A new giant branch clump structure in the Large Magellanic Cloud

We present Washington C and T-1 CCD photometry of 21 fields located in the northern part of the Large Magellanic Cloud (LMC), and spread over a region of more than 2.5 deg(2) approximately 6 degrees from the bar. The surveyed areas were chosen on the basis of their proximity to SL 388 and SL 509, wh ose fields showed the presence of a secondary giant clump, observationally detected by Bica et al. We also observed NGC 2209, located similar to 14 de grees away from SL 509. From the collected data, we found that most of the observed held CMDs do not show a separate secondary clump, but rather revea l a continuous vertical structure (VS), which is clearly seen for the first time. The VS also appears in the held of NGC 2209. Its position and size a re nearly the same throughout the surveyed regions: it lies below the red g iant clump (RGC) and extends from the bottom of the RGC to similar to 0.45 mag fainter, spanning the bluest color range of the RGC. In two fields in a ddition to the NGC 2209 held the RGC is slightly tilted, following approxim ately the reddening vector, while the VS maintains its verticality. We foun d that the number of stars in the VS box defined by Delta(C - T-1) = 1.45-1 .55 mag and Delta T-1 = 18.75-19.15 mag has a strong spatial variation, rea ching the highest VS star density just northeast of SL 509. Moreover, the m ore numerous the VS stars in a held, the larger the number of LMC giants in the same zone. We also found that, in addition to SL 509, two relatively m assive star clusters, SL 515 and NGC 2209, separated by more than 10 degree s from each other, develop giant clumps with a considerable number of VS st ars. 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, particul arly in those LMC regions with a noticeable large giant population. Our res ults are successfully predicted by the models of Girardi in the sense that a large proportion of 1-2 Gyr old stars mixed with older stars and with met allicities higher than [Fe/H] similar or equal to - 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 s tar density. Indeed, stars satisfying the requisites mentioned above are co mmonly found throughout the LMC, but the VS phenomenon is only clearly seen in some isolated regions. Finally, the fact that clump stars have an intri nsic luminosity dispersion further constrains the use of the clump magnitud e as a reliable distance indicator.