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Abstract W Serpentis is an eclipsing binary system and the prototype of the Serpentid class of variable stars. These are interacting binaries experiencing intense mass transfer and mass loss. However, the identities and properties of both stars in W Ser remain a mystery. Here, we present an observational analysis of high-quality, visible-band spectroscopy made with the Apache Point Observatory 3.5 m telescope and Astrophysical Research Consortium Echelle Spectrograph spectrograph plus the first near-IR, long-baseline interferometric observations obtained with the Center for High Angular Resolution Astronomy Array. We present examples of the appearance and radial velocities of the main spectral components: prominent emission lines, strong shell absorption lines, and weak absorption lines. We show that some of the weak absorption features are associated with the cool mass donor, and we present the first radial velocity curve for the donor star. The donor’s absorption lines are rotationally broadened, and we derive a ratio of donor to gainer mass of 0.36 ± 0.09 based on the assumptions that the donor fills its Roche lobe and that it rotates synchronously with the orbit. We use a fit of the All-Sky Automated Survey light curve to determine the orbital inclination and mass estimates of 2.0M_⊙and 5.7M_⊙for the donor and gainer, respectively. The partially resolved interferometric measurements of orbital motion are consistent with our derived orbital properties and the distance from Gaia EDR3. Spectroscopic evidence indicates that the gainer is enshrouded in an opaque disk that channels the mass transfer stream into an outflow through the L3 region and into a circumbinary disk. 

Abstract We examine the geometry of the post–asymptotic giant branch (AGB) star binary AC Her and its circumbinary disk. We show that the observations describe a binary orbit that is perpendicular to the disk with an angular momentum vector that is within 9° of the binary eccentricity vector, meaning that the disk is close to a stable polar alignment. The most likely explanation for the very large inner radius of the dust is a planet within the circumbinary disk. This is therefore both the first reported detection of a polar circumbinary disk around a post-AGB binary and the first evidence of a polar circumbinary planet. We consider the dynamical constraints on the circumbinary disk size and mass. The polar circumbinary disk feeds circumstellar disks with gas on orbits that are highly inclined with respect to the binary orbit plane. The resulting circumstellar disk inclination could be anywhere from coplanar to polar depending upon the competition between the mass accretion and binary torques. 

Abstract 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-infrared Center for High Angular Resolution Astronomy (CHARA) interferometry of 37 Be stars selected for spectroscopic indications of low-mass companions. From multiepochH- and/orK-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 Very Large Telescope Interferometer/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 withγCas–like X-ray emission, with sdOB and main-sequence companions of the expected spectroscopic mass being ruled out for the X-ray-prototypical starsγCas andπAqr, leaving elusive WDs as the most likely companions, as well as a likely explanation of the X-rays. No low-mass main-sequence close companions were identified for the other stars. 

Abstract Close binary interactions may play a critical role in the formation of the rapidly rotating Be stars. Mass transfer can result in a mass gainer star spun up by the accretion of mass and angular momentum, while the mass donor is stripped of its envelope to form a hot and faint helium star. Far-UV spectroscopy has led to the detection of about 20 such binary Be+sdO systems. Here we report on a 3 yr program of high-quality spectroscopy designed to determine the orbital periods and physical properties of five Be binary systems. These binaries are long orbital period systems withP= 95–237 days and small semiamplitudeK₁< 11 km s⁻¹. We combined the Be star velocities with prior sdO measurements to obtain mass ratios. A Doppler tomography algorithm shows the presence of the Heiiλ4686 line in the faint spectrum of the hot companion in four of the targets. We discuss the observed line variability and show evidence of phase-locked variations in the emission profiles of HD 157832, suggesting a possible disk spiral density wave due to the presence of the companion star. The stripped companions in HD 113120 and HD 137387 may have a mass larger than 1.4M_⊙, indicating that they could be progenitors of Type Ib and Ic supernovae. 

Abstract The origin of the bright and hard X-ray emission flux among theγCas subgroup of B-emission line (Be) stars may be caused by gas accretion onto an orbiting white dwarf (WD) companion. Such Be+WD binaries are the predicted outcome of a second stage of mass transfer from a helium star mass donor to a rapidly rotating mass gainer star. The stripped donor stars become small and hot white dwarfs that are extremely faint compared to their Be star companions. Here we discuss model predictions about the physical and orbital properties of Be+WD binaries, and we show that current observational results onγCas systems are consistent with the expected large binary frequency, companion faintness and small mass, and relatively high mass range of the Be star hosts. We determine that the companions are probably not stripped helium stars (hot subdwarf sdO stars), because these are bright enough to detect in ultraviolet spectroscopy, yet their spectroscopic signatures are not observed in studies ofγCas binaries. Interferometry of relatively nearby systems provides the means to detect very faint companions including hot subdwarf and cooler main-sequence stars. Preliminary observations of fiveγCas binaries with the CHARA Array interferometer show no evidence of the companion flux, leaving white dwarfs as the only viable candidates for the companions. 

Abstract We present a spectroscopic analysis of the most rapidly rotating stars currently known, VFTS 102 ( $v_e\sin i=649\pm 52$km s⁻¹; O9: Vnnne+) and VFTS 285 ( $v_e\sin i=610\pm 41$km s⁻¹; O7.5: Vnnn), both members of the 30 Dor complex in the Large Magellanic Cloud. This study is based on high-resolution ultraviolet spectra from Hubble Space Telescope/Cosmic Origins Spectrograph and optical spectra from the Very Large Telescope (VLT) X-shooter plus archival VLT GIRAFFE spectra. We utilize numerical simulations of their photospheres, rotationally distorted shape, and gravity darkening to calculate model spectral line profiles and predicted monochromatic absolute fluxes. We use a guided grid search to investigate parameters that yield best fits for the observed features and fluxes. These fits produce estimates of the physical parameters for these stars (plus a Galactic counterpart,ζOph) including the equatorial rotational velocity, inclination, radius, mass, gravity, temperature, and reddening. We find that both stars appear to be radial-velocity constant. VFTS 102 is rotating at critical velocity, has a modest He enrichment, and appears to share the motion of the nearby OB-association LH 99. These properties suggest that the star was spun up through a close binary merger. VFTS 285 is rotating at 95% of critical velocity, has a strong He enrichment, and is moving away from the R136 cluster at the center of 30 Dor. It is mostly likely a runaway star ejected by a supernova explosion that released the components of the natal binary system.