The puzzling properties of the magnetic O star Tr16-22

BAGNULO, S.; PETIT, V.; NAZÉ, Y.; MORRELL, N. I.; NEINER, C.; BARBÁ, R.; GAMEN, R. C.

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2016 vol. 596 p. 44 - 51

0004-6361

Context. The detection of bright, hard, and variable X-ray emission inTr16-22 prompted spectropolarimetric observations of this star, which inturn led to the discovery of a surface magnetic field. Aims: Wewant to further constrain the properties of this star, in particular toverify whether X-ray variations are correlated to changes in opticalemission lines and magnetic field strength, as expected from the obliquerotator model that is widely accepted for magnetic O stars. Methods: We have obtained new low-resolution spectropolarimetric andlong-term high-resolution spectroscopic monitoring of Tr16-22, and wealso analyse new, serendipitous X-ray data. Results: The newX-ray observations are consistent with previous data, but their additiondoes not help to solve the ambiguity in the variation timescale becauseof numerous aliases. No obvious periodicity or any large variations aredetected in the spectropolarimetric data of Tr16-22 obtained over threemonths. The derived field values appear to be in line with previousmeasurements, suggesting constancy of the field (though the possibilityof small, short-term field variations cannot be excluded). Variations inthe equivalent widths of Hα are very small, and they do not appearto be related to the X-ray timescale; the overall lack of largevariations in optical emission lines is consistent with the magneticfield constancy. In addition, variations of the radial velocitiesindicate that Tr16-22 is probably a SB1 binary with a very long period. Conclusions: Our new measurements of optical emission lines andmagnetic field strength do not show an obvious correlation with X-rayvariations. Our current data thus cannot be interpreted in terms of thecommon model, which assumes the electromagnetic emission associated witha wind confined by a dipolar field tilted with respect to the rotationaxis. However, the sampling is imperfect and new data are needed tofurther constrain the actual periodicity of the various observedphenomena. If inconsistencies are confirmed, then we will need toconsider alternative scenarios.