The evolutionary history of ultra-compact accreting binaries

Context. AM Canum Venaticorum (AM CVn) stars are ultra-compact binary systems composed of a white dwarf primary accreting from a hydrogen-deficient donor. They play a crucial role in astrophysics as potential progenitors of Type Ia supernovae and as laboratories for gravitational wave studies. However, their formation and evolutionary history remain incomplete. Three formation channels have been discussed in the literature: the white dwarf, He-star, and cataclysmic variable channels. Aims. The chemical composition of the accretor atmosphere reflects the material transferred from the donor. In this work we took the first accurate measurements of the fundamental parameters of the accreting white dwarf in ZTF J225237.05−051917.4, including the abundances of key elements such as carbon, nitrogen, and silicon, by analysing ultraviolet spectra obtained with the Hubble Space Telescope (HST). These measurements provide new insight into the evolutionary history of the system and, together with existing optical observations, establish it as a benchmark to develop our pipeline, paving the way for its application to a larger sample of AM CVn systems. Methods. We determined the binary parameters through photometric analysis and constrained the atmospheric parameters of the white dwarf accretor, including its effective temperature, surface gravity, and chemical abundances, by fitting the HST ultraviolet spectrum with synthetic spectral models. We then inferred the system’s formation channel by comparing the results with theoretical evolutionary models. Results. According to our measurements, the accretor’s effective temperature ( T eff ) is 23 300 ± 600 K and the surface gravity (log g ) is 8.4 ± 0.3, which imply an accretor mass ( M WD ) of 0.86 ± 0.16 M ⊙ . We find a high nitrogen-to-carbon abundance ratio by mass of > 153. Conclusions. The accretor is significantly hotter than previous estimates based on simplified blackbody fits to the spectral energy distribution, underscoring the importance of detailed spectral modelling for accurately determining system parameters. Our results show that ultraviolet spectroscopy is well suited to constraining the formation channels of AM CVn systems. Of the three proposed formation channels, the He-star channel can be excluded given the high nitrogen-to-carbon ratio. Our results are consistent with both the white dwarf and cataclysmic variable channels.

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Journal
Astronomy and Astrophysics
Published
2025-12-08
DOI
https://doi.org/10.1051/0004-6361/202557568
Primary Topic
Astrophysical Phenomena and Observations
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article
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article

The evolutionary history of ultra-compact accreting binaries

Martin J. Green, R. G. Farfán, A. F. Pala, P. Rodríguez-Gil et al.
Astronomy and Astrophysics
Astrophysical Phenomena and Observations
article

The evolutionary history of ultra-compact accreting binaries

Martin J. Green, R. G. Farfán, A. F. Pala, P. Rodríguez-Gil, Odette Toloza, Juan-Luis González-Carballo, R. Naves Nogues, Danny T. H. Steeghs, F. -J. Hambsch, P. Groot, Domitilla de Martino, Adam Popowicz, M. Morales-Aimar, Paula Szkody, G. Ramsay, Gordon Myers, G. Tovmassian, M. R. Schreiber, T. Kupfer, Krzysztof Bernacki, Szczepan Bednarz, Esteban Fernández Mañanes, C. Knigge, D. Koester, Jan van Roestel, E. M. Sion, Diogo Belloni, Alan Brown, E. O. Waagen, R. Poggiani, Boris T. Gänsicke, Tsing-Wai Wong, Yu Wang, Jose-Luis Martin-Velasco, Esteban Reina-Lorenz, José-Luis Salto-González
article en

Abstract

Context. AM Canum Venaticorum (AM CVn) stars are ultra-compact binary systems composed of a white dwarf primary accreting from a hydrogen-deficient donor. They play a crucial role in astrophysics as potential progenitors of Type Ia supernovae and as laboratories for gravitational wave studies. However, their formation and evolutionary history remain incomplete. Three formation channels have been discussed in the literature: the white dwarf, He-star, and cataclysmic variable channels. Aims. The chemical composition of the accretor atmosphere reflects the material transferred from the donor. In this work we took the first accurate measurements of the fundamental parameters of the accreting white dwarf in ZTF J225237.05−051917.4, including the abundances of key elements such as carbon, nitrogen, and silicon, by analysing ultraviolet spectra obtained with the Hubble Space Telescope (HST). These measurements provide new insight into the evolutionary history of the system and, together with existing optical observations, establish it as a benchmark to develop our pipeline, paving the way for its application to a larger sample of AM CVn systems. Methods. We determined the binary parameters through photometric analysis and constrained the atmospheric parameters of the white dwarf accretor, including its effective temperature, surface gravity, and chemical abundances, by fitting the HST ultraviolet spectrum with synthetic spectral models. We then inferred the system’s formation channel by comparing the results with theoretical evolutionary models. Results. According to our measurements, the accretor’s effective temperature ( T eff ) is 23 300 ± 600 K and the surface gravity (log g ) is 8.4 ± 0.3, which imply an accretor mass ( M WD ) of 0.86 ± 0.16 M ⊙ . We find a high nitrogen-to-carbon abundance ratio by mass of > 153. Conclusions. The accretor is significantly hotter than previous estimates based on simplified blackbody fits to the spectral energy distribution, underscoring the importance of detailed spectral modelling for accurately determining system parameters. Our results show that ultraviolet spectroscopy is well suited to constraining the formation channels of AM CVn systems. Of the three proposed formation channels, the He-star channel can be excluded given the high nitrogen-to-carbon ratio. Our results are consistent with both the white dwarf and cataclysmic variable channels.

Astronomy and AstrophysicsVol. 706
University of Pisa (IT), Silesian University of Technology (PL), Texas Tech University (US), National Institute of Standards and Technology (US), Radboud University Nijmegen (NL), Institute of Science and Technology Austria (AT), University of Cape Town (ZA), Universidad de La Laguna (ES), Universität Hamburg (DE), Carnegie Institution for Science (US), American Association of Variable Star Observers (US), University of Washington (US), Armagh Observatory & Planetarium (GB), Astronomical Observatory of Capodimonte (IT), Austrian Federal Railways (AT), University of Warwick (GB), Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne (DE), Max Planck Institute for Astrophysics (DE), South African Radio Astronomy Observatory (ZA), National Observatory (BR), Geotechnical Observations (United Kingdom) (GB), Instituto de Astrofísica de Canarias (ES), Astronomical Institute of the Slovak Academy of Sciences (SK), Observatori de l'Ebre (ES), Astronomical Observatory (RS), Stichting tot Steun VCVGZ (NL), University of Southampton (GB), Federico Santa María Technical University (CL), Villanova University (US), University of Oklahoma (US), Universidade Estadual Paulista (Unesp) (BR), University of Amsterdam (NL), Universidad Nacional Autónoma de México (MX)
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Astrophysical Phenomena and Observations
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