J/A+A/628/A62 Spectra of 25 carbon star envelopes (Massalkhi+, 2019)
Study of CS, SiO, and SiS abundances in carbon star envelopes:
assessing their role as gas-phase precursors of dust.
Massalkhi S., Agundez M., Cernicharo J.
<Astron. Astrophys. 628, A62 (2019)>
=2019A&A...628A..62M 2019A&A...628A..62M (SIMBAD/NED BibCode)
ADC_Keywords: Stars, carbon ; Abundances ; Spectra, radio
Keywords: astrochemistry - molecular processes - stars: abundances -
stars: AGB and post-AGB - circumstellar matter
Abstract:
We aim to determine the abundances of CS, SiO, and SiS in a large
sample of carbon star envelopes covering a wide range of mass loss
rates to investigate the potential role that these molecules could
play in the formation of dust in the surroundings of the central AGB
star. We surveyed a sample of 25 carbon-rich AGB stars in the
λ2mm band, more concretely in the J=3-2 line of CS and SiO and
in the J=7-6 and J=8-7 lines of SiS, using the IRAM 30m telescope. We
performed excitation and radiative transfer calculations based on the
LVG method to model the observed lines of the molecules and to derive
their fractional abundances in the observed envelopes. We also
assessed the effect of infrared pumping in the excitation of the
molecules. We detected CS in all 25 targeted envelopes, SiO in 24 of
them, and SiS in 17 sources. Remarkably, SiS is not detected in any
envelope with a mass loss rate below 10-6M☉/yr while it is
detected in all envelopes with mass loss rates above that threshold.
We found that CS and SiS have similar abundances in carbon stars
envelopes, while SiO is present with a lower abundance. We also found
a strong correlation in which the denser the envelope, the less
abundant CS and SiO are. The trend is however only tentatively seen
for SiS in the high mass loss rate range. Furthermore, we found a
relation in which the integrated flux of the MgS dust feature at 30um
increases as the fractional abundance of CS decreases. The decline in
the fractional abundance of CS with increasing density could be due to
gas phase chemistry in the inner envelope or to adsorption onto dust
grains. The latter possibility is favored by a correlation between the
CS fractional abundance and the 30um feature, which suggests that CS
is efficiently incorporated onto MgS dust around C-rich AGB stars. In
the case of SiO, the observed abundance depletion with increasing
density is most likely caused by an efficient incorporation onto dust
grains. The latter possibility is favored by a correlation between the
CS fractional abundance and the 30um feature, which suggests that CS
is efficiently incorporated onto MgS dust around C-rich AGB stars. In
the case of SiO, the observed abundance depletion with increasing
density is most likely caused by an efficient incorporation onto dust
grains. We conclude that CS and SiO (very likely) and SiS
(tentatively) are good candidates to act as gas-phase precursors of
dust in C-rich AGB envelopes.
Description:
The FITS file contains the spectra of the CS J=3-2, SiO J=3-2,
SiS J=7-6, and SiS=8-7 lines of the 25 carbon-rich AGB stars
observed with the IRAM 30m telescope that are shown in Figures 1, 2,
3, 4 in the article.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
list.dat 90 100 List of spectra
spectra.fits 2880 44 Spectra
--------------------------------------------------------------------------------
Byte-by-byte Description of file: list.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 A7 --- Line Observed line
9- 17 A9 --- Object Object name
19- 27 F9.5 deg RAdeg Right ascension (J2000)
29- 38 F10.6 deg DEdeg Declination (J2000)
40- 51 E12.6 Hz RestFreq Rest frequency
53- 64 E12.6 Hz ImgFreq Image frequency
65- 70 I6 m/s Vel Velocity
72- 79 F8.3 m/s DVel Velocity resolution
81- 90 A10 --- Obs.Date Observation date
--------------------------------------------------------------------------------
Acknowledgements:
Sarah Massalkhi, sarah.massalkhi(at)csic.es
(End) Sarah Massalkhi [Spain], Patricia Vannier [CDS] 18-Jun-2019