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Register a new userThe extreme colliding-wind system Apep: resolved imagery of the central binary and dust plume in the infrared
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The recent discovery of a spectacular dust plume in the system 2XMM J160050.7-514245 (referred to as Apep) suggested a physical origin in a colliding-wind binary by way of the Pinwheel mechanism. Observational data pointed to a hierarchical triple-star system, however several extreme and unexpected physical properties seem to defy the established physics of such objects. Most notably, a stark discrepancy was found in the observed outflow speed of the gas as measured spectroscopically in the line-of-sight direction compared to the proper motion expansion of the dust in the sky plane. This enigmatic behaviour arises at the wind base within the central Wolf-Rayet binary: a system that has so far remained spatially unresolved. Here we present an updated proper motion study deriving the expansion speed of Apeps dust plume over a two-year baseline that is four times slower than the spectroscopic wind speed, confirming and strengthening the previous finding. We also present the results from high-angular-resolution near-infrared imaging studies of the heart of the system, revealing a close binary with properties matching a Wolf-Rayet colliding-wind system. Based on these new observational constraints, an improved geometric model is presented yielding a close match to the data, constraining the orbital parameters of the Wolf-Rayet binary and lending further support to the anisotropic wind model.
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Infrared imaging of the colliding-wind binary Apep has revealed a spectacular dust plume with complicated internal dynamics that challenges standard colliding-wind binary physics. Such challenges can be potentially resolved if a rapidly-rotating Wolf-Rayet star is located at the heart of the system, implicating Apep as a Galactic progenitor system to long-duration gamma-ray bursts. One of the difficulties in interpreting the dynamics of Apep is that the spectral composition of the stars in the system was unclear. Here we present visual to near-infrared spectra that demonstrate that the central component of Apep is composed of two classical Wolf-Rayet stars of carbon- (WC8) and nitrogen-sequence (WN4-6b) subtypes. We argue that such an assignment represents the strongest case of a classical WR+WR binary system in the Milky Way. The terminal line-of-sight wind velocities of the WC8 and WN4-6b stars are measured to be $2100 pm 200$ and $3500 pm 100$ km s$^{-1}$, respectively. If the mass-loss rate of the two stars are typical for their spectral class, the momentum ratio of the colliding winds is expected to be $approx$ 0.4. Since the expansion velocity of the dust plume is significantly smaller than either of the measured terminal velocities, we explore the suggestion that one of the Wolf-Rayet winds is anisotropic. We can recover a shock-compressed wind velocity consistent with the observed dust expansion velocity if the WC8 star produces a significantly slow equatorial wind with a velocity of $approx$530 km s$^{-1}$. Such slow wind speeds can be driven by near-critical rotation of a Wolf-Rayet star.
We present high-resolution infrared (2--18 micron) images of the archetypal periodic dust-making Wolf-Rayet binary system WR140 (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 WR140 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 datasets constrains the optical properties of the dust. 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/y. Longer wavelength (4.68-micron and 12.5-micron) images shows dust emission from the corresponding features from the previous (1993) periastron passage and dust-formation episode. A third persistent dust concentration to the east of the binary (the `arm) was found to have a proper motion ~ 320 mas/y. Extrapolation of the motions of the concentrations back to the binary suggests that the eastern `arm began expansion 4--5 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. Comparison of model dust images and the observations constrain 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.
We present infrared photometry of the episodic dust-making Wolf-Rayet system WR19 (LS3), tracking its fading from a third observed dust-formation episode in 2007 and strengthening the view that these episodes are periodic (P = 10.1+/-0.1 y). Radial velocities of the O9 component observed between 2001 and 2008 show RV variations consistent with WC19 being a spectroscopic binary of high eccentricity (e=0.8), having periastron passage in 2007.14, shortly before the phase of dust formation. In this respect, WR19 resembles the archetypical episodic dust-making colliding-wind binary system WR140.
Observations of the WC9+OB system WR 65 in the infrared show variations of its dust emission consistent with a period near 4.8~yr, suggesting formation in a colliding-wind binary (CWB) having an elliptical orbit. If we adopt the IR maximum as zero phase, the times of X-ray maximum count and minimum extinction to the hard component measured by Oskinova & Hamann fall at phases 0.4--0.5, when the separation of the WC9 and OB stars is greatest. We consider WR 65 in the context of other WC8-9+OB stars showing dust emission.
We present results from a global view on the colliding-wind binary WR 147. We analysed new optical spectra of WR 147 obtained with Gran Telescopio CANARIAS and archive spectra from the Hubble Space Telescope by making use of modern atmosphere models accounting for optically thin clumping. We adopted a grid-modelling approach to derive some basic physical characteristics of both stellar components in WR 147. For the currently accepted distance of 630 pc to WR 147, the values of mass-loss rate derived from modelling its optical spectra are in acceptable correspondence with that from modelling its X-ray emission. However, they give a lower radio flux than observed. A plausible solution for this problem could be if the volume filling factor at large distances from the star (radio-formation region) is smaller than close to the star (optical-formation region). Adopting this, the model can match well both optical and thermal radio emission from WR 147. The global view on the colliding-wind binary WR 147 thus shows that its observational properties in different spectral domains can be explained in a self-consistent physical picture.
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