6. The metallicity and gravity effects
One problem of concern is to evaluate gravity and metallicity effects on the stellar parameters derived for field stars. In fact, at least for bolometric flux measurements, these effects become now detectable using the high-precision broad-band photometry of the ISO standards. In order to better see the different role played by these effects, I begin to plot the diagram of Fig. 11 for dwarfs and giants separately. In fact, the model-atmosphere calculations of BG make it clear that the gravity and metallicity will have negligible effects on the near-infrared colour adopted here as a practically unbiased temperature indicator. Also, the high-gravity/metal-poor dwarfs will have bluer colours than low-gravity/metal-normal giants with the same temperature (Kurucz 1991), as clearly appear in the actual two-colour diagram. Then, with reference to the residuals of Fig. 4, the smaller scatter observed on the high-gravity/metal-poor dwarfs quite consistent with the random errors on photometry alone indicates that the corresponding bolometric fluxes would be rather insensitive to the widely differing poorer metallicity and practically constant gravity of these stars, whereas the gravity rather than metallicity must be likely responsible for the increased scatter in the flux measurements of low-gravity/metal-normal giants. Also, the small average differential effects drawn in Fig. 5 are likely to be ascribed to gravity which would make the near-infrared colour of giants smaller than that of dwarfs according to the relation (8) and to the model-atmosphere results of BG.
The influence of metallicity on bolometric flux determinations of F-G-K dwarfs has been recently stressed by Alonso et al. (1995) using a sample of stars with most of metallicities measured by high dispersion spectroscopy. For a subset of 49 stars in common with the ISO standards, the effect of the metallicity gradient is clearly observed in the upper plot of Fig. 12 which shows the colour residuals from ridge-line representation drawn in Fig. 11. The middle and lower plots show the bolometric flux residuals from the average third-order polynomial function (AL) and second-order one (MI), respectively. As it can be seen, there is no evidence for any systematic deviation from the actual second-order regression line representing Class V stars, in contrast with the remarkable trend suggested by Alonso et al.. In fact, these authors accounted for the effects of metallicity by using a model-fitting approach rather than to attempt detection of such systematic variations readily observed in the diagram of the metallicity-sensitive colour . Notice also that the model-dependent flux variations adopted by Alonso et al. would imply temperature shifts by up to 2.5 % at stellar metallicities as poor as - 3.5 dex. Such large systematic effects indeed appear in the final temperature scales derived by the authors (Alonso et al. 1996).
© European Southern Observatory (ESO) 1998
Online publication: October 22, 1998