Roadmap on the theoretical work of BinaMIcS

We review the different theoretical challenges concerning magnetism in interacting binary or multiple stars that will be studied in the BinaMIcS (Binarity and Magnetic Interactions in various classes of Stars) project during the corresponding spectropolarimetric Large Programs at CFHT and TBL. We describe how completely new and innovative topics will be studied with BinaMIcS such as the complex interactions between tidal flows and stellar magnetic fields, the MHD star-star interactions, and the role of stellar magnetism in stellar formation and vice versa. This will strongly modify our vision of the evolution of interacting binary and multiple stars.Comment: 2 pages, proceeding of IAUS 302 Magnetic fields throughout stellar evolution, correct list of author

The dramatic change of the fossil magnetic field of HD 190073: evidence of the birth of the convective core in a Herbig star ?

In the context of the ESPaDOnS and Narval spectropolarimetric surveys of Herbig Ae/Be stars, we discovered and then monitored the magnetic field of HD 190073 over more than four years, from 2004 to 2009. Our observations all displayed similar Zeeman signatures in the Stokes V spectra, indicating that HD 190073 hosted an aligned dipole, stable over many years, consistent with a fossil origin. We obtained new observations of the star in 2011 and 2012 and detected clear variations of the Zeeman signature on timescales of days to weeks, indicating that the configuration of its field has changed between 2009 and 2011. Such a sudden change of external structure of a fossil field has never previously been observed in any intermediate or high-mass star. HD 190073 is an almost entirely radiative pre-main sequence star, probably hosting a growing convective core. We propose that this dramatic change is the result of the interaction between the fossil field and the ignition of a dynamo field generated in the newly-born convective core.Comment: 4 pages, 5 figures, accepted for publication in A&

Attempts to measure the magnetic field of the pulsating B star ν\nu Eridani

We report on attempts to measure the magnetic field of the pulsating B star $\nu$ Eridani with the Musicos spectropolarimeter attached to the 2m telescope at the Pic du Midi, France. This object is one of the most extensively studied stars for pulsation modes, and the existence of a magnetic field was suggested from the inequality of the frequency separations of a triplet in the stars' oscillation spectrum. We show that the inferred 5-10 kG field was not present during our observations, which cover about one year. We discuss the influence of the strong pulsations on the analysis of the magnetic field strength and set an upper limit to the effective longitudinal field strength and to the field strength for a dipolar configuration. We also find that the observed wind line variability is caused by the pulsations.Comment: 8 pages, 7 figures, accepted by A&

How unique is Plaskett's star? A search for organized magnetic fields in short period, interacting or post-interaction massive binary systems

Amongst O-type stars with detected magnetic fields, the fast rotator in the close binary called Plaskett's star shows a variety of unusual properties. Since strong binary interactions are believed to have occurred in this system, one may wonder about their potential role in generating magnetic fields. Stokes V spectra collected with the low-resolution FORS2 and high-resolution ESPaDOnS and Narval spectropolarimeters were therefore used to search for magnetic fields in 15 interacting or post-interaction massive binaries. No magnetic field was detected in any of them, with 0G always being within 2sigma of the derived values. For 17 out of 25 stars in the systems observed at high-resolution, the 90% upper limit on the individual dipolar fields is below the dipolar field strength of Plaskett's secondary; a similar result is found for five out of six systems observed at low resolution. If our sample is considered to form a group of stars sharing similar magnetic properties, a global statistical analysis results in a stringent upper limit of ~200G on the dipolar field strength. Moreover, the magnetic incidence rate in the full sample of interacting or post-interaction systems (our targets + Plaskett's star) is compatible with that measured from large surveys, showing that they are not significantly different from the general O-star population. These results suggest that binary interactions play no systematic role in the magnetism of such massive systems.Comment: 11 pages, accepted for publication in MNRA

gamma Peg: testing Vega-like magnetic fields in B stars

gam Peg is a bright B pulsator showing both p and g modes of beta Cep and SPB types. It has also been claimed to be a magnetic star by some authors while others do not detect a magnetic field. We aimed at checking for the presence of a field, characterise it if it exists or provide a firm upper limit of its strength if it is not detected. If gam Peg is magnetic, it would make an ideal asteroseismic target to test various theoretical scenarios. If it is very weakly magnetic, it would be the first observation of an extension of Vega-like fields to early B stars. Finally, if it is not magnetic and we can provide a very low upper limit on its non-detected field, it would make an important result for stellar evolution models. We acquired high resolution, high signal-to-noise spectropolarimetric Narval data at TBL. We also gathered existing dimaPol@DAO and Musicos@TBL spectropolarimetric data. We analysed the Narval and Musicos observations using the LSD technique to derive the longitudinal magnetic field and Zeeman signatures in lines. The longitudinal field strength was also extracted from the Hbeta line observed with the DAO. With a Monte Carlo simulation we derived the maximum strength of the field possibly hosted by gam Peg. We find that no magnetic signatures are visible in the very high quality spectropolarimetric data. The average longitudinal field measured in the Narval data is Bl=-0.1+/-0.4 G. We derive a very strict upper limit of the dipolar field strength of Bpol~40 G. We conclude that gamma Peg is not magnetic: it does not host a strong stable fossil field as observed in a fraction of massive stars, nor a very weak Vega-like field. There is therefore no evidence that Vega-like fields exist in B stars contrary to the predictions by fossil field dichotomy scenarios. These scenarios should thus be revised. Our results also provide strong constraints for stellar evolution models.Comment: 8 pages, accepted in A&

Discovery of Magnetic Fields in Slowly Pulsating B Stars

We present the first observations and conclusions of a magnetic survey with FORS 1 at the VLT of a sample of 25 Slowly Pulsating B stars. A clear mean longitudinal magnetic field of the order of a few hundred Gauss was detected in eleven SPBs. Among them several SPBs show a magnetic field that varies in time. It becomes clear that SPBs cannot be regarded anymore as non-magnetic stars

On the Hα\alpha emission from the β\beta 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 $\beta$ Cephei is a very slow rotator that still shows H$\alpha$ emission episodes like in other Be stars, contradicting current theories. We investigate the hypothesis that the H$\alpha$ emission stems from the spectroscopically unresolved companion of $\beta$ 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$\alpha$ emission is not related to the primary in $\beta$ 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 $-$400 to +400 km s$^{-1}$. The companion star in its 90-year orbit is likely to be a classical Be star with a spectral type around B6-8. By identifying its Be-star companion as the origin of the H$\alpha$ emission behaviour, the enigma behind the Be status of the slow rotator $\beta$ Cep has been resolved.Comment: 4 pages, 3 figures. Accepted by A&A Letter