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[24.06] Primordial Stars: The Next Generation

N. Panagia (ESA/STScI), M. Stiavelli, H.C. Ferguson, H.S. Stockman (STScI)

We present some results of a study on the expected properties of the first stellar generations in the Universe. In particular, we consider and discuss the diagnostics, based on the emission from associated HII regions, that permit one to discern bona fide primeval stellar generations from the ones formed after pollution by supernova explosions.

Since the main cooling processes in a low metallicity (Z\odot/100) regime are f-f and f-b transitions of ionized hydrogen and helium, plus collisional excitation of Lyman-\alpha and other hydrogen lines, the electron temperatures in primordial HII regions are expected to be virtually independent of the exciting star properties and the metal abundances, and to exceed 20000~K. As a consequence, collisional excitation processes become much more important than they are in the local Universe, and all emission lines are expected to be much stronger relative to the corresponding predictions from classical nebular theory at solar metallicity.

We show that a proper characterization of truly primeval stellar generations must be based on the simultaneous presence of high intensities and equivalent widths of non-resonant lines of Hydrogen and Helium, such as H\alpha and HeII 4686Å, and the absence of collisionally excited metal lines, mostly from the first heavy elements to be synthetized in large quantities, i.e. C and O, which are expected to have lines such CIII]~1909Å, OIII]~1666Å, [OIII]~5007Å, etc., with intensities above 10% the intensity of H\beta already for metallicities as low as 10^-3 solar.

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