HORIZONTAL-BRANCH MORPHOLOGY IN THE FIELD FROM THE LIGHT-TO-MASS RATIO OF RR LYRAE VARIABLES

Period and blue amplitude of the light curve are used to calculate the light-to-mass ratio of RRab variables in five Galactic fields with galactocentric distance from 0.5 to 19 kpc. Comparison of the observed light-to-mass ratios with the predicted ones from synthetic horizontal branch models suggests that the original helium abundance of field variables with [Fe/H] < -0.7 is nearly constant over the whole range of metallicity, and close to the value [Y(MS) = 0.23] derived from RR Lyrae variables in Galactic globular clusters. It is shown that, at a fixed galactocentric distance, the frequency of RRab variables with large value of the light-to-mass ratio increases as the metallicity decreases, while, at fixed metallicity, it increases as the galactocentric distance decreases. Such a behavior is the same as found for RRab stars in Galactic globular clusters, suggesting a similar evolutionary history for the two populations of variables. Interpreting the light-to-mass ratio of RRab variables in terms of the mean color distribution of RR Lyrae progenitors yields that, at fixed metallicity, the horizontal branch (HB) in the field moves from the red to the blue as the galactocentric distance decreases, a result which matches the radial variation in HB morphology observed within the Galactic globular cluster system. Such a conclusion is rather independent of the adopted enhancement of alpha elements with respect to iron. Finally, some evidence is presented suggesting that the original helium content of several RR Lyrae variables with [Fe/H] greater-than-or-equal-to -0. 7 is larger than Y(MS) = 0.23.