The dearth of dustless HdC stars in the Magellanic Clouds: A metallicity-driven white dwarf merger mass effect
Abstract
Hydrogen-deficient carbon (HdC) stars are rare supergiants believed to form through the merger of two white dwarfs (WDs). They comprise the dust-producing R Coronae Borealis (RCB) stars and the dustless HdC (dLHdC) stars. While dedicated Galactic surveys find the two subclasses in roughly equal numbers, no dLHdC star had previously been identified in the Magellanic Clouds, despite 30 known RCB stars in these two galaxies. We present the first systematic search for dLHdC stars in the Large and Small Magellanic Clouds, aiming to measure the dLHdC-to-RCB ratio in environments of distinct metallicities and well-constrained star formation histories. Candidates were selected from the Gaia DR3 Magellanic catalogue using colour-magnitude cuts combining Gaia, SkyMapper DR4, and VMC DR6 photometry, complemented by a random-forest classifier trained on known Galactic dLHdC stars. The search reached an overall detection efficiency of ~96%. All candidates were followed up spectroscopically with WiFeS on the ANU 2.3m telescope. We discovered the first two Magellanic dLHdC stars: SP77 29-5 in the LMC and SMSS 005551.43-723139.2 in the SMC, the latter belonging to the rare Sr-rich subclass. The resulting dLHdC-to-RCB number ratio of ~1:15 overall is far below the near-unity Galactic value. We argue this deficit is intrinsic rather than observational. Convolving the delay time distribution from a sub-class of white dwarf mergers with each Cloud's star formation history, we estimate that ~80% (LMC) and about two-thirds (SMC) of HdC stars originate from mergers with short delay times, which at low metallicity are biased towards higher total WD masses, and thus towards the luminous regime associated with RCB stars rather than the fainter dLHdC population.
Related
- New Optical and Mid-Infrared RR Lyrae Period-Luminosity-Metallicity Relations for Precision Distances and Metallicity Distributions in the Magellanic Clouds
- The independence of the mid-infrared RR Lyrae Period-Luminosity relation from metallicity from a study of three globular clusters in the Large Magellanic Cloud
- A High Resolution View of Three Massive Young Stellar Objects in the Magellanic Clouds
- Observing Co-Located Neutral and Ionized Gas-Phase Iron Depletion in the Magellanic Clouds
- Observing Colocated Neutral and Ionized Gas-phase Iron Depletion in the Magellanic Clouds
BibTeX
@misc{tisserand2026dearth,
title = {{The dearth of dustless HdC stars in the Magellanic Clouds: A metallicity-driven white dwarf merger mass effect}},
author = {Patrick Tisserand and Ashley J. Ruiter and Eden Cheng and Jamie Soon and Anais Möller and Courtney L. Crawford and George Bowden and Ivo R. Seitenzahl and Simon J. Murphy and Christopher A. Onken and Christian Wolf and Geoffrey C. Clayton},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.37404},
eprint = {2610.37404},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.37404},
note = {CC BY-NC-SA 4.0},
}
RIS
TY - UNPB
TI - The dearth of dustless HdC stars in the Magellanic Clouds: A metallicity-driven white dwarf merger mass effect
AU - Patrick Tisserand
AU - Ashley J. Ruiter
AU - Eden Cheng
AU - Jamie Soon
AU - Anais Möller
AU - Courtney L. Crawford
AU - George Bowden
AU - Ivo R. Seitenzahl
AU - Simon J. Murphy
AU - Christopher A. Onken
AU - Christian Wolf
AU - Geoffrey C. Clayton
PY - 2026
DA - 2026/10/27
PB - arXiv
DO - 10.48550/arxiv.2610.37404
UR - https://arxiv.org/abs/2610.37404
AB - Hydrogen-deficient carbon (HdC) stars are rare supergiants believed to form through the merger of two white dwarfs (WDs). They comprise the dust-producing R Coronae Borealis (RCB) stars and the dustless HdC (dLHdC) stars. While dedicated Galactic surveys find the two subclasses in roughly equal numbers, no dLHdC star had previously been identified in the Magellanic Clouds, despite 30 known RCB stars in these two galaxies. We present the first systematic search for dLHdC stars in the Large and Small Magellanic Clouds, aiming to measure the dLHdC-to-RCB ratio in environments of distinct metallicities and well-constrained star formation histories. Candidates were selected from the Gaia DR3 Magellanic catalogue using colour-magnitude cuts combining Gaia, SkyMapper DR4, and VMC DR6 photometry, complemented by a random-forest classifier trained on known Galactic dLHdC stars. The search reached an overall detection efficiency of ~96%. All candidates were followed up spectroscopically with WiFeS on the ANU 2.3m telescope. We discovered the first two Magellanic dLHdC stars: SP77 29-5 in the LMC and SMSS 005551.43-723139.2 in the SMC, the latter belonging to the rare Sr-rich subclass. The resulting dLHdC-to-RCB number ratio of ~1:15 overall is far below the near-unity Galactic value. We argue this deficit is intrinsic rather than observational. Convolving the delay time distribution from a sub-class of white dwarf mergers with each Cloud's star formation history, we estimate that ~80% (LMC) and about two-thirds (SMC) of HdC stars originate from mergers with short delay times, which at low metallicity are biased towards higher total WD masses, and thus towards the luminous regime associated with RCB stars rather than the fainter dLHdC population.
ER -