Magnetometry of a sample of massive stars in Carina

Yaël Nazé, Stefano Bagnulo, Véronique Petit, Thomas Rivinius, Gregg Wade, Gregor Rauw, Marc Gagné

Figure 1. Normalized Stokes V (in selected spectral windows, with rectification) as a function of x (= òÈÒg_eff C_Z λ² 1/I (dI/dλ), see Eq. (2)) for the two positive cases of Tr16-22 (left) and Tr16-13 (right). In these panels, the slope is directly linked to the intensity of the longitudinal field. The best-fit lines, derived from the least-squares analysis, are drawn with thick dashed lines, for both the actual Stokes V parameter (top panels) and the diagnostic null profile (bottom panels). In the latter case, or more generally in the case of non-detections, these lines should be horizontal. The line thickness is larger than the 1σ error bar on the constant term, while the two thin dotted lines show lines with slopes at ±1σ from the best-fit value.

Abstract

X-ray surveys of the Carina nebula have revealed a few hard and luminous sources associated with early-type stars. Such unusual characteristics for the high-energy emission may be related to magnetically-confined winds. To search for the presence of magnetic fields in these objects, we performed a limited spectropolarimetric survey using the FORS instrument. The multi-object mode was used, so that a total of 21 OB stars could be investigated during a one-night-long run. A magnetic field was detected in two objects of the sample, with a 6σ significance; Tr16-22 and 13. Such a detection was expected for Tr16-22, as its X-ray emission is too bright, variable and hard, compared to other late-type O or O+OB systems. It is more surprising for Tr16-13, a poorly known star which so far has never shown any peculiar characteristics. Subsequent monitoring is now needed to ascertain the physical properties of these objects and enable a full modelling of their magnetic atmospheres and winds.

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Last Revised: 2012 April 20th