o Puppis: another Be+sdO binary?

Abstract

The spectrum of the Be star o Pup is shown to vary periodically with a period of 28.9 days. A radial-velocity variable He i 6678 Å emission component suggests that the star is the same type as φ Per, 59 Cyg, and FY CMa, i.e. binaries consisting of a Be star and a hot companion that illuminates the Be star disk (also called φ Per-type binaries or Be+sdO binaries). The range of radial-velocity variations of the stronger emission component in the helium line observed in o Pup is about 270 km s −1 , a value in good agreement with the range derived for φ Per-type binaries. The radial velocity curves defined by the prominent emission peak in He i 6678 Å line and Hα + Paschen emission move in anti-phase. We suggest that o Pup is generically similar to φ Per-type systems and may represent the fourth case of a Be star with a hot subdwarf companion.

Stellar parameters of Be stars observed with X-shooter

“…This is in agreement with the reported number of about 20% of Be stars showing this feature (e.g., Koubský et al 2012). In absorption and possibly emission the IR Ca ii triplet is seen in another 8 stars.…”

“…It has been speculated that this is connected to binarity, in particular accretion onto a secondary (Polidan 1976). Koubský et al (2012) reject this, but acceded that the presence of this line must be due to some peculiarity in the circumstellar environment that is not further specified. The stars for which a clear or possible Ca ii triplet emission is observed are also flagged in Table 3.…”

“…Although our sample (heavily biased toward later type Be stars) does not include stars with Ca ii triplet emission earlier than B5, a literature search does reveal such stars (Polidan & Peters 1976;Polidan 1976;Briot 1981;Koubský et al 2012). …”

“…This emission strength can even be transient without a major change in the Balmer line emission properties; for example, the Ca ii triplet was not detected in γ Cas by Briot (1981), but was reported to be present by Koubský et al (2012) to be anticorrelated with the Balmer emission for the same star.…”

A Spectroscopic Orbit for the Late-type Be Star β CMi

“…This star becomes the dominant light source and a Be star with a small, 1.5M ⊙ companion with a long orbital period of 126 d. This situation appears to be common for a wide variety of Be stars. Similar direct ultraviolet detections of hot subdwarf companions have been accomplished for FY CMa (Peters et al 2008), 59 Cyg (Peters et al 2013), HR 2142 (Peters et al 2016), and o Pup (Rivinius et al 2012, Koubský et al 2012) all with small mass functions in the range of 2 × 10 −3 − 1.3 × 10 −2 M ⊙ . Other Be binaries that have not had direct detections of the companion stars such as γ Cas (Smith et al 2012) or ζ Tau (Ruždjak et al 2009) also show similar mass functions (f (M ) = 1 × 10 −3 and f (M ) = 6 × 10 −3 respectively) that indicate low-mass companions.…”