Blue giants and the age calibration of Magellanic Cloud clusters

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Session 49 -- Star Clusters in the Milky Way and Other Galaxies
Display presentation, Thursday, June 15, 1995, 9:20am - 4:00pm

[49.02] Blue giants and the age calibration of Magellanic Cloud clusters

E.K.Grebel (AI\,U\,Bonn), Wm.J.Roberts (Baltimore), W.Brandner (AI W\"urzburg)

The apparent upper main sequence of the young SMC cluster NGC 330 comprises $\approx 30$ stars more scattered in color than the main sequence (MS) stars immediately below them. Identifying these stars with the MS turnoff region is not consistent with the location of the blue and red supergiants. Fitting both apparent upper MS and supergiant loci, an age spread of 15 Myr for semiconvective models or 38 Myr for full overshoot was derived (Chiosi et al.\ 1995, A\&A 293, 710). Caloi et al.\ (1993, A\&A 271, 109) find combined high T$_{\rm eff}$ and low $\log$ g values from IUE spectra of ``upper MS'' stars in NGC 330, These and other properties cannot be reconciled with their being MS stars in the sense of being H burning. Our own spectral classifications and those of Lennon et al.\ (1994, Space Sci.\ Rev.\ 66, 169) show that these stars in fact belong to luminosity class II to IV. We find the earliest %main sequence MS stars to be B0\,V stars at V=15.6 in very good agreement with our isochrone fitting (Grebel et al.\ 1994, BAAS 26, 1397), while the previously assumed turnoff was at V=14.3.

We identify some of the blue giants as Be stars, but the majority of them does not currently show H$\alpha$ emission. We suggest that the blue giants may be (1) binaries with a complicated history of mass transfer making them more massive, or (2) evolved, He-burning post-red supergiants. In an HR diagram these stars lie in the Hertzsprung gap, so statistically many fewer stars should be found there if they were evolving toward their first red supergiant phase.

In young LMC clusters like NGC 2004 and NGC 1818, we find also several blue stars above the MS turnoff determined by isochrone fitting. %A study of IUE spectra of these stars In NGC 2004, Caloi \& Cassatella (1995, A\&A 295, 63) find high T$_{\rm eff}$ and low $\log$ g for them from IUE spectra. We suggest that the (1) ages of young MC clusters based on turnoff fitting have been systematically underestimated, (2) claimed large age spreads may not exist, and (3) evolutionary models for supergiants may show a too small blue loop excursion.

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