TY  - JOUR
A1  - Fossati, Luca
A1  - Castro, Norberto
A1  - Morel, Thierry
A1  - Langer, Norbert
A1  - Briquet, Maryline
A1  - Carroll, Thorsten Anthony
A1  - Hubrig, Swetlana
A1  - Nieva, Maria-Fernanda
A1  - Oskinova, Lida
A1  - Przybilla, Norbert
A1  - Schneider, Fabian R. N.
A1  - Schoeller, Magnus
A1  - Simon Díaz, Sergio
A1  - Ilyin, Ilya
A1  - de Koter, Alex
A1  - Reisenegger, Andreas
A1  - Sana, Hugues
T1  - B fields in OB stars (BOB): on the detection of weak magnetic fields in the two early B-type stars beta CMa and epsilon CMa Possible lack of a "magnetic desert" in massive stars
JF  - Astronomy and astrophysics : an international weekly journal
N2  - 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 beta CMa (HD 44743; B1 II/III) and epsilon 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 beta Cep star beta 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 degrees, 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 degrees and a dipolar magnetic field strength (B-d) of about 100 G (60 < B-d < 230 G within the 1 sigma 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 epsilon 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.
KW  - stars: atmospheres
KW  - stars: evolution
KW  - stars: magnetic field
KW  - stars: individual: epsilon CMa
KW  - stars: individual: beta CMa
KW  - stars: massive
Y1  - 2015
U6  - https://doi.org/10.1051/0004-6361/201424986
SN  - 0004-6361
SN  - 1432-0746
VL  - 574
PB  - EDP Sciences
CY  - Les Ulis
ER  -