2. Heliospheric, interstellar, and Siriospheric absorption towards Sirius
Bertin et al. (1995b) have suggested that the additional absorption on the blue side is due to neutral gas associated with Sirius's wind, a counterpart of Mg II absorption detected at this velocity. Here we consider another possibility, that the absorption is from the interaction area between Sirius's wind and the interstellar gas around the star.
In the following, we make a series of assumptions:
- Sirius is embedded in the "blue cloud" seen at the Doppler shift of 13 km s-1. This is a very reasonable assumption, owing to the very small length of the line-of-sight.
- Sirius has a wind with a terminal velocity of the same order of magnitude as the solar wind velocity (say 400-1500 km s-1), and a mass flux at least that of the solar wind. These assumptions are compatible with the predictions of radiatively driven wind models or coronal winds (see Bertin et al. 1995b).
- The gas near the Siriopause is not fully ionized by the EUV radiation from Sirius B. Using model results of Paerels et al. (1987) for a 25,000 K pure H white dwarf, the EUV flux of Sirius B balances the travel time associated to a star/ISM relative motion of 25 km s-1 at distances of about 200 AU, which implies that if the size of the siriosphere is of this order or larger, neutral atoms of the cloud can penetrate within it. Such a size is very likely reached, since the Sirius wind is probably stronger than the solar wind and then the equilibrium with the ISM is reached at larger distances.
- The axis of symmetry of the siriosphere, determined by the relative motion between the star and the ambient gas, makes an angle with the line-of-sight direction. We know the 3D motion of Sirius A from ephemerides for the orbital system, combined with the radial velocity of Sirius A at the time of the observations, vr= -5 km s-1, but we do not know the 3D motion of the cloud. Multiple clouds have been observed for many short lines of sight besides Sirius, but their projected velocities are never far from that of the LIC; the separation is only 6 km s-1 for the non-LIC cloud seen towards Sirius. Thus, assuming the motions of these additional clouds are identical to that of the LIC is a reasonable approximation, and making this assumption for the Sirius cloud leads to an estimated angle of .
Under these assumptions, we can estimate some characteristics of the HIA and HSWA populations around Sirius. The distance at which pressure equilibrium between the wind and the ISM is reached depends on the ISM pressure and the stellar wind momentum flux. If the mass flux and/or the velocity of the Sirius wind are larger than the solar wind flux and velocity, which is likely, the HIA component will be created at larger distances from the star in comparison with the solar case. But for the interstellar gas outside the discontinuity, the conditions of deceleration and heating should be about the same as for the heliosphere, since the gas has to decelerate in both cases by about the same quantity to be at rest with the star (the relative velocity between the star and the ISM). In the solar case, the relative velocity is 25.5 km s-1. For Sirius, we can estimate this velocity to be about 20-40 km s-1. The velocity is km s-1 if the surrounding cloud motion is assumed to be identical to the LIC. However, since it's projected velocity (13 km s-1) is lower than the LIC's projected velocity (19 km s-1), the actual relative velocity is likely to be lower than 31 km s-1 and therefore not too different from the solar case. In this instance, the absorption will be found at velocities between 13 km s-1 (projection of the cloud's velocity) and -5 km s-1 (the projection of the stellar motion). Fig. 1b shows the resulting theoretical absorption.
The compressed stellar wind should also have properties similar to the compressed solar wind, although possibly formed at larger distances and possibly hotter. We have computed the absorption in the solar frame for , which should be equivalent to what would be seen by an observer on Sirius. Then, we have changed its sign and added -5 km s-1 to represent what would be seen for an observer at rest with the Sun and looking towards Sirius. Fig. 1b shows the predicted absorption. It can be seen that the HIA and especially the HSWA absorptions fall at the location of the "missing" absorption in the blue wing.
Fig. 1d shows the consequences of this additional absorption on the simulated spectrum. In order to obtain a complete "filling" of the line we have multiplied by 2 the column density of the HIA and HSWA components, which corresponds to a cloud two times denser than the LIC, or distances in the siriosphere two times larger, or any combination. It is beyond the scope of this paper to investigate all solutions since there are too many. However, from this crude estimate we conclude that siriospheric absorption could possibly account for the extra absorption observed in the blue wing.
© European Southern Observatory (ESO) 1999
Online publication: December 22, 1998