Critical evaluation of magnetic field detections reported for pulsating B-type stars in the light of ESPaDOnS, Narval, and reanalyzed FORS1/2 observations

M. Shultz, G.A. Wade, J. Grunhut, S. Bagnulo, J.D. Landstreet, C. Neiner, A. Alecian, D. Hanes

Fig. 1. Examples of LSD profiles of stars observed with ESPaDOnS and Narval. Stokes I in black (bottom), diagnostic N in blue (middle), V in red (top). Both V and N have been scaled and shifted for display purposes. Vertical lines denote the integration range adopted for longitudinal field measurement. Top, leftòÀÓright: ε Lupi, V1449 Aql, α Pyx, Bottom, leftòÀÓright: 33 Eri, 15 CMa, HY Vel. Note the similarity between N and V in 5 of the 6 stars. Note also that the scaling of V and N varies substantially between stars. For each star, the LSD profile with the highest SNR is shown.

Abstract

Recent spectropolarimetric studies of 7 SPB and β Cep stars have suggested that photospheric magnetic fields are more common in B-type pulsators than in the general population of B stars, suggesting a significant connection between magnetic and pulsational phenomena. We present an analysis of new and previously published spectropolarimetric observations of these stars. New Stokes V observations obtained with the high-resolution ESPaDOnS and Narval instruments confirm the presence of a magnetic field in one of the stars (ε Lup), but find no evidence of magnetism in 5 others. A re-analysis of the published longitudinal field measurements obtained with the low-resolution FORS1/2 spectropolarimeters finds that the measurements of all stars show more scatter from zero than can be attributed to Gaussian noise, suggesting the presence of a signal and/or systematic underestimation of error bars. Re-reduction and re-measurement of the FORS1/2 spectra from the ESO archive demonstrates that small changes in reduction procedure lead to substantial changes in the inferred longitudinal field, and substantially reduces the number of field detections at the 3δ level. Furthermore, we find that the published periods are not unique solutions to the time series of either the original or the revised FORS1/2 data. We conclude that the reported field detections, proposed periods and field geometry models for ¾‘ Pyx, 15 CMa, 33 Eri and V1449 Aql are artefacts of the data analysis and reduction procedures, and that magnetic fields at the reported strength are no more common in SPB/β Cep stars than in the general population of B stars.

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Last Revised: 2012 February 13th