Orbitally modulated dust formation by the WC7+O5 colliding-wind binary WR 140

Abstract

We present high‐resolution infrared (2–18 μm) images of the archetypal periodic dust‐making Wolf–Rayet binary system WR 140 (HD 193793) taken between 2001 and 2005, and multi‐colour (J–[19.5]) photometry observed between 1989 and 2001. The images resolve the dust cloud formed by WR 140 in 2001, allowing us to track its expansion and cooling, while the photometry allows tracking the average temperature and total mass of the dust. The combination of the two data sets constrains the optical properties of the dust, and suggests that they differ from those of the dust made by the WC9 dust‐makers, including the classical ‘pinwheel’, WR 104. The photometry of individual dust emission features shows them to be significantly redder in (nbL′–[3.99]), but bluer in ([7.9]–[12.5]), than the binary, as expected from the spectra of heated dust and the stellar wind of a Wolf–Rayet star. The most persistent dust features, two concentrations at the ends of a ‘bar’ of emission to the south of the star, were observed to move with constant proper motions of 324 ± 8 and 243 ± 7 mas yr−1. Longer wavelength (4.68 and 12.5 μm) images show dust emission from the corresponding features from the previous (1993) periastron passage and dust formation episode, showing that the dust expanded freely in a low‐density void for over a decade, with dust features repeating from one cycle to the next. A third persistent dust concentration to the east of the binary (the ‘arm’) was found to have a proper motion ∼320 mas yr−1, and a dust mass about one‐quarter that of the ‘bar’. Extrapolation of the motions of the concentrations back to the binary suggests that the eastern ‘arm’ began expansion four to five months earlier than those in the southern ‘bar’, consistent with the projected rotation of the binary axis and wind‐collision region (WCR) on the sky. A comparison of model dust images and the observations constrains the intervals when the WCR was producing sufficiently compressed wind for dust nucleation in the WCR, and suggests that the distribution of this material was not uniform about the axis of the WCR, but more abundant in the following edge in the orbital plane.

Stagnant Shells in the Vicinity of the Dusty Wolf–Rayet–OB Binary WR 112

“…The model parameters assume an expansion velocity of 1200 km s −1 and the following orbital parameters for the central the binary system: i=35°, Ω=130°, ω=165°, e=0.40, and P=23.5 years. The upper limit derived for the expansion velocity is almost an order of magnitude less than the predicted and observed velocities of dusty outflows produced in colliding-wind WR binaries like WR140 (Williams et al 2009) and WR104 . Additionally, the mid-IR fluxes and inferred dust properties of WR112 (Table 2) do not exhibit significant variability, whereas the orbitally modulated dust production in WR140 shows an order of magnitude variation in 8.75 and 12.5 μm emission (Williams et al 2009).…”

“…The upper limit derived for the expansion velocity is almost an order of magnitude less than the predicted and observed velocities of dusty outflows produced in colliding-wind WR binaries like WR140 (Williams et al 2009) and WR104 . Additionally, the mid-IR fluxes and inferred dust properties of WR112 (Table 2) do not exhibit significant variability, whereas the orbitally modulated dust production in WR140 shows an order of magnitude variation in 8.75 and 12.5 μm emission (Williams et al 2009). It is therefore unlikely that the nebula originated from the high velocity outflows of the central WR binary.…”

“…The late-type WC+OB binary WR112 (Massey & Conti 1983;van der Hucht 2001) is one of the few dusty WC systems whose nebula has been detected and resolved in the mid-infrared (8-18μm; Marchenko et al 2002. Others known are WR48a (Marchenko & Moffat 2007) and WR140 Williams et al 2009). The well-studied WR140 is the only one of these systems with kinematically measured masses, hosting a 15 M e WC7 star and a 36M e O5 star (Fahed et al 2011;Monnier et al 2011).…”

Impact of orbital motion on the structure and stability of adiabatic shocks in colliding wind binaries

“…Williams et al (2009) argued that stronger dust emission in the trailing arm would explain better the IR high-resolution images of WR 140, and attributed this to density variations. The winds have nearly identical velocities but the WR has a mass-loss rate ten times higher than its O companion.…”

Radio emission from the massive stars in the galactic super star cluster Westerlund 1

“…Williams 1999;Tuthill et al 1999;Monnier et al 1999;Williams et al 2009) A synthesis of the IR, X-ray and these radio observations therefore argues for a high binary fraction amongst the WR stars, with C08 suggesting that at least 70%, and possibly the complete population, of WR stars in Wd 1 are in massive binary systems. However, given the lack of radial velocity variations from spectral line observations this assertion has yet to be verified, and may be exceedingly challenging to verify.…”