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
Hubrig, S., ..., Oskinova L.M., ... (BOB collaboration)
Abstract:
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.
In the framework of our ESO Large Program, we carried out low-resolution
spectropolarimetric observations with FORS2 in 2013 April of the three
most massive central stars in the Trifid nebula, HD164492A, HD164492C,
and HD164492D. These observations indicated a strong longitudinal
magnetic field of about 500-600G in the poorly studied component
HD164492C. To confirm this detection, we used HARPS in
spectropolarimetric mode on two consecutive nights in 2013 June.
Our HARPS observations confirmed the longitudinal magnetic field in
HD164492C. Furthermore, the HARPS observations revealed that HD164492C
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-B1.5V. 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
HD164492C 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.
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