A new giant branch clump structure in the Large Magellanic Cloud

Abstract

We present Washington C and T1 CCD photometry of 21 Ðelds located in the northern part of the Large Magellanic Cloud (LMC), and spread over a region of more than 2.5 deg² approximately 6° from the bar. The surveyed areas were chosen on the basis of their proximity to SL 388 and SL 509, whose Ðelds showed the presence of a secondary giant clump, observationally detected by Bica et al. We also observed NGC 2209, located ~14° away from SL 509. From the collected data, we found that most of the observed Ðeld CMDs do not show a separate secondary clump, but rather reveal a continuous vertical structure (VS), which is clearly seen for the Ðrst time. The VS also appears in the field of NGC 2209. 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. In two Ðelds in addition to the NGC 2209 field the RGC is slightly tilted, following approximately the reddening vector, while the VS maintains its verticality. We found that the number of stars in the VS box deÐned by ∆(C−T1)=1.45−1.55 mag and ∆T1 18.75−19.15 mag has a strong spatial variation, reaching the highest VS star density just northeast of SL 509. Moreover, the more numerous the VS stars in a field, the larger the number of LMC giants in the same zone. We also found that, in addition to SL 509, two relatively massive star clusters, SL 515 and NGC 2209, separated by more than 10º from each other, develop giant clumps with a considerable number of VS stars. This result demonstrates that VS stars belong to the LMC and are most likely the result of some kind of evolutionary process in the LMC, particularly in those LMC regions with a noticeable large giant population. Our results are successfully predicted by the models of Girardi in the sense that a large proportion of 1È2 Gyr old stars mixed with older stars and with metallicities higher than [Fe/H]-~[0.7 should result in a fainter and bluer secondary clump near the mass at which degenerate core He burning takes place. However, our results apparently suggest that in order to trigger the formation of VS stars, there should be other conditions in addition to the appropriate age, metallicity, and the necessary red giant star density. Indeed, stars satisfying the requisites mentioned above are commonly found throughout the LMC, but the VS phenomenon is only clearly seen in some isolated regions. Finally, the fact that clump stars have an intrinsic luminosity dispersion further constrains the use of the clump magnitude as a reliable distance indicator.