We present a simple approach for obtaining robust values of astrophysical parameters from the observed colour-magnitude diagrams (CMDs) of star clusters. The basic inputs are the Hess diagram built with the photometric measurements of a star cluster
and a set of isochrones covering wide ranges of age and metallicity. In short, each isochrone is shifted in apparent distance modulus and colour excess until it crosses over the maximum possible Hess density. Repeating this step for all available isochrones leads to the construction of the solution map, in which the optimum values of age and metallicity - as well as foreground/background reddening and distance from the Sun - can be searched for. Controlled tests with simulated CMDs show that the approach is efficient in recovering the input values. We apply the approach to the open clusters M,67, NGC,6791, and NGC,2635, which are characterised by different ages, metallicities and distances from the Sun.
We study how the spectral fitting of galaxies, in terms of light fractions derived in one spectral region translates into another region, by using results from evolutionary synthesis models. In particular, we examine propagation dependencies on Evolu
tionary Population Synthesis (EPS, {sc grasil}, {sc galev}, Maraston and {sc galaxev}) models, age, metallicity, and stellar evolution tracks over the near-UV---near infrared (NUV---NIR, 3500AA to 2.5mc) spectral region. Our main results are: as expected, young ($t lesssim$ 400 Myr) stellar population fractions derived in the optical cannot be directly compared to those derived in the NIR, and vice versa. In contrast, intermediate to old age ($t gtrsim$ 500 Myr) fractions are similar over the whole spectral region studied. The metallicity has a negligible effect on the propagation of the stellar population fractions derived from NUV --- NIR. The same applies to the different EPS models, but restricted to the range between 3800 AA and 9000 AA. However, a discrepancy between {sc galev}/Maraston and {sc grasil}/{sc galaxev} models occurs in the NIR. Also, the initial mass function (IMF) is not important for the synthesis propagation. Compared to {sc starlight} synthesis results, our propagation predictions agree at $sim$95% confidence level in the optical, and $sim$85% in the NIR. {bf In summary, spectral fitting} performed in a restricted spectral range should not be directly propagated from the NIR to the UV/Optical, or vice versa. We provide equations and an on-line form ({bf Pa}nchromatic {bf A}veraged {bf S}tellar {bf P}opulation - paasp) to be used for this purpose.
We derive fundamental parameters of the embedded cluster DBSB48 in the southern nebula Hoffleit18 and the very young open cluster Trumpler14, by means of deep JHKs infrared photometry. We build colour-magnitude and colour-colour diagrams to derive re
ddening and age, based on main sequence and pre-main sequence distributions. Radial stellar density profiles are used to study cluster structure and guide photometric diagram extractions. Field-star decontamination is applied to uncover the intrinsic cluster sequences in the diagrams. Ages are inferred from K-excess fractions. A prominent pre-main-sequence population is present in DBSB48, and the K-excess fraction f_K=55+/-6% gives an age of 1.1+/-0.5Myr. A mean reddening of A_K_s=0.9+/-0.03 was found, corresponding to $A_V=8.2pm0.3$. The cluster CMD is consistent with the far kinematic distance of 5 kpc for Hoffleit 18. For Trumpler 14 we derived similar parameters as in previous studies in the optical, in particular an age of $1.7pm0.7$ Myr. The fraction of stars with infrared excess in Trumpler 14 is $f_K=28pm4%$. Despite the young ages, both clusters are described by a King profile with core radii $rc=0.46pm0.05$ pc and $rc=0.35pm0.04$ pc, respectively for DBSB 48 and Trumpler 14. Such cores are smaller than those of typical open clusters. Small cores are probably related to the cluster formation and/or parent molecular cloud fragmentation. In DBSB 48, the magnitude extent of the upper main sequence is $Delta ksapprox2$ mag, while in Trumpler 14 it is $Delta ksapprox5$ mag, consistent with the estimated ages.
