J/A+A/667/A85 Peculiar Hydrogen-deficient Carbon stars (Crawford +, 2022)
Peculiar hydrogen-deficient carbon stars:
strontium-rich stars and the s-process.
Crawford C.L., Tisserand P., Clayton G.C., Munson B.
<Astron. Astrophys. 667, A85 (2022)>
=2022A&A...667A..85C 2022A&A...667A..85C (SIMBAD/NED BibCode)
ADC_Keywords: Stars, carbon ; Equivalent widths ; Optical
Keywords: stars: abundances - methods: observational - stars: carbon -
stars: chemically peculiar - stars: evolution - supergiants
Abstract:
R Coronae Borealis (RCB) variables and their non-variable
counterparts, the dustless Hydrogen-Deficient Carbon (dLHdC)
stars have been known to exhibit enhanced s-processed material on
their surfaces, especially Sr, Y, and Ba. No comprehensive work has
been done to explore the s-process in these types of stars, however
one particular RCB star, U Aqr, has been under scrutiny for its
extraordinary Sr enhancement.
We aim to identify RCB and dLHdC stars that have significantly
enhanced Sr abundances, such as U Aqr, and use stellar evolution
models to begin to estimate the type of neutron exposure that occurs
in a typical HdC star.
We compare the strength of the SrII 4077Å spectral line to CaII H
to identify the new subclass of Sr-rich HdCs. We additionally use the
structural and abundance information from existing RCB MESA models to
calculate the neutron exposure parameter, τ.
We identify six stars in the Sr-rich class. Two are RCBs, and four are
dLHdCs. We additionally find that the preferred RCB MESA model has a
neutron exposure τ~=0.1mb-1, which is lower than the estimated
between 0.15 and 0.6mb-1 for the Sr-rich star U Aqr found in the
literature. We find trends in the neutron exposure corresponding to
He-burning shell temperature, metallicity, and assumed s-processing
site.
We have found a sub-class of 6 HdCs known as the Sr-rich class, which
tend to lie in the halo, outside the typical distribution of RCBs and
dLHdCs. We find that dLHdC stars are more likely to be Sr-rich than
RCBs, with an occurrence rate of ∼13% for dLHdCs and ∼2% for RCBs.
This is one of the first potential spectroscopic differences between
RCBs and dLHdCs, along with dLHdCs having stronger surface abundances
of 18O. We additionally find neutron exposure trends in our RCB models
that will aide in understanding the interplay between model parameters
and surface s-process elements.
Description:
This table contains the pseudo-equivalent widths for many s-process
spectral features in Hydrogen-deficient Carbon star spectra. These
were used to identify stars that were rich in Strontium and Barium.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
pews.dat 52 149 Pseudo-equivalent widths of HdC spectral features
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See also:
May/22 : Hydrogen-deficient Carbon stars spectra (Tisserand+, 2022)
Byte-by-byte Description of file: pews.dat
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Bytes Format Units Label Explanations
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1- 24 A24 --- Name Star name
26- 31 F6.2 0.1nm Cah ?=-99 Pseudo-EW for Ca II H (1)
33- 38 F6.2 0.1nm Cak ?=-99 Pseudo-EW for Ca II K (1)
40- 45 F6.2 0.1nm Sr4077 ?=-99 Pseudo-EW for Sr II 4077 line (1)
47- 52 F6.2 0.1nm Ba4554 ?=-99 Pseudo-EW for Ba II 4554 line (1)
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Note (1): -99 indicates no spectral data.
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Acknowledgements:
Courtney Crawford, ccour14(at)lsu.edu
(End) C. Crawford [Louisiana State Univ], P. Vannier [CDS] 11-Feb-2022