B fields in OB stars (BOB): on the detection of weak magnetic fields in the two early B-type starsβ CMa andϵ CMa

Abstract

Only a small fraction of massive stars seem to host a measurable structured magnetic field, whose origin is still unknown and whose implications for stellar evolution still need to be assessed. Within the context of the "B fields in OB stars (BOB)" collaboration, we used the HARPSpol spectropolarimeter to observe the early B-type stars β CMa (HD 44743; B1 II/III) and CMa (HD 52089; B1.5II) in December 2013 and April 2014. For both stars, we consistently detected the signature of a weak (<30 G in absolute value) longitudinal magnetic field, approximately constant with time. We determined the physical parameters of both stars and characterise their X-ray spectrum. For the β Cep star β CMa, our mode identification analysis led to determining a rotation period of 13.6± 1.2 days and of an inclination angle of the rotation axis of 57.6 ± 1.7• , with respect to the line of sight. On the basis of these measurements and assuming a dipolar field geometry, we derived a best fitting obliquity of about 22• and a dipolar magnetic field strength (B d ) of about 100 G (60 < B d < 230 G within the 1σ level), below what is typically found for other magnetic massive stars. This conclusion is strengthened further by considerations of the star's X-ray spectrum. For CMa we could only determine a lower limit on the dipolar magnetic field strength of 13 G. For this star, we determine that the rotation period ranges between 1.3 and 24 days. Our results imply that both stars are expected to have a dynamical magnetosphere, so the magnetic field is not able to support a circumstellar disk. We also conclude that both stars are most likely core hydrogen burning and that they have spent more than 2/3 of their main sequence lifetime. A histogram of the distribution of the dipolar magnetic field strength for the magnetic massive stars known to date does not show the magnetic field "desert" observed instead for intermediate-mass stars. The biases involved in the detection of (weak) magnetic fields in massive stars with the currently available instrumentation and techniques imply that weak fields might be more common than currently observed. Our results show that, if present, even relatively weak magnetic fields are detectable in massive stars and that more observational effort is probably still needed to properly access the magnetic field incidence.

B fields in OB stars (BOB): Detection of a magnetic field in the He-strong star CPD −57° 3509

“…This is consistent with an evolutionary age (see Table 3) that compares reasonably well with the age of the parent open cluster NGC 3293, 10.7 Myr (Lotkin & Matkin 1994). There is also reasonably good agreement between the spectroscopic distance of CPD −57 • 3509 (d spec = 2630 ± 370 pc) and the cluster distance of ∼2460 pc (Briquet et al 2013;Petit et al 2013;Alecian et al 2014;Neiner et al 2014;Fossati et al 2014Fossati et al , 2015aSikora et al 2015;Castro et al 2015, grey circles), and CPD −57 • 3509 (black dot). Error bars are 1σ uncertainties.…”

The magnetic field of the double-lined spectroscopic binary system HD 5550

“…On the other hand, the Ap component of HD 5550 has a higher temperature than the samples analysed in those works. Polar field strengths of 10-100 G have been recently found in early-B type stars (above B3) (Fossati et al 2015;Briquet et al 2016). While the limit of 100 G appears to be strict at temperatures lower than 10 000 K, it may decrease with increasing temperatures.…”

How unique is Plaskett’s star? A search for organized magnetic fields in short period, interacting or post-interaction massive binary systems

“…Such fields are generally considered to be (mostly) dipolar. In addition, except for ζ Ori A (a supergiant O-star with an extremely low field, Blazère et al 2015) and β CMa (Fossati et al 2015a), the detected fields are strong, with B d larger than 0.5 kG for O and B0.5-1 stars (e.g. Petit et al 2013).…”