Attempts to measure the magnetic field of the pulsating B starνEridani

Abstract

We report on attempts to measure the magnetic field of the pulsating B star ν Eridani with the Musicos spectropolarimeter attached to the 2 m 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. Key words. stars: magnetic fields -stars: early-type -stars: individual: ν Eri -stars: oscillations -stars: activity -line: profiles IntroductionThe B2III star ν Eridani (HD 29248, V = 3.93) is known to show radial velocity variations for more than a century (Frost & Adams 1903). It was found to be a multi-mode non-radial pulsator belonging to the class of β Cephei variables with a main frequency of 5.76 c d −1 , identified as a = 0, p 1 mode. Handler et al. (2004) and Jerzykiewicz et al. (2005) detected two independent low frequency, high-order g modes, demonstrating that the star also belongs to the class of Slowly Pulsating B (SPB) stars. The star has the richest known oscillation spectrum of all β Cephei stars. From a very extensive campaign (see Handler et al. 2004;Aerts et al. 2004;De Ridder et al. 2004;Jerzykiewicz et al. 2005, hereafter Papers I, II, III and IV), 34 photometric and 20 spectroscopic frequencies were detected, corresponding to 14 different pulsation frequencies.Among these 14, 12 are high-frequency modes, out of which 9 form three triplets, which are slightly asymmetric. The symmetric part is attributed to the effect of stellar rotation, whereas the asymmetric parts could be due to higher order rotational effects or to a magnetic field. DJ found that the asymmetry of this triplet, as measured from data of van Hoof (1961), could only partly be explained by the quadratic effects of rotation (see, e.g., Saio 1981) and suggested that a strong magnetic dipole field of the order of 5−10 kG could explain this discrepancy. In a more recent analysis (Paper I) the asymmetry was found to be a factor of 2 smaller than before, and Pamyatnykh et al. (2004) could entirely account for the asymmetry in terms of quadratic rotational effects. However, in Papers III and IV the asymmetry was again found to be larger, and a second and third triplet were detected around 6.24 c d −1 and 7.91 c d −1 . More advanced modeling of the different separations and asymmetries in all three triplets is still needed.It is clear that an observational limit for the magnetic field strength will constrain such models, but until now no magnetic measurements of this star are available. The specific prediction by DJ moti...

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“…The question of how pulsations affect the magnetic field measurements is not yet solved in spite of the fact that the number of magnetic studies of pulsating stars is gradually increasing. The effect of pulsations on magnetic field measurements was for the first time discussed by Schnerr et al (2006) and Hubrig et al (2011a). Schnerr et al (2006) discussed the influence of pulsations on the analysis of the magnetic field strength in the β Cephei star ν Eri in MUlti-SIte COntinuous Spectroscopy (MUSICOS) spectra and tried to model the signatures found in Stokes V and N spectra.…”

“…The effect of pulsations on magnetic field measurements was for the first time discussed by Schnerr et al (2006) and Hubrig et al (2011a). Schnerr et al (2006) discussed the influence of pulsations on the analysis of the magnetic field strength in the β Cephei star ν Eri in MUlti-SIte COntinuous Spectroscopy (MUSICOS) spectra and tried to model the signatures found in Stokes V and N spectra. Although the authors reported that they are able to quantitatively establish the influence of pulsations on the magnetic field determination through some modelling, they still detect profiles in Stokes V and N, which are the result of the combined effects of the pulsations and the inaccuracies in wavelength calibration that were not removed by their imperfect modelling of these effects.…”