Non-thermal radio emission from O-type stars. II. HD 167971

R. Blomme(1), M. De Becker(2), M.C. Runacres(3), S. Van Loo(4), D.Y.A. Setia Gunawan(5)

(1) Royal Observatory of Belgium; (2) Institut d'Astrophysique, Liege, Belgium; (3) Vrije Universiteit Brussel, Belgium; (4) University of Leeds, UK; (5) ATNF, Australia

HD 167971 is a triple system consisting of a 3.3-day eclipsing binary
(O5-8 V + O5-8 V) and an O8 supergiant. It is also a well known non-thermal
radio emitter. We observed the radio emission of HD 167971 with the Very
Large Array (VLA) and the Australia Telescope Compact Array (ATCA). By
combining these data with VLA archive observations we constructed a radio
lightcurve covering a 20-yr time-range. We searched for, but failed to find,
the 3.3-day spectroscopic period of the binary in the radio data. This could
be due to the absence of intrinsic synchrotron radiation at the colliding-wind
region between the two components of the eclipsing binary, or due to the large
amount of free-free absorption that blocks the synchrotron radiation.
We are able to explain many of the observed characteristics of the radio data
if the non-thermal emission is produced in a colliding-wind region between
the supergiant and the combined winds of the binary. Furthermore, if the
system is gravitationally bound, the orbital motion occurs over a period of
~ 20 years, or longer, as suggested by the long-term variability in the radio
data. We argue that the variability is due to the free-free absorption that
changes with orbital phase or may also in part be due to changes in
separation, should the orbit be eccentric.

Reference: A&A, in press
Status: Manuscript has been accepted