The Magnetic Early B-type Stars I: Magnetometry and Rotation

Abstract

The rotational and magnetic properties of many magnetic hot stars are poorly characterized, therefore the MiMeS and BinaMIcS collaborations have collected extensive high-dispersion spectropolarimetric datasets of these targets. We present longitudinal magnetic field measurements $⟨B_{\mathrm{z}}⟩$ for 52 early B-type stars (B5 to B0), with which we attempt to determine their rotational periods $P_{\mathrm{rot}}$. Supplemented with high-resolution spectroscopy, low-resolution DAO circular spectropolarimetry, and archival Hipparcos photometry, we determined $P_{\mathrm{rot}}$ for 10 stars, leaving only 5 stars for which $P_{\mathrm{rot}}$ could not be determined. Rotational ephemerides for 14 stars were refined via comparison of new to historical magnetic measurements. The distribution of $P_{\mathrm{rot}}$ is very similar to that observed for the cooler Ap/Bp stars. We also measured $v\sin i$ and $v_{\mathrm{mac}}$ for all stars. Comparison to non-magnetic stars shows that $v\sin i$ is much lower for magnetic stars, an expected consequence of magnetic braking. We also find evidence that $v_{\mathrm{mac}}$ is lower for magnetic stars. LSD profiles extracted using single-element masks revealed widespread, systematic discrepancies in $⟨B_{\mathrm{z}}⟩$ between different elements: this effect is apparent only for chemically peculiar stars, suggesting it is a consequence of chemical spots. Sinusoidal fits to H line $⟨B_{\mathrm{z}}⟩$ measurements (which should be minimally affected by chemical spots), yielded evidence of surface magnetic fields more complex than simple dipoles in 6 stars for which this has not previously been reported; however, in all 6 cases the second- and third-order amplitudes are small relative to the first-order (dipolar) amplitudes.

38 pages, 47 figures, accepted for publication in MNRAS