196 research outputs found
The mass-loss rates and molecular abundances of S-type AGB stars
The S-type stars are believed to have a C/O-ratio close to unity (within a
few percent). They are considered to represent an intermediate evolutionary
stage as AGB stars evolve from oxygen-rich M-type stars into carbon stars. As
possible transition objects the S-type stars could give important clues to the
mass-loss mechanism(s) and to the chemical evolution along the AGB. Using
observations of circumstellar radio line emission in combination with a
detailed radiative transfer analysis, we have estimated mass-loss rates and
abundances of chemically important molecules (SiO, HCN) for a sample of 40
S-type AGB stars. The results will be compared to previous results for M-type
and carbon stars.Comment: To appear in the proceedings of Why Galaxies Care About AGB stars I
The abundance of SiS in circumstellar envelopes around AGB stars
New SiS multi-transition (sub-)millimetre line observations of a sample of
AGB stars with varying photospheric C/O-ratios and mass-loss rates are
presented. A combination of low- and high-energy lines are important in
constraining the circumstellar distribution of SiS molecules. A detailed
radiative transfer modelling of the observed SiS line emission is performed,
including the effect of thermal dust grains in the excitation analysis. We find
that the circumstellar fractional abundance of SiS in these environments has a
strong dependence on the photospheric C/O-ratio as expected from chemical
models. The carbon stars (C/O>1) have a mean fractional abundance of 3.1E-6,
about an order of magnitude higher than found for the M-type AGB stars (C/O<1)
where the mean value is 2.7E-7. These numbers are in reasonable agreement with
photospheric LTE chemical models. SiS appears to behave similar to SiO in terms
of photodissociation in the outer part of the circumstellar envelope. In
contrast to previous results for the related molecule SiO, there is no strong
correlation of the fractional abundance with density in the CSE, as would be
the case if freeze-out onto dust grains were important. However, possible
time-variability of the line emission in the lower J transitions and the
sensitivity of the line emission to abundance gradients in the inner part of
the CSE may mask a correlation with the density of the wind. There are
indications that the SiS fractional abundance could be significantly higher
closer to the star which, at least in the case of M-type AGB stars, would
require non-equilibrium chemical processes.Comment: Accepted for publication in A&A (14 pages, 7 figures
On the origin of H_2CO abundance enhancements in low-mass protostars
High angular resolution H_2CO 218 GHz line observations have been carried out toward the low-mass protostars IRAS 16293-2422 and L1448-C using the Owens Valley Millimeter Array at ~2" resolution. Simultaneous 1.37 mm continuum data reveal extended emission which is compared with that predicted by model envelopes constrained from single-dish data. For L1448-C the model density structure works well down to the 400 AU scale to which the interferometer is sensitive. For IRAS 16293-2422 , a known proto-binary object, the interferometer observations indicate that the binary has cleared much of the material in the inner part of the envelope, out to the binary separation of ~800 AU. For both sources there is excess unresolved compact emission centered on the sources, most likely due to accretion disks âŸ200 AU in size with masses of âł0.02 M_â (L1448-C) and âł0.1 M_â (IRAS 16293-2422). The H_2CO data for both sources are dominated by emission from gas close to the positions of the continuum peaks. The morphology and velocity structure of the H_2CO array data have been used to investigate whether the abundance enhancements inferred from single-dish modelling are due to thermal evaporation of ices or due to liberation of the ice mantles by shocks in the inner envelope. For IRAS 16293-2422 the H_2CO interferometer observations indicate the presence of rotation roughly perpendicular to the large scale CO outflow. The H_2CO distribution differs from that of C^(18)O, with C^(18)O emission peaking near MM1 and H_2CO stronger near MM2. For L1448-C, the region of enhanced H_2CO emission extends over a much larger scale >1" than the radius of 50-100 K (0."6-0".15) where thermal evaporation can occur. The red-blue asymmetry of the emission is consistent with the outflow; however the velocities are significantly lower. The H_2CO 3_(22)-2_(21)/3_(03)-2_(02) flux ratio derived from the interferometer data is significantly higher than that found from single-dish observations for both objects, suggesting that the compact emission arises from warmer gas. Detailed radiative transfer modeling shows, however, that the ratio is affected by abundance gradients and optical depth in the 3_(03)-2_(02) line. It is concluded that a constant H_2CO abundance throughout the envelope cannot fit the interferometer data of the two H_2CO lines simultaneously on the longest and shortest baselines. A scenario in which the H_2CO abundance drops in the cold dense part of the envelope where CO is frozen out but is undepleted in the outermost region provides good fits to the single-dish and interferometer data on short baselines for both sources. Emission on the longer baselines is best reproduced if the H_2CO abundance is increased by about an order of magnitude from ~ 10^(-10) to ~ 10^(-9) in the inner parts of the envelope due to thermal evaporation when the temperature exceeds ~50 K. The presence of additional H_2CO abundance jumps in the innermost hot core region or in the disk cannot be firmly established, however, with the present sensitivity and resolution. Other scenarios, including weak outflow-envelope interactions and photon heating of the envelope, are discussed and predictions for future generation interferometers are presented, illustrating their potential in distinguishing these competing scenarios
Circumstellar molecular line emission from S-type AGB stars: Mass-loss rates and SiO abundances
The main aim is to derive reliable mass-loss rates and circumstellar SiO
abundances for a sample of 40 S-type AGB stars based on new multi-transitional
CO and SiO radio line observations. In addition, the results are compared to
previous results for M-type AGB stars and carbon stars to look for trends with
chemical type. The circumstellar envelopes are assumed to be spherically
symmetric and formed by a constant mass-loss rate. The mass-loss rates are
estimated from fitting the CO observations using a non-local, non-LTE radiative
transfer code. Once the physical properties of the circumstellar envelopes are
determined, the same radiative transfer code is used to model the observed SiO
lines in order to derive circumstellar abundances and the sizes of the SiO
line-emitting regions. We have estimated mass-loss rates of 40 S-type AGB stars
and find that the derived mass-loss rates have a distribution that resembles
those previously derived for similar samples of M-type AGB stars and carbon
stars. The estimated mass-loss rates also correlate well with the corresponding
expansion velocity. In all, this indicates that the mass loss is driven by the
same mechanism in all three chemical types of AGB stars. In addition, we have
estimated the circumstellar fractional abundance of SiO relative to H2 in 26 of
the sample S-type AGB stars. The derived SiO abundances are, on average, about
an order of magnitude higher than predicted by stellar atmosphere thermal
equilibrium chemistry, indicating that non-equilibrium chemical processes
determines the abundance of SiO in the circumstellar envelope. Moreover, a
comparison with the results for M-type AGB stars and carbon stars show that for
a certain mass-loss rate, the circumstellar SiO abundance seems independent
(although with a large scatter) of the C/O-ratio.Comment: 24 pages, 11 figure
The abundance of HCN in circumstellar envelopes of AGB stars of different chemical types
A multi-transition survey of HCN (sub-) millimeter line emission from a large
sample of AGB stars of different chemical type is presented. The data are
analysed and circumstellar HCN abundances are estimated. The sample stars span
a large range of properties such as mass-loss rate and photospheric C/O-ratio.
The analysis of the new data allows for more accurate estimates of the
circumstellar HCN abundances and puts new constraints on chemical models. In
order to constrain the circumstellar HCN abundance distribution a detailed
non-LTE excitation analysis, based on the Monte Carlo method, is performed.
Effects of line overlaps and radiative excitation from dust grains are
included. The median values for the derived abundances of HCN (with respect to
H2) are 3x10-5, 7x10-7 and 10-7 for carbon stars (25 stars), S-type AGB stars
(19 stars) and M-type AGB stars (25 stars), respectively. The estimated sizes
of the HCN envelopes are similar to those obtained in the case of SiO for the
same sample of sources and agree well with previous results from
interferometric observations, when these are available. We find that there is a
clear dependence of the derived circumstellar HCN abundance on the C/O-ratio of
the star, in that carbon stars have about two orders of magnitude higher
abundances than M-type AGB stars, on average. The derived HCN abundances of the
S-type AGB stars have a larger spread and typically fall in between those of
the two other types, however, slightly closer to the values for the M-type AGB
stars. For the M-type stars, the estimated abundances are much higher than what
would be expected if HCN is formed in thermal equilibrium. However, the results
are also in contrast to predictions from recent non-LTE chemical models, where
very little difference is expected in the HCN abundances between the various
types of AGB stars.Comment: Accepted for publication in A&
SiO in C-rich circumstellar envelopes of AGB stars: effects of non-LTE chemistry and grain adsorption
New SiO multi-transition millimetre line observations of a sample of carbon
stars, including J=8-7 observations with the APEX telescope, are used to probe
the role of non-equilibrium chemistry and the influence of grains in
circumstellar envelopes of carbon stars. A detailed radiative transfer
modelling, including the effect of dust emission in the excitation analysis, of
the observed SiO line emission is performed. A combination of low- and
high-energy lines are important in constraining the abundance distribution. It
is found that the fractional abundance of SiO in these C-rich environments can
be several orders of magnitude higher than predicted by equilibrium stellar
atmosphere chemistry. In fact, the SiO abundance distribution of carbon stars
closely mimic that of M-type (O-rich) AGB stars. A possible explanation for
this behaviour is a shock-induced chemistry, but also the influence of dust
grains, both as a source for depletion as well as production of SiO, needs to
be further investigated. As observed for M-type AGB stars, a clear trend that
the SiO fractional abundance decreases as the mass-loss rate of the star
increases is found for the carbon stars. This indicates that SiO is accreted
onto dust grains in the circumstellar envelopes.Comment: Accepted for publication in A&A, 11 pages, 7 figure
Low-mass star formation in R Coronae Australis: Observations of organic molecules with the APEX telescope
This paper presents new APEX submillimetre molecular line observations of
three low-mass protostars, IRS7A, IRS7B, and IRAS32, in the R Coronae Australis
molecular cloud complex. The molecular excitation analysis is performed using a
statistical equilibrium radiative transfer code. The derived beam averaged
fractional abundances vary by less than a factor of two among the three
sources, except those of H2CO and CH3OH, which show differences of about an
order of magnitude. The molecular abundances are similar to those typically
found in other star-forming regions in the Galaxy, such as the ~Oph and
Perseus molecular clouds. There is a marked difference in the kinetic
temperatures derived for the protobinary source IRS7 from H2CO (40-60 K) and
CH3OH (20 K), possibly indicating a difference in origin of the emission from
these two molecules.Comment: Accepted for A&A special issue on APEX first results. 4 pages, 2
figure
Circumstellar water vapour in M-type AGB stars: Constraints from H2O(1_10 - 1_01) lines obtained with Odin
Aims: Spectrally resolved circumstellar H2O(1_10 - 1_01) lines have been
obtained towards three M-type AGB stars using the Odin satellite. This provides
additional strong constrains on the properties of circumstellar H2O and the
circumstellar envelope. Methods: ISO and Odin satellite H2O line data are used
as constraints for radiative transfer models. Special consideration is taken to
the spectrally resolved Odin line profiles, and the effect of excitation to the
first excited vibrational states of the stretching modes (nu1=1 and nu3=1) on
the derived abundances is estimated. A non-local, radiative transfer code based
on the ALI formalism is used. Results: The H2O abundance estimates are in
agreement with previous estimates. The inclusion of the Odin data sets stronger
constraints on the size of the H2O envelope. The H2O(1_10 - 1_01) line profiles
require a significant reduction in expansion velocity compared to the terminal
gas expansion velocity determined in models of CO radio line emission,
indicating that the H2O emission lines probe a region where the wind is still
being accelerated. Including the nu3=1 state significantly lowers the estimated
abundances for the low-mass-loss-rate objects. This shows the importance of
detailed modelling, in particular the details of the infrared spectrum in the
range 3 to 6 micron, to estimate accurate circumstellar H2O abundances.
Conclusions: Spectrally resolved circumstellar H2O emission lines are important
probes of the physics and chemistry in the inner regions of circumstellar
envelopes around asymptotic giant branch stars. Predictions for H2O emission
lines in the spectral range of the upcoming Herschel/HIFI mission indicate that
these observations will be very important in this context.Comment: accepted in A&A, 10 pages, 8 figure
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