5. Atomic diffusion and the value of
The ratio of helium supplied to the interstellar medium by stars relative to their supply of heavy elements is an important quantity to test theoretical stellar yields and for deriving the slope of the relation between helium and oxygen in extra-galactic HII regions, a fundamental ingredient for determining the primordial helium abundance.
One possible approach to the determination of this quantity is the study of the MS width of local subdwarfs. Since changes of the initial values of Y and Z push the MS in opposite directions (increasing Y makes the MS bluer, while increasing Z makes it redder), the width of the local subdwarfs MS for a fixed metallicity range is a function of the ratio in the interstellar medium. One can therefore consider two indicators: either the vertical (usually in ) width at a fixed value of , or the horizontal width (in or log)) at a fixed value of (see, e.g., Castellani et al. 1999 and references therein). Usually the lower MS (corresponding to subdwarfs with 5.5-6.0) is used for the analysis to avoid (as in the MS-fitting technique) evolutionary effects and the influence of the mixing length calibration.
As it has been shown before, one of the effects of atomic diffusion on MS subdwarfs is to increase the MS width for a fixed metallicity interval and assumed initial value. This is due to the fact that the colour difference between the diffusive C isochrones and the standard ones is metallicity dependent, and is larger at larger metallicities.
As an example, we have considered a value for subdwarfs effective temperature log()=3.70 (corresponding to 6); we then computed the MS broadening due to diffusion, in the interval between [Fe/H]=-2.3 and -0.7 - a metallicity range typical of Halo subdwarfs - and for subdwarfs ages equal to 8 and 12 Gyr, by means of comparisons with the SW98 models. As expected, results to be larger for C isochrones with respect to standard ones, the exact value depending on the subdwarfs age since the entire MS location of C isochrones does depend on age; this means that standard isochrones underestimate with respect to the calibrated diffusive ones.
The amount of this systematic difference was derived by computing additional C isochrones and varying the initial ratio in the range between 1 and 5. We found that C isochrones (in the explored range) lead to initial ratios larger by 1-2, the exact amount depending on the subdwarfs ages. Moreover, we found that the dependence on the initial helium abundance of the values of at a fixed log along the lower MS, is in broad agreement with the results from standard models by Castellani et al. (1999).
© European Southern Observatory (ESO) 2000
Online publication: March 17, 2000