The long-period AM CVn star SDSS J155252.48+320150.9

TL;DR

This paper reports the discovery and characterization of SDSS J155252.48+320150.9 as a new AM CVn star with a 56.3-minute orbital period, confirming its ultracompact binary nature and providing insights into its temperature and component masses.

Contribution

The study confirms SDSS J1552 as a new AM CVn star with the second-longest orbital period, expanding the known sample of such ultracompact binaries.

Findings

Orbital period of 56.272 minutes confirmed.

No hydrogen detected in the spectra.

White dwarf temperature estimated at 12,000-15,000 K.

Abstract

The Sloan Digital Sky Survey has been instrumental in obtaining a homogeneous sample of the rare AM CVn stars: mass-transferring binary white dwarfs. As part of a campaign of spectroscopic follow-up on candidate AM CVn stars from the Sloan Digital Sky Survey, we have obtained time-resolved spectra of the g=20.2 candidate SDSS J155252.48+320150.9 on the Very Large Telescope of the European Southern Observatory. We report an orbital period of 3376.3+/-0.3 s, or 56.272+/-0.005 min, based on an observed `S-wave' in the helium emission lines of the spectra. This confirms the ultracompact nature of the binary. Despite its relative closeness to the orbital period minimum for hydrogen-rich donors, there is no evidence for hydrogen in the spectra. We thus classify SDSS J1552 as a new bona fide AM CVn star, with the second-longest orbital period after V396 Hya (P=65.5 min). The continuum of SDSS J1552 is compatible with either a blackbody or helium atmosphere of 12,000-15,000 K. If this represents the photosphere of the accreting white dwarf, as is expected, it puts the accretor at the upper end of the temperature range predicted by thermal evolution models. This suggests that SDSS J1552 consists of (or formerly consisted of) relatively high-mass components.