J/A+A/642/A20 W Aql APEX spectrum (De Beck+, 2020)
The surprisingly carbon-rich environment of the S-type star W Aql.
De Beck E., Olofsson H.
<Astron. Astrophys. 642, A20 (2020)>
=2020A&A...642A..20D 2020A&A...642A..20D (SIMBAD/NED BibCode)
ADC_Keywords: Stars, giant ; Spectra, radio
Keywords: stars: AGB and post-AGB - stars: mass loss - astrochemistry -
stars: individual: W Aql
Abstract:
W Aql is an asymptotic giant branch (AGB) star with an atmospheric
elemental abundance ratio C/O~=0.98. It has previously been reported to
have circumstellar molecular abundances intermediate between those of
M-type and C-type AGB stars, which respectively have C/O<1 and C/O>1.
This intermediate status is considered typical for S-type stars,
although our understanding of the chemical content of their
circumstellar envelopes is currently rather limited.
We wish to assess the reported intermediate status of W Aql by
analysing the line emission of molecules that have not been observed
towards this star before.
We have performed observations in the frequency range 159-268GHz
with the SEPIA/B5 and PI230 instruments on the APEX telescope. We make
abundance estimates through direct comparison to available spectra
towards a number of well-studied AGB stars and based on rotational
diagram analysis in the case of one molecule.
From a compilation of our abundance estimates and those found in the
literature for two M-type (R Dor, IK Tau), two S-type (χ Cyg,
W Aql), and two C-type stars (V Aql, IRC +10 216), we conclude that W
Aql's circumstellar environment appears considerably closer to that of
a C-type AGB star than to that of an M-type AGB star. In particular,
we detect emission from C2H, SiC2, SiN, and HC3N, molecules
previously only detected towards the circumstellar environment of
C-type stars. This conclusion, based on the chemistry of the gaseous
component of the circumstellar environment, is further supported by
reports in the literature on the presence of atmospheric molecular
bands and spectral features of dust species which are typical for
C-type AGB stars. Although our observations mainly trace species in
the outer regions of the circumstellar environment, our conclusion
matches closely that based on recent chemical equilibrium models for
the inner wind of S-type stars: the atmospheric and circumstellar
chemistry of S-type stars likely resembles that of C-type AGB stars
much more closely than that of M-type AGB stars.
Further observational investigation of the gaseous circumstellar
chemistry of S-type stars is required to characterise its dependence
on the atmospheric C/O. Non-equilibrium chemical models of the
circumstellar environment of AGB stars need to address the particular
class of S-type stars and the chemical variety that is induced by the
range in atmospheric C/O.
Description:
The FITS file WAQLAPEXTA_DeBeck2020.fits contains the fully reduced
spectrum of W Aql in the range 159.0-270.0GHz obtained with
APEX-SEPIA/B5 and APEX-PI230.
The intensity is given in corrected antenna temperature, TA*, as
described in Sect. 2 of the paper.
Objects:
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RA (2000) DE Designation(s)
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19 15 23.36 -07 02 50.3 W Aql = V* W Aql
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 138 1 Information on fits spectrum
fits/* . 1 Fits spectrum
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 26 I7 --- Nx Number of pixels along X-axis
28- 50 A23 "datime" Obs.date Observation date
52- 63 E12.6 Hz bFreq Lower value of relative frequency
65- 75 E11.6 Hz BFreq Upper value of relative frequency
77- 83 F7.1 Hz dFreq Frequency resolution
85- 89 I5 Kibyte size Size of FITS file
91-118 A28 --- FileName Name of FITS file, in subdirectory fits
120-138 A19 --- Title Title of the FITS file
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Acknowledgements:
Elvire De Beck, elvire.debeck(at)chalmers.se
(End) Patricia Vannier [CDS] 08-Jul-2020