The evolutionary history of ultra-compact accreting binaries. II. HST spectroscopy of two eclipsing AM CVn stars
Year: 2026
Type: preprint
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 $\pm$ 1500 K and surface gravities logg = 8.36 $\pm$ 0.05 for YZ LMi, and Teff = 29900 $\pm$ 2200 K and logg = 8.37 $\pm$ 0.05 for ZTF J0407-00. The inferred masses of the two WDs are 0.84 $\pm$ 0.10 and 0.86 $\pm$ 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.
Source: arXiv (Cornell University)
Authors: Yu, W., Pala, A. F., Kupfer, T., Gänsicke, B. T., Wong, T. L. S., Belloni, D., Brown, A. J., Koester, D., Green, M. J., van Roestel, J., Schreiber, M. R., Huff, W. A., Waagen, E. O., González-Carballo, J. -L., Bolzoni, S., Botana-Albá, C., Fernández, F. J. Calvo, Campos-Cucarella, F., Castillo-García, R., Cejudo-Fernández, D., De Martino, D., Dvorak, S., Fernández-andújar, J. M., Fernández-Mañanes, E., Galdies, C., García-Cuesta, F., Farfán, R. González, Gout, J. -F., Knigge, C., Martin-Velasco, J. -L., Morales-Aimar, M., Naves-Nogues, R., Oksanen, A., Poggiani, R., Poyner, G., Ramsay, G., Reina-Lorenz, E., Rodríguez-Gil, P., Romanov, F. D., Rutherford, T., Salto-González, J. -L., Shears, J., Sion, E. M., Solaz, J., Steeghs, D., Stephens, R., Stubbings, R., Szkody, P., Toloza, O., Tordai, T., Tovmassian, G., Tremosa-Espasa, L., Young, B.
Institutions: East Tennessee State University, Tetoora Road Observatory, British Astronomical Association, Armagh Observatory & Planetarium, Universidad de La Laguna, Instituto de Astrofísica de Canarias, Villanova University, American Association of Variable Star Observers, University of Washington, Hungarian Astronomical Association, Universidad Nacional Autónoma de México, Perth Observatory
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funders: National Aeronautics and Space Administration, Space Telescope Science Institute, European Commission, Deutsche Forschungsgemeinschaft, National Natural Science Foundation of China, Chinese Academy of Sciences, Dirección General de Asuntos del Personal Académico, Universidad Nacional Autónoma de México
OpenAlex ID: W7224115366