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DOI: 10.1007/s10509-022-04127-5

Ultraviolet Spectropolarimetry: on the origin of rapidly rotating B stars

C.E. JonesJ. Labadie-BartzD.V. Cotton ...+11 P.A. Scowen
Nov 2021
摘要
UV spectroscopy and spectropolarimetry hold the key to understanding certainaspects of massive stars that are largely inaccessible with optical or longerwavelength observations. This is especially true for the rapidly-rotating Beand Bn stars, owing to their high temperatures, geometric asymmetries, binaryproperties, and evolutionary history. UV spectropolarimetric observations areextremely sensitive to the photospheric consequences of rapid rotation (i.e.oblateness, temperature, and surface gravity gradients). Our polarizedradiative-transfer modelling predicts that with low-resolution UVspectropolarimetry covering 120 -- 300 nm the inclination angle of a rapidrotator can be determined to within 5 degrees, and the rotation rate to within1%. The origin of rapid rotation in Be/n stars can be explained by eithersingle-star or binary evolution, but their relative importance is largelyunknown. Some Be stars have hot sub-luminous (sdO) companions, which at anearlier phase transferred their envelope (and with it mass and angularmomentum) to the present-day rapid rotator. Through spectral modelling of awide range of simulated Be/n+sdO configurations, we demonstrate thathigh-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 degreeof sensitivity is needed to more fully explore the parameter space of Be/n+sdObinaries, which so far has been limited to about a dozen systems withrelatively luminous sdO stars. We suggest that a UV spectropolarimetric surveyof Be/n stars is the next step forward in understanding this population. Such adataset would, when combined with population synthesis models, allow for thedetermination of the relative importance of the possible evolutionary pathwaystraversed by these stars, which is also crucial for understanding their futureevolution and fate.
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