Non-thermal radio emission from O-type stars. I. HD~168112

R. Blomme$^1$, S. Van Loo$^1$, M. De Becker$^2$, G. Rauw$^2$, M.C. Runacres$^1$, D. Y. A. Setia Gunawan$^3$ and J.M. Chapman$^3$

1 Royal Observatory of Belgium, Ringlaan 3, B-1180 Brussel, Belgium
2 Institut d'Astrophysique, Universit\'e de Li\`ege,
All\'ee du 6 Ao\^ut, B\^at B5c, B-4000 Li\`ege (Sart-Tilman), Belgium
3 Australia Telescope National Facility, PO Box 76, Epping, NSW 2121, Australia

We present a radio lightcurve of the O5.5 III(f$^+$) star HD~168112,
based on archive data from the Very Large Array (VLA) and the Australia
Telescope Compact Array (ATCA).
The fluxes show considerable variability and a negative spectral index,
thereby confirming that HD~168112 is a non-thermal radio emitter.
The non-thermal radio emission is
believed to be due to synchrotron radiation
from relativistic electrons that have been Fermi accelerated in
shocks. For HD~168112, it is not known whether these shocks are due to
a wind-wind collision in a binary system or to
the intrinsic instability of the stellar wind driving mechanism.
Assuming HD~168112 to be a single star,
our synchrotron model shows
that the velocity jump of the shocks should be very high, or there
should be a very large number of shocks in the wind. Neither
of these is compatible with time-dependent hydrodynamical calculations
of O star winds.
If, on the other hand, we assume that HD~168112 is a binary, the high
velocity jump is easily explained by ascribing it to the wind-wind
collision. By further assuming the star to be an eccentric binary,
we can explain the observed radio variability by the colliding-wind
region moving in and out of the region where
free-free absorption is important. The radio data presented here
show that the binary has a period of between one and two years.
By combining the radio data with X-ray data, we find that
the most likely period is $\sim$~1.4~yr.

Reference: A\&A (in press)
Status: Manuscript has been accepted