Detección de una nueva fase en la evolución de estrellas gigantes rojas de la Nube Mayor de Magallanes

PIATTI, ANDRÉS E.; GEISLER, DOUG; BICA, EDUARDO; CLARIA OLMEDO, JUAN JOSÉ; SANTOS JR, JOÂO F.C.; SARAJEDINI, A.; DOTTORI, HORACIO

Rosario

Congreso; Reunión Anual de la Asoc. Argentina de Astronomía; 1999

We present Washington C,T1 CCD photometry of 21 fields located in the northern part of the Large Magellanic Cloud (LMC), and spread over a region of nearly 2o (~ 6o from the bar). The surveyed areas were chosen on the basis of the presence of a secondary giant clump in the CMDs of the fields of SL 388 and SL 509, observationally detected by Bica et al., 1998, AJ 116, 723. From the collected data we found that most of the observed field CMDs do not show a separate secondary clump, but rather a vertical structure (VS). The VS also appears in the field of NGC 2209 (~14o far away), but it is negligible in the remaining observed cluster fields. Its position and size are nearly the same throughout the surveyed regions: it lies below the Red Giant Clump (RGC) and extends from the bottom of the RGC to ~ 0.45 mag fainter, spanning the bluest color range of the RGC. Our results demonstrate that the VS stars belong to the LMC and that they are not composed of old objects in the LMC or of background population of RGC stars. The existence of VS stars in some relatively massive clusters, like NGC 2209, is also a clear indicator of the evolutionary origin of VS stars. We also determined that VS stars are only clearly found in those fields which satisfy some particular conditions, such as containing a significant number of 1-2 Gyr old stars and with metallicities higher than [Fe/H]≈ -0.7. These conditions constrain the VS phenomenon to clearly appear only in some isolated parts of the LMC, particularly those with a noticeable large giant population. However, a large number of RG stars, in addition to age and metallicity, is not a sufficient requisite for finding VS stars. Nevertheless, the non-uniform spatial distribution of VS stars in the LMC reveals that non-homogeneously distributed star formation events occurred in this galaxy about 1-2 Gyrs ago.