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Astron. Astrophys. 325, 1025-1030 (1997)

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2. [FORMULA]

The variation of [FORMULA] with stellar spectral type and luminosity class has been investigated by Hamilton (1985). We have compared his results with Kurucz's (1979) models of solar metallicity using the spectral type-effective temperature and luminosity class-surface gravity relations derived by Schmidt-Kaler (Landolt-Börnstein 1982, Tables 4.1.4.3 and 4.1.5.23). The comparison shows that the theoretical values fit nicely the observed ones. Using a later version of Kurucz's models does not alter the results here presented.

The behaviour of [FORMULA] as a function of the stellar parameters (Fig. 1) has been derived from Kurucz's models. For stars with [FORMULA] (not included in Fig. 1) [FORMULA] has been computed from the stellar spectra of Straizys & Sviderskiene (1972) and keeps always values above 2, reaching a maximum of 3 at [FORMULA] K.

[FIGURE] Fig. 1a-c. [FORMULA] indices of Kurucz's stellar models: a) as a function of the stellar effective temperature for solar metallicity and two different gravities (filled circles and solid line Log g=4; empty circles and dotted line Log g=1.5); b) as a function of the gravity for solar metallicity stars and 4 different effective temperatures; c) as a function of the metallicity for Log g =4 stars and two different effective temperatures

The [FORMULA] values for SSPs, computed using Kurucz's spectra, are shown in Fig. 2; the typical uncertainty is 0.1. While this index raises to high values for old solar metallicity SSPs, its value remains much lower in the case of low metallicity SSPs even after 15 Gyr.

[FIGURE] Fig. 2. [FORMULA] values of single stellar populations

The results obtained from the integrated spectra of elliptical models with different average metallicities are shown in Table 1, while those of other Hubble types are given in Table 2. It must be stressed that the results presented in the tables include both metallicity and age effects: in computing the integrated spectrum, stellar populations of different metallicities are taken into account, according to a standard chemical evolutionary model that uses the instantaneous recycling approximation. A full description of the adopted galactic models is given in Barbaro & Poggianti (1997).


[TABLE]

Table 1. [FORMULA] of the elliptical model as a function of the average metallicity



[TABLE]

Table 2. Results for [FORMULA] and [FORMULA] including the gaseous emission (total) and only for the stellar component (only abs.)


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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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