On the Impact of Helium Content on the RR Lyrae Distance Scale
  We constructed new sets of He-enhanced (Y = 0.30, Y = 0.40) nonlinear,
time-dependent convective hydrodynamical models of RR Lyrae (RRL) stars
covering a broad range in metal abundances (Z from 0.0001 to 0.02). The
increase in He content from the canonical value (Y = 0.245) to Y = 0.30 and
0.40 causes a simultaneous increase in stellar luminosity and in pulsation
period. To investigate the dependence of the RRL distance scale on the He
abundance, we computed new optical (RI) and near-infrared (JHK ) period
luminosity metallicity helium relations. Interestingly enough, the increase in
He content causes a minimal change in the coefficients of both period and
metallicity terms, since canonical and He-enhanced models obey similar PLZ
relations. On the contrary, the classical B and V band mean magnitude
metallicity relations and the R band PLZ relation display a significant
dependence on the He content. The He enhanced models are, at fixed metal
content, 0.2 to 0.5 mag brighter than canonical ones. This variation is only
marginally affected by evolutionary effects. The quoted distance diagnostics
once calibrated with trigonometric parallaxes (Gaia) will provide the
opportunity to estimate the He content of field and cluster RRLs. Moreover, the
use of either spectroscopic or photometric metal abundances will pave the way
to new empirical constraints on the universality of the helium to metal
enrichment ratio in old stellar tracers.
