6 research outputs found
The variable mass loss of the AGB star WX Psc as traced by the CO J=1-0 through 7-6 lines and the dust emission
Low and intermediate mass stars lose a significant fraction of their mass
through a dust-driven wind during the Asymptotic Giant Branch (AGB) phase.
Recent studies show that winds from late-type stars are far from being smooth.
Mass-loss variations occur on different time scales, from years to tens of
thousands of years. The variations appear to be particularly prominent towards
the end of the AGB evolution. The occurrence, amplitude and time scale of these
variations are still not well understood.
The goal of our study is to gain insight into the structure of the
circumstellar envelope (CSE) of WX Psc and map the possible variability of the
late-AGB mass-loss phenomenon.
We have performed an in-depth analysis of the extreme infrared AGB star WX
Psc by modeling (1) the CO J=1-0 through 7-6 rotational line profiles and the
full spectral energy distribution (SED) ranging from 0.7 to 1300 micron. We
hence are able to trace a geometrically extended region of the CSE.
Both mass-loss diagnostics bear evidence of the occurrence of mass-loss
modulations during the last ~2000 yr. In particular, WX Psc went through a high
mass-loss phase (Mdot~5e-5 Msun/yr) some 800 yr ago. This phase lasted about
600 yr and was followed by a long period of low mass loss (Mdot~5e-8 Msun/yr).
The present day mass-loss rate is estimated to be ~6e-6 Msun/yr.
The AGB star WX Psc has undergone strong mass-loss rate variability on a time
scale of several hundred years during the last few thousand years. These
variations are traced in the strength and profile of the CO rotational lines
and in the SED. We have consistently simulated the behaviour of both tracers
using radiative transfer codes that allow for non-constant mass-loss rates.Comment: 12 pages, accepted for publication in A&
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
Imaging the circumstellar dust around AGB stars with PolCor
The aim of this paper is to investigate how the new imaging Polarimeter and
Coronograph (PolCor) at the Nordic Optical Telescope (NOT) can be used in the
study of circumstellar structures around AGB stars, with the purpose of
preparing for a study of a larger sample. We have observed two types of AGB
stars using the PolCor instrument on the NOT: the binary S-type star W Aql and
two carbon stars with detached shells, U Cam and DR Ser. The polarized light
traces the dust distribution around the stars. From the polarimeter images the
polarized intensity, the polarization degree, and the polarization angle over
the images are calculated. The location and extent of dust structures are
examined in the images. The total dust mass and the dust-to-gas ratios of the
detached shells are also calculated. The images of the circumstellar envelope
of W Aql show what seems to be an elongated structure in the south-west
direction. The detached shells of U Cam and DR Ser are clearly seen in the
images. This is the first time the detached shell around DR Ser has been
imaged. The radii and widths of the shells are determined and found to be 7.9"
and 7.6", and 0.9" and 1.2", for U Cam and DR Ser, respectively. This is
consistent with previous results. The dust masses of the feature south-west of
W Aql, and in the shells of U Cam and DR Ser are also estimated and found to be
1x10^-6, 5x10^-7, and 2x10^-6 Msun, respectively. W Aql is a known binary and
the shape of the circumstellar envelope seems to be in line with what could be
expected from binary interaction on these scales. For the shells, the results
are in agreement with previous investigations. Ages and formation time-scales
are also estimated for the detached shells and found to be consistent with the
thermal-pulse-formation scenario.Comment: Accepted for publication in A&
A HIFI preview of warm molecular gas around X Cygni: first detection of H2O emission toward an S-type AGB star
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