# # Program information file # PROGRAM_ID 2022B065 PROGRAM_TITLE Telling them apart : Identifying the first chemical differences between R Coronae Borealis and dustless Hydrogen-deficient Carbon stars PROGRAM_INV1 Viraj Karambelkar PROGRAM_INV2 Mansi Kasliwal PROGRAM_INV3 Courtney Crawford PROGRAM_INV4 Geoff Clayton PROGRAM_INV5 Patrick Tisserand PROGRAM_SCICAT stellar PROGRAM_ABSTRACT_BEG The remnants of low-mass white dwarf [WD] mergers are expected to spend a significant part of their lives as puffed-up supergiant stars. Hydrogen-deficient Carbon [HdC] stars are an enigmatic class of supergiants that have chemical compositions consistent with being WD merger remnants. This class comprises of two sub-types with similar chemical compositions - R Coronae Borealis [RCB] and dustless HdC [dLHdC] stars. RCB stars show rapid brightness declines attributed to dust-formation, while dustless HdC stars [dLHdC] do not show any dust formation. There is no confirmed chemical difference between these two sub-classes. Recent discoveries have significantly increased the sample of RCB and dLHdC stars in the Milky Way. Medium resolution near-infrared spectroscopic observations of these stars identified a possible chemical difference between these stars - their oxygen isotope ratios [16O/18O]. However, oxygen isotope ratios derived from medium resolution spectra have large uncertainties and in some cases are known to underestimate the true ratio. We propose to obtain high resolution [R=75000] observations of a sample of dLHdC and RCB stars using IRTF/iShell to precisely measure their oxygen isotope ratios. These observations will confirm 16O/18O as the first chemical difference between RCB and dLHdC stars. They will also constrain properties of the progenitor white dwarfs of these stars, and provide useful insights into white dwarf merger physics. PROGRAM_ABSTRACT_END