Emission line stars in low metallicity environments Estrellas con emisión en ambientes de baja metalicidad

Abstract

The Large Magellanic Cloud (LMC) is an excellent laboratory to study the stellar evolution and the ongoing stellar formation in low metallicity environments. Its distance is relatively well known and there is only a small interstellar extinction toward the LMC. We present in this thesis the observational study of a sample of Emission-line stars (ELSs) in the LMC, for which we examine star formation episodes associated with advancing ionization fronts and expanding supergiant shells (SGSs) in local and larger scales. SGSs are the largest and most energetic shells in the LMC, and are therefore expected to have a dramatic e ect on the Interestellar Medium (ISM). Their regions of influence are large enough to accumulate the amount of material needed for molecular cloud formation. On the other hand, the cumulative energy input from stellar winds, supernovae, and ionizing radiation is able to form molecular clouds via the accumulation, compression, and cooling of the interstellar medium in giant (100 1000 pc) supershells around OB clusters. Thus, advancing ionization fronts and expanding supergiant shells or giant shells compress nearby clouds, increasing their density and causing the collapse of deeply embedded cores, leading to new star formation. In this thesis, we investigate this issue in the open cluster NGC 1850 and within its vicinity. Here, we found a possible relationship between the location of ELS and the ionized gas structures with the SGS complex. In addition to this and to make a more relevant study on this hypothesis and to determine the influence of SGS on new star-formation and evolutionary stage of these stellar populations, we used the instrument Wide Field Imager (WFI) in its slitless spectroscopic mode, as well as photometry from di erent database in order to identify the H emission-line objects (from low to high mass, not evolved to evolved) in the LMC star-formation region NGC 1850 and its surroundin field. In this region 64 H emission-line stars were identified and these ELS are concentrated inside or around the SGS borders. We present the results of a detailed spectroscopic and photometric study of two cases of interacting binaries in the LMC bar, including the analysis of high-resolution spectra and the application of a sophisticated light curve model for the systems. In the first case, new high-resolution spectroscopic data were obtained for OGLE05155332����6925581, one of the brightest members in the LMC of the enigmatic group of the Double Period Variables (DPV). We found that the system is best modelled with a geometrically thin and optically thick disc around the primary star. The analysis of the photometric data has allowed us to derive improved orbital parameters and stellar parameters for this DPV system. Taking advantage of our new and improved system and stellar parameters, we explored the evolutionary stage of OGLE05155332����6925581 with the aid of published grids of evolutionary routes for binary systems of similar masses, considering conservative and non-conservative evolution. A comparison with these synthetic binary-star evolutionary models indicates that the system has an age of 4:76 107 yr, and is in the phase of rapid mass transfer, the second one in the life of this binary. This evolutionary stage is consistent with the existence of the circumprimary accretion disc found for the system, but the model that best fits the observations shows the system with a relatively large mass transfer rate of ˙M = 3:1 10����6 M yr����1. However, the orbital period remains relatively stable during almost 15 yr. We speculate that outflows could be extracting angular momentum from the system, keeping constant the orbital period. Furthermore, we show that under certain conditions of mass and angular momentum loss, the system orbital period can be kept constant even in high mass transfer rate regimes. In the second case, we establish the true nature of the luminous star ELHC 10, concluding that it is a long-period eclipsing binary composed of a post-asymptotic giant branch (post-AGB) star showing signatures of s-process nucleosynthesis, and an unseen early B-type stellar companion surrounded by an eclipsing circumstellar disc. Additional to this, we discovered line splitting of metallic lines in ELHC 10, characterized by discrete absorption components observed alternatively at the blue and red side of the photospheric line profiles during the orbital cycle, which can be interpreted as evidence for gas streams, leading to the formation of a circumstellar disc and a circumbinary disc in the system. Our study provides evidence that circumbinary discs can be formed by binary star interaction and outflows in post-AGB stars. It also suggests that low-mass post-AGB stars can be formed by an evolutionary channel di erent from single stars, specifically by depletion of an initially massive star by mass transfer in a semi-detached binary system. As a result of this thesis, three papers were produced, one of them is already published “Physical parameters and evolutionary route for the Large Magellanic Cloud interacting binary OGLE 05155332����6925581”, one has been submitted “The complex eclipsing binary ELHC 10 to post-AGB candidate in the Large Magellanic Cloud” and other is in the final stage of preparation “The LMC NGC 1850 region history: Emission-line stars as tracers of stars formation episodes.”