Massive pre-main sequence stars in the Magellanic Clouds

Abstract

We give an overview of the search and discovery ofSequence (PMS)stars in the Magellanic Clouds. For the first time theirregular optical brightness variability of Pre-Main Sequence stars,generally attributed to variable dust obscuration of the central star,has been used as an initial selection of young stellar objects.We discuss 21 PMS candidates in the Large Magellanic Cloud (LMC)and 7 PMS candidates in the Small Magellanic Cloud (SMC) detectedin this way. These stars have bolometric luminosities whichcorrespond to the most massive Galactic Herbig AeBe (HAeBe) stars, or evenmore massive.The location of the LMC and SMC objects is correlated withhigher densities of cold interstellar dust as measured by IRAS inthe far infrared. We derive the fundamental parameters fromspectroscopy and find that there location in the HR-diagram would indicatethat the stars are more luminous when compared to Galactic HAeBe stars ofthe same spectral type. There may be a trend with metallicity, in that theSMC PMS stars can be even more luminous than the LMC PMS stars. Itindicates a possible increase of the proto-stellar accretion rate withdecreasing metallicity.In the optical these HAeBe candidates have a higher probabilityto be found in small (10 pc) clusters then other field stars of similarcolour and magnitude. We present a case study of a cluster aroundone of the LMC HAeBe stars, using high resolution SUSI imaging. TheHα-emitters detected in this cluster are also located in anHR-diagram above the Galactic Palla-Stahler birthline corresponding to anaverage protostellar mass accretion rate of 10^-5 M_⊙ yr^-1.