A Method for Determining the Star Formation History of a Mixed Stellar Population

The code IAC-star is presented. It generates synthetic HR and color-magnitude diagrams (CMDs) and is mainly aimed to star formation history studies in nearby galaxies. Composite stellar populations are calculated on a star by star basis, by computing the luminosity, effective temperature and gravity of each star by direct bi-logarithmic interpolation in the metallicity and age grid of a library of stellar evolution tracks. Visual (broad band and HST) and infrared magnitudes a...

We present preliminary results from a new HST archival program aimed at tightly constraining the ancient (>4 Gyr ago) star formation histories (SFHs) of the field populations of the SMC and LMC. We demonstrate the quality of the archival data by constructing HST/WFPC2-based color-magnitude diagrams (CMDs; M_{F555W} ~ +8) for 7 spatially diverse fields in the SMC and 8 fields in the LMC. The HST-based CMDs are >2 magnitudes deeper than any from ground based observations, and a...

A new method, AMORE - based on a genetic algorithm optimizer, is presented for the automated study of colour-magnitude diagrams. The method combines several stellar population synthesis tools developed in the last decade by or in collaboration with the Padova group. Our method is able to recover, within the uncertainties, the parameters -- distance, extinction, age, metallicity, index of a power-law initial mass function and the index of an exponential star formation rate -- ...

We use the SMASH survey to obtain unprecedented deep photometry reaching down to the oldest main sequence turn-offs in the colour-magnitude diagrams (CMDs) of the Small Magellanic Cloud (SMC) and quantitatively derive its star formation history (SFH) using CMD fitting techniques. We identify five distinctive peaks of star formation in the last 3.5 Gyr, at $\sim $3, $\sim$2, $\sim$1.1, $\sim $0.45 Gyr ago, and one presently. We compare these to the SFH of the Large Magellanic ...

We present derivations of star formation histories based on color-magnitude diagrams of three fields in the LMC from HST/WFPC2 observations. A significant component of stars older than 4 Gyr is required to match the observed color-magnitude diagrams. Models with a dispersion-free age-metallicity relation are unable to reproduce the width of the observed main sequence; models with a range of metallicity at a given age provide a much better fit. Such models allow us to construc...

We analyzed HST/WFPC2 colour-magnitude diagrams (CMDs) of 15 populous Large Magellanic Cloud (LMC) stellar clusters with ages between ~ 0.3 Gyr and ~ 3 Gyr. These (V, B-V) CMDs are photometrically homogeneous and typically reach V \~ 22. Accurate and self-consistent physical parameters (age, metallicity, distance modulus and reddening) were extracted for each cluster by comparing the observed CMDs with synthetic ones. These determinations involved simultaneous statistical com...

Accurate star formation histories (SFHs) of galaxies are fundamental for understanding the build-up of their stellar content. However, the most accurate SFHs - those obtained from colour-magnitude diagrams (CMDs) of resolved stars reaching the oldest main sequence turnoffs (oMSTO) - are presently limited to a few systems in the Local Group. It is therefore crucial to determine the reliability and range of applicability of SFHs derived from integrated light spectroscopy, as th...

A quantitative method is presented to compare observed and synthetic colour-magnitude diagrams (CMDs). The method is based on a chi^2 merit function for a point (c_i,m_i) in the observed CMD, which has a corresponding point in the simulated CMD within n*sigma(c_i,m_i) of the error ellipse. The chi^2 merit function is then combined with the Poisson merit function of the points for which no corresponding point was found within the n*sigma(c_i,m_i) error ellipse boundary. Monte-...

We present HST photometry for three fields in the outer disk of the LMC extending approximately four magnitudes below the faintest main sequence turnoff. We cannot detect any strongly significant differences in the stellar populations of the three fields based on the morphologies of the color-magnitude diagrams, the luminosity functions, and the relative numbers of stars in different evolutionary stages. Our observations therefore suggest similar star formation histories in t...

Using WFPC2 aboard the Hubble Space Telescope, we have created deep color-magnitude diagrams in the V and I passbands for approximately 100,000 stars in a field at the center of the LMC bar and another in the disk. The main--sequence luminosity functions (LFs) from 19 mag < V < 23.5 mag, the red clump and horizontal branch morphologies, and the differential Hess diagram of the two fields all strongly imply that the disk and bar have significantly different star-formation hist...