Non-Thermal Emission in Wolf±Rayet Stars: Are Massive Companions Required?

Mon. Not. R. Astron. Soc. 319, 1005±1010 (2000) Non-thermal emission in Wolf±Rayet stars: are massive companions required? S. M. Dougherty1,2 and P. M. Williams3 1National Research Council of Canada, Herzberg Institute for Astrophysics, Dominion Radio Astrophysical Observatory, PO Box 248, White Lake Rd, Penticton, British Columbia V2A 6K3, Canada 2Department of Physics and Astronomy, University of Calgary, 2500 University Dr. NW, Calgary, Alberta T2N 1N4, Canada 3Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ Accepted 2000 June 27. Received 2000 June 13; in original form 2000 May 2 ABSTRACT We examine the radio spectral indices of 23 Wolf±Rayet (WR) stars to identify the nature of their radio emission. We identify nine systems as non-thermal emitters. In seven of these systems the non-thermal emission dominates the radio spectrum, while in the remaining two it is of comparable strength to the thermal, stellar wind emission, giving `composite' spectra. Among these nine systems, seven have known spectroscopic or visual binary companions. The companions are all massive O or early B-type stars, strongly supporting a connection between the appearance of non-thermal emission in WR stars and the presence of a massive companion. In three of these binaries, the origin of non-thermal emission in a wind-collision region between the stars has been well established in earlier work. The binary systems that exhibit only thermal emission are all short-period systems where a wind-collision zone is deep within the opaque region of the stellar wind of the WR star. To detect non-thermal emission in these systems requires optically thin lines of sight to the wind-collision region. Key words: stars: individual: WR 86 ± stars: Wolf±Rayet ± radio continuum: stars. tron emission requires a population of relativistic electrons. The 1 INTRODUCTION acceleration of the free electrons is widely attributed to first-order Wolf±Rayet (WR) stars have dense circumstellar envelopes, with Fermi acceleration in shocks within the stellar winds. The shocks outflow velocities ,1000±3000 km s21. These stellar winds are are thought to arise either from wind instabilities (Lucy & White photo-ionized by the strong UV radiation fields from the 1980; Chen & White 1994), or, in the case of massive binary underlying WR star, giving rise to a free±free continuum emission systems, close to the contact discontinuity where the stellar winds spectrum that is easily observed from IR to radio wavelengths. of two stars collide (Eichler & Usov 1993), the colliding-wind The brightness temperature of the free±free emission is typically binary (CWB) model. In a binary system, an alternative to shock ,104 K, as expected from a photo-ionized envelope in thermal acceleration is acceleration due to magnetic field compression in equilibrium. The emission is partially optically thick, and between the wind-collision region (Jardine, Allen & Pollock 1996). a mid-IR and radio frequencies has a power-law spectrum Sn / n ; Moreover, since the characteristic dense stellar winds of these with spectral index (a) typically 10:7 ! 10:8 for WR stars (e.g. stars are opaque out to large radii [,1000 Rp at n 5 GHz Williams et al. 1990a; Leitherer & Robert 1991). This is a little (Williams 1996)], observation of non-thermal emission implies steeper than the canonical value of 10.6 expected for a steady- that either relativistic electrons must be in situ at these large radii state, isothermal, radially symmetric wind (e.g Wright & Barlow or there must be some low-opacity lines of sight in the wind 1975), but can be readily accounted having the ion density through which we can observe the emission. Certainly, a CWB distribution decrease more rapidly than r22. could satisfy the first condition if the separation of the stars A number of WR stars have radio emission properties that differ exceeded a few 1000 Rp. from this typical picture: they exhibit high brightness temperatures The orbitally modulated non-thermal radio emission from (,106±107 K), and have flat or negative spectral indices, proper- WR 140 and the resolution of the radio emission from WR 147 ties characteristic of non-thermal synchrotron emission and high- into two components provide strong evidence for a CWB origin of energy phenomena in the stellar winds (Abbott, Bieging & the accelerated electrons. The radio flux and spectral-index Churchwell 1984; Abbott et al. 1986). The presence of synchro- variations of WR 140 are attributed to the varying line-of-sight opacity through the stellar wind to a non-thermal source moving w E-mail: [email protected] as the orbit progresses (Williams et al. 1990b). The resolution of q 2000 RAS 1006 S. M. Dougherty and P. M. Williams the radio emission from WR 147 into a thermal component Table 1. WR stars with radio spectral indices. coincident with the optical image of the WR star and a non- thermal component 000.6 away (Moran et al. 1989; Churchwell WR Sp. Typea Period Radio ab Class Referencesc et al. 1992) pointed to a wide CWB in which the interaction region 6 WN4 10.72 ^ 0.20 T 11 occurs at large distance from the WR star. 9 WC51O7 14.3 d .11.07 T 1 These results prompted van der Hucht et al. (1992) to suggest a 11 WC81O8.5 78.5 d 10.33 ^ 0.04 T=NT 2 CWB origin for all non-thermal radio emission by WR (and OB) 10.54 ^ 0.01* T=NT 12 .1 stars. This suggestion has been strongly supported by recent high- 15 WC6 1.25 T 1 16 WN8 10.63 ^ 0.15 T 2 resolution studies of WR 147, which found the postulated OB to 10.67 ^ 0.12* T 12 be a companion to the WR star and confirmed that the non- 22 WN71O7.5 80.4 d 10.14 ^ 0.19 T 2 thermal emission occurred between the two stars at a position 24 WN6 .11.67 T 2 1 .2 consistent with the CWB model (Williams et al. 1997; Niemela 25 WN6 O4? 1.26 ? 2 39 WC7 20.37 ^ 0.34 T 1 et al. 1998). The radio emission from another source, WR 146, 20.21 ^ 0.18* T=NT 12 was resolved into two components by Dougherty et al. (1996, 40 WN8 10.68 ^ 0.12 T 2 2000) and the optical image was resolved by Niemela et al. They 10.62 ^ 0.08* T 12 found that the relative positions of the stellar and non-thermal 48 WC61O9.5 18.3 d 20.39 ^ 0.15 NT 2 2 ^ radio components were also consistent with a CWB origin for the 0.36 0.07* NT 12 78 WN7 10.45 ^ 0.09 T 3 non-thermal emission. 86 WC71OB Visual 10.18 ^ 0.20 T 7,8 In the light of these results, and the extension of radio studies of 89 WN81a 10.74 ^ 0.18 T 2 massive stellar winds to the southern hemisphere (Leitherer, 90 WC7 10.02 ^ 0.24 T 1 1 ^ Chapman & Koribalski 1997; Chapman et al. 1999), it is timely to 0.29 0.18* T 12 105 WN9 20.27 ^ 0.09 NT 1 examine critically the suggestion by van der Hucht et al. (1992) 112 WC9 .0.95 T 1 that all WR stars showing non-thermal emission are in long- 21.34 ^ 0.15 NTd 12 period, colliding-wind binary systems. 125 WC71O9 .15 yr 20:5 ! 10:7 NT&T 3,13 134 WN6 10.93 ^ 0.25 T 11 136 WN6 10.70 ^ 0.14 T 3 137 WC71OB .13 yr 10.0 ^ 0.2 T=NT 6 2 DATA, ANALYSIS AND RESULTS 140 WC71O5 7.9 yr 20:5 ! 10:7 NT&T 9,10 145 WN71OB? 22.5 d 10.79 ^ 0.19 T 3 This study aims to identify WR stars that exhibit non-thermal 146 WC51O8 Visual 20.62 ^ 0.04 NTe 4 emission in addition to the expected thermal emission from their 10.74 ^ 0.20 T 4 stellar winds. There are a number of indicators for the presence of 147 WN81B0.5 Visual 20.37 ^ 0.07 NTe 5 non-thermal emission: brightness temperatures in excess of 10.66 ^ 0.02 T 5 , 6 1 a 10 K, radio spectral indices lower than 0.6, and variations Spectral types taken from van der Hucht (2000). on time scales of the order of 1 d. In this study any of these criteria b Calculated from equations (1) and (2). For those marked (*), more than are used to identify the presence of non-thermal emission. two data points were available and weighted linear regression was However, we rely most heavily on the spectral index of continuum possible. c (1) Leitherer et al. (1997); (2) Leitherer, Chapman & Koribalski (1995). emission to determine the presence of non-thermal emission. Values of a calculated from radiometry listed in table 1 of each of these Thermal emission from stellar winds is partially optically thick papers; (3) Abbot et al (1986); (4) Dougherty, Williams & Pollacco (2000); due to the radial density distribution of ions, resulting in a radio (5) Williams et al. (1997); (6) D. Florkowski, private communication; (7) continuum spectrum with an index of 10.6 or steeper, depending This paper; (8) Niemela et al. (1998); (9) Williams et al. (1990a); (10) on the details of the ion distribution. On the other hand, optically White & Becker (1995); (11) Hogg (1982); (12) Chapman et al. (1999); (13) Williams et al. (1992). thin non-thermal emission has a negative or flat spectral index d Variability at 2.4 GHz is a more reliable indicator of non-thermal with values as low as 20.5.

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