HD 152248: Evidence for interaction

8 Wind-wind interaction

We now attempt to provide a consistent explanation to all our observations. It is obvious, from the results presented above, that both components of the HD152248 system are very similar. They indeed have similar polar radii, masses, luminosities and spectral types. Therefore, we can expect that the stars also have similar wind properties (mass loss rates and asymptotic velocities). Howarth et al. (1997) derived terminal velocities of $v_{\infty, 1} = 2420$kms^-1 and $v_{\infty, 2} = 2010$kms^-1 for the primary and secondary respectively. We can use the stellar parameters derived above in conjunction with the theoretical mass loss recipe of Vink et al. (2000) to estimate the mass-loss rates of the two components. In this way, we obtain $\log{\dot{M}_1} = -5.53$ and $\log{\dot{M}_2} = -5.48$ ($M_{\odot}$yr^-1) yielding a wind momentum ratio (Stevens et al. 1992) of

$${\cal R} = \left(\frac{\dot{M}_1\,v_{\infty, 1}}{\dot{M}_2\,v_{\infty, 2}} \right)^{1/2} \simeq 1.03 \nonumber$$ (1)

From these simple arguments, we expect therefore that the stagnation point of a wind-wind collision should be located near the center of mass of the HD152248 system. Adopting a ``standard'' $\beta$ velocity law (with $\beta = 1$), we find that, depending on the orbital separation, the winds of both stars should reach between about 0.34 and $0.50 \times v_{\infty}$ before they collide. Using these pre-shock velocities, we obtain rather low values of the cooling parameter $\chi \leq 0.02$-0.15 (see Stevens et al. 1992 for the definition of $\chi$). Radiative cooling could therefore play an important role in the interaction region and cool the shock-heated material to temperatures where the recombination process can efficiently contribute to the formation of the H$\alpha$ and He II$\lambda$4686 emission lines.

A very simple model that can match most of the observations consists therefore in assuming the existence of a wind-wind interaction region located mid-way between both stars and limited by two roughly planar hydrodynamic shocks. This high density region, where the He II$\lambda$4686 and H$\alpha$ emission components are assumed to be produced, would (partly) be occulted at conjunction phases ($\phi=0.01$ and 0.52) which explains why the absorption is overwhelming near these phases. Similarly, this simple scenario also accounts for the phase-locked variations of the width of the emission component. Indeed, we expect from our model that the inner shock region would be almost perpendicular to the axis of the system, so that the distribution of the radial velocities of the particles escaping from the wind interaction region should be broader when our line of sight is aligned with the interaction zone (i.e. around $\phi=0.22$ and 0.80). On the other hand, this velocity distribution should be narrower at conjunction phases when our line of sight is forming an angle of about $i \approx 70\hbox{$^\circ$ }$ with the interaction region. Near conjunction, we thus expect to observe a narrower emission line than around quadrature.

Part of the difference of the relative strengthening of the emission near quadrature phases might be explained by assuming that the emission process is depending of the separation D between the two stars following some power of 1/D. Indeed this distance is slightly larger at $\phi=0.22$ than at $\phi=0.80$. The difference between the two absorption peaks at conjunction phases might also result from a difference in the separation between the two stars since the phases $\phi=0.0$ and $\phi=0.5$ respectively coincide with the periastron and the apastron passage. The unequal depths of the primary and secondary eclipses might also account for part of this difference, as the continuum level is lower at primary eclipse and the dilution of the emission is thus weaker. We might expect that the forthcoming X-ray observations will help to address those questions and will further allow us to refine the wind-wind interaction model of the HD152248 system.

HD 152248: Evidence for interaction