Prevalence of SED Turndown among Classical Be Stars: Are All Be Stars Close Binaries?

Abstract

Rapid rotation is a fundamental characteristic of classical Be stars and a crucial property allowing for the formation of their circumstellar disks. Past evolution in a mass and angular momentum transferring binary system offers a plausible solution to how Be stars attained their fast rotation. Although the subdwarf remnants of mass donors in such systems should exist in abundance, only a few have been confirmed due to tight observational constraints. An indirect method of detecting otherwise hidden companions is offered by their effect on the outer parts of Be star disks, which are expected to be disrupted or truncated. In the context of the IR and radio continuum excess radiation originating in the disk, the disk truncation can be revealed by a turndown in the spectral energy distribution due to reduced radio flux levels. In this work we search for signs of spectral turndown in a sample of 57 classical Be stars with radio data, 2 Klement et al.which include new data for 23 stars and the longest wavelength detections so far (λ ≈ 10 cm) for 2 stars. We confidently detect the turndown for all 26 stars with sufficient data coverage (20 of which are not known to have close binary companions). For the remaining 31 stars, data are inconclusive as to whether the turndown is present or not. The analysis suggests that many if not all Be stars have close companions influencing their outer disks. If confirmed to be subdwarf companions, the mass transfer spin-up scenario might explain the existence of the vast majority of classical Be stars.

Effects of Close Binary Evolution on the Main-sequence Morphology of Young Star Clusters

“…According to McSwain & Gies (2005), the number of such stars appears to be small. We conclude that it appears likely that binary evolution provides a significant, perhaps dominant, contribution to the Be star population in young star clusters, as suggested by Schootemeijer et al (2018) and Klement et al (2019). Figure 5.…”

Optical spectroscopy of Galactic field classical Be stars

“…Likewise, many other surveys have been performed both in the optical (e.g. Andrillat & Fehrenbach (1982); Hanuschik (1986); Dachs et al (1986); Banerjee et al (2000); Koubský et al (2012); Arcos et al (2017); Klement et al (2019), etc.) and in the near-infrared (Clark & Steele 2000;Steele & Clark 2001;Granada et al 2011;Mennickent et al 2009) regime to characterize CBe star discs.…”

Surveying the Bright Stars by Optical Interferometry. II. A Volume-limited Multiplicity Survey of Main-sequence F Stars

“…More recently, Jarad et al (1989) and Dulaney et al (2017) report SB1 orbits, but they differ widely in period, eccentricity, and RV amplitude, and Harmanec et al (2019), using a large set of spectra, concluded there was no convincing evidence of significant RV changes. However, Klement et al (2019) defend the results of Dulaney et al (2017), claiming many of the observations used in Harmanec et al (2019) lacked sufficient spectral resolution. Wang et al (2018) searched for hot sdO companions to Be stars in archival IUE spectra, but were unable to detect such a companion to β CMi.…”

“…However, binaries at even closer separations are not precluded. For example, studies of classical Be stars in the radio that show spectral energy distribution (SED) turndowns, thought to be indicative of disk truncation due to close binary companions (Klement et al 2019), point to a population of binary companions at separations of only a few tens to perhaps several hundred radii of the primary Be star (equivalent to < 10 AU). At distances typical of the brighter Be stars, these separations are equivalent to an-gular separations ( 30 mas) that are generally inaccessible to either speckle interferometry or AO.…”

“…At distances typical of the brighter Be stars, these separations are equivalent to an-gular separations ( 30 mas) that are generally inaccessible to either speckle interferometry or AO. Likewise, the rotationally broadened spectral lines of classical Be stars may make it difficult to detect the low-amplitude radial velocity (RV) shifts ( 10 km s −1 ) caused by such low-mass companions (Klement et al 2019). Therefore, long baseline optical interferometry (LBOI) appears ideally suited to the detection of very close companions to Be star primaries.…”