On the nature of the blue giants in NGC 330

Abstract

The young SMC cluster NGC 330 contains a number of blue stars that lie above the main-sequence turnoff found from our isochrone fitting and below the position of the blue supergiants. We used our own, new spectroscopy and published data on these stars to investigate their possible nature. Problems in interpreting the evolutionary status of the blue giants have been found in several preceding studies. In theoretical H-R diagrams, these stars lie in the rapidly traversed post main-sequence gap, similar to the unexpected concentration found by Fitzpatrick & Garmany (1990) in the H-R diagram of the LMC. We argue that these stars probably are core H burning main-sequence stars that appear as blue stragglers resulting from binary evolution as described in the simulations of Pols \& Marinus (1994) and effects of rapid rotation. Many of the blue stragglers are Be stars and likely rapid rotators. We suggest that there is evidence for the presence of blue stragglers also in NGC 1818, NGC 2004, and NGC 2100. We point out that blue stragglers may be a general phenomenon in the CMDs of young clusters in the Magellanic Clouds and discuss the implications for IMF and age determinations.