A New Giant Branch Clump Structure Towards the Large Magellanic Cloud

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

Estados Unidos

Congreso; 194th Meeting of the American Astronomical Society; 1999

We present Washington C,T_1 CCD photometry of 21 fields located in the northern part of the Large Magellanic Cloud (LMC), and spread over a region of nearly 2({Box }({o})) ( ~ 6(o) 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 ( ~ 10(o) 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 Main Giant Clump (MGC) and extends from the bottom of the MGC to ~ 0.45 mag fainter, spanning the bluest color range of the MGC. Moreover, VS stars are uniformly distributed in their corresponding fields. In two fields, as well as in the NGC 2209 field, the MGC is slightly tilted, following approximately the reddening vector, while the VS maintains its verticality. Finally, assuming that there is roughly the same number of MGC stars in each field, we found that the number of stars in the box defined by Delta (C-T_1)=1.44-1.53 mag and Delta T_1=18.8-19.7 mag has a strong spatial variation, reaching the highest VS star density just East of SL 509. Further studies are required in order to determine whether the VS is caused by the presence of an old stellar population or by an evolutionary effect as predicted by the theoretical models of Girardi et al. 1998 MNRAS 301, 723, or even by a layer of stars located behind the LMC.