291 research outputs found
A MiMeS analysis of the magnetic field and circumstellar environment of the weak-wind O9 sub-giant star HD 57682
I will review our recent analysis of the magnetic properties of the O9IV star
HD 57682, using spectropolarimetric observations obtained with ESPaDOnS at the
Canada-France-Hawaii telescope within the context of the Magnetism in Massive
Stars (MiMeS) Large Program. I discuss our most recent determination of the
rotational period from longitudinal magnetic field measurements and Halpha
variability - the latter obtained from over a decade's worth of professional
and amateur spectroscopic observations. Lastly, I will report on our
investigation of the magnetic field geometry and the effects of the field on
the circumstellar environment.Comment: 2 pages, 2 figures, IAUS272 - Active OB Stars: Structure, Evolution,
Mass Loss and Critical Limit
O-star mass-loss rates at low metallicity
Mass fluxes J are computed for the extragalactic O stars investigated by
Tramper et al. (2011; TSKK). For one early-type O star, computed and observed
rates agree within errors. However, for two late-type O stars, theoretical
mass-loss rates underpredict observed rates by ~ 1.6 dex, far exceeding
observational errors. A likely cause of the discrepancy is overestimated
observed rates due to the neglect of wind-clumping. A less likely but
intriguing possibility is that, in observing O stars with Z/Z_sun ~ 1/7, TSKK
have serendipitously discovered an additional mass-loss mechanism not evident
in the spectra of Galactic O stars with powerful radiation-driven winds.
Constraints on this unknown mechanism are discussed.
In establishing that the discrepancies, if real, are inescapable for purely
radiation-driven winds, failed searches for high-J solutions are reported and
the importance of a numerical technique that cannot spuriously create or
destroy momentum stressed.
The Z-dependences of the computed rates for Z/Z_sun in the interval (1/30, 2)
show significant departures from a single power law, and these are attributed
to curve-of-growth effects in the differentially-expanding reversing layers.
The best-fitting power-law exponents range from 0.68-0.97.Comment: 6 pages, 2 figure
Discovery of the first tau Sco analogues: HD 66665 and HD 63425
The B0.2 V magnetic star tau Sco stands out from the larger population of
massive OB stars due to its high X-ray activity, peculiar wind diagnostics and
highly complex magnetic field. This paper presents the discovery of the first
two tau Sco analogues - HD 66665 and HD 63425, identified by the striking
similarity of their UV spectra to that of tau Sco. ESPaDOnS spectropolarimetric
observations were secured by the Magnetism in Massive Stars CFHT Large Program,
in order to characterize the stellar and magnetic properties of these stars.
CMFGEN modelling of optical ESPaDOnS spectra and archived IUE UV spectra showed
that these stars have stellar parameters similar to those of tau Sco. A
magnetic field of similar surface strength is found on both stars, reinforcing
the connection between the presence of a magnetic field and wind peculiarities.
However, additional phase-resolved observations will be required in order to
assess the potential complexity of the magnetic fields, and verify if the wind
anomalies are linked to this property.Comment: 6 pages, 2 tables, 3 figures. Accepted for publication in MNRAS. The
definitive version will be available at www.blackwel-synergy.co
Mass fluxes for hot stars
In an attempt to understand the extraordinarily small mass-loss rates of
late-type O dwarfs, mass fluxes in the relevant part of (T_{eff}, g)-space are
derived from first principles using a previously-described code for
constructing moving reversing layers. From these mass fluxes, a weak-wind
domain is identified within which a star's rate of mass loss by a
radiatively-driven wind is less than that due to nuclear burning. The five
weak-wind stars recently analysed by Marcolino et al. (2009) fall within or at
the edge of this domain. But although the theoretical mass fluxes for these
stars are ~ 1.4 dex lower than those derived with the formula of Vink et al.
(2000), the observed rates are still not matched, a failure that may reflect
our poor understanding of low-density supersonic outflows.
Mass fluxes are also computed for two strong-wind O4 stars analysed by Bouret
et al. (2005). The predictions agree with the sharply reduced mass loss rates
found when Bouret et al. take wind clumping into account.Comment: Accepted by A&A; 6 pages, 5 figures; minor changes from v
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