R Coronae Borealis Stars Are Viable Factories of Pre-Solar Grains

Karakas, Amanda I.; Ruiter, Ashley J.; Hampel, Melanie

doi:10.1088/0004-637x/809/2/184

Cited by 25 publications

(31 citation statements)

References 65 publications

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“…Qualitatively, one can link this to rotation and other processes in the merger. An extension of this work to lower metallicity (Menon et al 2019) finds 16 O/ 18 O and 12 C/ 13 C ratios consistent with observed R CrBs and also with the possibility that these stars are the sources of some graphitic grains (Karakas et al 2015). The R CrB models of Lauer et al (2019) also use initial conditions motivated by WD mergers.…”

Section: Review Of Previous Worksupporting

confidence: 71%

Evolutionary Models for R Coronae Borealis Stars

Schwab

2019

ApJ

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We use Modules for Experiments in Stellar Astrophysics (MESA) to construct stellar evolution models that reach a hydrogen-deficient, carbon-rich giant phase like the R Coronae Borealis (R CrB) stars. These models use opacities from OPAL and AESOPUS that cover the conditions in the cool, H-deficient, CNO-enhanced envelopes of these stars. We compare models that begin from homogeneous He stars with models constructed to reproduce the remnant structure shortly after the merger of a He and a CO white dwarf (WD). We emphasize that models originating from merger scenarios have a thermal reconfiguration phase that can last up to ≈ 1 kyr post merger, suggesting some galactic objects should be in this phase. We illustrate the important role of mass loss in setting the lifetimes of the R CrB stars. Using AGB-like mass loss prescriptions, models with CO WD primaries 0.7 M typically leave the R CrB phase with total masses ≈ 0.6 − 0.7 M , roughly independent of their total mass immediately post-merger. This implies that the descendants of the R CrB stars may have a relatively narrow range in mass and luminosity as extreme He stars and a relatively narrow range in mass as single WDs.

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Section: Review Of Previous Worksupporting

confidence: 71%

Evolutionary Models for R Coronae Borealis Stars

Schwab

2019

ApJ

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“…There are only 5 low-gravity He-sdO stars and 4 O(He) stars known (Jeffery 2008a). Population synthesis calculations indicate an RCB birthrate ∼10 −2 -10 −3 yr −1 (Han 1998;Nelemans et al 2001;Tisserand et al 2013;Zhang et al 2014;Karakas et al 2015;Yungelson & Kuranov 2016;Brown et al 2016). RCB stars are thought to be ∼0.8-0.9 M from pulsation modeling (Saio 2008), and this mass agrees well with the predicted mass of the merger products of a CO-and a He-WD (Han 1998).…”

Section: Lifetimes and The Expected Number Of Rcb Starssupporting

confidence: 61%

“…Using a value of 0.0018 yr −1 for the birthrate of RCB stars (Karakas et al 2015), we can estimate the number of RCB and EHe stars in the Galaxy by multiplying this value times their typical lifetimes. Table 4 gives the lifetimes as RCB stars (t 2 ∼ 10 5 yr) and the lifetimes of the EHe stars (t 3t 2 ∼ 10 4 yr) as calculated by our MESA models.…”

Section: Lifetimes and The Expected Number Of Rcb Starsmentioning

confidence: 99%

Evolving R Coronae Borealis stars with mesa

Lauer

Chatzopoulos

Clayton

et al. 2019

Monthly Notices of the Royal Astronomical Society

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The R Coronae Borealis (RCB) stars are rare hydrogen-deficient, carbon-rich supergiants. They undergo extreme, irregular declines in brightness of many magnitudes due to the formation of thick clouds of carbon dust. It is thought that RCB stars result from the mergers of CO/He white dwarf (WD) binaries. We constructed postmerger spherically-symmetric models computed with the MESA code, and then followed the evolution into the region of the HR diagram where the RCB stars are located. We also investigated nucleosynthesis in the dynamically accreting material of CO/He WD mergers which may provide a suitable environment for significant production of 18 O and the very low 16 O/ 18 O values observed. We have also discovered that the N abundance depends sensitively on the peak temperature in the He-burning shell. Our MESA modeling consists of engineering the star by adding He-WD material to an initial CO-WD model, and then following the post-merger evolution using a nuclearreaction network to match the observed RCB abundances as it expands and cools to become an RCB star. These new models are more physical because they include rotation, mixing, mass-loss, and nucleosynthesis within MESA. We follow the later evolution beyond the RCB phase to determine the stars' likely lifetimes. The relative numbers of known RCB and Extreme Helium (EHe) stars correspond well to the lifetimes predicted from the MESA models. In addition, most of computed abundances agree very well with the observed range of abundances for the RCB class.

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“…This result is in excellent agreement with theoretical estimates made from population synthesis. Indeed, with He-CO white dwarfs merger birthrate ranging between ∼ 10 −3 and ∼ 5×10 −3 per year (Nelemans et al 2001;Ruiter et al 2009;Karakas et al 2015) and an RCB phase lifetime of about 10 5 years, as predicted by theoretical evolution models (Saio & Jeffery 2002;Lauer et al 2018), the expected theoretical number of Galactic RCB stars range between 100 and 500. These estimates are also consistent between the Milky Way and the Magellanic Clouds.…”

Section: How Many Galactic Rcb Stars Exist Out There?mentioning

confidence: 92%

A plethora of new R Coronae Borealis stars discovered from a dedicated spectroscopic follow-up survey

Tisserand

Clayton

Bessell

et al. 2020

A&A

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Context. It is increasingly suspected that R Coronae Borealis (RCB) stars -rare hydrogen-deficient and carbon-rich supergiant stars -are the product of mergers of white-dwarfs in the intermediary mass regime (0.6 < M T ot < 1.2M ). They show a wide range of effective temperatures and chemical abundances. Aims. Only 78 RCB stars are currently known in our Galaxy while up to 1000 were expected. It is necessary to find more of these peculiar and diverse stars to understand their origin and evolutionary path. We are undertaking such a dedicated search. We plan to follow up spectroscopically 2356 targets of interest that were carefully selected using the all sky 2MASS and WISE surveys. They have similar brightnesses and shell temperatures to known RCB stars and only a spectrum can reveal their real nature. Methods. We have observed nearly 500 of these targets using optical low-resolution spectrographs. These spectra were compared to synthetic spectra from a new grid of MARCs hydrogen-deficient atmospheric models. Classical RCB stars' photospheric temperatures range mostly from 4000 K to about 8500 K, and therefore their spectra look very different showing the presence of carbon molecules C 2 and CN up to ∼6800 K and solely atomic absorption lines above that. From the study of both observed and synthetic spectra, we have put in place a series of criteria to distinguish RCB stars from other AGB carbon-rich stars, mainly using estimates of the C/N ratio and 13 C strength, intensities of potential Balmer lines, and estimates of T eff using the Ca II triplet lines strength. Results. We found 45 new RCB stars, including 30 Cold (4000 < T e f f < 6800 K), 14 Warm (6800 < T e f f < 8500 K) and one hot RCB (T e f f > 10000 K). Forty of these belong to the Milky Way and five are located in the Magellanic Clouds. We also confirmed with the same spectroscopic method that the long lasting candidate KDM 5651 is indeed a new Magellanic RCB star, increasing the total number of Magellanic Cloud RCB stars to 30. Our method is particularly efficient at detecting Warm RCB stars as we have increased their number by a factor 4. Conclusions. We have increased by ∼50% the total number of RCB stars known, now reaching 147. We also include a list of 14 strong RCB candidates, most certainly observed during a dust obscuration phase. From the detection efficiency and success rate so far, we estimate that there should be no more than 500 RCB stars/HdC stars in the Milky Way.

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R Coronae Borealis Stars Are Viable Factories of Pre-Solar Grains

Cited by 25 publications

References 65 publications

Evolutionary Models for R Coronae Borealis Stars

Evolutionary Models for R Coronae Borealis Stars

Evolving R Coronae Borealis stars with mesa

A plethora of new R Coronae Borealis stars discovered from a dedicated spectroscopic follow-up survey

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