Oxygen Isotope Ratios in Hydrogen-Deficient Carbon Stars: A Correlation with Effective Temperature and Implications for White Dwarf Merger Outcomes

Abstract

Hydrogen-deficient Carbon (HdC) stars are a class of supergiants with anomalous chemical compositions, suggesting that they are remnants of CO-He white dwarf (WD) mergers. This class comprises two spectroscopically similar subclasses - dusty R Coronae Borealis (RCB) and dustless Hydrogen-deficient Carbon (dLHdC) stars. Both subclasses have a stark overabundance of ${}^{18}\text{O}$ in their atmospheres, but spectroscopic differences between them remain poorly studied. We present high-resolution ($R\approx 75000$) K-band spectra of six RCB and six dLHdC stars, including four newly discovered dLHdC stars, making this the largest sample to date. We develop a semi-automated fitting routine to measure ${}^{16}\text{O}{/}^{18}\text{O}$ ratios for this sample, tripling the number of dLHdC stars with oxygen isotope ratios measured from high resolution spectra. All six dLHdC stars have ${}^{16}\text{O}{/}^{18}\text{O}<1$, while the RCB stars have ${}^{16}\text{O}{/}^{18}\text{O}>4$. Additionally, for the first time, we find a trend of decreasing ${}^{16}\text{O}{/}^{18}\text{O}$ ratios with increasing effective temperature for HdC stars, consistent with predictions of theoretical WD merger models. However, we note that current models overpredict the low ${}^{16}\text{O}{/}^{18}\text{O}$ ratios of dLHdC stars by two orders of magnitude. We also measure abundances of C, N, O, Fe, S, Si, Mg, Na, and Ca for these stars. We observe a correlation between the abundances of ${}^{14}\text{N}$ and ${}^{18}\text{O}$ in our sample, suggesting that a fixed fraction of the ${}^{14}\text{N}$ is converted to ${}^{18}\text{O}$ in these stars via $\alpha$-capture. Our results affirm the emerging picture that the mass ratio/total mass of the WD binary determine whether an RCB or dLHdC is formed post-merger.

Published in Publications of the Astronomical Society of the Pacific. 14 pages, 6 figures

References in corpus (28)

• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• A Method of Correcting Near-Infrared Spectra for Telluric Absorption
• Initial Performance of the NEOWISE Reactivation Mission
• Observational constraints on the origin of the elements. IV: The standard composition of the Sun
• Very Large Excesses of 18O in Hydrogen-Deficient Carbon and R Coronae Borealis Stars: Evidence for White Dwarf Mergers
• Double white dwarf mergers and elemental surface abundances in extreme helium and R Coronae Borealis stars
• ExoMol line lists XXXI: Spectroscopy of lowest eights electronic states of C${}_2$
• Non-LTE Radiative Transfer with Turbospectrum
• MARCS model atmospheres
• Near-infrared narrow-band photometry of M-giant and Mira stars: models meet observations
• An analysis of ultraviolet spectra of Extreme Helium Stars and new clues to their origins
• Oxygen isotopic ratios in cool R Coronae Borealis stars
• An Extremely Large Excess of 18O in the Hydrogen-Deficient Carbon Star, HD 137613
• The 1995-1996 Decline of R Coronae Borealis - High Resolution Optical Spectroscopy
• Tracking down R Coronae Borealis stars from their mid-infrared WISE colours
• Optical and Ultraviolet Observations of A Low-Velocity Type II-Plateau Supernova 2013am in M65
• Observational constraints on the origin of the elements. VII. NLTE analysis of Y II lines in spectra of cool stars and implications for Y as a Galactic chemical clock
• A plethora of new R Coronae Borealis stars discovered from a dedicated spectroscopic follow-up survey
• CNO abundances of HdC and RCB stars: a view of the nucleosynthesis in a white dwarf merger
• The Galactic R Coronae Borealis stars : the C2 Swan bands, the carbon problem, and the 12C/13C ratio
• The dawn of a new era for dustless HdC stars with GAIA eDR3
• Census of R Coronae Borealis stars I: Infrared light curves from Palomar Gattini IR
• R Coronae Borealis Star Evolution: Simulating 3D Merger Events to 1D Stellar Evolution Including Large Scale Nucleosynthesis
• A Spectral Classification System for Hydrogen-deficient Carbon Stars
• R Coronae Borealis and dustless Hydrogen-deficient Carbon stars have different oxygen isotope ratios
• The carbon star DY Persei may be a cool R Coronae Borealis variable
• Abundance analyses of the new R Coronae Borealis stars: ASAS-RCB-8 and ASAS-RCB-10
• R Coronae Borealis Stars at Minimum Light -- UW Cen