1,416 research outputs found
Recurrent shell infall events in a B0.5e star: HD 58978 1979-1988
Infall from the circumstellar envelope onto the bright B0.5 IVe star, HD 58978 was studied. The IUE data indicate that the star was surrounded by a low and moderately ionized circumstellar shell at least 12 times between 1979 and 1988. During 6 of these episodes, the signatures of cool circumstellar material were redshifted with respect to the photosphere by 20 to 80 km/sec. The data indicate that the transition from infall to minimal shell absorption can occur in under 10 days, and are consistent either with infall phases lasting up to 6 months, or with infall episodes shorter than 10 to 15 days. The long term behavior of the shell episodes is compared with variability in the stellar wind
Magnetic fields in O-type stars measured with FORS1 at the VLT
The presence of magnetic fields in O-type stars has been suspected for a long
time. The discovery of such fields would explain a wide range of well
documented enigmatic phenomena in massive stars, in particular cyclical wind
variability, Halpha emission variations, chemical peculiarity, narrow X-ray
emission lines and non-thermal radio/X-ray emission. Here we present the
results of our studies of magnetic fields in O-type stars, carried out over the
last years.Comment: 2 pages, 1 figure, to appear in Proceedings of IAU Symposium 259
"Cosmic Magnetic Fields: from Planets, to Stars and Galaxies", Tenerife,
Spain, November 3-7, 200
Search for the magnetic field of the O7.5 III star xi Persei
Cyclical wind variability is an ubiquitous but as yet unexplained feature
among OB stars. The O7.5 III(n)((f)) star xi Persei is the brightest
representative of this class on the Northern hemisphere. As its prominent
cyclical wind properties vary on a rotational time scale (2 or 4 days) the star
has been already for a long time a serious magnetic candidate. As the cause of
this enigmatic behavior non-radial pulsations and/or a surface magnetic field
are suggested. We present a preliminary report on our attempts to detect a
magnetic field in this star with high-resolution measurements obtained with the
spectropolarimeter Narval at TBL, France during 2 observing runs of 5 nights in
2006 and 5 nights in 2007. Only upper limits could be obtained, even with the
longest possible exposure times. If the star hosts a magnetic field, its
surface strength should be less than about 300 G. This would still be enough to
disturb the stellar wind significantly. From our new data it seems that the
amplitude of the known non-radial pulsations has changed within less than a
year, which needs further investigation.Comment: 2 pages, 6 figures, contributed poster at IAU Symposium 259 "Cosmic
Magnetic Fields: from Planets, to Stars and Galaxies", Tenerife, Spain,
November 3-7, 200
On the H emission from the Cephei system
Be stars, which are characterised by intermittent emission in their hydrogen
lines, are known to be fast rotators. This fast rotation is a requirement for
the formation of a Keplerian disk, which in turn gives rise to the emission.
However, the pulsating, magnetic B1IV star Cephei is a very slow
rotator that still shows H emission episodes like in other Be stars,
contradicting current theories. We investigate the hypothesis that the
H emission stems from the spectroscopically unresolved companion of
Cep. Spectra of the two unresolved components have been separated in
the 6350-6850\AA range with spectro-astrometric techniques, using 11 longslit
spectra obtained with ALFOSC at the Nordic Optical Telescope, La Palma. We find
that the H emission is not related to the primary in Cep, but
is due to its 3.4 magnitudes fainter companion. This companion has been
resolved by speckle techniques, but it remains unresolved by traditional
spectroscopy. The emission extends from about 400 to +400 km s. The
companion star in its 90-year orbit is likely to be a classical Be star with a
spectral type around B6-8. By identifying its Be-star companion as the origin
of the H emission behaviour, the enigma behind the Be status of the
slow rotator Cep has been resolved.Comment: 4 pages, 3 figures. Accepted by A&A Letter
The magnetic characteristics of Galactic OB stars from the MiMeS survey of magnetism in massive stars
The Magnetism in Massive Stars (MiMeS) project represents the largest
systematic survey of stellar magnetism ever undertaken. Based on a sample of
over 550 Galactic B and O-type stars, the MiMeS project has derived the basic
characteristics of magnetism in hot, massive stars. Herein we report
preliminary results.Comment: Proceedings of IAUS 302: Magnetic fields throughout stellar evolutio
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