New Symbiotic Stars Monitored with the Hermes Spectrograph: A Case Study of Orbital Hα Modulation

Abstract

We present new orbital elements for two symbiotic systems involving a giant of spectral type S, namely V420 Hya and ER Del. These orbital elements are compared with existing elements for S-type binaries and for symbiotic binaries with M-giant primaries. It is shown that among the S-type binaries, most of the short-period systems (with P in the range 300 - 1000 d) exhibit some kind of symbiotic activity (Hα emission, UV continuum, hard X-rays), but symbiotic systems are not restricted to the short-period systems. the complex and varying Hα profile of V420 Hya has been decomposed to several components (broad emission with σ ~ 140 km/s, narrow emission with σ ~ 60 km/s, and narrow absorption components). Their orbital modulation reveals that the broad emission is located close to the companion (under the hypothesis of a system with a mass ratio Mg/Mc = 2), and that this broad emission is mutilated by absorption from matter located along the line of sight and flowing towards the observer (faster than the giant) at all orbital phases.

Keywords

• stars
• binaries
• symbiotic