Browsing by Subject "stars: luminosity function, mass function"

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Gravitational Fragmentation In Turbulent Primordial Gas And The Initial Mass Function Of Population III Stars
(2011-02) Clark, Paul C.; Glover, Simon C. O.; Klessen, Ralf S.; Bromm, Volker; Bromm, Volker
We report results from numerical simulations of star formation in the early universe that focus on the dynamical behavior of metal-free gas under different initial and environmental conditions. In particular we investigate the role of turbulence, which is thought to ubiquitously accompany the collapse of high-redshift halos. We distinguish between two main cases: the birth of Population III. 1 stars-those which form in the pristine halos unaffected by prior star formation-and the formation of Population III. 2 stars-those forming in halos where the gas has an increased ionization fraction. We find that turbulent primordial gas is highly susceptible to fragmentation in both cases, even for turbulence in the subsonic regime, i.e., for rms velocity dispersions as low as 20% of the sound speed. Fragmentation is more vigorous and more widespread in pristine halos compared to pre-ionized ones. If such levels of turbulent motions were indeed present in star-forming minihalos, Population III. 1 stars would be on average of somewhat lower mass, and form in larger groups, than Population III. 2 stars. We find that fragment masses cover over two orders of magnitude, suggesting that the Population III initial mass function may have been much broader than previously thought. This prompts the need for a large, high-resolution study of the formation of dark matter minihalos that is capable of resolving the turbulent flows in the gas at the moment when the baryons become self-gravitating. This would help to determine the applicability of our results to primordial star formation.
The Physics of Crystallization from Globular Cluster White Dwarf Stars in NGC 6397
(2009-03) Winget, D. E.; Kepler, S. O.; Campos, Fabiola; Montgomery, M. H.; Girardi, Leo; Bergeron, P.; Williams, Kurtis; Winget, D. E.; Montgomery, M. H.; Williams, Kurtis
We explore the physics of crystallization in the deep interiors of white dwarf (WD) stars using the color-magnitude diagram and luminosity function constructed from proper- motion cleaned Hubble Space Telescope photometry of the globular cluster NGC 6397. We demonstrate that the data are consistent with the theory of crystallization of the ions in the interior of WD stars and provide the first empirical evidence that the phase transition is first order: latent heat is released in the process of crystallization as predicted by van Horn. We outline how these data can be used to observationally constrain the value of Gamma = E(Coulomb)/E(thermal) near the onset of crystallization, the central carbon/oxygen abundance, and the importance of phase separation.