Issue |
A&A
Volume 367, Number 2, February IV 2001
|
|
---|---|---|
Page(s) | 652 - 673 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20000434 | |
Published online | 15 February 2001 |
Low-excitation atomic gas around evolved stars*
I. ISO observations of C-rich nebulae
1
University of Illinois, 1002 W. Green St., Urbana, IL 61801, USA
2
Observatorio Astonómico Nacional, Apartado 1143, 28800 Alcalá de Henares, Spain
3
CalTech, SIRTF Science Center, MS 314-6, Pasadena, CA 91125, USA
4
Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
5
University of Arizona, Steward Observatory, Tucson, AZ 85721, USA
Corresponding author: D. Fong, d-fong@astro.uiuc.edu
Received:
11
July
2000
Accepted:
24
November
2000
We present ISO LWS and SWS spectra of far-infrared (FIR) atomic fine structure
lines in 12 carbon-rich evolved stars including asymptotic giant branch (AGB) stars,
proto-planetary nebulae (PPNe) and planetary nebulae (PNe). The spectra include
grating and Fabry-Perot measurements of the line emission of
[Oi], [Cii], [Sii], [Siii], [Si],
[Fei], [Feii], [Neii] and [Nii].
Only 5 out of our 12 object sample have been detected in at least one of these FIR
lines. When we include the 12 oxygen-rich evolved stars from Castro-Carrizo et al.
([CITE], Paper II), we find that atomic line emission is observed only in those sources
in which the central star's Teff
≥ 10 000 K. Above this cutoff, the number
of detectable lines and the intensity of the line emission increase as Teff
increases. These trends suggest that the atomic lines originate from photodissociation
regions (PDRs). In general, the kinematics of the atomic gas, derived from line fits
to the Fabry-Perot data, are comparable to the molecular expansion velocities. These
kinematics are expected for atomic cooling lines associated with circumstellar PDRs.
AFGL 618, however, appears exceptional with dual velocity components: a narrow
component (<20 km s-1) that may be associated with a PDR, and a broad component
(∼66 km s-1) that may be produced in post-shocked, accelerated gas. A new PDR code
which properly treats enhanced carbon abundances was used to model the observations of
our carbon-rich objects. The predicted line intensities agree reasonably well with the
observations. Shock models, however, do not compare well with the observed line
intensities. PDR mass estimates ranging from ∼0.01-0.2 were derived
from the [Cii] 158 μm line emission. The atomic gas constitutes only a small
fraction of the total mass for young planetary nebulae, but its importance grows
significantly as the nebulae evolve. Our overall analysis shows that photodissociation,
and not shocks, dominates the evolution of the circumstellar envelope by transforming
the initially molecular asymptotic giant branch envelopes into the atomic gas found in
proto-planetary and planetary nebulae.
Key words: atomic data / stars: AGB and post-AGB / stars: circumstellar matter / stars: mass-loss / ISM: planetary nebulae
© ESO, 2001
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