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The evolutionary history of ultra-compact accreting binaries. I. Chemical abundances and formation channel of the eclipsing AM CVn system ZTF J225237.05-051917.4 from HST spectroscopy

W. Yu, A. F. Pala, T. Kupfer, B. T. Gänsicke, D. Koester, D. Belloni, T. L. S. Wong, M. R. Schreiber, J. van Roestel, A. J. Brown, E. O. Waagen, J. -L. González-Carballo, S. Bednarz, K. Bernacki, D. De Martino, E. Fernández Mañanes, R. González Farfán, M. J. Green, P. J. Groot, F. -J. Hambsch, C. Knigge, J. -L. Martin-Velasco, M. Morales-Aimar, G. Myers, R. Naves Nogues, R. Poggiani, A. Popowicz, G. Ramsay, E. Reina-Lorenz, P. Rodríguez-Gil, J. -L. Salto-González, E. M. Sion, D. Steeghs, P. Szkody, O. Toloza, G. Tovmassian

TL;DR

This work presents a detailed UV spectroscopic analysis of the eclipsing AM CVn ZTF J225237.05-051917.4 to derive the accretor’s fundamental parameters and surface composition. Through time-tagged HST COS data and hydrogen-deficient atmosphere modelling, the authors measure Teff = 23{,}300 ± 600 K, log g = 8.4 ± 0.3, and M_WD = 0.86 ± 0.16 M_⊙, with N/C > 153 and C < -5.00, enabling formation-channel discrimination. The nitrogen-rich signature disfavors the He-star channel, while the WD and CV channels remain consistent with the observations; comparisons with MESA evolutionary tracks reinforce this conclusion and illustrate how UV spectroscopy constrains binary evolution histories. The paper establishes a robust pipeline for applying UV spectroscopic analysis to a larger AM CVn sample, advancing the understanding of their origins and their role as gravitational-wave sources and SN Ia progenitors.

Abstract

AM Canum Venaticorum (AM CVn) stars are ultra-compact binary systems composed of a white dwarf (WD) primary accreting from a H-deficient donor. They are important as potential progenitors of Type Ia supernovae and laboratories for gravitational-wave studies, yet their evolutionary history remains unsolved. Three formation channels have been proposed: the WD channel, the He-star channel, and the cataclysmic variable (CV) channel. We aim to provide the first accurate measurements of the fundamental parameters of the accretor in ZTFJ225237.05-051917.4, including the abundances of key elements such as C, N, and Si, by analysing UV spectra obtained with the Hubble Space Telescope. These measurements provide new insight into the system's evolutionary history and establish it as a benchmark to develop our pipeline for application to a larger sample of AM CVns. We determine the binary parameters from photometric modelling and constrain the atmospheric parameters of the WD accretor, including Teff, logg, and chemical abundances, by fitting the UV spectrum with synthetic spectral models. We then infer the system's formation channel by comparing our results with theoretical evolutionary models. We measure a Teff=23300$\pm$600K and a surface gravity of logg=8.4$\pm$0.3, which implies an accretor mass of 0.86$\pm$0.16 solar masses. We find a high N/C abundance ratio by mass of >153. 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 determining accurate system parameters. Our results show that UV spectroscopy is well-suited to constraining the formation channels of AM CVn systems. We conclude that the He-star channel can be excluded based on the high N/C ratio, while the WD and CV channels remain consistent with the observations.

The evolutionary history of ultra-compact accreting binaries. I. Chemical abundances and formation channel of the eclipsing AM CVn system ZTF J225237.05-051917.4 from HST spectroscopy

TL;DR

This work presents a detailed UV spectroscopic analysis of the eclipsing AM CVn ZTF J225237.05-051917.4 to derive the accretor’s fundamental parameters and surface composition. Through time-tagged HST COS data and hydrogen-deficient atmosphere modelling, the authors measure Teff = 23{,}300 ± 600 K, log g = 8.4 ± 0.3, and M_WD = 0.86 ± 0.16 M_⊙, with N/C > 153 and C < -5.00, enabling formation-channel discrimination. The nitrogen-rich signature disfavors the He-star channel, while the WD and CV channels remain consistent with the observations; comparisons with MESA evolutionary tracks reinforce this conclusion and illustrate how UV spectroscopy constrains binary evolution histories. The paper establishes a robust pipeline for applying UV spectroscopic analysis to a larger AM CVn sample, advancing the understanding of their origins and their role as gravitational-wave sources and SN Ia progenitors.

Abstract

AM Canum Venaticorum (AM CVn) stars are ultra-compact binary systems composed of a white dwarf (WD) primary accreting from a H-deficient donor. They are important as potential progenitors of Type Ia supernovae and laboratories for gravitational-wave studies, yet their evolutionary history remains unsolved. Three formation channels have been proposed: the WD channel, the He-star channel, and the cataclysmic variable (CV) channel. We aim to provide the first accurate measurements of the fundamental parameters of the accretor in ZTFJ225237.05-051917.4, including the abundances of key elements such as C, N, and Si, by analysing UV spectra obtained with the Hubble Space Telescope. These measurements provide new insight into the system's evolutionary history and establish it as a benchmark to develop our pipeline for application to a larger sample of AM CVns. We determine the binary parameters from photometric modelling and constrain the atmospheric parameters of the WD accretor, including Teff, logg, and chemical abundances, by fitting the UV spectrum with synthetic spectral models. We then infer the system's formation channel by comparing our results with theoretical evolutionary models. We measure a Teff=23300600K and a surface gravity of logg=8.40.3, which implies an accretor mass of 0.860.16 solar masses. We find a high N/C abundance ratio by mass of >153. 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 determining accurate system parameters. Our results show that UV spectroscopy is well-suited to constraining the formation channels of AM CVn systems. We conclude that the He-star channel can be excluded based on the high N/C ratio, while the WD and CV channels remain consistent with the observations.

Paper Structure

This paper contains 18 sections, 1 equation, 12 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. Ultraviolet observations
  4. Brightness monitoring
  5. Methods
  6. Light curve analysis
  7. Spectral analysis
  8. Interstellar medium contamination
  9. Spectral fitting
  10. Uncertainty estimation
  11. Carbon and hydrogen abundance constrain
  12. Results
  13. Discussion
  14. Formation channel identification
  15. Chemical abundances
  16. ...and 3 more sections

Figures (12)

  • Figure 1: Average HST/COS spectrum of ZTF J2252$-$05 covering 900--2050 Å. Masked regions include areas of low sensitivity ($<1100$ and $>1850$ Å; grey) and geocoronal Ly$\alpha$ at 1215 Å (shaded). Night-only data were used to reduce airglow contamination. The error of the averaged flux is shown in red.
  • Figure 2: Normalised UV light curve of ZTF J2252$-$05, extracted from time-resolved COS observations. The total exposure time was 11 563 s, covering five eclipses. The gaps correspond to intervals when the target was behind the Earth and not visible to HST.
  • Figure 3: Photometric monitoring of ZTF J2252$-$05, including observations from LCO, AAVSO and ObSN. The vertical dashed line highlights the time of the HST observations (2022 November 17). Fluctuations can occur during monitoring, as a single exposure may coincide with, or partially cover, an eclipse ($\simeq1.5$ min) of the WD.
  • Figure 4: Fitted UV light curve of ZTF J2252$-$05. The black points show the light curve binned into 3 s intervals. The best-fit model (red) includes contributions from the accreting WD (dashed line), the bright spot (dash-dotted line), and the accretion disc (dotted line). The residuals are shown below the light curve.
  • Figure 5: Spectral comparison between ZTF J2252$-$05 (black) and WD 2253$-$062 (red). The lower panel shows the spectrum of WD 2253$-$062 obtained with the G130M grating, degraded to match the resolution of the ZTF J2252$-$05 observation with the G140L grating. The upper panel presents a zoom-in on the potential ISM-contaminated regions around OI $1304$ and CII $1335$ Å. Both spectra were continuum normalised.
  • ...and 7 more figures