26 research outputs found
Discovery of a strong magnetic field on the O star HD 191612: new clues to the future of theta1 Orionis C?
From observations made with the ESPaDOnS spectropolarimeter, recently
installed on the 3.6-m Canada--France--Hawaii Telescope, we report the
discovery of a strong magnetic field in the Of?p spectrum variable HD 191612 --
only the second known magnetic O star (following theta1 Ori C). The stability
of the observed Zeeman signature over four nights of observation, together with
the non-rotational shape of line profiles, argue that the rotation period of HD
191612 is significantly longer than the 9-d value previously proposed. We
suggest that the recently identified 538-d spectral-variability period is the
rotation period, in which case the observed line-of-sight magnetic field of
-220+-38 G implies a large-scale field (assumed dipolar) with a polar strength
of about -1.5 kG. If confirmed, this scenario suggests that HD 191612 is,
essentially, an evolved version of the near-ZAMS magnetic O star theta1 Ori C,
but with an even stronger field (about 15 kG at an age similar to that of
theta1Ori C). We suggest that the rotation rate of HD 191612, which is
exceptionally slow by accepted O-star standards, could be due to
angular-momentum dissipation through a magnetically confined wind.Comment: Accepted by MNRAS Letters, 5 pages, 2 figures, 2 table
The kink-type instability of toroidal stellar magnetic fields with thermal diffusion
The stability of toroidal magnetic fields in rotating radiative stellar zones
is studied for realistic values of both the Prandtl numbers. The two considered
models for the magnetic geometry represent fields with odd and even symmetry
with respect to the equator. In the linear theory in Boussinesq approximation
the resulting complex eigenfrequency (including growth rate and drift rate) are
calculated for a given radial wavenumber of a nonaxisymmetric perturbation with
m=1. The ratio of the Alfven frequency, \Omega_A, to the rate of the basic
rotation, \Omega, controls the eigenfrequency of the solution. For strong
fields with \Omega_A > \Omega the solutions do not feel the thermal diffusion.
The growth rate runs with \Omega_A and the drift rate is close to -\Omega so
that the magnetic pattern will rest in the laboratory system. For weaker fields
with \Omega_A < \Omega the growth rate strongly depends on the thermal
conductivity. For fields with dipolar parity and for typical values of the heat
conductivity the resulting very small growth rates are almost identical with
those for vanishing gravity. For fields with dipolar symmetry the differential
rotation of any stellar radiative zone (like the solar tachocline) is shown as
basically stabilizing the instability independent of the sign of the shear.
Finally, the current-driven kink-type instability of a toroidal background
field is proposed as a model for the magnetism of Ap stars. The recent
observation of a lower magnetic field treshold of about 300 Gauss for Ap stars
is understood as corresponding to the minimum magnetic field producing the
instability.Comment: 11 pages, 7 figures, acc. for publicatio
A test of the planet-star unipolar inductor for magnetic white dwarfs
Despite thousands of spectroscopic detections, only four isolated white dwarfs exhibit Balmer emission lines. The temperature inversion mechanism is a puzzle over 30 years old that has defied conventional explanations. One hypothesis is a unipolar inductor that achieves surface heating via ohmic dissipation of a current loop between a conducting planet and a magnetic white dwarf. To investigate this model, new time-resolved spectroscopy, spectropolarimetry, and photometry of the prototype GD 356 are studied. The emission features vary in strength on the rotational period, but in antiphase with the light curve, consistent with a cool surface spot beneath an optically thin chromosphere. Possible changes in the line profiles are observed at the same photometric phase, potentially suggesting modest evolution of the emission region, while the magnetic field varies by 10 per cent over a full rotation. These comprehensive data reveal neither changes to the photometric period, nor additional signals such as might be expected from an orbiting body. A closer examination of the unipolar inductor model finds points of potential failure: the observed rapid stellar rotation will inhibit current carriers due to the centrifugal force, there may be no supply of magnetospheric ions, and no antiphase flux changes are expected from ohmic surface heating. Together with the highly similar properties of the four cool, emission-line white dwarfs, these facts indicate that the chromospheric emission is intrinsic. A tantalizing possibility is that intrinsic chromospheres may manifest in (magnetic) white dwarfs, and in distinct parts of the Hertzsprung–Russell diagram based on structure and composition
Magnetic chemically peculiar stars
Chemically peculiar (CP) stars are main-sequence A and B stars with
abnormally strong or weak lines for certain elements. They generally have
magnetic fields and all observables tend to vary with the same period.
Chemically peculiar stars provide a wealth of information; they are natural
atomic and magnetic laboratories. After a brief historical overview, we discuss
the general properties of the magnetic fields in CP stars, describe the oblique
rotator model, explain the dependence of the magnetic field strength on the
rotation, and concentrate at the end on HgMn stars.Comment: 9 pages, 4 figures, 2 tables, chapter in "Determination of
Atmospheric Parameters of B-, A-, F- and G-Type Stars", Springer (2014), eds.
E. Niemczura, B. Smalley, W. Pyc
A prevalence of dynamo-generated magnetic fields in the cores of intermediate-mass stars
Magnetic fields play a part in almost all stages of stellar evolution. Most low-mass stars, including the Sun, show surface fields that are generated by dynamo processes in their convective envelopes. Intermediate-mass stars do not have deep convective envelopes, although 10 per cent exhibit strong surface fields that are presumed to be residuals from the star formation process. These stars do have convective cores that might produce internal magnetic fields, and these fields might survive into later stages of stellar evolution, but information has been limited by our inability to measure the fields below the stellar surface. Here we report the strength of dipolar oscillation modes for a sample of 3,600 red giant stars. About 20 per cent of our sample show mode suppression, by strong magnetic fields in the cores, but this fraction is a strong function of mass. Strong core fields occur only in red giants heavier than 1.1 solar masses, and the occurrence rate is at least 50 per cent for intermediate-mass stars (1.6–2.0 solar masses), indicating that powerful dynamos were very common in the previously convective cores of these stars
Discovery of a magnetic field in the early B-type star sigma Lupi
Context. Magnetic early B-type stars are rare. Indirect indicators are needed to identify them before investing in time-intensive spectropolarimetric observations.
Aims: We use the strongest indirect indicator of a magnetic field in B stars, which is periodic variability of ultraviolet (UV) stellar wind lines occurring symmetric about the approximate rest wavelength. Our aim is to identify probable magnetic candidates which would become targets for follow-up spectropolarimetry to search for a magnetic field.
Methods: From the UV wind line variability the B1/B2V star sigma Lupi emerged as a new magnetic candidate star. AAT spectropolarimetric measurements with SEMPOL were obtained. The longitudinal component of the magnetic field integrated over the visible surface of the star was determined with the least-squares deconvolution method.
Results: The UV line variations of sigma Lupi are similar to what is known in magnetic B stars, but no periodicity could be determined. We detected a varying longitudinal magnetic field with 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 photometric period of 3.02 d, which we identify with the rotation period of the star. Additional observations with ESPaDOnS attached to the CFHT confirmed this discovery, and allowed the determination of a precise magnetic period. Analysis revealed that sigma Lupi is a helium-strong star, with an enhanced nitrogen abundance and an underabundance of carbon, and has a chemically spotted surface. Conclusions.sigma Lupi is a magnetic oblique rotator, and is a He-strong star. Like in other magnetic B stars the UV wind emission appears to originate close to the magnetic equatorial plane, with maximum emission occurring when a magnetic pole points towards the Earth. The 3.01972 ± 0.00043 d magnetic rotation period is consistent with the photometric period, with maximum light corresponding to maximum magnetic field.
Based on observations obtained by the International Ultraviolet Explorer, collected at NASA Goddard Space Flight Center and Villafranca Satellite Tracking Station of the European Space Agency. Optical observations have been obtained at the Anglo Australian Telescope (AAT) and at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii.The SMARTS 0.9 m photometry is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/545/A11912 pages, 9 figuresstatus: publishe