Bibcode
DOI
Gallart, Carme; Freedman, Wendy L.; Aparicio, A.; Bertelli, Giampaolo; Chiosi, Cesare
Referencia bibliográfica
The Astronomical Journal, Volume 118, Issue 5, pp. 2245-2261.
Fecha de publicación:
11
1999
Número de citas
199
Número de citas referidas
153
Descripción
We present a quantitative analysis of the star formation history (SFH)
of the Local Group dSph galaxy Leo I, from the information in its Hubble
Space Telescope [(V-I),I] color-magnitude diagram (CMD). It reaches the
level of the oldest main-sequence turnoffs, and this allows us to
retrieve the SFH in considerable detail. The method we use is based on
comparing, via synthetic CMDs, the expected distribution of stars in the
CMD for different evolutionary scenarios with the observed distribution.
We consider the SFH to be composed by the SFR(t), the chemical
enrichment law Z(t), the initial mass function (IMF), and a function
β(f,q) controlling the fraction f and mass ratio distribution q of
binary stars. We analyze a set of ~=50 combinations of four Z(t), three
IMFs, and more than four β(f,q). For each of them, the best SFR(t)
is searched for among ~=6x107 models. The comparison between
the observed CMD and the model CMDs is done through
χ2ν minimization of the differences in the
number of stars in a set of regions of the CMD, chosen to sample stars
of different ages or in specific stellar evolutionary phases. We
empirically determine the range of χ2ν
values that indicate acceptable models for our set of data using tests
with models with known SFHs. Our solution for the SFH of Leo I defines a
minimum of χ2ν in a well-defined position
of the parameter space, and the derived SFR(t) is robust, in the sense
that its main characteristics are unchanged for different combinations
of the remaining parameters. However, only a narrow range of assumptions
for Z(t), IMF, and β(f,q) result in a good agreement between the
data and the models, namely, Z=0.0004, a IMF Kroupa et al. or slightly
steeper, and a relatively large fraction of binary stars, with
f=0.3-0.6, q>0.6, and an approximately flat IMF for the secondaries,
or particular combinations of these parameters that would produce a like
fraction of similar mass binaries. Most star formation activity (70% to
80%) occurred between 7 and 1 Gyr ago. At 1 Gyr ago, it abruptly dropped
to a negligible value, but seems to have been active until at least
~=300 million years ago. Our results do not unambiguously answer the
question of whether Leo I began forming stars around 15 Gyr ago, but it
appears that the amount of this star formation, if it existed at all,
would be small.