The evolutionary history of ultra-compact accreting binaries

I. Chemical abundances and the formation channel of the eclipsing AM CVn system ZTF J225237.05−051917.4 from HST spectroscopy

¹ Hamburg Observatory, University of Hamburg Gojenbergsweg 112 21029 Hamburg, Germany
² European Southern Observatory Karl-Schwarzschild-Strasse 2 85748 Garching bei München, Germany
³ Department of Physics and Astronomy, Texas Tech University 2500 Broadway Lubbock TX 79409, USA
⁴ Department of Physics, University of Warwick Coventry CV4 7AL, UK
⁵ Institut für Theoretische Physik und Astrophysik, University of Kiel 24098 Kiel, Germany
⁶ São Paulo State University (UNESP), School of Engineering and Sciences Guaratinguetá, Brazil
⁷ The Observatories of the Carnegie Institution for Science Pasadena CA 91101, USA
⁸ Department of Physics, University of California Santa Barbara CA 93106, USA
⁹ Departamento de Física, Universidad Técnica Federico Santa María Av. España 1680 Valparaíso, Chile
¹⁰ Institute of Science and Technology Austria Am Campus 1 3400 Klosterneuburg, Austria
¹¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam 1090 GE Amsterdam, The Netherlands
¹² American Association of Variable Star Observers (AAVSO), 185 Alewife Brook Pkwy Suite 410 Cambridge MA 02138, USA
¹³ Observadores de Supernovas (ObSN), Observatorio Cerro del Viento, MPC I84, Pl. Fernández Pirfano 3-5A Badajoz 06010, Spain
¹⁴ Silesian University of Technology Akademicka 16 Gliwice, Poland
¹⁵ INAF - Osservatorio Astronomico di Capodimonte Salita Moiariello 16 80131 Naples, Italy
¹⁶ Observadores de Supernovas (ObSN), Observatorio Estelia, MPC Y90, C/ Ladines 12 Ladines Asturias 33993, Spain
¹⁷ Observadores de Supernovas (ObSN), Observatorio Uraniborg C/ Antequera 8 41400 Écija Sevilla, Spain
¹⁸ Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma 440 W. Brooks Street Norman OK 73019, USA
¹⁹ JILA, University of Colorado and National Institute of Standards and Technology 440 UCB Boulder CO 80309-0440, USA
²⁰ Department of Astrophysics/IMAPP, Radboud University PO Box 9010 6500 GL Nijmegen, The Netherlands
²¹ Department of Astronomy, University of Cape Town Private Bag X3 Rondebosch 7701, South Africa
²² South African Astronomical Observatory PO Box 9 Observatory 7935, South Africa
²³ Vereniging Voor Sterrenkunde (VVS) Zeeweg 96 8200 Brugge, Belgium
²⁴ Groupe Européen d’Observations Stellaires (GEOS) 23 Parc de Levesville 28300 Bailleau l’Eveque, France
²⁵ Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne (BAV) Munsterdamm 90 12169 Berlin, Germany
²⁶ School of Physics and Astronomy, University of Southampton Highfield Southampton SO17 1BJ, UK
²⁷ Observadores de Supernovas (ObSN), Observatorio Carpe Noctem, MPC I72 Paseo de la Maliciosa 11 Collado Mediano Madrid 28027, Spain
²⁸ Observadores de Supernovas (ObSN), Observatorio de Sencelles, MPC K14 Camí de Sonfred 1 Sencelles Islas Baleares 07140, Spain
²⁹ American Association of Variable Star Observers (AAVSO) 5 Inverness Way Hillsborough CA 94010, USA
³⁰ Observadores de Supernovas (ObSN), Observatorio Montcabrer, MPC 213 C/ Jaume Balmes 24 Cabrils Barcelona 08348, Spain
³¹ Dipartimento di Fisica, Università di Pisa 56127 Pisa, Italy
³² Armagh Observatory & Planetarium College Hill Armagh BT61 9DG, UK
³³ Observadores de Supernovas (ObSN), Observatorio de Masquefa, MPC 232 Av. Can Marcet 41 Masquefa Barcelona 08783, Spain
³⁴ Instituto de Astrofísica de Canarias E-38205 La Laguna Tenerife, Spain
³⁵ Departamento de Astrofísica, Universidad de La Laguna E-38206 La Laguna Tenerife, Spain
³⁶ Observadores de Supernovas (ObSN), Cal Maciarol mòdul 8 Observatory, MPC A02, Masia Cal Maciarol Camí de l’Observatori s/n Áger Lleida 25691, Spain
³⁷ Department of Astrophysics and Planetary Science, Villanova University Villanova PA 19085, USA
³⁸ Department of Astronomy, University of Washington Seattle WA 98195, USA
³⁹ Instituto de Astronomía, Universidad Nacional Autónoma de México Ciudad Universitaria 04510 CDMX, México.

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Received: 6 October 2025
Accepted: 29 November 2025

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.