A study of transients from ground-based surveys reveals new AM CVn stars

TL;DR

This work targets 15 transient sources identified as AM CVn candidates and combines ground-based spectroscopy with photometry and TESS observations to robustly classify each system. The study confirms nine AM CVn stars (including eight new spectroscopic confirmations), identifies three hydrogen-rich CVs, and reveals three evolved CV candidates, with several targets yielding orbital or superhump periods from TESS data. By measuring emission-line widths, line separations, and blackbody temperatures, the authors connect spectral properties to accretion-disc dynamics and donor types, and they compute X-ray and UV-derived mass accretion rates that are broadly consistent with disc-instability-driven outbursts. The results refine identification criteria for AM CVn stars in all-sky surveys and demonstrate the value of leveraging TESS and Gaia data to constrain system parameters, advancing population and evolutionary studies of ultra-compact binaries.$

Abstract

AM CVn stars are ultra-compact semi-detached binaries consisting of a white dwarf primary and a hydrogen-depleted secondary. In this paper we present spectroscopic and photometric results of 15 transient sources pre-classified as AM CVn candidates. Our analysis confirms 9 systems of the type AM CVn, 3 hydrogen-rich cataclysmic variables (accreting white dwarfs with near-main-sequence stars for donors) and 3 systems that could be evolved cataclysmic variables. Eight of the AM CVn stars are analysed spectroscopically for the first time, which increases the number of spectroscopically confirmed AM CVns by about $10\%$. TESS data revealed the orbital period of the AM CVn star ASASSN-20pv to be $P_\mathrm{orb}=27.282\,\mathrm{min}$, which helps to constrain the possible values of its mass ratio. TESS also helped to determine the superhump periods of one AM CVn star (ASASSN-19ct, $P_\mathrm{sh}=30.94\,\mathrm{min}$) and two cataclysmic variables we classify as WZ Sge stars ($P_\mathrm{sh}=90.77\,\mathrm{min}$ for ZTF18aaaasnn and $P_\mathrm{sh}=91.6\,\mathrm{min}$ for ASASSN-15na). We identified very different abundances in the spectra of the AM CVns binaries ASASSN-15kf and ASASSN-20pv (both $P_\mathrm{orb}\sim 27.5$ min), suggesting different type of donors. Six of the studied AM CVns are X-ray sources, which helped to determine their mass accretion rates. Photometry shows that the duration of all the superoutbursts detected in the AM CVns is consistent with expectations from the disc instability model. Finally, we provide refined criteria for the identification of new systems using all-sky surveys such as LSST.

Figures (13)

• Figure 1: Finding charts for observed targets, the images are blue bands from the Digitized Sky Survey 2 1996ASPC..101...88L.
• Figure 1: Flux-calibrated Gemini spectra. Regions with unreliable flux-calibration are plotted in light colour.
• Figure 1: Long-term light curves based on ground based photometric observations.
• Figure 2: TESS light curves of four targets showing various types of outburst behaviour and quiescent states. Different superoutbursts phases of ASASSN-19ct are highlighted by green, orange, and brown colours and correspondingly labelled, measurements with low-quality flags are shown in light blue colour.
• Figure 3: normalised spectra of AM CVn stars. Positions of prominent spectral lines are marked by vertical lines, blends of multiple lines are marked by thick lines. The dashed red vertical line shows the potential position of H$\alpha$ line which is absent in all of the presented spectra.