Physical parameters of rich LMC clusters from modeling of deep HST colour–magnitude diagrams

Abstract

Abstract.We present the analysis of deep colour-magnitude diagrams (CMDs) of five rich LMC clusters. The data were obtained with HST/WFPC2 in the F555W (∼V) and F814W (∼I) filters, reaching V 555 ∼ 25. The sample of clusters is composed of NGC 1805 and NGC 1818, the youngest ones (τ < 100 Myr), NGC 1831 and NGC 1868, of intermediate-age (400 < τ < 1000 Myr), and Hodge 14, the oldest (τ > 1200 Myr). We discuss and apply a statistical method for correcting the CMD for sampling incompleteness and field star contamination. Efficient use of the CMD data was made by means of direct comparisons of the observed to model CMDs. The CMD modeling process generates a synthetic Main Sequence (MS), where we introduce as model inputs the information about age, chemical composition, present day mass function (PDMF), fraction of unresolved binaries, distance modulus and light extinction. The photometric uncertainties were empirically determined from the data and incorporated into the model as well. Statistical techniques of CMD comparisons using 1 and 2 dimensions are presented and applied as an objective method to assess the compatibility between model and data CMDs. By modeling the CMDs from the central region we infer the metallicity (Z), the intrinsic distance modulus ((m− M) 0 ) and the reddening value (E(B− V)) for each cluster. We also determined the age for the clusters with τ > 400 Myr. By means of two-dimensional CMD comparisons we Taking into account the uncertainties, these values are in accordance with the ones obtained applying the one-dimensional CMD analysis, adding reliability to these determinations.

ASteCA: Automated Stellar Cluster Analysis

“…This can be seen for example in the series of articles by Kerber et al (Kerber et al 2002;Kerber & Santiago 2005) where an estimate of the density of field stars in the cluster region is used to implement a field star removal process together with a cluster CMD modeling strategy that selects the best observed vs. artificial fit via a statistical tool; the white dwarf based Bayesian CMD inversion technique developed in von Hippel et al (2006) expanded and coupled with a basic field star cleaning process in van Dyk et al (2009); the synthetic cluster fitting method introduced in Monteiro et al (2010;MDC10, further developed in the articles Dias et al 2012;Oliveira et al 2013), which includes a likelihood-based decontamination algorithm; the work by Pavani et al (2011) where CMD density-based membership probabilities are given to stars within the cluster region to later apply a very basic isochrone fitting process that makes use of stars close to a given isochrone in CMD space; and the articles by Alves et al (2012) and Dias et al (2014), who employ the same membership probability assignment method used in Pavani et al (2011) coupled with a slightly improved isochrone fitting algorithm based on the one developed by MDC10, but applied to a Hess diagram of the CMD instead of the full CMD. In Buckner & Froebrich (2013) the membership assignment method presented in Froebrich et al (2010), a variation of the Bonatto & Bica (2007) algorithm, is used in conjunction with the Besançon model of the galaxy 16 to derive distances to OCs, based on foreground stars density estimations.…”

SMC west halo: a slice of the galaxy that is being tidally stripped?

“…This method was extensively explained in previous works of our group, where it was applied to LMC clusters observed with HST/WFPC2 (Kerber et al 2002(Kerber et al , 2007Kerber & Santiago 2005) and Galactic open clusters from 2MASS (Alves et al 2012). In Paper I we analysed the CMDs of five SMC stellar clusters observed with SOAR/SOI using the same setup as in this paper.…”

“…5 we show the completeness curves for the reference cluster NGC 152 for different radii and V magnitudes; the curves for the other clusters are presented in Appendix A. Cluster member stars were estimated statistically following the method of Paper I developed by Kerber & Santiago (2005). The idea is to compare the CMD of stars in the direction of the cluster with another made of nearby field stars.…”

Hot subdwarfs from the stable Roche lobe overflow channel

“…These results are similar to ours, but binary evolution calculations with the onset of mass transfer at the main sequence stage have been included in our models. Elson et al (1998) discovered a blue star with T eff ≈ 31 500 K, log(g) ≈ 4.4, logL/L ∼ 3.0 (Burleigh et al 1999) in the young cluster NGC 1818 in the Large Magellanic Cloud (LMC), which has a main-sequence turnoff mass of ∼7.5-9.5 M , an age of ∼2-4 × 10 7 yr ) and a metallicity of ∼0.005 (Kerber & Santiago 2005). This star was considered as a candidate of luminous WD.…”