TY - JOUR A1 - Castro, Norberto A1 - Fossati, Luca A1 - Hubrig, Swetlana A1 - Simon Díaz, Sergio A1 - Schoeller, Markus A1 - Ilyin, Ilya A1 - Carrol, Thorsten A. A1 - Langer, Norbert A1 - Morel, Thierry A1 - Schneider, Fabian R. N. A1 - Przybilla, Norbert A1 - Herrero, Artemio A1 - de Koter, Alex A1 - Oskinova, Lida A1 - Reisenegger, Andreas A1 - Sana, Hugues T1 - B fields in OB stars (BOB) Detection of a strong magnetic field in the O9.7 V star HD 54879 JF - Astronomy and astrophysics : an international weekly journal N2 - The number of magnetic stars detected among massive stars is small; nevertheless, the role played by the magnetic field in stellar evolution cannot be disregarded. Links between line profile variability, enhancements/depletions of surface chemical abundances, and magnetic fields have been identified for low-mass B-stars, but for the O-type domain this is almost unexplored. Based on FORS 2 and HARPS spectropolarimetric data, we present the first detection of a magnetic field in HD54879, a single slowly rotating O9.7 V star. Using two independent and different techniques we obtained the firm detection of a surface average longitudinal magnetic field with a maximum amplitude of about 600 G, in modulus. A quantitative spectroscopic analysis of the star with the stellar atmosphere code FASTWIND results in an effective temperature and a surface gravity of 33 000 +/- 1000K and 4.0 +/- 0.1 dex. The abundances of carbon, nitrogen, oxygen, silicon, and magnesium are found to be slightly lower than solar, but compatible within the errors. We investigate line-profile variability in HD54879 by complementing our spectra with spectroscopic data from other recent OB-star surveys. The photospheric lines remain constant in shape between 2009 and 2014, although H alpha shows a variable emission. The H alpha emission is too strong for a standard O9.7 V and is probably linked to the magnetic field and the presence of circumstellar material. Its normal chemical composition and the absence of photospheric line profile variations make HD54879 the most strongly magnetic, non-variable single O-star detected to date. KW - stars: atmospheres KW - stars: evolution KW - stars: magnetic field KW - stars: massive KW - stars: individual: HD 54879 Y1 - 2015 U6 - https://doi.org/10.1051/0004-6361/201425354 SN - 1432-0746 VL - 581 PB - EDP Sciences CY - Les Ulis ER - 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 - TY - JOUR A1 - Przybilla, Norbert A1 - Fossati, Luca A1 - Hubrig, Swetlana A1 - Nieva, M. -F. A1 - Jaervinen, S. P. A1 - Castro, Norberto A1 - Schoeller, M. A1 - Ilyin, Ilya A1 - Butler, Keith A1 - Schneider, F. R. N. A1 - Oskinova, Lida A1 - Morel, T. A1 - Langer, N. A1 - de Koter, A. T1 - B fields in OB stars (BOB): Detection of a magnetic field in the He-strong star CPD-57 degrees 3509 JF - Organic letters N2 - Methods. Spectropolarimetric observations with FORS2 and HARPSpol are analysed using two independent approaches to quantify the magnetic field strength. A high-S/N FLAMES/GIRAFFE spectrum is analysed using a hybrid non-LTE model atmosphere technique. Comparison with stellar evolution models constrains the fundamental parameters of the star. Results. We obtain a firm detection of a surface averaged longitudinal magnetic field with a maximum amplitude of about 1 kG. Assuming a dipolar configuration of the magnetic field, this implies a dipolar field strength larger than 3.3 kG. Moreover, the large amplitude and fast variation (within about 1 day) of the longitudinal magnetic field implies that CPD-57 degrees 3509 is spinning very fast despite its apparently slow projected rotational velocity. The star should be able to support a centrifugal magnetosphere, yet the spectrum shows no sign of magnetically confined material; in particular, emission in H alpha is not observed. Apparently, the wind is either not strong enough for enough material to accumulate in the magnetosphere to become observable or, alternatively, some leakage process leads to loss of material from the magnetosphere. The quantitative spectroscopic analysis of the star yields an effective temperature and a logarithmic surface gravity of 23 750 +/- 250 K and 4.05 +/- 0.10, respectively, and a surface helium fraction of 0.28 +/- 0.02 by number. The surface abundances of C, N, O, Ne, S, and Ar are compatible with the cosmic abundance standard, whereas Mg, Al, Si, and Fe are depleted by about a factor of 2. This abundance pattern can be understood as the consequence of a fractionated stellar wind. CPD-57 degrees 3509 is one of the most evolved He-strong stars known with an independent age constraint due to its cluster membership. KW - stars: abundances KW - stars: atmospheres KW - stars: evolution KW - stars: magnetic field KW - stars: individual: CPD-57 degrees 3509 KW - stars: massive Y1 - 2016 U6 - https://doi.org/10.1051/0004-6361/201527646 SN - 1432-0746 VL - 587 PB - EDP Sciences CY - Les Ulis ER -