V4334 Sgr (Sakurai's Object): The Distance Problem

The central star V4334 Sgr (Sakurai's Object) of the planetary nebula PN G010.4+04.4 underwent in 1995–1996 the rare event of a very late helium flash. It represents only one out of two such events during the era of modern astronomy (the other event was V605 Aql = Nova Aql 1919). All the other prominent objects of that type originate from events occurring several thousands of years ago (e.g. A30, A78). Thus it is of special interest for stellar evolution theory to model the detailed observations obtained during the last four years. Those models depend essentially on basic stellar parameters like effective temperature, surface gravity and stellar radius. Most of them depend strongly on the assumed distance to the object. Some models may give some constraints on this parameter, but most of them depend on the assumption as input parameter. Hence to determine a reliable distance is of considerable significance. This should be obtained through models that give us lower and upper boundaries, or through means which are independent of models. The detailed review, by using every kind of determination available up to now, leads to a Galactic foreground extinction of E _B–V =0^m75 ±0.05 and a distance of D = 2.0_-0.6 ^+1.0 kpc.