The evolutionary history of ultra-compact accreting binaries. II. HST spectroscopy of two eclipsing AM CVn stars
Abstract
AM CVn stars are ultra-compact binaries in which a white dwarf (WD) accretes helium-rich material from a hydrogen-deficient donor. They are promising mHz gravitational-wave sources and potential progenitors of Type Ia supernovae. However, their evolutionary pathways remain uncertain. Three formation channels have been proposed: the WD, helium star, and cataclysmic variable channels. We aim to determine the atmospheric chemical composition of the underlying accreting WDs in two eclipsing AM CVn systems, YZ LMi and ZTF J0407-00, in order to constrain their formation channels. Using ultraviolet spectra obtained with the Hubble Space Telescope, we modelled the atmospheres of the accreting WDs, whose surface compositions are enriched by material transferred from the donor and therefore trace the donor's composition. In particular, the nitrogen-to-carbon abundance ratio provides a diagnostic of the donor's nuclear burning history and hence of the system's formation channel. We measure effective temperatures Teff = 24400 ± 1500 K and surface gravities logg = 8.36 ± 0.05 for YZ LMi, and Teff = 29900 ± 2200 K and logg = 8.37 ± 0.05 for ZTF J0407-00. The inferred masses of the two WDs are 0.84 ± 0.10 and 0.86 ± 0.12 solar masses, respectively. Carbon is not detected in either system, yielding lower limits on the nitrogen-to-carbon mass ratio of N/C > 117 and N/C > 185. We increase from one to three the number of AM CVn systems whose accreting WDs and chemical abundances have been analysed using detailed UV atmosphere models. The high N/C ratios rule out the helium star formation channel for both systems. The WD channel is favoured by the absence of detectable hydrogen at all observed wavelengths, although the evolved cataclysmic variable channel cannot be conclusively excluded.
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BibTeX
@misc{yu2026evolutionary,
title = {{The evolutionary history of ultra-compact accreting binaries. II. HST spectroscopy of two eclipsing AM CVn stars}},
author = {W. Yu and A. F. Pala and T. Kupfer and B. T. Gänsicke and T. L. S. Wong and D. Belloni and A. J. Brown and D. Koester and M. J. Green and J. van Roestel and M. R. Schreiber and W. A. Huff and E. O. Waagen and J. -L. González-Carballo and S. Bolzoni and C. Botana-Albá and F. J. Calvo Fernández and F. Campos-Cucarella and R. Castillo-García and D. Cejudo-Fernández and D. De Martino and S. Dvorak and J. M. Fernández-andújar and E. Fernández-Mañanes and C. Galdies and F. García-Cuesta and R. González Farfán and J. -F. Gout and C. Knigge and J. -L. Martin-Velasco and M. Morales-Aimar and R. Naves-Nogues and A. Oksanen and R. Poggiani and G. Poyner and G. Ramsay and E. Reina-Lorenz and P. Rodríguez-Gil and F. D. Romanov and T. Rutherford and J. -L. Salto-González and J. Shears and E. M. Sion and J. Solaz and D. Steeghs and R. Stephens and R. Stubbings and P. Szkody and O. Toloza and T. Tordai and G. Tovmassian and L. Tremosa-Espasa and B. Young},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.36066},
eprint = {2610.36066},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.36066},
}
RIS
TY - UNPB
TI - The evolutionary history of ultra-compact accreting binaries. II. HST spectroscopy of two eclipsing AM CVn stars
AU - W. Yu
AU - A. F. Pala
AU - T. Kupfer
AU - B. T. Gänsicke
AU - T. L. S. Wong
AU - D. Belloni
AU - A. J. Brown
AU - D. Koester
AU - M. J. Green
AU - J. van Roestel
AU - M. R. Schreiber
AU - W. A. Huff
AU - E. O. Waagen
AU - J. -L. González-Carballo
AU - S. Bolzoni
AU - C. Botana-Albá
AU - F. J. Calvo Fernández
AU - F. Campos-Cucarella
AU - R. Castillo-García
AU - D. Cejudo-Fernández
AU - D. De Martino
AU - S. Dvorak
AU - J. M. Fernández-andújar
AU - E. Fernández-Mañanes
AU - C. Galdies
AU - F. García-Cuesta
AU - R. González Farfán
AU - J. -F. Gout
AU - C. Knigge
AU - J. -L. Martin-Velasco
AU - M. Morales-Aimar
AU - R. Naves-Nogues
AU - A. Oksanen
AU - R. Poggiani
AU - G. Poyner
AU - G. Ramsay
AU - E. Reina-Lorenz
AU - P. Rodríguez-Gil
AU - F. D. Romanov
AU - T. Rutherford
AU - J. -L. Salto-González
AU - J. Shears
AU - E. M. Sion
AU - J. Solaz
AU - D. Steeghs
AU - R. Stephens
AU - R. Stubbings
AU - P. Szkody
AU - O. Toloza
AU - T. Tordai
AU - G. Tovmassian
AU - L. Tremosa-Espasa
AU - B. Young
PY - 2026
DA - 2026/10/26
PB - arXiv
DO - 10.48550/arxiv.2610.36066
UR - https://arxiv.org/abs/2610.36066
AB - AM CVn stars are ultra-compact binaries in which a white dwarf (WD) accretes helium-rich material from a hydrogen-deficient donor. They are promising mHz gravitational-wave sources and potential progenitors of Type Ia supernovae. However, their evolutionary pathways remain uncertain. Three formation channels have been proposed: the WD, helium star, and cataclysmic variable channels. We aim to determine the atmospheric chemical composition of the underlying accreting WDs in two eclipsing AM CVn systems, YZ LMi and ZTF J0407-00, in order to constrain their formation channels. Using ultraviolet spectra obtained with the Hubble Space Telescope, we modelled the atmospheres of the accreting WDs, whose surface compositions are enriched by material transferred from the donor and therefore trace the donor's composition. In particular, the nitrogen-to-carbon abundance ratio provides a diagnostic of the donor's nuclear burning history and hence of the system's formation channel. We measure effective temperatures Teff = 24400 ± 1500 K and surface gravities logg = 8.36 ± 0.05 for YZ LMi, and Teff = 29900 ± 2200 K and logg = 8.37 ± 0.05 for ZTF J0407-00. The inferred masses of the two WDs are 0.84 ± 0.10 and 0.86 ± 0.12 solar masses, respectively. Carbon is not detected in either system, yielding lower limits on the nitrogen-to-carbon mass ratio of N/C > 117 and N/C > 185. We increase from one to three the number of AM CVn systems whose accreting WDs and chemical abundances have been analysed using detailed UV atmosphere models. The high N/C ratios rule out the helium star formation channel for both systems. The WD channel is favoured by the absence of detectable hydrogen at all observed wavelengths, although the evolved cataclysmic variable channel cannot be conclusively excluded.
ER -