Interacting binaries W Serpentids and Double Periodic Variables

TL;DR

This paper reports the discovery of 7 new Galactic Double Periodic Variables (DPVs), analyzes their properties, and compares them with W Serpentids, revealing differences in impact status, disc stability, and stellar characteristics in interacting binary systems.

Contribution

It provides the first comprehensive analysis of DPVs' impact status, disc stability, and stellar parameters, and increases the known Galactic DPV population by 50%.

Findings

Discovered 7 new Galactic DPVs, increasing known count by 50%.

DPVs are tangential-impact systems with stable discs smaller than the tidal radius.

W Serpentids show larger stellar mass range and impact/non-impact behavior.

Abstract

W Serpentids and Double Periodic Variables (DPVs) are candidates for close interacting binaries in a non-conservative evolutionary stage; while W Serpentids are defined by high-excitation ultraviolet emission lines present during most orbital phases, and by usually showing variable orbital periods, DPVs are characterized by a long photometric cycle lasting roughly 33 times the (practically constant) orbital period. We report the discovery of 7 new Galactic DPVs, increasing the number of known DPVs in our Galaxy by 50%. We find that DPVs are tangential-impact systems, i.e. their primaries have radii barely larger than the critical Lubow-Shu radius. These systems are expected to show transient discs, but we find that they host stable discs with radii smaller than the tidal radius. Among tangential-impact systems including DPVs and semi-detached Algols, only DPVs have primaries with masses between 7 and 10 $M_{\odot}$. We find that DPVs are in a Case-B mass transfer stage with donor masses between 1 and 2 M$_{\odot}$ and with primaries resembling Be stars. W Serpentids are impact and non-impact systems, their discs extend until the last non-intersecting orbit and show a larger range of stellar mass and mass ratio than DPVs. Infrared photometry reveals significant color excesses in many DPVs and W Serpentids, usually larger for the latter ones, suggesting variable amounts of circumstellar matter.