1,320 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
Calculation of the average Green's function of electrons in a stochastic medium via higher-dimensional bosonization
The disorder averaged single-particle Green's function of electrons subject
to a time-dependent random potential with long-range spatial correlations is
calculated by means of bosonization in arbitrary dimensions. For static
disorder our method is equivalent with conventional perturbation theory based
on the lowest order Born approximation. For dynamic disorder, however, we
obtain a new non-perturbative expression for the average Green's function.
Bosonization also provides a solid microscopic basis for the description of the
quantum dynamics of an interacting many-body system via an effective stochastic
model with Gaussian probability distribution.Comment: RevTex, no figure
Discovery of the magnetic field of the B1/B2V star \sigma Lupi
In our search for new magnetic massive stars we use the strongest indirect
indicator of a magnetic field in B stars, which is periodic variability of UV
stellar wind lines occurring in a velocity range symmetric around zero. Our aim
is to obtain follow-up spectropolarimetry to search for a magnetic field in
magnetic candidate stars. We quantify UV wind line variability, and analyse its
time behaviour. The B1/B2V star sigma Lup emerged as a new magnetic candidate
star. AAT spectropolarimetric measurements with SEMPOL were obtained.
The stellar wind line variations of sigma Lup are similar to what is known in
magnetic B stars, but no periodicity could be determined. We detected a
longitudinal magnetic field with varying strength and amplitude of about 100 G
with error bars of typically 20 G, which supports an oblique magnetic-rotator
configuration. The equivalent width variations of the UV lines, the magnetic
and the optical line variations are consistent with the well-known photometric
period of 3.02 days, which we identify with the rotation period of the star.
Additional observations with ESPaDOnS at CFHT strongly confirmed this
discovery, and allowed to determine a precise magnetic period. Further analysis
revealed that Lupi is a helium-strong star, with an enhanced nitrogen
abundance and an underabundance of carbon, and has a spotted surface.
We conclude that sigma Lup is a magnetic oblique rotator, and is a He-strong
star. It is the 4th B star for which a magnetic field is discovered from
studying only its wind variability. Like in the other magnetic B stars the wind
emission originates in the magnetic equator, with maximum emission occurring
when a magnetic pole points towards the Earth. The 3.02 d magnetic rotation
period is consistent with the photometric period, with maximum light
corresponding to maximum magnetic field. A full paper will be submitted to A&A.Comment: 4 pages, 5 figures, to appear in proceedings with AIP. Stellar
polarimetry: From birth to death, Eds. Jennifer Hoffman, Barb Whitney, and
Jon Bjorkma
Magnetic field measurements and wind-line variability of OB-type stars
Context. The first magnetic fields in O- and B-type stars that do not belong
to the Bp-star class, have been discovered. The cyclic UV wind-line
variability, which has been observed in a significant fraction of early-type
stars, is likely to be related to such magnetic fields. Aims. We attempt to
improve our understanding of massive-star magnetic fields, and observe
twenty-five carefully-selected, OB-type stars. Methods. Of these stars we
obtain 136 magnetic field strength measurements. We present the UV wind-line
variability of all selected targets and summarise spectropolarimetric
observations acquired using the MUSICOS spectropolarimeter, mounted at the TBL,
Pic du Midi, between December 1998 and November 2004. From the average Stokes I
and V line profiles, derived using the LSD method, we measure the magnetic
field strengths, radial velocities, and first moment of the line profiles.
Results. No significant magnetic field is detected in any OB-type star that we
observed. Typical 1{\sigma} errors are between 15 and 200 G. A possible
magnetic-field detection for the O9V star 10 Lac remains uncertain, because the
field measurements depend critically on the fringe- effect correction in the
Stokes V spectra. We find excess emission in UV-wind lines, centred about the
rest wavelength, to be a new indirect indicator of the presence of a magnetic
field in early B-type stars. The most promising candidates to host magnetic
fields are the B-type stars {\delta} Cet and 6 Cep, and a number of O stars.
Conclusions. Although some O and B stars have strong dipolar field, which cause
periodic variability in the UV wind-lines, such strong fields are not
widespread. If the variability observed in the UV wind-lines of OB stars is
generally caused by surface magnetic fields, these fields are either weak
(<~few hundred G) or localised.Comment: A&A publishe
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