Issue |
A&A
Volume 368, Number 3, March IV 2001
|
|
---|---|---|
Page(s) | 969 - 993 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20010072 | |
Published online | 15 March 2001 |
Models of circumstellar molecular radio line emission*
Mass loss rates for a sample of bright carbon stars
Stockholm Observatory, 133 36 Saltsjöbaden, Sweden
Corresponding author: F. L. Schöier, fredrik@strw.leidenuniv.nl
Received:
8
May
2000
Accepted:
4
January
2001
Using a detailed radiative transfer analysis, combined with an
energy balance equation for the gas, we have performed extensive
modelling of circumstellar CO radio line emission from a large sample
of optically bright carbon stars, originally observed by Olofsson et
al. (ApJS, 87, 267). Some new observational results are presented
here. We determine some of the basic parameters that characterize
circumstellar envelopes (CSEs), e.g., the stellar mass loss rate, the
gas expansion velocity, and the kinetic temperature structure of the
gas. Assuming a spherically symmetric CSE with a smooth gas density
distribution, created by a continuous mass loss, which expands with a
constant velocity we are able to model reasonably well 61 of our 69
sample stars. The derived mass loss rates depend crucially on the
assumptions in the circumstellar model, of which some can be
constrained if enough observational data exist. Therefore, a reliable
mass loss rate determination for an individual star requires, in
addition to a detailed radiative transfer analysis, good observational
constraints in the form of multi-line observations and radial
brightness distributions. In our analysis we use the results of a
model for the photodissociation of circumstellar CO by Mamon et al.
(1988). This leads to model fits to observed radial brightness
profiles that are, in general, very good, but there are also a few
cases with clear deviations, which suggest departures from a simple
r-2 density law. The derived mass loss rates span almost four orders of magnitude, from
~5 10 yr-1 up to
~2 10
yr-1, with the median mass
loss rate being ~3 10
yr-1.
We estimate that the mass loss rates are typically accurate to
~50% within the adopted circumstellar model. The physical conditions
prevailing in the CSEs vary considerably over such a large range of
mass loss rates. Among other things, it appears that the dust-to-gas
mass ratio and/or the dust properties change with the mass loss rate.
We find that the mass loss rate and the gas expansion velocity are
well correlated, and that both of them clearly depend on the pulsational
period and (with larger scatter) the stellar luminosity. Moreover, the mass
loss rate correlates weakly with the stellar effective temperature, in
the sense that the cooler stars tend to have higher mass loss rates,
but there seems to be no correlation with the stellar C/O-ratio.
We conclude that the mass loss rate increases with increased
regular pulsation and/or luminosity, and that the expansion velocity
increases as an effect of increasing mass loss rate (for low mass
loss rates) and luminosity.
Five, of the remaining eight, sample stars have detached CSEs in the form of
geometrically thin CO shells. The present mass loss rates and shell
masses of these sources are estimated. Finally, in three cases we encounter
problems using our model. For two of these sources there are
indications of significant departures from overall spherical symmetry
of the CSEs. Carbon stars on the AGB are probably important in returning processed gas to
the ISM. We estimate that carbon stars of the type considered here
annually return ~0.05
of gas to the Galaxy, but more
extreme carbon stars may contribute an order of magnitude more.
However, as for the total carbon budget of the Galaxy, carbon stars
appear to be of only minor importance.
Key words: stars: AGB and post-AGB / stars: carbon / circumstellar matter / stars: late-type / radio lines: stars
© ESO, 2001
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.