Constraints on the distance moduli, helium and metal abundances, and
  ages of globular clusters from their RR Lyrae and non-variable
  horizontal-branch stars. II. Multiple stellar populations in 47Tuc, M3, and
  M13
  We present a new set of horizontal-branch (HB) models computed with the MESA
stellar evolution code. The models adopt $\alpha$-enhanced \cite{ags09} metals
mixtures and include the gravitational settling of He. They are used in our HB
population synthesis tool to generate theoretical distributions of HB stars in
order to describe the multiple stellar populations in the globular clusters
47Tuc, M3, and M13. The observed HB in 47Tuc is reproduced very well by our
simulations for [Fe/H] $= -0.70$ and [$\alpha$/Fe] $= +0.4$ if the initial
helium mass fraction varies by $\Delta Y_0 \sim 0.03$ and approximately 21%,
37%, and 42% of the stars have $Y_0 = 0.257$, 0.270, and 0.287, respectively.
These simulations yield $(m-M)_V = 13.27$, implying an age near 13.0 Gyr. In
the case of M3 and M13, our synthetic HBs for [Fe/H] $= -1.55$ and
[$\alpha$/Fe] $= 0.4$ match the observed ones quite well if M3 has $\Delta Y_0
\sim 0.01$ and $(m-M)_V = 15.02$, resulting in an age of 12.6 Gyr, whereas M13
has $\Delta Y_0 \sim 0.08$ and $(m-M)_V = 14.42$, implying an age of 12.9 Gyr.
Mass loss during giant-branch evolution and $\Delta Y_0$ appear to be the
primary second parameters for M3 and M13. New observations for 7 of the 9 known
RR Lyrae in M13 are also reported. Surprisingly, periods predicted for the
$c$-type variables tend to be too high (by up to $\sim 0.1$~d).
