3D modelling of the colliding winds in η Carinae - evidence for radiative inhibition

Abstract

The X‐ray emission from the supermassive star η Car is simulated using a 3D model of the wind–wind collision. In the model the intrinsic X‐ray emission is spatially extended and energy dependent. Absorption due to the unshocked stellar winds and the cooled post‐shock material from the primary LBV star is calculated as the intrinsic emission is ray traced along multiple sightlines through the 3D spiral structure of the circumstellar environment. The observable emission is then compared to available X‐ray data, including the light curve observed by the Rossi X‐ray Timing Explorer (RXTE) and spectra observed by XMM–Newton. The orientation and eccentricity of the orbit are explored, as are the wind parameters of the stars and the nature and physics of their close approach. Our modelling supports a viewing angle with an inclination of ≃42°, consistent with the polar axis of the Homunculus nebula, and the projection of the observer's line of sight on to the orbital plane has an angle of ≃0°–30° in the prograde direction on the apastron side of the semimajor axis. However, there are significant discrepancies between the observed and model light curves and spectra through the X‐ray minimum. In particular, the hard flux in our synthetic spectra is an order of magnitude greater than observed. This suggests that the hard X‐ray emission near the apex of the wind–wind collision region (WCR) ‘switches off’ from periastron until two months afterwards. Further calculations reveal that radiative inhibition significantly reduces the pre‐shock velocity of the companion wind. As a consequence the hard X‐ray emission is quenched, but it is unclear whether the long duration of the minimum is due solely to this mechanism alone. For instance, it is possible that the collapse of the WCR on to the surface of the companion star, which would be aided by significant inhibition of the companion wind, could cause an extended minimum as the companion wind struggles to re‐establish itself as the stars recede. For orbital eccentricities, e≲ 0.95, radiative braking prevents a wind collision with the companion star's surface. Models incorporating a collapse/disruption of the WCR and/or reduced pre‐shock companion wind velocities bring the predicted emission and the observations into much better agreement.

ηCarinae: a very large hadron collider

“…The X-ray spectrum of η Carinae supports the colliding-wind binary scenario (Usov 1992;Stevens et al 1992;Corcoran 2005;Pittard 2007;Parkin et al 2009). The basic idea is that the LBV emits a rather slow and dense stellar wind (v ∞,1 500 km s −1 , M 1 2.5 × 10 −4 M /yr, Pittard & Corcoran 2002), which collides into the faster, shallower wind originating in the secondary component (v ∞,2 3000 km s −1 ,Ṁ 2 1.0 × 10 −5 M /yr, Pittard & Corcoran 2002).…”

“…Pittard & Corcoran 2002) and could be affected by many phenomena (Parkin et al 2009). These simulations can reproduce an X-ray luminosity of ≈20 L above a few keV.…”

VLTI-AMBER velocity-resolved aperture-synthesis imaging ofηCarinae with a spectral resolution of 12 000

“…The wind collision leads to a wind-wind collision cavity (e.g., Pittard & Corcoran 2002;Humphreys et al 2008;Okazaki et al 2008;Parkin et al 2009Parkin et al , 2011Gull et al 2009Gull et al , 2011Madura & Owocki 2010;Madura et al 2012Madura et al , 2013Groh et al 2010Groh et al , 2012a. Radiative transfer models of the wind-wind collision zone were reported by Clementel et al (2015a,b).…”

“…Studies of the binary or binary wind-wind collision zone were reported by many authors (e.g., Damineli 1996;Damineli et al 1997Damineli et al , 1998Damineli et al , 2000Damineli et al , 2008bDavidson 1997;Davidson et al 2005Davidson et al , 2015Smith & Gehrz 2000;Smith et al 2004;Smith 2010;Pittard & Corcoran 2002;Corcoran et al 1997Corcoran et al , 2001Corcoran 2005;Ishibashi et al 1999;Soker 2003Soker , 2007Weis et al 2005;Gull et al 2006Gull et al , 2009Gull et al , 2011Gull et al , 2016Nielsen et al 2007;Weigelt et al 2007; Kashi & Soker 2007, 2009aParkin et al 2009;Groh et al 2010Groh et al , 2012bMadura & Owocki 2010;Madura et al 2012Madura et al , 2013Mehner et al 2010aMehner et al , 2011Mehner et al , 2012Mehner et al , 2015Richardson et al 2010;Teodoro et al 2013Teodoro et al , 2016Clementel et al 2015a,b;Hamaguchi et al 2016). The X-ray...…”

ETA CARINAE’S THERMAL X-RAY TAIL MEASURED WITH XMM-NEWTON AND NuSTAR

“…Since the primary star drives a dense, slow (V∼420 km s −1 , Groh et al 2012) wind, the companion must have a very fast wind of ∼3000 km s −1 in order for the WWC to produce the observed hot X-ray plasmas (Pittard & Corcoran 2002). The unseen companion should be, therefore, a massive O star or a WolfRayet star (Verner et al 2005;Parkin et al 2009;Mehner et al 2010).…”