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B fields in OB stars (BOB)
(2017)
Aims. The B fields in OB stars (BOB) Collaboration is based on an ESO Large Programme to study the occurrence rate, properties, and ultimately the origin of magnetic fields in massive stars. Methods. In the framework of this program, we carried out low-resolution spectropolarimetric observations of a large sample of massive stars using FORS2 installed at the ESO VLT 8m telescope. Results. We determined the magnetic field values with two completely independent reduction and analysis pipelines. Our in-depth study of the magnetic field measurements shows that differences between our two pipelines are usually well within 3 sigma errors. From the 32 observations of 28 OB stars, we were able to monitor the magnetic fields in CPD -57 degrees 3509 and HD164492C, confirm the magnetic field in HD54879, and detect a magnetic field in CPD -62 degrees 2124. We obtain a magnetic field detection rate of 6 +/- 3% for the full sample of 69 OB stars observed with FORS 2 within the BOB program. For the preselected objects with a nu sin i below 60 km s(-1), we obtain a magnetic field detection rate of 5 +/- 5%. We also discuss X-ray properties and multiplicity of the objects in our FORS2 sample with respect to the magnetic field detections.
Aims. Recent magnetic field surveys in O- and B-type stars revealed that about 10% of the core-hydrogen-burning massive stars host large-scale magnetic fields. The physical origin of these fields is highly debated. To identify and model the physical processes responsible for the generation of magnetic fields in massive stars, it is important to establish whether magnetic massive stars are found in very young star-forming regions or whether they are formed in close interacting binary systems.
Methods. In the framework of our ESO Large Program, we carried out low-resolution spectropolarimetric observations with FORS 2 in 2013 April of the three most massive central stars in the Trifid nebula, HD 164492A, HD 164492C, and HD 164492D. These observations indicated a strong longitudinal magnetic field of about 500-600 G in the poorly studied component HD 164492C. To confirm this detection, we used HARPS in spectropolarimetric mode on two consecutive nights in 2013 June.
Results. Our HARPS observations confirmed the longitudinal magnetic field in HD 164492C. Furthermore, the HARPS observations revealed that HD 164492C cannot be considered as a single star as it possesses one or two companions. The spectral appearance indicates that the primary is most likely of spectral type B1-B1.5 V. Since in both observing nights most spectral lines appear blended, it is currently unclear which components are magnetic. Long-term monitoring using high-resolution spectropolarimetry is necessary to separate the contribution of each component to the magnetic signal. Given the location of the system HD 164492C in one of the youngest star formation regions, this system can be considered as a Rosetta Stone for our understanding of the origin of magnetic fields in massive stars.
B fields in OB stars (BOB): Detection of a magnetic field in the He-strong star CPD-57 degrees 3509
(2016)
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.
The B fields in OB stars (BOB) survey is an ESO large programme collecting spectropolarimetric observations for a large number of early-type stars in order to study the occurrence rate, properties, and ultimately the origin of magnetic fields in massive stars. As of July 2014, a total of 98 objects were observed over 20 nights with FORS2 and HARPSpol. Our preliminary results indicate that the fraction of magnetic OB stars with an organised, detectable field is low. This conclusion, now independently reached by two different surveys, has profound implications for any theoretical model attempting to explain the field formation in these objects. We discuss in this contribution some important issues addressed by our observations (e.g., the lower bound of the field strength) and the discovery of some remarkable objects.
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.
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.