Análise astrométrica e espectroscópica de estrelas do aglomerado jovem NGC 6530

Abstract

In this work, we have studied NGC 6530, a young (∼ 2.3 Myr) open cluster located at a distance of 1325 pc from the Sun. The study of young clusters provides crucial information about the star formation process, because they harbor from tens to thousands of stars, like the Classical T Tauri Stars (CTTSs), which are young low mass stars surrounded by a circumstellar disc from which they accrete gas and with which they interact. CTTSs present photometric and spectroscopic variability and their spectra have intense and wide emissions from the Hα line originated in the circumstellar environment. NGC 6530 was observed for 2 years by the Gaia satellite and with these data we have conducted a study of cluster membership based on measurements of proper motion and parallax of stars in the NGC 6530 region. We have found 86 members for the cluster using decontamination methods and isochrones fitting and we obtained the distance modulus, the color excess and the age of the cluster, as (m − M)o = 10.55, with errors +0.25 and −0.30 , E(B−V)= 0.48 ± 0.07 and 2.8 Myrs, with the value varying between 2.0 and 3.5 Myrs, respectively. NGC 6530 was also observed in 2016 by the VLT (ESO) telescope equipped with the FLAMES spectrograph, enabling a spectroscopic study of several stars in the cluster. Our spectroscopic sample consists of 61 stars, ranging from spectral type K4 to F1. Among these, 17 were classified as members by our astrometric analysis. We have carried out a study of the characteristics and variability of the Hα emission line for each of the 61 stars. We have removed the nebular contribution from the Hα line and estimated the radial velocity from the photospheric lines present in the stellar spectra. We have calculated the equivalent width and the width at 10% of the maximum intensity of the Hα line. With these parameters, 54% of the stars in our sample were classified as CTTSs and 24% as weak-line T Tauri stars (WTTSs). We were not able to classify 21% of our targets due to the presence of strong nebular contribution and/or uncertainties in the removal of the nebular contribution. We have searched for periodic variations in the Hα line of the CTTSs and have shown that most of these stars have no periodicity in this line. We have morphologically classified the average Hα line profiles and have showen that our CTTSs sample has the Hα line profile dominated by magnetospheric accretion. We have calculated the equivalent width of the LiI line and related it to the radial velocities and to the astrometric study, to analyse the membership to NGC 6530 of the stars observed with the VLT/FLAMES. We have determined 16 more stars in our sample as possible members of the NGC 6530 cluster, since the parameters obtained with the spectroscopy are compatible with the cluster data.