Distances to planetary nebulae are very difficult to obtain by trigonometric methods because the PN are far away. This has led to the development of a host of methods to `obtain' individual distances. Some of these methods make use of a property of the nebula, which is given a fixed value. An example of this is the "Shklovsky method", which assigns a fixed value to the ionized nebular mass. Such methods, apart from probably being unreliable, preclude the possibility of discussing the evolution of the PN. Other methods which do not make use of nebular parameters are also in use. The most promising of these is the "extinction distance" and the "spectroscopic distance". This last method is at present applicable to all central stars which show an absorption line spectrum, and has been applied extensively in the past 15 years. For example, a summary of the distances obtained in this way for 42 central stars (with references) is shown in Pottasch (1996, Table 6). A comparison of some of these distances with the most recent parallax measurements (Harris, 1997) has been given by Pottasch (1997) and reasonable agreement is obtained. However, a comparison of the distances obtained by Hajian et al, (1995; see also Terzian, 1997) from the (VLA) measured nebular expansion, with spectroscopic distances, shows a poor agreement: the expansion distances are smaller by a factor of 3 than the spectroscopic values. While the number of cases involved is small, the comparison with the trigonometric parallaxes (7 objects) is always made with higher gravity stars (white dwarfs). The expansion distances (2 objects) are compared with distances obtained from analysis of "lower gravity" stars with NLTE models (Mendez et al., 1988). Hipparcos is able to shed some light on this problem: parallaxes have been measured for 3 PN central stars with spectroscopic distances, and for which low surface gravities have been found. In this paper the Hipparcos measurements are presented and a comparison is made with the spectroscopic distances. The significance of the comparison is discussed.
© European Southern Observatory (ESO) 1998
Online publication: November 24, 1997