A New Giant Branch Clump Structure in the Large Magellanic Cloud

We present Washington C, T1 CCD photometry of 21 fields located in thenorthern part of the Large Magellanic Cloud (LMC), and spread over a region ofmore than 2.52 degrees approximately 6 degrees from the bar. The surveyed areaswere chosen on the basis of their proximity to SL 388 and SL 509, whose fieldsshowed the presence of a secondary giant clump, observationally detected byBica et al. (1998, AJ, 116, 723). From the collected data we found that most ofthe observed field CMDs do not show a separate secondary clump, but rather acontinuous vertical structure (VS), which is clearly seen for the first time.Its position and size are nearly the same throughout the surveyed regions: itlies below the Red Giant Clump (RGC) and extends from the bottom of the RGC toapproximately 0.45 mag fainter, spanning the bluest color range of the RGC. Themore numerous the VS stars in a field, the larger the number of LMC giants inthe same zone. Our analysis demonstrate that VS stars belong to the LMC and aremost likely the consequence of some kind of evolutionary process in the LMC,particularly in those LMC regions with a noticeable large giant population. Ourresults suggest that in order to trigger the formation of VS stars, thereshould be other conditions besides the appropriate age, metallicity, and thenecessary red giant star density. Indeed, stars satisfying the requisitesmentioned above are commonly found throughout the LMC, but the VS phenomenon isonly clearly seen in some isolated regions. Finally, the fact that clump starshave an intrinsic luminosity dispersion further constrains the use of the clumpmagnitude as a reliable distance indicator.Comment: 25 pages, 9 figures, 3 tables; to be published in AJ, Dec. issu