Multiple variations in the radio light-curve of the colliding wind binary WR 146 (WC6+O): evidence for a third component *
D.Y.A. Setia Gunawan 1,2,
A.G. de Bruyn 3,1,
K.A. van der Hucht 2 and
P.M. Williams 4
Received 2 September 1999 / Accepted 12 January 2000
The Wolf-Rayet star WR 146 (HM19-3, WC6+O) is the brightest WR star at radio wavelengths. We have been monitoring this system with the Westerbork Synthesis Radio Telescope (WSRT ) at 1.4 and 5 GHz (21 and 6 cm) since 1989. The time-averaged spectral index -0.62 clearly points to a domination by non-thermal radiation, which we associate with colliding winds in this binary system. The non-thermal radio flux distribution shows a turn-over at low frequency, which we suggest to be due to free-free absorption of the synchrotron emission from the colliding wind region by plasma around the system.
In the period 1989 - 1997 the average 1.4-GHz flux density increased from 61 to 73 mJy; in the the period 1989 - 1999 the average 5-GHz flux density increased from 29 to 37 mJy. The light-curves show three different kinds of variations: (i) a slow linear rise in a time-span of a decade; (ii) a 3.38 yr periodic variation; and, (iii) rapid non-periodic variations on a time-scale of weeks.
We examine whether the slow rise of the flux density could be explained by decreasing free-free absorption in the line-of-sight through the radiophotosphere of the O component, while moving in an eccentric orbit around the WR component. However, the similarity of the amplitudes ( 22% in 10 yr) of the rises at 1.4 and 5 GHz argues against a change in free-free absorption, expected to be strongly wavelength dependent. This points to an intrinsic flux-density variation, possibly due to modulation of the magnetic field strength resulting from orbital motion in a very-long-period eccentric binary system. The relation between the flux-density increase and orbital motion is supported by positional measurements of the 5-GHz data.
We detect a possible motion of the shock zone relative to one of the control sources (Control A) of in the 10 yr observing span. At a distance of 1250 pc this motion corresponds to a projected tangential velocity of about 30 km s-1, which is a plausible orbital velocity for a system like WR 146.
Superimposed on the 1.4-GHz slow rise, we find a sinusoidal variation with a period P = 3.38 0.02 yr and a semi-amplitude of 4.3 0.2 mJy. Adopting a distance of 1250 pc to the system and a 162 mas WR+O separation, we consider the observed 3.38 yr period too short to be the WR+O binary period by at least two orders of magnitude. We suggest that the periodic variability is caused by a third, low-mass object, modulating the mass flow and/or the magnetic-field of the O component. Unfortunately, our 5-GHz data are far too few and not adequately spread over the whole phase to confirm that they consistently follow the 3.38 yr period found in the 1.4-GHz data.
The erratic `micro'-variation in the 1.4-GHz light-curve is about 4 of the typical 0.5 mJy observational uncertainty, on a time-scale of weeks to months. When irregularities in the mass flow (clumps, inhomogeneities and/or turbulence in the O and/or WR star winds) reach the wind collision region, variation in the non-thermal emission can be expected. Such irregularities can also affect the free-free line-of-sight absorption at the lowest observing frequencies.
Key words: stars: binaries: general stars: Wolf-Rayet stars: individual: WR 146 radio continuum: stars
* Based on observations made with the Westerbork Synthesis Radio Telescope (WSRT). The WSRT is operated by the Netherlands Foundation for Research in Astronomy (NFRA) which is financially supported by the Netherlands Organization for Scientific Research (NWO).
Send offprint requests to: (D.Y.A.Setia-Gunawan@astro.rug.nl)
© European Southern Observatory (ESO) 2000
Online publication: April 10, 2000