Recent Publications

The CHARA Array interferometric program on the multiplicity of classical Be stars: new detections and orbits of stripped subdwarf companions

Robert Klement (1,2), Thomas Rivinius (2), Douglas R. Gies (3), Dietrich Baade (4), Antoine Merand (4), John D. Monnier (5), Gail H. Schaefer (1), Cyprien Lanthermann (1), Narsireddy Anugu (1), Stefan Kraus (6), Tyler Gardner (6)

((1) The CHARA Array of Georgia State University, Mount Wilson, CA, USA, (2) European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Santiago, Chile, (3) Center for High Angular Resolution Astronomy, Department of Physics and Astronomy, Georgia State University, Atlanta, GA, USA, (4) European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Garching bei Munchen, Germany, (5) Department of Astronomy, University of Michigan, Ann Arbor, MI, USA, (6) Astrophysics Group, School of Physics and Astronomy, University of Exeter, Exeter, UK)

Rapid rotation and nonradial pulsations enable Be stars to build decretion disks, where the characteristic line emission forms. A major but unconstrained fraction of Be stars owe their rapid rotation to mass and angular-momentum transfer in a binary. The faint, stripped companions can be helium-burning subdwarf OB-type stars (sdOBs), white dwarfs (WDs), or neutron stars. We present optical/near-IR CHARA interferometry of 37 Be stars selected for spectroscopic indications of low-mass companions. From multi-epoch H- and/or K-band interferometry plus radial velocities and parallaxes collected elsewhere, we constructed 3D orbits and derived flux ratios and absolute dynamical masses of both components for six objects, quadrupling the number of anchor points for evolutionary models. In addition, a new wider companion was identified for the known Be + sdO binary 59 Cyg, while auxiliary VLTI/GRAVITY spectrointerferometry confirmed circumstellar matter around the sdO companion to HR 2142. On the other hand, we failed to detect any companion to the six Be stars with gamma Cas-like X-ray emission, with sdOB and main-sequence companions of the expected spectroscopic mass being ruled out for the X-ray-prototypical stars gamma Cas and pi Aqr, leaving the elusive WD companions as the most likely companions, as well as a likely explanation of the X-rays. No low-mass main-sequence close companions were identified in the other stars.

Comments: Accepted to ApJ

Link: https://doi.org/10.48550/arXiv.2312.08252

TSRG/eROSITA survey of Be stars

Yael Naze (FNRS/ULiege), Jan Robrade (Hamburger Sternwarte)

Massive stars are known X-ray emitters and those belonging to the Be category are no exception. One type of X-ray emission even appears specific to that category, the γ Cas phenomenon. Its actual incidence has been particularly difficult to assess. Thanks to four semesters of sky survey data taken by the Spectrum-Roentgen-Gamma (SRG)/extended ROentgen Survey with an Imaging Telescope Array (eROSITA), we revisit the question of the X-ray properties of Be stars. Amongst a large catalogue of Be stars, eROSITA achieved 170 detections (20 per cent of the sample), mostly corresponding to the earliest spectral types and/or close objects. While X-ray luminosities show an uninterrupted increasing trend with the X-ray-to-bolometric luminosity ratios, the X-ray hardness was split between a large group of soft (and fainter on average) sources and a smaller group of hard (and brighter on average) sources. The latter category gathers at least 34 sources, nearly all displaying early spectral types. Only a third of them were known before to display such X-ray properties. The actual incidence of hard and bright X-rays amongst early-type Be stars within 100-1000 pc appears to be ~12 per cent, which is far from negligible. At the other extreme, no bright supersoft X-ray emission seems to be associated with any of our targets.

Comments: accepted for publication by MNRAS

Link: https://doi.org/10.48550/arXiv.2307.13308