3. Mean stellar parameters
No empirical effective temperature in the sense of the method elaborated by Code et al. (1976) is available for Cet. There are several indirect ways of estimating the effective temperatures of stars. These methods use some properties of the spectrum such as the slope of continuum over a limited wavelength interval or the relative strengths of absorption lines from elements present in two or more stages of ionization to identify a star with a model atmosphere. The effective temperature of the model atmosphere which gives the best representation of the observed data is said to be equivalent to the effective temperature of a star.
One of the efficient methods of estimating the atmospheric parameters is based on the Strömgren-Crawford photometric system. Two indices, and [ ], are used to derive the effective temperature of early-type stars. Although the [ ] index is a more sensitive indicator of , there are reasons for preferring the -index as a temperature indicator, cf. Shobbrook (1976) and Davis & Shobbrook (1977). These reasons appear to originate in a metal-line index involved in the definition of [ ] = 2 [ ] + [ ]. A further argument for as an effective temperature indicator follows from an inspection of the theoretical indices calculated for the older (Kurucz 1979a, b ; Lester et al. 1986) and the latest (Kurucz 1991) line-blanketed models of atmospheres. Since the new models were obtained for upgraded stellar opacities, such a comparison gives insight to sensitivity of the photometric indices to uncertainties in the line blanketing effects. As one can expect, the index is much less sensitive to changes in stellar opacities than the [ ] one. We found the difference of between the old and new models, which corresponds to K for a star with = 25000 K and log g = 4.0. This effect is small in the context of the present paper, but using the new -indices has an important advantage: they are calculated for a grid of models with step 1000 K in near 25000 K, whereas the step in the older grid is equal to 2500 K. We therefore derived the effective temperature and surface gravity from the original data of Cet using - and -indices given by Kurucz (1991) and Smalley & Dworetsky (1995), respectively. Unfortunately, this is not entirely a consistent approach, because the index corresponds to the older grid of models (the new Kurucz (1991) data do not contain the index). However, due to the fact that the theoretical indices are calibrated to observations of standard stars (cf. Smalley & Dworetsky 1995), one can expect that the final result is only little influenced by this quasi-homogeneous approach. The results are shown in Table 1. Following Shobbrook (1978), we adopted the relations and E( ) = 0.24 E( ) in the de-reddening procedure. Stellar parameters derived from the calibration formulae given by Napiwotzki et al. (1993) and Balona (1994), which are based on the [ ] and indices, respectively, are also shown in Table 1. As one can see, for a given set of observations, Napiwotzki's et al. (1993) formula and our approach give the same results for within an error box of dex. Similar scatter is present when different data are adopted for the mean photospheric indices of Cet with the exception of numbers obtained from the two-parametric formula given by Balona (1994), cf. Table 1. A direct interpolation in the theoretical grid of indices is recommended. It is interesting to note that the effective temperature derived from the photometric indices is in good agreement (again within the error box of dex.) with the temperature obtained from the nonadiabatic observables discussed in Sect. 2.2.
Table 1. log , log g and E( ) derived from photometry.
Recently, Kolb & Baade (1994) reported a log g determination for Cet from an analysis of the line. They found log g = 3.70 0.25 for = 21000 K. The error 0.25 in log g is mainly due to assumed uncertainty in the effective temperature of the order of 2000 K.
© European Southern Observatory (ESO) 1997
Online publication: October 15, 1997