2,571 research outputs found
The physics and chemistry of circumstellar envelopes of S-stars on the AGB
The S-stars have been suggested to be a brief transitional phase as stars
evolve from oxygen-rich M-type stars into carbon stars, through the dredge up
of carbon from He-shell burning. As possible transition objects, S-stars might
help achieve a deeper understanding of the chemical evolution as a star ascends
the AGB, as well as shed more light on the mass-loss mechanism. We have
initiated a large survey of 40 S-stars to observe line emission in common
molecules such as CO, SiO, HCN, CS and SiS. Detailed radiative transfer
modelling of multi-transition CO radio line observations towards a sample of 40
S-stars shows that the mass-loss rate distribution of S-stars is consistent
with those found for M-type AGB stars and carbon stars. Initial results from
modelling of the circumstellar SiO emission are also presented.Comment: 2 pages, 1 figure, to appear in Proceedings from 'Why Galaxies Care
About AGB stars
Probing the mass loss history of carbon stars using CO line and dust continuum emission
An extensive modelling of CO line emission from the circumstellar envelopes
around a number of carbon stars is performed. By combining radio observations
and infrared observations obtained by ISO the circumstellar envelope
characteristics are probed over a large radial range. In the radiative transfer
analysis the observational data are consistently reproduced assuming a
spherically symmetric and smooth wind expanding at a constant velocity. The
combined data set gives better determined envelope parameters, and puts
constraints on the mass loss history of these carbon stars. The importance of
dust in the excitation of CO is addressed using a radiative transfer analysis
of the observed continuum emission, and it is found to have only minor effects
on the derived line intensities. The analysis of the dust emission also puts
further constraints on the mass loss rate history. The stars presented here are
not likely to have experienced any drastic long-term mass loss rate
modulations, at least less than a factor of about 5, over the past thousands of
years. Only three, out of nine, carbon stars were observed long enough by ISO
to allow a detection of CO far-infrared rotational lines.Comment: 11pages, 7 figures, accepted by A&
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 detached dust and gas shells around the carbon star U Ant
Context: Geometrically thin, detached shells of gas have been found around a
handful of carbon stars. --Aims: Previous observations of scattered stellar
light in the circumstellar medium around the carbon star U Ant were taken
through filters centred on the resonance lines of K and Na. These observations
could not separate the scattering by dust and atoms. The aim of this paper is
to remedy this situation. --Methods: We have obtained polarization data on
stellar light scattered in the circumstellar medium around U Ant through
filters which contain no strong lines, making it possible to differentiate
between the two scattering agents. Kinematic, as well as spatial, information
on the gas shells were obtained through high-resolution echelle spectrograph
observations of the KI and NaD lines. --Results: We confirm the existence of
two detached shells around U Ant. The inner shell (at a radius of approx 43"
and a width of approx 2") consists mainly of gas, while the outer shell (at a
radius of approx 50" and a width of approx 7") appears to consist exclusively
of dust. Both shells appear to have an over-all spherical geometry. The gas
shell mass is estimated to be 2x10^-3 M(Sun), while the mass of the dust shell
is estimated to be 5x10^-5 M(Sun). The derived expansion velocity, from the KI
and NaD lines, of the gas shell, 19.5 km/s, agrees with that obtained from CO
radio line data. The inferred shell age is 2700 years. There is structure, e.g.
in the form of arcs, inside the gas shell, but it is not clear whether these
are due to additional shells. --Conclusions: Our results support the hypothesis
that the observed geometrically thin, detached shells around carbon stars are
the results of brief periods of intense mass loss, probably associated with
thermal pulses, and subsequent wind-wind interactions
Mass Loss Evolution and the Formation of Detached Shells around TP-AGB Stars
The origin of the so called 'detached shells' around AGB stars is not fully
understood, but two common hypotheses state that these shells form either
through the interaction of distinct wind phases or an eruptive mass loss
associated with a He-shell flash. We present a model of the formation of
detached shells around thermal pulse asymptotic giant branch (TP-AGB) stars,
based on detailed modelling of mass loss and stellar evolution, leading to a
combination of eruptive mass loss and wind interaction.
The purpose of this paper is first of all to connect stellar evolution with
wind and mass loss evolution and demonstrate its consistency with observations,
but also to show how thin detached shells around TP-AGB stars can be formed.
Previous attempts to link mass loss evolution with the formation of detached
shells were based on approximate prescriptions for the mass loss and have not
included detailed modelling of the wind formation as we do here. (abridged)Comment: 16 pages, 15 figures. Accepted for publication in Astronomy &
Astrophysic
Mass loss rates of a sample of irregular and semiregular M-type AGB-variables
We have determined mass loss rates and gas expansion velocities for a sample
of 69 M-type irregular (IRV; 22 objects) and semiregular (SRV; 47 objects)
AGB-variables using a radiative transfer code to model their circumstellar CO
radio line emission. We believe that this sample is representative for the mass
losing stars of this type. The (molecular hydrogen) mass loss rate distribution
has a median value of 2.0E-7 solar masses per year. M-type IRVs and SRVs with a
mass loss rate in excess of 5E-7 solar masses per year must be very rare, and
among these mass losing stars the number of sources with mass loss rates below
a few 10E-8 solar masses per year must be small. We find no significant
difference between the IRVs and the SRVs in terms of their mass loss
characteristics. Among the SRVs the mass loss rate shows no dependence on the
period. Likewise the mass loss rate shows no correlation with the stellar
temperature. The gas expansion velocity distribution has a median of 7.0 km/s.
The mass loss rate and the gas expansion velocity correlate well, a result in
line with theoretical predictions for an optically thin, dust-driven wind. In
general, the model produces line profiles which acceptably fit the observed
ones.
We have compared the results of this M-star sample with a similar C-star
sample analysed in the same way. The mass loss rate characteristics are very
similar for the two samples. On the contrary, the gas expansion velocity
distributions are clearly different. In particular, the number of low-velocity
sources is much higher in the M-star sample. We found no example of the sharply
double-peaked CO line profile, which is evidence of a large, detached CO-shell,
among the M-stars. About 10% of the C-stars show this phenomenon.Comment: 16 pages, 11 figures, accepted by A&
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