
inferred HI density appears consistent with but is slightly higher than the
value expected in steady state equilibrium between formation of HI via cosmic
ray destruction of H and destruction via formation of H on grain
surfaces. The distribution and abundance of atomic hydrogen in molecular clouds
is a critical test of dark cloud chemistry and structure, including the issues
of grain surface reaction rates, PDRs, circulation, and turbulent diffusion.Comment: 40 pages, 10 figures, accepted by Ap
CO line observations of OH/IR stars in the inner Galactic Bulge: Characteristics of stars at the tip of the AGB
12CO and 13CO lines, as well as a mm-wave continuum, have been observed for a
sample of 22 OH/IR stars in directions within 2 degrees of the Galactic Centre.
Photometry data have been gathered from the literature to construct SEDs and to
determine pulsational variability. Radiative transfer models have been used to
interpret the data. All stars in the sample were detected in at least one CO
line, and 8 objects were detected in 324 GHz continuum. Based on luminosity
criteria, the sample is divided into 17 objects that most likely lie within the
inner Galactic Bulge, and 5 objects that are most likely foreground objects.
The median luminosity of the inner-Galactic-Bulge objects, 5600 Lsun,
corresponds to an initial mass in the range 1.2-1.6 Msun, indicating that these
OH/IR stars descend from solar-type stars. The objects in this sub-sample are
further divided into two classes based on their SED characteristics: 11 objects
have SEDs that are well matched by models invoking dust envelopes extending
from a few stellar radii and outwards, while 6 objects are better modelled as
having detached dust envelopes with inner radii in the range 200-600 au and
warmer central stars. The former objects have periodic variability, while the
latter objects are predominantly non-periodic. The median gas-mass-loss rate,
gas terminal expansion velocity, gas-to-dust mass ratio, and circumstellar
12CO/13CO abundance ratio have been estimated to be 2x10{-5} Msun/yr, 18 km/s,
200 (excluding the sources with detached dust envelopes, which show markedly
lower gas-to-dust ratios), and 5, respectively, for the inner-Galactic-Bulge
objects. The inner-Galactic-Bulge OH/IR stars studied here constitute an
excellent sample of equidistant objects for the purpose of understanding the
evolution of the mass-loss-rate characteristics at the tip of the AGB.Comment: 32 pages, 17 figure
Shock-Excited Maser Emission from Supernova Remnants: G32.8-0.1, G337.8-0.1, G346.6-0.2, and the HB3/W3 Complex
We present the results of VLA observations in the ground-state hydroxyl (OH)
transition at 1720 MHz toward 20 supernova remnants (SNRs). We detect compact
emission from four objects. For three of these objects (G32.8-0.1, G337.8-0.1,
and G346.6-0.2), we argue that the emission results from masers which are
shock-excited due to the interaction of the SNR and an adjacent molecular
cloud. We observe a characteristic Zeeman profile in the Stokes V spectrum,
which allows us to derive a magnetic field of 1.5 and 1.7 mG for G32.8-0.1 and
G346.6-0.2, respectively. The velocity of the masers also allows us to
determine a kinematic distance to the SNR. Our criteria for a maser to be
associated with an SNR along the line of sight are that the position and
velocity of the maser and SNR must agree, and the OH(1720) emission must be
unaccompanied by other OH lines.Comment: Accepted to the Astronomical Journal (19 pages total, 6 figures, 4
tables
Absolute positions of 6.7-GHz methanol masers
The ATCA, MERLIN and VLA interferometers were used to measure the absolute
positions of 35 6.7 GHz methanol masers to subarcsecond or higher accuracy. Our
measurements represent essential preparatory data for Very Long Baseline
Interferometry, which can provide accurate parallax and proper motion
determinations of the star-forming regions harboring the masers. Our data also
allow associations to be established with infrared sources at different
wavelengths. Our findings support the view that the 6.7 GHz masers are
associated with the earliest phases of high-mass star formation.Comment: A&A accepted 29 pages, 6 figures, 5 table
Evolved star water maser cloud size determined by star size
Cool, evolved stars undergo copious mass loss but the details of how the
matter is returned to the ISM are still under debate. We investigated the
structure and evolution of the wind at 5 to 50 stellar radii from Asymptotic
Giant Branch and Red Supergiant stars. 22-GHz water masers around seven evolved
stars were imaged using MERLIN, at sub-AU resolution. Each source was observed
at between 2 and 7 epochs (several stellar periods). We compared our results
with long-term Pushchino single dish monitoring. The 22-GHz emission is located
in ~spherical, thick, unevenly filled shells. The outflow velocity doubles
between the inner and outer shell limits. Water maser clumps could be matched
at successive epochs separated by <2 years for AGB stars, or at least 5 years
for RSG. This is much shorter than the decades taken for the wind to cross the
maser shell, and comparison with spectral monitoring shows that some features
fade and reappear. In 5 sources, most of the matched features brighten or dim
in concert from one epoch to the next. One cloud in W Hya was caught in the act
of passing in front of a background cloud leading to 50-fold, transient
amplification. The masing clouds are 1-2 orders of magnitude denser than the
wind average and contain a substantial fraction of the mass loss in this
region, with a filling factor <1%. The RSG clouds are ~10x bigger than those
round the AGB stars. Proper motions are dominated by expansion, with no
systematic rotation. The maser clouds survive for decades (the shell crossing
time) but the masers are not always beamed in our direction. Radiative effects
cause changes in flux density throughout the maser shells on short timescales.
Cloud size is proportional to parent star size; clouds have a similar radius to
the star in the 22-GHz maser shell. Stellar properties such as convection cells
must determine the clumping scale.Comment: Accepted by A&A 2012 July 10 Main text 29 pages, 62 figures Appendix
44 pages, 23 figure
Observations of the circumstellar water 110 → 110 and ammonia 110 → 00 lines in IRC +10216 by the Odin satellite
Submillimeter lines of H 2O and NH 3 have been detected in the carbon star IRC +10216 (CW Leo) with the Odin submillimeter satellite. The detection of the J K-, K+ = 1 10 → 1 10 557 GHz line of ortho-H 2O confirms the earlier detection in the same source with SWAS. The detection of the J K = 1 0 rarr; 0 0 572 GHz line represents the first observation of the ground-state rotational transition of NH 3 in a stellar envelope. By fitting a molecular line transfer model to the observed lines, we derive an ortho-H 20 abundance of 2.4 × 10 -6, which is consistent with estimates from the SWAS observation. The derived ortho-NH 3 abundance of 1 × 10 -6 relative to H 2 is significantly higher than those derived from 24 GHz inversion transitions and is slightly higher than those from vibrational transitions in the infrared band. The high H 2O and NH 3 abundances in the carbon-rich star IRC+10216 underscore shortcomings in the conventional gas-phase LTE and non-LTE chemical models. © 2006. The American Astronomical Society. All rights reserved.published_or_final_versio
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