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 Magnetism in Massive Stars and Binarity and Magnetic Interactions in various classes of Stars collaborations have collected extensive high-dispersion spectropolarimetric data sets of these targets. We present longitudinal magnetic field measurements 〈Bz〉 for 52 early B-type stars (B5–B0), with which we attempt to determine their rotational periods Prot. Supplemented with high-resolution spectroscopy, low-resolution Dominion Astrophysical Observatory circular spectropolarimetry, and archival Hipparcos photometry, we determined Prot for 10 stars, leaving only five stars for which Prot could not be determined. Rotational ephemerides for 14 stars were refined via comparison of new to historical magnetic measurements. The distribution of Prot is very similar to that observed for the cooler Ap/Bp stars. We also measured v sin i and vmac 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 vmac is lower for magnetic stars. Least-squares deconvolution profiles extracted using single-element masks revealed widespread, systematic discrepancies in 〈Bz〉 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 〈Bz〉 measurements (which should be minimally affected by chemical spots), yielded evidence of surface magnetic fields more complex than simple dipoles in six stars for which this has not previously been reported; however, in all six cases, the second- and third-order amplitudes are small relative to the first-order (dipolar) amplitudes.

The effects of surface fossil magnetic fields on massive star evolution – II. Implementation of magnetic braking in mesa and implications for the evolution of surface rotation in OB stars

“…We computed a representative grid of stellar evolution models to compare with observations and parameters presented by Shultz et al (2018) (see Section 6). This grid of models encompasses a range of initial stellar masses between 5 and 15 M , initial rotational rates of Ω ini /Ω crit,ini between 0.2 and 0.8, and initial polar magnetic field strengths between 1 and 30 kG.…”

“…For reference, at half-way through their main sequence evolution (shown with purple markers on Figure 3) the magnetic (SURF) and non-magnetic models with Ω ini /Ω ini,crit = 0.8 have stellar ages of 6.9 Myr and 7.3 Myr, while at the TAMS their ages are 13.9 Myr and 14.7 Myr, respectively (see Tables C1 and C2). This may lead to systematic shifts in the estimated stellar ages of observed stars, therefore it has to be considered when comparing observed magnetic massive stars with evolutionary models that do not account for surface magnetic fields Schneider et al 2016;Shultz et al 2018). Shultz et al (2018) presented high-resolution magnetometry and obtained rotation periods for the known population (55) of main sequence magnetic B-type stars with spectral types between B5 and B0.…”

“…This may lead to systematic shifts in the estimated stellar ages of observed stars, therefore it has to be considered when comparing observed magnetic massive stars with evolutionary models that do not account for surface magnetic fields Schneider et al 2016;Shultz et al 2018). Shultz et al (2018) presented high-resolution magnetometry and obtained rotation periods for the known population (55) of main sequence magnetic B-type stars with spectral types between B5 and B0. We use this sample of magnetic stars to evaluate our magnetic B-star evolutionary models.…”

“…The stars in the 15 M mass bin from the Shultz et al (2018) sample have a DM, therefore they are expected to be evolved. However, they are not matched with the computed models on the R K − R A plane.…”

“…Magnetic fields are routinely detected in stars across the entire Hertzsprung-Russell diagram (HRD), from early to late evolutionary phases (Donati & Landstreet 2009). Surface magnetic fields are detected in 7% of hot, massive, OB stars (Wade et al 2014(Wade et al , 2016Martins et al 2015;Neiner et al 2015;Morel et al 2015;Fossati et al 2015;Grunhut et al 2017;Shultz et al 2018;Petit et al 2019). Unlike those E-mail: z.keszthelyi@uva.nl detected in cool stars, these surface fields are likely not being actively generated by a dynamo mechanism, especially because there is no evidence that extended convection zones 1 1 Hot stars do have thin sub-surface layers where inefficient convection (accounting for usually ≈ 3% of the energy transport) occurs due to the iron opacity bump (Cantiello et al 2009), and while there might perhaps be dynamo activity in those layers (Cantiello & Braithwaite 2011), that would not give rise to the strong, globally organized fields which are observed in magnetic OB stars.…”