Cepheid theoretical models and observations in HST/WFC3 filters: the
  effect on the Hubble constant Ho
  We present a complete theoretical scenario for classical Cepheids in the most
commonly used HST/WFC3 filters, going from optical (F555W, F606W and F814W) to
near-infrared (F160W) bands. The importance of such a study is related to the
recent release of new classical Cepheids observed with HST/WFC3 in 8 distant
galaxies where SNIa are hosted. These observations have posed sound constraints
to the current distance scale with uncertainties on the Hubble constant Ho
smaller than 3%. Our models explore a large range of metallicity and Helium
content, thus providing a robust and unique theoretical tool for describing
these new and future HST/WFC3 observations. As expected, the Period-Luminosity
(PL) relation in F160W filter is linear and slightly dependent on the
metallicity when compared with optical bands, thus it seems the most accurate
tool to constrain extragalactic distances with Cepheids. We compare the
pulsation properties of Cepheids observed with HST/WFC3-IR with our theoretical
scenario and we discuss the agreement with the predicted Instability Strip for
all the investigated galaxy samples including the case of NGC4258. Finally,
adopting our theoretical F160W PL relation for Z=0.02 and log P>1.0, we derive
new distance moduli. In particular, for NGC 4258, we derive a distance modulus
mu0 = 29.345 +- 0.004 mag with a sigma = 0.34 mag, which is in very good
agreement with the geometrical maser value. Moreover, using the obtained
distance moduli, we estimate the Hubble constant value, Ho=76.0 +- 1.9 km s-1
Mpc-1 in excellent agreement with the most recent literature values.
