Caracterização e evolução estrutural de aglomerados abertos da galáxia

Abstract

Open clusters are excellent tools for the investigation of large numberof astrophysical phenomena. While a cluster is still young, the gas anddust remaining from its formation makes difficult the identification of theemerging stellar population. After the ejection of the gas and dust fromits parental cloud, the surviving cluster orbits the Galactic disk graduallylosing its stars due to external forces and to its own dynamical evolution,until dispersion. In these final stages, the small number of cluster stars and the spatial fluctuations of the disk stellar population are the main limiting factors on the identification and characterization of open clusters. The field star contamination of the cluster stellar population is a constant difficulty in the study of open clusters. Consequently, we designed a method to separate stars pertaining to an open cluster from the field stars, based on the photometric differences between these populations. Tested on two well studied clusters and on a simulated cluster, the method showed consistent results, independently of the physicalproperties of the target or of the properties of its surrounding field. Theseresults pointed to a convergent parameter region, common to all targets studied. Its efficiency, however, depends on the selection of a field sample that is representative of the field population in the cluster region, according to the color-magnitude diagram stellar distribution.To contribute for a better understanding of the evolution of open clusters,we selected for our study two targets in very distinct evolutionary states.NGC1981 is a young open cluster that, despite still showing dust and gas remaining from its formation stage and having various massive stars, already shows signs of disruption. On the contrary, ESO442SC04 presents only a small stellar overdensity over the field and does not show any hint in its stellar population that allows it to be readily distinguished from the field. Investigation of NGC1981 with the photometric decontamination method revealed a large population of cluster pre-main sequence stars, detected for the first time. Solar metallicity isochrone fittings on color-magnitude diagrams built from optical and near-infrared data resulted an age t = 5 ± 1 Myr, distance d = 380±17 pc and reddening E(B-V ) = 0,07±0,03. Analysis of its structural parameters by fittings of King profiles revealed a compact (RC = 0,09 ± 0,04 pc) but of low density (0 = 2,4 ± 1,3 stars/arcmin2) core, presenting a limiting radius of Rlim = 1,21 ± 0,11 pc. The relaxation time, calculated from the stars kinematics, trelax = 0,42 Myr, shows that the cluster has had time to dynamically evolve, possibly leading to a core collapse and to the evaporation of stars. Indeed, the mass distribution of the cluster shows a depletion of low mass stars when compared to the Salpeters mass function. The derived total mass is small (M = 137 ± 14 M), and the kinetic and potential energies calculated for the stars inside the limiting radius indicate that the system is gravitationally unbound. In the detailed study of the stellar population of ESO442SC04, we combined photometric data (2MASS and SOI/SOAR), kinematic data (UCAC3) and spectroscopy of the target inner stars. Analysis of the stellar density profile confirms the presence of a small compact core, with an obvious stellar overdensity over the field. In the color-magnitude diagram, these stars distinguish themselves from an adjacent field mimicking the population of an old, poor populated cluster. We found, however, that the kinematics of the stars in the target surroundings do not reveal a co-moving population, when the fluctuations of the Galactic field proper motions are taken into account. Besides, the determined stellar parameters and the proper motions of its stars showed a large dispersion, particularly among the brighter stars. Finally, the distribution of the redder stars in the two-color diagram are different from that of the bluer stars, suggesting that they belong to distinct populations. We conclude that NGC1981 is a young open cluster about to disperse itself. The recent ejection of the gas remaining from its formation, allied to its small stellar population have generated a reduced gravitational potential, unable to hold its stellar population. We unveiled the physical nature of ESO442SC04 which, although similar to an open cluster remnant, has proved to be an asterism. We emphasize however that the analysis of this class of objects can be misleading when based solely on photometric or proper motion data.