2022A&A...667A..84K

em Context. R Coronae Borealis (RCB) and dustless Hydrogen-deficient Carbon (dLHdC) stars are believed to be remnants of low mass white dwarf mergers. These supergiant stars have peculiar hydrogen-deficient carbon-rich chemistries and stark overabundances of ¹⁸O. RCB stars undergo dust formation episodes resulting in large-amplitude photometric variations that are not seen in dLHdC stars. Recently, the sample of known dLHdC stars in the Milky Way has more than quintupled with the discovery of 27 new dLHdC stars.

em Aims. It has been suggested that dLHdC stars have lower ¹⁶O/¹⁸O than RCB stars. We aim to compare the ¹⁶O/¹⁸O ratios for a large sample of dLHdC and RCB stars to examine this claim.

em Methods. We present medium resolution (R ≈ 3000) near-infrared spectra of 20 newly discovered dLHdC stars. We also present medium resolution (R ≈ 3000 - 8000) K-band spectra for 49 RCB stars. Due to the several free parameters and assumptions in our fitting strategy, we provide wide range estimates on the ¹⁶O/¹⁸O ratios of seven dLHdC and 33 RCB stars that show ¹²C¹⁶O and ¹²C¹⁸O absorption bands, and present the largest sample of such ¹⁶O/¹⁸O wide-range values for dLHdC and RCB stars to date.

em Results. We find that six of the seven dLHdC stars have ¹⁶O/¹⁸O < 0.5, while 28 of the 33 RCB stars have ¹⁶O/¹⁸O > 1. We also confirm that unlike RCB stars, dLHdC stars do not show strong blueshifted (> 200 km s^–1) He I 10 833 Å absorption, suggesting the absence of strong, dust-driven winds around them. Conclusions. Although we only can place wide range estimates on the ¹⁶O/¹⁸O and these are more uncertain in cool RCBs, our medium resolution spectra suggest that most dLHdC stars have lower ¹⁶O/¹⁸O than most RCB stars. This confirms one of the first, long-suspected spectroscopic differences between RCB and dLHdC stars. The different oxygen isotope ratios rule out the existing picture that RCB stars represent an evolved stage of dLHdC stars. Instead, we suggest that whether the white dwarf merger remnant is a dLHdC or RCB star depends on the mass ratios, masses and compositions of the merging white dwarfs. Future high resolution spectroscopic observations will help confirm and more precisely quantify the difference between the oxygen isotope ratios of the two classes.