| 1. |
- Garcia-Hernandez, D. A., et al.
(författare)
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Oxygen Isotopic Ratios in Cool R Coronae Borealis Stars
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 714:1, s. 144-154
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the relationship between R Coronae Borealis (RCB) stars and hydrogen-deficient carbon (HdC) stars by measuring precise O-16/O-18 ratios for five cool RCB stars. The O-16/O-18 ratios are derived by spectrum synthesis from high-resolution (R similar to 50,000) K-band spectra. Lower limits to the O-16/O-17 and N-14/N-15 ratios as well as Na and S abundances (when possible) are also given. RCB stars in our sample generally display less O-18 than HdC stars-the derived O-16/O-18 ratios range from 3 to 20. The only exception is the RCB star WX CrA, which seems to be an HdC-like star with O-16/O-18 = 0.3. Our result of a higher O-16/O-18 ratio for the RCB stars must be accounted for by a theory of the formation and evolution of HdC and RCB stars. We speculate that a late dredge-up of products of He burning, principally C-12 and O-16, may convert an O-18-rich HdC star into an O-18-poor RCB star as the H-deficient star begins its final evolution from a cool supergiant to the top of the white dwarf cooling track.
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| 2. |
- Garcia-Hernandez, D. A., et al.
(författare)
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The Carbon Star DY Persei May Be a Cool R Coronae Borealis Variable
- 2023
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Ingår i: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 948:1
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Tidskriftsartikel (refereegranskat)abstract
- Optical and near-IR photometry suggests that the carbon star DY Persei exhibits fadings similar to those of R Coronae Borealis (RCB) variables. Photometric surveys of the Galaxy and Magellanic Clouds uncovered new DY Per variables with infrared photometry identifying them with cool carbon stars, perhaps, with an unusual tendency to shed mass. In an attempt to resolve DY Per's identity crisis-a cool carbon giant or a cool RCB variable?-we analyze a high-resolution IGRINS H&K-band spectrum of DY Per. The CO first-overtone bands in the K band of DY Per show a high abundance of O-18 such that O-16/O-18 = 4 +/- 1, a ratio sharply at odds with published results for regular cool carbon giants with O-16/O-18 similar to 1000 but this exceptionally low ratio is characteristic of RCB variables and HdC stars. This similarity suggests that DY Per indeed may be a cool RCB variable. Current opinion considers RCB variables to result from the merger of a He onto a CO white dwarf; observed abundances of these H-deficient stars including the exceptionally low O-16/O-18 ratios are in fair accord with predicted compositions for white dwarf merger products. An H-deficiency for DY Per is not directly observable but is suggested from the strength of an HF line and an assumption that F may be overabundant, as observed and predicted for RCB stars.
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