Determining the temperature of a hot planetary nebula central star is difficult. The methods used are well known, the most reliable being the Zanstra method, followed by the Energy Balance method. All the methods have been critically discussed by Stasinska and Tylenda (1986), (hereafter ST) using idealized nebular models. We shall apply them to the case of NGC 6537.
To measure the Zanstra temperature it is necessary to know, in addition to the hydrogen and helium nebular line flux, the stellar continuum at some low energy wavelength. Usually the magnitude of the star in the visual is used. But as a star becomes hotter, the visual magnitude of the star becomes fainter compared to the nebular emission. At some point the star can no longer be distinguished from the nebular emission. The standard technique is to use a filter which can exclude the stronger nebular lines, but the nebular continuum always remains. An image should be made in very good seeing.
This has been done for NGC 6537 by Gathier and Pottasch (1988) and Jacoby and Kaler (1989). Neither of these groups was able to detect the central star. Gathier and Pottasch report that it must be fainter than mv=18.9, corresponding to a HeII Zanstra temperature of 210 000 K. Jacoby and Kaler find a very similar limit to the magnitude, 18.78. These latter authors predict that a `crossover' magnitude, (that magnitude for which the H and the HeII Zanstra temperatures will be the same), is 19.67, corresponding to a stellar temperature of 250 000 K. Heap et al. (1990) also fail to detect the star in conditions of very good seeing. They report that the star must be fainter than mv=20.0.
The Hubble Space Telescope (HST) permits a much improved measurement of the magnitude. This is because the stellar image is an order of magnitude smaller, due to the fact that there is no seeing to broaden the image. This greatly increases the contrast, by about a factor of 100. We shall report on these measurements here.
© European Southern Observatory (ESO) 2000
Online publication: October 24, 2000