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

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5. Temperature of the central star

Before discussing the consequences of having the nebula at the distance of the visual companions, it is necessary to discuss the temperature of the central star. There are two good ways of determining the temperature: 1) from the spectrum of the central star, and 2) from the Zanstra method. The spectrum of the central star has been discussed by Napiwotzki (1993) and Napiwotzki & Schönberner (1995). Napiwotzki (1993) classifies it as a PG 1159 star, because of the absence of hydrogen lines and the presence of H II and C IV lines. The spectrum is very similar to spectra analysed by Werner et al. (1991). These authors find a value of temperature T and a log g = 7.0 for these stars, which include PG 1159 itself. Napiwotzki adopts these parameters for the central star of NGC 650, which appears spectroscopically very similar to PG 1159. The uncertainty in the temperature is about 10%. Several PN central stars show a similar spectrum: A 21, Jn 1, PG 1520+525. As far as the present observations allow a comparison, they appear identical to PG 1159. Kawaler & Bradley (1994) have made a careful analysis of the non-radial pulsations in this star and have been able to derive very accurate values of temperature, luminosity and mass for this star. These values agree within the limits of error with the less accurate values of Werner et al. (1991). The temperature they find is T. We shall return to the other parameters in Sect. 7. The Zanstra method can also be used to determine the temperature. The resultant values differ from values given earlier in the literature because of the improved value of the V magnitude given in Table 2. Using the same extinction, an H[FORMULA] flux of log H[FORMULA] = -10.67 and a value of the ratio He II  4686/H[FORMULA] = 0.6 (Cahn et al., 1992), we find the hydrogen and He II Zanstra temperatures to be: T(H) = 160,000 K, and T(He II ) = 145,000 K. The rather close agreement of these temperatures indicates that the nebula is optically thick both in hydrogen and He II ionizing radiation. We adopt T in the following discussion.

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© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998