Astron. Astrophys. 342, L13-L16 (1999) 


Letter to the Editor 

Heliospheric and astrospheric hydrogen absorption towards Sirius: no need for interstellar hot gas


V.V. Izmodenov  **  1, R. Lallement 1 and Y.G. Malama 2  

1 Service d'Aeronomie, CNRS, F-91371 Verrieres le Buisson, France (e-mail:; 
2 Institute for Problems in Mechanics, Russian Academy of Science, Moscow, Russia 

Received 13 November 1998 / Accepted 4 December 1998 


We use an updated self-consistent kinetic/gasdynamic model of the solar wind/interstellar flow interaction to compute the Lyman-[FORMULA] absorption by hydrogen atoms of both interstellar and solar origin generated in the heliospheric interface. We apply this model to the direction of the star Sirius. We show that the neutralized, compressed solar wind from the heliosheath explains the extra absorption at positive redshifts observed in the Lyman-[FORMULA] line with the HST/GHRS. This removes the need for the interstellar hot gas previously proposed to explain this extra absorption. We also show that extra absorption in the blue wing can possibly be explained by H atoms formed in an astrosphere around Sirius, providing the stellar wind is at least as massive and fast as the solar wind. 

Key words: ISM: abundances - ISM: clouds - ISM: structure - stars: winds, outflows - Sun: solar wind - ultraviolet: ISM 

* Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute which is operated by the Association of Universities for Research in Astronomy Inc., under NASA Contract NASA5-26555 
** Permanently at Moscow State University, Department of Aeromechanics and Gas dynamics, Faculty of Mechanics and Mathematics (e-mail: