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*Astron. Astrophys. 326, 620-628 (1997)*
## 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
1979*a*, *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
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