Abstract by Charles Proffitt
We discuss new HST/STIS observations of the B III resonance doublet in 26 early-B stars with projected rotational velocities between 50 and 220 km/s and compare the measured boron abundances with predictions of stellar evolution models that include rotationally induced mixing. Our results suggest that some stars show significantly less mixing than predicted by the models. When combined with previous results for stars with low projected rotational velocities (below 50 km/s) the distribution of [B/H] with V sin(i) bears a strong resemblance to that for nitrogen, the so-called Hunter diagram (Hunter et al 2008). Especially surprising is the apparent detection of substantial boron in two Be stars that are believed to be very rapid rotators viewed at small inclination angles for which the true rotational velocities are at 75 - 80% of the breakup limit.
Relevant Papers
"Testing Rotational Mixing Predictions with New Boron Abundances in Main-Sequence B-Type Stars"
Mendel et al 2006
"Nitrogen enrichment, boron depletion and magnetic fields in slowly-rotating B-type dwarfs"
Morel et al 2008
"Rotating Massive Main-Sequence Stars I: Grids of Evolutionary Models and Isochrones"
Bortt et al. 2011a
"Rotating Massive Main-Sequence Stars II: Simulating a Population of LMC early B-type Stars as a Test of Rotational Mixing"
Brott et al. 2011b
Talk Slides
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Have B-type stars been observed with sounding rockets?
ReplyDeleteMassive B-type "He-rich" stars do not have global He abundance enhancements, but rather diffusion (in either direction, depending on Teff) and inhomogeneity effects related to strong magnetic fields. The He spectra are usually variable on the rotational periods, because the magnetic and rotational axes are not aligned ("oblique rotators"). Also, any nuclear He enhancements would occur on a much longer timescale than those of N and B, because the latter abundances are altered at the onset of nuclear burning. Finally, it is appearing likely that most "classical" OBN V-III objects (i.e., with large, morphological CNO anomalies) are mass-transfer binaries.
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