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Astron. Astrophys. 335, 277-280 (1998)

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6. The gravity distance

We make use here of the spectral similarity of the central star of NGC 650 and PG 1159. Werner et al. (1991) find spectroscopically a value of log g = 7.0 [FORMULA] 0.5. Kawaler & Bradley (1994) find log g = 7.38 [FORMULA] 0.1 and M = 0.59 [FORMULA] from non-linear pulsations. Using the relation of Mendez et al (1988)

[EQUATION]

where M is the stellar mass, g is the surface gravity (cm s-2), V is the visual magnitude of the star corrected for extinction, and d is the distance in kpc. F is the stellar flux from a model atmosphere, and for the temperature of this star has a value of 24 in units of 108 erg cm-2 s-1 Å-1. The distance to NGC 650 is then 1.2 kpc. If NGC 650 were placed at a distance of 5 kpc, its gravity would be log g = 6.15 which is too low for the spectrum found. Another way of obtaining the same result, is by making use of the distance to PG 1159 of 440 pc (Kawaler & Bradley, 1994). Since its V magnitude is 14.83 compared to 17.00 for the central star of NGC 650, the distance to the latter must be 1.2 kpc if all the other characteristics of the star are the same.

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© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998

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