6. Analysis of line profiles
6.1. Mean spectrum of Cet
High-resolution images were used to study line profiles of selected species. First, we constructed the mean spectrum of Cet, because one can expect small pulsational effects in moderate 0.2 Å -resolution observations collected by the IUE satellite. The procedure was similar to the one described in Sect. 5, but now small corrections to wavelengths derived by a cross-correlation method were applied for individual spectra. Having already established photospheric parameters log and log , we examined how LTE theoretical spectra fit the mean high-resolution spectrum of Cet. For this purpose, the solar chemical composition of elements was adopted from Kurucz's (1979a ) data, i.e., the carbon and silicon abundances were taken as equal to log = -3.48 and log = -4.50, respectively. The results are plotted in Fig. 4 for hydrogen, carbon and silicon lines listed in Table 3. Damping constants for the Stark broadening of the line absorption coefficients used in the calculations were taken from Stehle (1994) for the H I line and Sahal-Brechot & Segre (1971) for carbon and silicon lines. We assumed the microturbulent velocity =0 and the rotational velocity of Cet equal to sin i = 15 (cf. Gies & Lambert 1992). An increase in to 2 as in Kurucz's models of atmospheres has negligible effect on the analysed lines, which are located at the damping part of the curve of growth. The dotted line in Fig. 4a shows the photospheric flux near 1200 Å, whereas the synthetic spectrum including the interstellar component of H I is shown as the solid line. The additional interstellar component was taken into account as described by Jenkins (1970). We found the hydrogen interstellar column density equal to N(H I) = . This is in good agreement with the E( ) value discussed in Sect. 4.1. Using the calibration of E( ) vs. log N(H I) given by Bohlin et al. (1978), one can find E( ) = .
Table 3. Atomic data of selected lines.
6.2. Line profile behaviour during pulsation cycle
In this section, we report the comparison of the individual high-resolution images of Cet (cf. Table 2) with the model predictions for different pulsational phases. Theoretical profiles were calculated as described by Cugier (1993), taking into account geometrical, temperature and pressure effects in specific intensity variations on the stellar surface during the pulsation cycle. The Doppler broadening due to the velocity field of a pulsating and rotating star is also included. In this paper, the original computing code was modified in agreement with the nonadiabatic (temperature and pressure) effects described by Cugier et al. (1994). The amplitude of the stellar radius variations of Cet was obtained by normalizing the predicted amplitude to the observed one. Thus we have self-consistent data for both continuum and line profile variations as functions of pulsation phase. Having already established the stellar nonadiabatic model (cf. Sect. 4) and parameters involved in the analysis of the mean line profiles (cf. Sect. 6.1), no additional parameters in model calculations for different pulsation phases are needed. We examined the behaviour of Si III 1300 Å and Si IV 1400 Å lines and found satisfactory agreement with the observations. As an example, Fig. 5 shows the Si III 1300 Å lines for the first 8 high-resolution images listed in Table 2. As one can see, the fit quality is basically the same as in Fig. 4, although the noise in the observed data is larger.
In Sect. 6.1, we analysed the mean spectrum of Cet averaged over the pulsating phases. The question is how well this spectrum describes the steady-state model corresponding to this star. We repeated exactly the same procedure as for the observed mean spectrum of Cet, but for the theoretical spectra presented here. The mean theoretical spectrum was then compared with the steady-state model. We found the differences in the line profiles less than 0.1 per cent, with the exception of the line cores where the differences reach about 3 per cent. It results in changes of total equivalent widths of the Si III lines near 1300 Å (as measured from 1292 Å to 1306 Å) by about 0.5 per cent. This indicates that the procedure used in Sect. 6.1 is well justified to study the steady-state model corresponding to Cet.
© European Southern Observatory (ESO) 1997
Online publication: October 15, 1997