Ultraviolet Spectropolarimetry: on the origin of rapidly rotating B stars

Abstract

UV spectroscopy and spectropolarimetry hold the key to understanding certain aspects of massive stars that are largely inaccessible with optical or longer wavelength observations. This is especially true for the rapidly-rotating Be and Bn stars, owing to their high temperatures, geometric asymmetries, binary properties, and evolutionary history. UV spectropolarimetric observations are extremely sensitive to the photospheric consequences of rapid rotation (i.e. oblateness, temperature, and surface gravity gradients). Our polarized radiative-transfer modelling predicts that with low-resolution UV spectropolarimetry covering 120 -- 300 nm the inclination angle of a rapid rotator can be determined to within 5 degrees, and the rotation rate to within 1%. The origin of rapid rotation in Be/n stars can be explained by either single-star or binary evolution, but their relative importance is largely unknown. Some Be stars have hot sub-luminous (sdO) companions, which at an earlier phase transferred their envelope (and with it mass and angular momentum) to the present-day rapid rotator. Through spectral modelling of a wide range of simulated Be/n+sdO configurations, we demonstrate that high-resolution high-SNR UV spectroscopy can detect an sdO star even when $\sim$1,000 times fainter in the UV than its Be/n star companion. This degree of sensitivity is needed to more fully explore the parameter space of Be/n+sdO binaries, which so far has been limited to about a dozen systems with relatively luminous sdO stars. We suggest that a UV spectropolarimetric survey of Be/n stars is the next step forward in understanding this population. Such a dataset would, when combined with population synthesis models, allow for the determination of the relative importance of the possible evolutionary pathways traversed by these stars, which is also crucial for understanding their future evolution and fate.

38 pages, 16 figures, 1 table, topical collection in Astrophysics and Space Science: UV Spectropolarimetry for Stellar, Interstellar, and Exoplanetary Astrophysics with Polstar

References in corpus (44)

• The Transiting Exoplanet Survey Satellite
• Binary Population and Spectral Synthesis Version 2.1: construction, observational verification and new results
• Effects of gravitational darkening on the determination of fundamental parameters in fast rotating B-type stars
• On the formation of Be stars through binary interaction
• Imaging and Modeling Rapidly Rotating Stars: Alpha Cephei and Alpha Ophiuchi
• The Detection and Characterization of Be+sdO Binaries from HST/STIS FUV Spectroscopy
• Non-LTE Monte Carlo Radiative Transfer: II. Non-Isothermal Solutions for Viscous Keplerian Disks
• Investigating the lack of main-sequence companions to massive Be stars
• Common envelopes in massive stars: Towards the role of radiation pressure and recombination energy in ejecting red supergiant envelopes
• Spectral and spatial imaging of the Be+sdO binary phi Persei
• The life cycles of Be viscous decretion discs: time-dependent modelling of infrared continuum observations
• Detection of a Hot Subdwarf Companion to the Be Star FY Canis Majoris
• Detection of the Ultraviolet Spectrum of the Hot Subdwarf Companion of 60 Cygni (B1 Ve) from a Survey of IUE Spectra of Be Stars
• Hoki: Making BPASS accessible through Python
• The Hot Companion and Circumbinary Disk of the Be Star HR 2142
• The Single Star Path to Be Stars
• Velocity monitoring of gamma-Cas stars reveals their binarity status
• gamma Cas stars: Normal Be stars with disks impacted by the wind of a helium-star companion?
• A propelling neutron star in the enigmatic Be-star $\gamma$~Cassiopeia
• Outbursts and Disk Variability in Be Stars
• Transport of angular momentum by stochastically excited waves as an explanation for the outburst of the rapidly rotating Be star HD49330
• TESS observations of Be stars: a new interpretation
• Growth and Dissipation of Be Star Discs in Misaligned Binary Systems
• Critical angular velocity and anisotropic mass loss of rotating stars with radiation-driven winds
• The Role of Evolutionary Age and Metallicity in the Formation of Classical Be Circumstellar Disks II. Assessing the Evolutionary Nature of Candidate Disk Systems
• The Be Star 66 Ophiuchi: 60 Years of Disk Evolution
• A Multi-observing-technique Study of the Dynamical Evolution of the Viscous Disk around the Be Star $\omega$ CMa
• Outbursts and stellar properties of the classical Be star HD 6226
• Centrifugally Driven Mass Loss and Outbursts of Massive Stars
• Be and Bn stars: Balmer discontinuity and stellar-class relationship
• The Polstar High Resolution Spectropolarimetry MIDEX Mission
• The Asymmetrical Wind of the Candidate Luminous Blue Variable MWC 314
• Spiral Density Enhancements in Be Binary Systems
• Spectroscopic Detection of the Pre-White Dwarf Companion of Regulus
• USuRPER: Unit-Sphere Representation PERiodogram for full spectra
• A new study of the spectroscopic binary 7 Vul with a Be star primary
• The Massive Binary System 9 Sgr Revisited: New Insights into Disentangling Methods
• Magnetic fields of Be stars: preliminary results from a hybrid analysis of the MiMeS sample
• Polarization simulations of stellar wind bow shock nebulae. II. The case of dust scattering
• Evolution of the disk of pi Aqr: from near-disappearance to a strong maximum
• A study of the F-giant star $\theta$ Scorpii A: a post-merger rapid rotator?
• Ultraviolet Spectropolarimetry with Polstar: Interstellar Medium Science
• The peculiar fast-rotating star 51 Oph probed by VEGA/CHARA
• Physics of rotation: problems and challenges

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