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The Genesis of Super Star Clusters as Self-Gravitating HII Regions.pdf

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The Genesis of Super Star Clusters as Self-Gravitating HII Regions

a r X i v : a s t r o - p h / 0 0 1 2 0 0 5 v 1 1 D e c 2 0 0 0 The Genesis of Super Star Clusters as Self-Gravitating HII Regions Jonathan C. Tan1 and Christopher F. McKee2 1 Dept. of Astronomy, UC Berkeley, Berkeley, CA 94720, USA 2 Depts. of Physics and of Astronomy, UC Berkeley, Berkeley, CA 94720, USA Abstract. We examine the effects of ionization, radiation pressure and main sequence winds from massive stars on self-gravitating, clumpy molecular clouds, thereby mod- eling the formation and pre-supernova feedback of massive star clusters. We find the process of “turbulent mass loading” is effective in confining HII regions. Extrapolating typical Galactic high-mass star forming regions to greater initial gas cloud masses and assuming steady star formation rates, we determine the timescales for cloud disruption. We find that a dense (nc ? 2 × 10 5 cm?3) cloud with initial mass Mc ? 4 × 10 5 M⊙ is able to form ～ 2 × 105 M⊙ of stars (50% efficiency) before feedback disperses the gas after ～ 3 Myr. This mass and age are typical of young, optically visible super star clusters (SSCs). The high efficiency permits the creation of a bound stellar system. 1 Introduction Most Galactic stars are born in highly clustered regions [12,5], where the disrup- tive effects of massive stars are paramount. The multitude of dusty high redshift sources and the intensity of the far infrared background they produce also imply that a major fraction (～ 1/2) of total cosmic star formation has occurred in star- bursts, replete with massive stars [20]. A significant fraction of star formation in local starbursts occurs via the creation of super star clusters (SSCs) [22,24], each with hundreds to thousands of OB stars crammed into a few parsecs. At least some SSCs are gravitationally bound [8,9] and their masses (～ 105 ? 106 M⊙ [19,15]) and sizes suggest we may have found globular clusters in their infancy. Massive stars violently disrupt their surroundings with ionizing and non- ionizin