There is growing evidence that some Be stars were spun up through mass transfer in a close binary system, leaving the former mass donor star as a hot, stripped-down object. There are five known cases of Be stars with hot subdwarf (sdO) companions that were discovered through International Ultraviolet Explorer (IUE) spectroscopy. Here we expand the search for Be+sdO candidates using archival FUV spectra from IUE. We collected IUE spectra for 264 stars and formed cross-correlation functions (CCFs) with a model spectrum for a hot subdwarf. Twelve new candidate Be+sdO systems were found, and eight of these display radial velocity variations associated with orbital motion. The new plus known Be+sdO systems have Be stars with spectral subtypes of B0 to B3, and the lack of later-type systems is surprising given the large number of cooler B-stars in our sample. We discuss explanations for the observed number and spectral type distribution of the Be+sdO systems, and we argue that there are probably many Be systems with stripped companions that are too faint for detection through our analysis.

Detection of additional Be+sdO systems from IUE spectroscopy

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 infrared 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, which include new data for 23 stars and the longest-wavelength detections so far (λ ≈ 10 cm) for two 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, the 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.

/pdf/prevalence-of-sed-turndown-among-classical-be-stars-are-all-3v8xmfqeoi.pdf

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

Context. The structure of the inner parts of Be star disks (≲ 20 stellar radii) is well explained by the viscous decretion disk (VDD) model, which is able to reproduce the observable properties of most of the objects studied so far. The outer parts, on the other hand, are not observationally well-explored, as they are observable only at radio wavelengths. A steepening of the spectral slope somewhere between infrared and radio wavelengths was reported for several Be stars that were previously detected in the radio, but a convincing physical explanation for this trend has not yet been provided.Aims. We test the VDD model predictions for the extended parts of a sample of six Be disks that have been observed in the radio to address the question of whether the observed turndown in the spectral energy distribution (SED) can be explained in the framework of the VDD model, including recent theoretical development for truncated Be disks in binary systems.Methods. We combine new multi-wavelength radio observations from the Karl. G. Jansky Very Large Array (JVLA) and Atacama Pathfinder Experiment (APEX) with previously published radio data and archival SED measurements at ultraviolet, visual, and infrared wavelengths. The density structure of the disks, including their outer parts, is constrained by radiative transfer modeling of the observed spectrum using VDD model predictions. In the VDD model we include the presumed effects of possible tidal influence from faint binary companions. Results. For 5 out of 6 studied stars, the observed SED shows strong signs of SED turndown between far-IR and radio wavelengths. A VDD model that extends to large distances closely reproduces the observed SEDs up to far IR wavelengths, but fails to reproduce the radio SED. Using a truncated VDD model improves the fit, leading to a successful explanation of the SED turndown observed for the stars in our sample. The slope of the observed SEDs in the radio is however not well reproduced by disks that are simply cut off at a certain distance. Rather, some matter seems to extend beyond the truncation radius, where it still contributes to the observed SEDs, making the spectral slope in the radio shallower. This finding is in agreement with our current understanding of binary truncation from hydrodynamical simulations, in which the disk does extend past the truncation radius. Therefore, the most probable cause for the SED turndown is the presence of binary companions that remain undetected for most of our sources.

/pdf/revealing-the-structure-of-the-outer-disks-of-be-stars-3wdrnf9eas.pdf

Revealing the structure of the outer disks of Be stars

HR 6819 is a bright ($V=536$), blue star recently proposed to be a triple containing a detached black hole (BH) We show that the system is a binary and does not contain a BH Using spectral decomposition, we disentangle the observed composite spectra into two components: a rapidly rotating Be star and a slowly rotating B star with low surface gravity $(\log g \approx 275)$ Both stars show periodic radial velocity (RV) variability, but the RV semi-amplitude of the B star's orbit is $K_{\rm B}= (627 \pm 1)\,\rm km\,s^{-1}$, while that of the Be star is only $K_{\rm Be} = (45\pm 2)\,\rm km\,s^{-1}$ This implies that the B star is less massive by at least a factor of 10 The surface abundances of the B star bear imprints of CNO burning We argue that the B star is a bloated, recently stripped helium star with mass $\approx 05\,M_{\odot}$ that is currently contracting to become a hot subdwarf The orbital motion of the Be star obviates the need for a BH to explain the B star's motion A stripped-star model reproduces the observed luminosity of the system, while a normal star with the B star's temperature and gravity would be more than 10 times too luminous HR 6819 and the binary LB-1 probably formed through similar channels We use MESA models to investigate their evolutionary history, finding that they likely formed from intermediate-mass ($3-7\,M_{\odot}$) primaries stripped by slightly lower-mass secondaries and are progenitors to Be + sdOB binaries such as $\phi$ Persei The lifetime of their current evolutionary phase is on average $2\times 10^5$ years, of order half a percent of the total lifetime of the Be phase This implies that many Be stars have hot subdwarf and white dwarf companions, and that a substantial fraction ($20-100\%$) of field Be stars form through accretion of material from a binary companion

A stripped-companion origin for Be stars: clues from the putative black holes HR 6819 and LB-1

High-dispersion spectroscopic monitoring of HESS J0632+057 has been carried out over four orbital cycles in order to search for orbital modulation, covering the entire orbital phase. We have measured radial velocity of H$\alpha$ emission line with the method introduced by Shafter et al. (1986), which has been successfully applied to some Be stars. The velocity is seen to increase much earlier than expected for the orbital period of 315 days, and much more steeply than expected at around "apastron". The period of the H$\alpha$ modulation is found to be as $308^{+26}_{-23}$ days. We have also analyzed Swift/XRT data from 2009 to 2015 to study the orbital modulation, selecting the data with good statistics ($\geq$ 30 counts). With additional two-year data to the previous works, the orbital period has been updated to $313^{+11}_{-8}$ days, which is consistent with the previous X-ray periods and the spectroscopic one. The past XMM-Newton and Chandra observations prefer the period of 313 days. With the new period, assuming that H$\alpha$ velocities accurately trace the motion of the Be star, we have derived a new set of the orbital parameters. In the new orbit, which is less eccentric ($e \sim 0.6$), two outbursts occur after apastron, and just after periastron. Besides, the column density in bright phase ($4.7^{+0.9}_{-08}\times10^{21}\;\mathrm{cm^{-2}}$) is higher than in faint phase ($2.2\pm0.5\times10^{21}\;\mathrm{cm^{-2}}$). These facts suggest that outbursts occur when the compact object passes nearby/through the Be disk. The mass function implies that mass of the compact object is less than 2.5 $\mathrm{M_{sun}}$ assuming that the mass of the Be star is 13.2--18.2 $\mathrm{M_{sun}}$ (Aragona at al. 2010) unless the inclination is extremely small. The photon index indicates that the spectra becomes softer when the system is bright. These suggest that the compact object is a pulsar.

/pdf/orbital-solution-leading-to-an-acceptable-interpretation-for-1wwziln4eo.pdf

Orbital solution leading to an acceptable interpretation for the enigmatic gamma-ray binary HESS J0632+057

The late-type Be star β CMi is remarkably stable compared to other Be stars that have been studied. This has led to a realistic model of the outflowing Be disk by Klement et al. These results showed that the disk is likely truncated at a finite radius from the star, which Klement et al. suggest is evidence for an unseen binary companion in orbit. Here we report on an analysis of the Ritter Observatory spectroscopic archive of β CMi to search for evidence of the elusive companion. We detect periodic Doppler shifts in the wings of the Hα line with a period of 170 days and an amplitude of 2.25 km s−1, consistent with a low-mass binary companion (M ≈ 0.42 M ⊙). We then compared small changes in the violet-to-red peak height changes (V/R) with the orbital motion. We find weak evidence that it does follow the orbital motion, as suggested by recent Be binary models by Panoglou et al. Our results, which are similar to those for several other Be stars, suggest that β CMi may be a product of binary evolution where Roche lobe overflow has spun up the current Be star, likely leaving a hot subdwarf or white dwarf in orbit around the star. Unfortunately, no direct sign of this companion star is found in the very limited archive of International Ultraviolet Explorer spectra.

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

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