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Astron. Astrophys. 330, L41-L44 (1998)

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2. The problem of distance determination

Conventional distance methods, like spectroscopic parallaxes can not be applied in this case as our current understanding of stellar evolution can not be used to predict the physical properties of such a very late He-flash with any degree of certainty. To the contrary, once a distance is known, accurate values can be derived to check the validity of theory. The existence of an old PN around Sakurai's Object gives us an alternative means of determining the distance. The physical connection between the star and the PN is confirmed by measurements of the radial velocity ([FORMULA] 100 km s-1) which give very similar results for both (Duerbeck & Benetti 1996, Kerber et al. 1997c). In addition, Sakurai's Object is almost perfectly located in the centre of the PN. Using a PN for distance determination may sound like a losing proposition when one considers that PN distances are notoriously difficult to obtain and that the results often remain unreliable (Terzian 1993). In fact statistical means of distance determination - like a distance scale derived from the Shklovsky method - contain large uncertainties. Van de Steene & Zijlstra (1995) explicitly warn against using them for distance determination of individual objects. Individual distance methods generally are more reliable but often make use of a special property of the object, like e.g. cluster membership (Dufour 1984). An individual method that does not make any assumptions about the object itself is the extinction or reddening method. It has successfully been employed by a number of authors (Acker 1978, Gathier et al. 1986, Martin 1994, Saurer 1995) This method requires that the reddening as a function of distance be determined from photometry of stars in the field surrounding the PN. The details of the method including potential problems - in particular the influence of the inhomogeneity of the interstellar medium - are discussed e.g. in Gathier et al. (1986).

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© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998