Thermally Pulsing Asymptotic Giant Branch Stars in the Cluster NGC 419.

In this work we study the Thermally Pulsing Asymptotic Giant Branch (TP-AGB) stars in the Small Magellanic Cloud (SMC) star cluster NGC 419. This is the known star cluster which is richest in TP-AGB and carbon stars. The aim of this work is twofold: i) to test available stellar models originally calculated for field TP-AGB stars of the SMC, and ii) to calibrate the processes involved during TP-AGB phase (mass loss and third dredge-up) to reproduce the observed properties of TP-AGB stars of the cluster (star counts and luminosities). First we consider photometrical data by High Resolution Channel and by Wide Field Channel of the Advanced Camera for Surveys (respectively ACS/HRC and ACS/WFC) to estimate the global properties of the cluster (age, mass). From the color-magnitude diagram (CMD) of NGC 419 we can note some dispersion, especially in the main sequence and in the turn-off. This is likely due to rotation but, since this aspect is not treated in the models, in order to have a satisfactory description of star counts related to pre-AGB phases, we analyze it as due to an age spread. The population synthesis code used is TRILEGAL. First of all we use ACS/HRC data and we consider the isochrone fitting method. In the estimated age range we build the star formation history (SFH) of the cluster, trying to reproduce the luminosity function of the red clump region. In this way we are able to reproduce well the CMD of the red giants, and we expect to have a reasonable distribution of the core masses of stars entering the AGB phase. We are also able to estimate the stellar mass comprised in the HRC field. Afterwards we use ACS/WFC data to estimate the total mass of the cluster: we consider the number density profile in order to assess the background density and the radius of the cluster. Once determined the global properties of the cluster, in the second part of this work we consider near-IR data (2MASS and CASPIR photometry) of AGB stars of NGC 419. We first consider a complete list of AGB stars of the cluster and then we discard the ones which are outside the fixed radius and the ones which are below the tip of red giant branch. We test eighteen models for TP-AGB stars, obtaining median values for the adopted quantities and 1σ error bars using the cumulative function. A general result of all models is a deficit of predicted M-stars and a too faint M/C transition luminosity (these quantities are mainly related to the onset of third dredge-up). There is instead a good agreement of most of the models with the data in reproducing the number of C-stars and the AGB tip luminosity. We discuss the three most different models. We analyze their diffencences concerning mass loss regimes, characteristic of third dredge-up and the Hertzsprung-Russel diagrams. The general aspect required by all models for a better description of the data and for a following work is a later onset of the third dredge-up which would imply a longer M-phase and a brighter M/C transition luminosity