Revisiting the extremely long-period cataclysmic variables V479 Andromedae and V1082 Sagitarii

TL;DR

This paper investigates two extremely long-period cataclysmic variables, V479 And and V1082 Sgr, whose donors are nuclearly evolved subgiants and whose orbital periods challenge standard CV evolution. By combining new UV and IR spectroscopy, TESS photometry, refined Gaia distances, and MESA simulations using the CARB magnetic-braking prescription, the authors demonstrate that both systems harbor Roche-lobe–filling subgiant donors and can be reproduced by convergent evolution driven by strong magnetic braking. The results reveal unusual chemical compositions in the donors, consistent with a prior thermal-timescale mass-transfer episode, and favor a scenario in which magnetic braking is stronger for evolved donors than for unevolved ones. The findings have implications for CV evolution and suggest that extremely long-period CVs can contribute to the progenitors of close double white dwarf binaries, highlighting the need for further detailed studies of similar systems.

Abstract

The overwhelming majority of CVs have orbital periods shorter than 10 hr. However, a few have much longer periods, and their formation and existence pose challenges for the CV evolution models. These extremely long-period CVs must host nuclearly evolved donor stars, as otherwise, the companion of the white dwarf would be too small to fill its Roche lobe. This makes them natural laboratories for testing binary evolution models and accretion processes with subgiant donors. To shed light on the formation and evolution of accreting compact objects with subgiant companions, we investigated two extremely long-period CVs in detail, namely V479 And and V1082 Sgr. We searched for reasonable formation pathways to explain their refined stellar and binary parameters. We used a broad set of new observations, including ultraviolet and infrared spectroscopy, results of circular polarimetry, and improved Gaia distance estimates to determine fundamental parameters to be confronted with numerical simulations. Furthermore, we utilized the MESA code to conduct numerical simulations, employing state-of-the-art prescriptions, such as the CARB model for strong magnetic braking. Both systems have unusual chemical compositions and very low masses for their assigned spectral classes. This most likely indicates that they underwent thermal timescale mass transfer. We found models for both that can reasonably reproduce their properties. We conclude that the donor stars in both V479 And and V1082 Sgr are filling their Roche lobes. Our findings suggest that orbital angular momentum loss is stronger due to magnetic braking in CVs with subgiant donors compared to those with unevolved donors. In addition, our findings suggest that extremely long-period CVs could significantly contribute to the population of double white dwarf binaries in close orbits.

Figures (12)

• Figure 1: The global TESS light curve of V479 And comprises three sectors (marked as 17, 57, and 84), each $\approx28$-day long. The original data has a cadence of 120 s. For better visualization, we binned the data to 600 points. The X-axes have similar lengths.
• Figure 2: The power spectra of V479 And for individual sectors, i.e., 57 (pink), and 84 (green). The horizontal lines indicate false alarm probability levels corresponding to 0.1 (dash-dotted; yellow), 0.05 (dotted; orange), and 0.01 (dashed; dark orange). The vertical dotted lines indicate meaningful frequencies: the red is the double orbital frequency, the green is the orbital frequency.
• Figure 3: The HST COS UV spectrum of V479 And. The major emission lines are marked. The geocoronal lines are masked but displayed in a bleak color.
• Figure 4: The SED of V479 And comprised of the HST/COS ultraviolet spectrum (dark grey) and the SDSS optical spectrum (black). In blue and red are shown the upper limits for a white dwarf and a main-sequence star, respectively (see Section \ref{['subsec:HST']}). The donor's model assumes a distance of 1850 pc. The white dwarf model is scaled to the same distance, and both are plotted on an absolute flux scale.
• Figure 5: The radial velocities measured from a complex of absorption lines in J (blue), H (magenta), K (red) bands of V1082 Sgr spectra. The curve fits the radial velocities obtained from high-resolution optical data shown as light-gray points in the upper panel. The residuals of J and K measurements from the radial velocity curve obtained from optical data are presented in the bottom panel.