Recent results for classical Be stars are reviewed and links to general astrophysics are presented. Classical Be stars are B-type stars close to the main sequence that exhibit line emission over the photospheric spectrum. The excess is attributed to a circumstellar gaseous component that is commonly accepted to be in the form of an equatorial disk. Since 1988, when the last such review was published, major progress has been made. The geometry and kinematics of the circumstellar environment can be best explained by a rotationally supported relatively thin disk with very little outflow, consistent with interferometric observations. The presence of short-term periodic variability is restricted to the earlier type Be stars. This variation for at least some of these objects has been shown to be due to nonradial pulsation. For at least one star, evidence for a magnetic field has been observed. The mechanisms responsible for the production and dynamics of the circumstellar gas are still not constrained. Observations of nonradial pulsation beating phenomena connected to outbursts point toward a relevance of pulsation, but this mechanism cannot be generalized. Either the evidence that Be stars do not form a homogeneous group with respect to disk formation is growing or the short-term periodic variability is less important than previously thought. The statistics of Be stars investigated in open clusters of the Milky Way and the Magellanic Clouds has reopened the question of the evolutionary status of Be stars. The central B star is a fast rotator, although theoretical developments have revived the question of how high rotational rates are, so the commonly quoted mean value of about 70%-80% of the critical velocity may just be a lower limit. Be stars are in a unique position to make contributions to several important branches of stellar physics, e.g., asymmetric mass-loss processes, stellar angular momentum distribution evolution, astroseismology, and magnetic field evolution.

http://iopscience.iop.org/article/10.1086/378307/pdf

Classical Be Stars

Magnetic fields are present in a wide variety of stars throughout the HR diagram and play a role at basically all evolutionary stages, from very-low-mass dwarfs to very massive stars, and from young star-forming molecular clouds and protostellar accretion discs to evolved giants/supergiants and magnetic white dwarfs/neutron stars. These fields range from a few μG (e.g., in molecular clouds) to TG and more (e.g., in magnetic neutron stars); in nondegenerate stars in particular, they feature large-scale topologies varying from simple nearly axisymmetric dipoles to complex nonaxsymmetric structures, and from mainly poloidal to mainly toroidal topologies. After recalling the main techniques of detecting and modeling stellar magnetic fields, we review the existing properties of magnetic fields reported in cool, hot, and young nondegenerate stars and protostars, and discuss our understanding of the origin of these fields and their impact on the birth and life of stars.

Magnetic Fields of Nondegenerate Stars

Magnetic fields are present in a wide variety of stars through out the HR diagram and play a role at basically all evolutionary stages, from very-low- mass dwarfs to very massive stars, and from young star-forming molecular clouds and protostellar accretion discs to evolved gi- ants/supergiants and magnetic white dwarfs/neutron stars. These fields range from a fewG (e.g., in molecular clouds) to TG and more (e.g., in magnetic neutron stars); in non-degenerate stars in particular, they feature large-scale topologies varying f rom simple nearly-axisymmetric dipoles to complex non-axsymmetric structures, and from mainly poloidal to mainly toroidal topology. After recalling the main techniques of detecting and modelling stellar magnetic fields, we review the existing properties of magnetic fields reported in cool, hot and young non-degenerate stars and protostars, and discuss our understanding of the origin of these fields and their impact on the birth and life of stars.

Magnetic fields of non-degenerate stars

We present here the final results of the first spectropolarimetric survey of a small sample of active M dwarfs, aimed at providing observational constraints on dynamo action on both sides of the full-convection threshold (spectral type M4). Our two previous studies (Donati et al. 2008b; Morin et al. 2008b) were focused on early and mid M dwarfs. The present paper examines 11 fully convective late M dwarfs (spectral types M5–M8). Tomographic imaging techniques were applied to time-series of circularly polarised profiles of 6 stars, in order to infer their large-scale magnetic topologies. For 3 other stars we could not produce such magnetic maps, because of low variability of the Stokes V signatures, but were able to derive some properties of the magnetic fields. We find 2 distinct categories of magnetic topologies: on the one hand strong axisymmetric dipolar fields (similar to mid M dwarfs), and on the other hand weak fields generally featuring a significant non-axisymmetric component, and sometimes a significant toroidal one. Comparison with unsigned magnetic fluxes demonstrates that the second category of magnetic fields shows less organization (less energy in the large scales), similarly to partly convective early M dwarfs. Stars in both categories have similar stellar parameters, our data do not evidence a separation between these 2 categories in the mass-rotation plane. We also report marginal detection of a large-scale magnetic field on the M8 star VB 10 featuring a significant toroidal axisymmetric component, whereas no field is detectable on VB 8 (M7).

Large-scale magnetic topologies of late M dwarfs★: Magnetic topologies of late M dwarfs

In the past decade, a consensus has emerged regarding the nature of classical Be stars: They are very rapidly rotating main sequence B stars, which, through a still unknown, but increasingly constrained process, form an outwardly diffusing gaseous, dust-free Keplerian disk. In this work, first the definition of Be stars is contrasted to similar classes, and common observables obtained for Be stars are introduced and the respective formation mechanisms explained. We then review the current state of knowledge concerning the central stars as non-radially pulsating objects and non-magnetic stars, as far as it concerns large-scale, i.e., mostly dipolar, global fields. Localized, weak magnetic fields remain possible, but are as of yet unproven. The Be-phenomenon, linked with one or more mass-ejection processes, acts on top of a rotation rate of about 75 % of critical or above. The properties of the process can be well constrained, leaving only few options, most importantly, but not exclusively, non-radial pulsation and small-scale magnetic fields. Of these, it is well possible that all are realized: In different stars, different processes may be acting. Once the material has been lifted into Keplerian orbit, memory of the details of the ejection process is lost, and the material is governed by viscosity. The disks are fairly well understood in the theoretical framework of the viscous decretion disk model. This is not only true for the disk structure, but as well for its variability, both cyclic and secular. Be binaries are reviewed under the aspect of the various types of interactions a companion can have with the circumstellar disk. Finally, extragalactic Be stars, at lower metallicities, seem more common and more rapidly rotating.

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Classical Be stars Rapidly rotating B stars with viscous Keplerian decretion disks

In this paper we report a new series of high-precision (median  G) longitudinal magnetic field measurements of RR Lyrae, the brightest Blazhko star, obtained with the MuSiCoS spectropolarimeter over a period of almost 4 years from 1999-2002. These data provide no evidence whatsoever for a strong magnetic field in the photosphere of RR Lyrae, a result consistent with Preston's ([CITE]) results, but inconsistent with apparent magnetic field detections by Babcock ([CITE]) and Romanov et al. ([CITE], [CITE]). Following discussion of these disparate results, we conclude that RR Lyrae is a bona fide non-magnetic star, a conclusion which leads to the general falsification of models of the Blazkho effect requiring strong photospheric magnetic fields. Furthermore, due to the 4 year baseline of our observations, we can also dismiss the hypothesis that RR Lyrae undergoes a 4-year “magnetic cycle”. Therefore the origin of the observed 4–year modulation of RR Lyrae remains unexplained, and we stress the need for additional theoretical efforts to come to a better understanding of this phenomenon.

/pdf/no-evidence-of-a-strong-magnetic-field-in-the-blazhko-star-3p69yb0rup.pdf

No evidence of a strong magnetic field in the Blazhko star RR Lyrae

Context. Although the star itself is not helium enriched, the periodicity and the variability in the UV wind lines of the pulsating B1 IV star β Cephei are similar to what is observed in magnetic helium-peculiar B stars, suggesting that β Cep is magnetic. Aims. We searched for a magnetic field using high-resolution spectropolarimetry. From UV spectroscopy, we analysed the wind variability and investigated the correlation with the magnetic data. Methods. We used 130 time-resolved circular polarisation spectra that were obtained from 1998 (when β Cep was discovered to be magnetic) to 2005, with the MuSiCoS echelle spectropolarimeter at the 2 m Telescope Bernard Lyot. We applied the least-square deconvolution method on the Stokes V spectra and derived the longitudinal component of the integrated magnetic field over the visible hemisphere of the star. We performed a period analysis on the magnetic data and on equivalent-width measurements of UV wind lines obtained over 17 years. We also analysed the short- and long-term radial velocity variations, which are due to the pulsations and the 90-year binary motion, respectively. Results. β Cep hosts a sinusoidally varying magnetic field with an amplitude 97 ± 4Ga nd an average value−6 ± 3G . From the UV wind line variability, we derive a period of 12.00075(11) days, which is the rotation period of the star, and is compatible with the observed magnetic modulation. Phases of maximum and minimum field match those of maximum emission in the UV wind lines, strongly supporting an oblique magnetic-rotator model. We discuss the magnetic behaviour as a function of pulsation behaviour and UV line variability. Conclusions. This paper presents the analysis of the first confirmed detection of a dipolar magnetic field in an upper main-sequence pulsating star. Maximum wind absorption originates in the magnetic equatorial plane. Maximum emission occurs when the magnetic north pole points to the Earth. Radial velocities agree with the ∼90-year orbit around its Be-star binary companion.

/pdf/discovery-of-the-magnetic-field-in-the-pulsating-b-star-b-4751xh13fm.pdf

Discovery of the magnetic field in the pulsating B star β Cephei

Emission in the 4d → 4f transitions of Mn II (multiplet 13, λλ6122-6132), in the 4f → 6g transitions of P II, and in λ6149.5 of Hg II has been detected in the spectrum of the helium-weak star 3 Centauri A (B5 III-IVp). Weaker emission from the same Mn II multiplet is also seen in the hot, mild HgMn star 46 Aquilae (B9 III). It is suggested that the emission is of photospheric origin and may be evidence for the stratification of manganese, phosphorus, and mercury in the photosphere of 3 Cen A and of manganese in 46 Aql.

Emission Lines in the Spectrum of the 3He Star 3 Centauri A.

Emission in the 4d - 4f transitions of MnII (multiplet 13, 6122-6132 Ang), in the 4f - 6g transitions of PII, and in 6149.5 Ang of HgII has been detected in the spectrum of the helium weak star 3 Centauri A (B5 III-IVp). Weaker emission from the same MnII multiplet is also seen in the hot, mild HgMn star 46 Aquila (B9 III).It is suggested that the emission is of photospheric origin and may be evidence for the stratification of manganese, phosphorus and mercury in the photosphere of 3 CenA, and of manganese in 46Aql.

Emission Lines in the Spectrum of the 3He Star 3 Cen A

New circular spectropolarimetric observations of the B1 IIIe star β Cep (υsini = 25 km s−1) show a sinusoidally varying weak longitudinal magnetic field (~ 200 G peak-to-peak). The period corresponds to the 12 day period in the stellar wind variations observed in ultraviolet spectral lines. Maximum field occurs at maximum emission in the UV wind lines. This gives compelling evidence for a magnetic-rotator model for this star, with an unambiguous rotation period of 12 days. The similarity between the Hα emission phases in β Cep and in Be stars suggests that the origin of the Be phenomenon does not have to be rapid rotation: we propose that in β Cep the velocity to bring material in (Keplerian) orbit is provided by the high corotation velocity at the Alfven radius (~10 R*), whereas in Be stars this is done by the rapid rotation of the surface. In both cases the cause of the emission phases has still to be found. Weak temporary magnetic fields remain the strongest candidate. A full paper, with results including additional measurements in June and July 1999, will appear in A & A.

/pdf/the-magnetic-field-of-b-cep-and-the-be-phenomenon-2nqyucr4yw.pdf

S. L. S. Shorlin

Papers

No evidence of a strong magnetic field in the Blazhko star RR Lyrae

Discovery of the magnetic field in the pulsating B star β Cephei

Emission Lines in the Spectrum of the 3He Star 3 Centauri A.

Emission Lines in the Spectrum of the 3He Star 3 Cen A

The Magnetic Field of β Cep and the Be Phenomenon