Gamma Pegasi : testing Vega-like magnetic fields in B stars

Abstract : Context. The bright B pulsator γ Peg shows both p and g modes of β Cep and SPB types. It has also been claimed that it is a magnetic star, while others do not detect any magnetic field.Aims. We check for the presence of a magnetic field, with the aim to characterise it if it exists, or else provide a firm upper limit of its strength if it is not detected. If γ Peg is magnetic as claimed, it would make an ideal asteroseismic target for testing 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.Methods. We acquired high resolution, high signal-to-noise spectropolarimetric Narval data at Telescope Bernard Lyot (TBL). We also gathered existing dimaPol spectropolarimetric data from the Dominion Astrophysical Observatory (DAO) and Musicos spectropolarimetric data from TBL. We analysed the Narval and Musicos observations using the least-squares deconvolution (LSD) technique to derive the longitudinal magnetic field and Zeeman signatures in lines. The longitudinal field strength was also extracted from the Hβ line observed with the DAO. With a Monte Carlo simulation we derived the maximum strength of the field possibly hosted by γ Peg.Results. 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.Conclusions. We conclude that γ Peg is not magnetic: it hosts neither 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.
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C. Neiner, D. Monin, B. Leroy, S. Mathis, D. Bohlender. Gamma Pegasi : testing Vega-like magnetic fields in B stars. Astronomy and Astrophysics - A&A, EDP Sciences, 2014, 562, pp.A59. ⟨10.1051/0004-6361/201323093⟩. ⟨cea-01162433⟩

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