Super Star Clusters

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Session 37 -- Massive Hot Stars with the Hubble Space Telescope Part II
Oral presentation, Tuesday, 31, 1994, 2:00-5:30

[37.03] Super Star Clusters

R.W. O'Connell (UVa)

Super star clusters represent an extreme in the star formation process. They are very luminous, compact objects with $L_V > 10^6 L_{V,\odot}$ and diameters $\leq 10$ pc. Their surface brightnesses are $\geq 100$ times higher than normal OB associations and clusters in ``giant H II regions''. Prior to HST about a dozen such objects had been identified in nearby galaxies, but at ground-based resolution they are nearly point sources. We review recent HST observations of individual super star clusters in NGC 1140, 1569, and 1705. They have half-light radii of only 2--3.5 pc, and some show evidence of substructure which should be resolvable with the repaired HST. After allowing for age differences, the surface brightness of NGC 1569-A is over 65 times higher than the core of 30 Doradus in the LMC and 1200 times higher than the mean rich LMC star cluster. In some cases, the energy released by the clusters into their surroundings is sufficient to drive galaxy-wide winds. Their properties make super star clusters good analogues of young globular clusters.

In some, though not all, cases super star clusters appear to form in the aftermath of a merger or accretion event. The most impressive examples are the clusters detected by HST in NGC 1275 and 7252, one of which has the extraordinary luminosity $\sim 6 \times 10^8 L_{V,\odot}$. M82 affords a nearby view of a post-interaction system. HST imaging has identified over 80 super star clusters in its central regions with mean luminosities of $\sim 3 \times 10^6 L_{V,\odot}$. Their close packing and signs of interaction with the well-known supernova-driven wind suggest that they do not evolve independently. Super cluster evolution in starbursts is probably a collective phenomenon.

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