751 research outputs found
The 12CO/13CO ratio in AGB stars of different chemical type-Connection to the 12C/13C ratio and the evolution along the AGB
The aim of this paper is to investigate the evolution of the 12C/13C ratio
along the AGB through the circumstellar 12CO/13CO ratio. This is the first time
a sample including a significant number of M- and S-type stars is analysed
together with a carbon-star sample of equal size, making it possible to
investigate trends among the different types and establish evolutionary
effects. The circumstellar 12CO/13CO abundance ratios are estimated through a
detailed radiative transfer analysis of single-dish radio line emission
observations. First, the 12CO radiative transfer is solved, assuming an
abundance (dependent on the chemical type of the star), to give the physical
parameters of the gas, i.e. mass-loss rate, gas expansion velocity, and gas
temperature distribution. Then, the 13CO radiative transfer is solved using the
results of the 12CO model giving the 13CO abundance. Finally, the 12CO/13CO
abundance ratio is calculated. The circumstellar 12CO/13CO abundance ratio
differs between the three spectral types. This is consistent with what is
expected from stellar evolutionary models assuming that the spectral types
constitute an evolutionary sequence; however, this is the first time this has
been shown observationally for a relatively large sample covering all three
spectral types. The median value of the 13CO abundance in the inner
circumstellar envelope is 1.6x10^-5, 2.3x10^-5, and 3.0x10^-5 for the M-type,
S-type, and carbon stars of the sample, respectively, corresponding to
12CO/13CO abundance ratios of 13, 26, and 34, respectively. Interestingly, the
abundance ratio spread of the carbon stars is much larger than for the M- and
S-type stars, even when excluding J-type carbon stars, in line with what could
be expected from evolution on the AGB. We find no correlation between the
isotopologue ratio and the mass-loss rate, as would be expected if both
increase as the star evolves.Comment: 11 pages, 5 figures, accepted for publication in A&
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
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
ALMA observations of the vibrationally-excited rotational CO transition towards five AGB stars
We report the serendipitous detection with ALMA of the vibrationally-excited
pure-rotational CO transition towards five asymptotic giant branch
(AGB) stars, Cet, R Aqr, R Scl, W Aql, and Gru. The observed lines
are formed in the poorly-understood region located between the stellar surface
and the region where the wind starts, the so-called warm molecular layer. We
successfully reproduce the observed lines profiles using a simple model. We
constrain the extents, densities, and kinematics of the region where the lines
are produced. R Aqr and R Scl show inverse P-Cygni line profiles which indicate
infall of material onto the stars. The line profiles of Cet and R Scl show
variability. The serendipitous detection towards these five sources shows that
vibrationally-excited rotational lines can be observed towards a large number
of nearby AGB stars using ALMA. This opens a new possibility for the study of
the innermost regions of AGB circumstellar envelopes.Comment: 6 pages, 2 figures, 2 tables, 2016MNRAS.463L..74
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&
Detailed modelling of the circumstellar molecular line emission of the S-type AGB star W Aquilae
S-type AGB stars have a C/O ratio which suggests that they are transition
objects between oxygen-rich M-type stars and carbon-rich C-type stars. As such,
their circumstellar compositions of gas and dust are thought to be sensitive to
their precise C/O ratio, and it is therefore of particular interest to examine
their circumstellar properties.
We present new Herschel HIFI and PACS sub-millimetre and far-infrared line
observations of several molecular species towards the S-type AGB star W Aql. We
use these observations, which probe a wide range of gas temperatures, to
constrain the circumstellar properties of W Aql, including mass-loss rate and
molecular abundances. We used radiative transfer codes to model the
circumstellar dust and molecular line emission to determine circumstellar
properties and molecular abundances. We assumed a spherically symmetric
envelope formed by a constant mass-loss rate driven by an accelerating wind.
Our model includes fully integrated H2O line cooling as part of the solution of
the energy balance. We detect circumstellar molecular lines from CO, H2O, SiO,
HCN, and, for the first time in an S-type AGB star, NH3. The radiative transfer
calculations result in an estimated mass-loss rate for W Aql of 4.0e-6 Msol
yr-1 based on the 12CO lines. The estimated 12CO/13CO ratio is 29, which is in
line with ratios previously derived for S-type AGB stars. We find an H2O
abundance of 1.5e-5, which is intermediate to the abundances expected for M and
C stars, and an ortho/para ratio for H2O that is consistent with formation at
warm temperatures. We find an HCN abundance of 3e-6, and, although no CN lines
are detected using HIFI, we are able to put some constraints on the abundance,
6e-6, and distribution of CN in W Aql's circumstellar envelope using
ground-based data. We find an SiO abundance of 3e-6, and an NH3 abundance of
1.7e-5, confined to a small envelope.Comment: 17 pages, 15 figure
CO and HCN isotopologue ratios in the outflows of AGB stars
Isotopologue line intensity ratios of circumstellar molecules have been
widely used to trace the photospheric elemental isotopic ratios of evolved
stars. However, depending on the molecular species and the physical conditions
of the environment, the circumstellar isotopologue ratio may deviate
considerably from the stellar atmospheric value. In this paper, we aim to
examine how the CO and HCN abundance ratios vary radially due to chemical
reactions in the outflows of AGB stars and the effect of excitation and optical
depth on the resulting line intensity ratios. We find that the circumstellar
12CO/13CO can deviate from its atmospheric value by up to 25-94% and 6-60% for
C- and O-type CSEs, respectively. We show that variations of the intensity of
the ISRF and the gas kinetic temperature can significantly influence the CO
isotopologue ratio in the outer CSEs. On the contrary, the H12CN/H13CN ratio is
stable for all tested mass-loss rates. The RT modeling shows that the
integrated line intensity ratio of CO of different rotational transitions
varies significantly for stars with intermediate mass-loss rates due to
combined chemical and excitation effects. In contrast, the excitation
conditions for the both HCN isotopologues are the same. We demonstrate the
importance of using the isotopologue abundance profiles from chemical models as
inputs to RT models in the interpretation of isotopologue observations.
Previous studies of CO isotopologue ratios are based on multi-transition data
for individual sources and it is difficult to estimate the errors in the
reported values due to assumptions that are not entirely correct according to
this study. If anything, previous studies may have overestimated the
circumstellar 12CO/13CO abundance ratio. The use of the HCN as a tracer of C
isotope ratios is affected by fewer complicating problems, provided one
accounts corrections for high optical depths.Comment: 14 pages, 11 figure
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