B fields in OB stars (BOB): The discovery of a magnetic field in a multiple system in the Trifid nebula, one of the youngest star forming regions

Abstract

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° 3509

“…Evolutionary tracks of Brott et al (2011) for initial rotational velocity of ∼300 km s −1 are shown (full lines), labelled by initial stellar mass. The magnetic star in the Trifid nebula and HD 345439 (Hubrig et al 2014(Hubrig et al , 2015b are not included here because of difficulties in establishing their properties. (Lotkin & Matkin 1994).…”

B fields in OB stars (BOB): Low-resolution FORS2 spectropolarimetry of the first sample of 50 massive stars

“…Hubrig et al (2014a) reported the detection of a rather strong magnetic field on the basis of FORS2 and HARPSpol data. Figure 7 illustrates the clear detection of the magnetic field at the ∼9σ level, already reported by Hubrig et al (2014a) 6 .…”

“…Hubrig et al (2014a) reported the detection of a rather strong magnetic field on the basis of FORS2 and HARPSpol data. Figure 7 illustrates the clear detection of the magnetic field at the ∼9σ level, already reported by Hubrig et al (2014a) 6 . The high-resolution HARPSpol observations and further high-resolution UVES spectra revealed that HD 164492C is in fact a multiple system, composed of at least two stars.…”

B field in OB stars (BOB): The outstandingly strong magnetic field in the evolved He-strong star CPD −62° 2124

“…This setting led to a resolving power of approximately 1700 and a wavelength coverage from 3250-6215 Å. Eight consecutive exposures of 600 s were carried out with a total exposure time of 4800 s. More details on the instrument settings and exposure sequence are provided by Hubrig et al (2014a;see also Fossati et al 2015b).…”

“…The HARPSpol data were independently reduced and analysed using different techniques by the Bonn and Potsdam groups, as described in previous BOB works (Hubrig et al 2014a;Przybilla et al 2016).…”