SALT revisits DY Cen: a rapidly evolving strontium-rich single helium star

Jeffery, C. S.; Rao, N. Kameswara; Lambert, David L.

doi:10.1093/mnras/staa406

Cited by 11 publications

(6 citation statements)

References 39 publications

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“…It is not clear whether this kind of Sr enrichment is exclusive to cool stars or if it is a small sample bias. Jeffery et al (2020) points out that the hot RCB star DY Cen shows very strong Sr II enhancement in its 1987 spectrum, indicating that this kind of Sr enhancement can be seen up to surface temperatures of 18800 K. We note for clarity that the hot RCB stars are not included in this study (De Marco et al 2002;Tisserand et al 2020). Compared to the total population of RCBs and dLHdCs, we find that two out of the 87 RCBs with blue optical spectra and four out of 32 dLHdCs have this kind of Sr enhancement.…”

Section: Discussionmentioning

confidence: 99%

Peculiar hydrogen-deficient carbon stars: strontium-rich stars and the s-process

Crawford

Tisserand

Clayton

et al. 2022

A&A

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Context. 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. Aims. 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. Methods. We compared the strength of the Sr II 4077 Å spectral line to Ca II H to identify the new subclass of Sr-rich HdCs. We additionally used the structural and abundance information from existing RCB MESA models to calculate the neutron exposure parameter, τ. Results. We identified six stars in the Sr-rich class. Two are RCBs, and four are dLHdCs. We additionally found that the preferred RCB MESA model has a neutron exposure τ 0.1 mb −1 , which is lower than the estimated τ between 0.15 and 0.6 mb −1 for the Sr-rich star U Aqr found in the literature. We found trends in the neutron exposure corresponding to He-burning shell temperature, metallicity, and assumed s-processing site. Conclusions. We have found a sub-class of six HdCs known as the Sr-rich class, which tend to lie in the halo, outside the typical distribution of RCBs and dLHdCs. We found 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 18 O. We additionally found neutron exposure trends in our RCB models that will aide in understanding the interplay between model parameters and surface s-process elements.

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Section: Discussionmentioning

confidence: 99%

Peculiar hydrogen-deficient carbon stars: strontium-rich stars and the s-process

Crawford

Tisserand

Clayton

et al. 2022

A&A

View full text Add to dashboard Cite

show abstract

“…It is not clear whether this kind of Sr enrichment is exclusive to cool stars or if it is a small sample bias. Jeffery et al (2020) points out that the hot RCB star DY Cen shows very strong Sr II enhancement in its 1987 spectrum, indicating that this kind of Sr enhancement can be seen up to surface temperatures of 18800 K. We note for clarity that the hot RCB stars are not included in this study (De Marco et al 2002;Tisserand et al 2020). Compared to the total population of RCBs and dLHdCs, we find that 2 out of the 87 RCBs with blue optical spectra and 4 out of 32 dLHdCs have this kind of Sr enhancement.…”

Section: Discussionmentioning

confidence: 99%

Peculiar Hydrogen-deficient Carbon Stars: Strontium-Rich Stars and the s-Process

Crawford¹,

Tisserand²,

Clayton³

et al. 2021

Preprint

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Context. 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. Aims. 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. Methods. We compare the strength of the Sr II 4077 Å spectral line to Ca II 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. Results. 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 0.1 mb −1 , which is lower than the estimated τ between 0.15 and 0.6 mb −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. Conclusions. 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 18 O. 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.

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“…We do, however, have a very good constraint of the realtime evolution of a hot RCB star, DY Cen. Archival plates have allowed us to watch the evolution of DY Cen from a cool RCB-like star in 1970 to a hot EHe-like star in 2014 (De Marco et al 2002;Schaefer 2016;Jeffery et al 2020). DY Cen has evolved through the EHe portion of the HR diagram, from roughly log(T eff ) = 4.28 in 1987 to log(T eff ) = 4.39 in 2015, in a timescale of about 30 years.…”

Section: Discussionmentioning

confidence: 99%

Modeling R Coronae Borealis Stars: Effects of He-Burning Shell Temperature and Metallicity

Crawford,

Clayton,

Munson

et al. 2020

Preprint

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The R Coronae Borealis (RCB) stars are extremely hydrogen-deficient carbon stars which produce large amounts of dust, causing sudden deep declines in brightness. They are believed to be formed primarily through white dwarf mergers. In this paper, we use MESA to investigate how post-merger objects with a range of initial He-burning shell temperatures from 2.1 -5.4 ×10 8 K with solar and subsolar metallicities evolve into RCB stars. The most successful model of these has subsolar metallicity and an initial temperature near 3 ×10 8 K. We find a strong dependence on initial He-burning shell temperature for surface abundances of elements involved in the CNO cycle, as well as differences in effective temperature and radius of RCBs. Elements involved in nucleosynthesis present around 1 dex diminished surface abundances in the 10% solar metallicity models, with the exception of carbon and lithium which are discussed in detail. Models with subsolar metallicities also exhibit longer lifetimes than their solar counterparts. Additionally, we find that convective mixing of the burned material occurs only in the first few years of post-merger evolution, after which the surface abundances are constant during and after the RCB phase, providing evidence for why these stars show a strong enhancement of partial He-burning products.

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SALT revisits DY Cen: a rapidly evolving strontium-rich single helium star

Cited by 11 publications

References 39 publications

Peculiar hydrogen-deficient carbon stars: strontium-rich stars and the s-process

Peculiar hydrogen-deficient carbon stars: strontium-rich stars and the s-process

Peculiar Hydrogen-deficient Carbon Stars: Strontium-Rich Stars and the s-Process

Modeling R Coronae Borealis Stars: Effects of He-Burning Shell Temperature and Metallicity

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