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UID:UW-Physics-Event-8180
DTSTART:20230202T213000Z
DTEND:20230202T223000Z
DTSTAMP:20260414T054149Z
LAST-MODIFIED:20230127T205338Z
LOCATION:4421 Sterling Hall 
SUMMARY:The Discovery and Properties of Binary-Stripped Helium Stars\,
  Astronomy Colloquium\, Dr. Ylva Götberg \, Carnegie Observatories
DESCRIPTION:Massive stars stripped of their H-rich envelopes through m
 ass transfer or common envelope ejection are thought to be the main pr
 ogenitors of H-poor supernovae\, to emit large amounts of hard ionizin
 g radiation\, and constitute two necessary steps in the binary evoluti
 on pathways towards compact objects merging in gravitational wave even
 ts. Despite their importance\, these stripped helium stars have remain
 ed elusive. With new UV photometry combined with optical magnitudes\, 
 and follow-up optical spectroscopy\, we identified the first sample of
  dozens of such stripped star systems in the Magellanic Clouds. We obt
 ain estimates for their stellar parameters by fitting their optical sp
 ectra to a newly computed grid of helium star atmosphere models. Align
 ed with theoretical expectations\, we find that stripped stars are hot
  (Teff~50-100 kK)\, compact (log g ~ 5)\, He-rich (Y_surf ~0.6-1)\, an
 d H-poor (X_surf ~0-0.4). Furthermore\, by matching the spectroscopic 
 fits with the photometrical data\, we find small radii (~1 Rsun)\, a r
 ange of luminosities (L ~ 1\,000-100\,000 Lsun)\, and masses that are 
 sufficient to lead to core-collapse (~2-8 Msun). There are strong indi
 cations that the stellar winds are surprisingly weak\, suggesting that
  binary-stripped helium stars are the main ones responsible for both I
 Ib and Ib supernovae. Apart from providing an observational anchor for
  both binary evolution models and simulations of common envelope eject
 ion\, this sample of stars proves that the full mass range of helium s
 tars exists\, forming a bridge between subdwarfs and Wolf-Rayet stars.
URL:https://www.physics.wisc.edu/events/?id=8180
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