Age spreads and the temperature dependence of age estimates in Upper Sco
  Past estimates for the age of the Upper Sco Association are typically 11-13
Myr for intermediate-mass stars and 4-5 Myr for low-mass stars. In this study,
we simulate populations of young stars to investigate whether this apparent
dependence of estimated age on spectral type may be explained by the star
formation history of the association. Solar and intermediate mass stars begin
their pre-main sequence evolution on the Hayashi track, with fully convective
interiors and cool photospheres. Intermediate mass stars quickly heat up and
transition onto the radiative Henyey track. As a consequence, for clusters in
which star formation occurs on a similar timescale as the transition from a
convective to a radiative interior, discrepancies in ages will arise when ages
are calculated as a function of temperature instead of mass. Simple simulations
of a cluster with constant star formation over several Myr may explain about
half of the difference in inferred ages versus photospheric temperature;
speculative constructions that consist of a constant star formation followed by
a large supernova-driven burst could fully explain the differences, including
those between F and G stars where evolutionary tracks may be more accurate. The
age spreads of low-mass stars predicted from these prescriptions for star
formation are consistent with the observed luminosity spread of Upper Sco. The
conclusion that a lengthy star formation history will yield a temperature
dependence in ages is expected from the basic physics of pre-main sequence
evolution and is qualitatively robust to the large uncertainties in pre-main
sequence evolutionary models.
