A Spectroscopic and Interferometric Study of W Serpentis Stars. I. Circumbinary Outflow in the Interacting Binary W Serpentis

TL;DR

This study combines spectroscopic and interferometric observations to analyze the W Serpentis binary system, revealing the properties of its components, mass transfer mechanisms, and circumbinary outflow, advancing understanding of interacting binary stars.

Contribution

It provides the first radial velocity curve for the donor star and combines spectroscopic and interferometric data to characterize the system's mass transfer and circumbinary outflow.

Findings

Donor star's radial velocity curve obtained.

Mass ratio of donor to gainer determined as 0.36.

Evidence of an opaque disk around the gainer and circumbinary outflow.

Abstract

W Serpentis is an eclipsing binary system and the prototype of the Serpentid class of variable stars. These are interacting binaries experiencing intense mass transfer and mass loss. However, the identities and properties of both stars in W Ser remain a mystery. Here we present an observational analysis of high quality, visible-band spectroscopy made with the Apache Point Observatory 3.5 m telescope and ARCES spectrograph plus the first near-IR, long-baseline interferometric observations obtained with the CHARA Array. We present examples of the appearance and radial velocities of the main spectral components: prominent emission lines, strong shell absorption lines, and weak absorption lines. We show that some of the weak absorption features are associated with the cool mass donor, and we present the first radial velocity curve for the donor star. The donor's absorption lines are rotationally broadened, and we derive a ratio of donor to gainer mass of 0.36 +/- 0.09 based on the assumptions that the donor fills its Roche lobe and rotates synchronously with the orbit. We use a fit of the ASAS light curve to determine the orbital inclination and mass estimates of 2.0 and 5.7 solar masses for the donor and gainer, respectively. The partially resolved interferometric measurements of orbital motion are consistent with our derived orbital properties and the distance from Gaia EDR3. Spectroscopic evidence indicates that the gainer is enshrouded in an opaque disk that channels the mass transfer stream into an outflow through the L3 region and into a circumbinary disk.