Browsing by Subject "stellar clusters"

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Fragmentation And Evolution Of Molecular Clouds. II. The Effect Of Dust Heating
(2010-02) Urban, Andrea; Martel, Hugo; Evans, Neal J.; Urban, Andrea; Evans, Neal J.
We investigate the effect of heating by luminosity sources in a simulation of clustered star formation. Our heating method involves a simplified continuum radiative transfer method that calculates the dust temperature. The gas temperature is set by the dust temperature. We present the results of four simulations; two simulations assume an isothermal equation of Stateand the two other simulations include dust heating. We investigate two mass regimes, i. e., 84 M(circle dot) and 671 M(circle dot), using these two different energetics algorithms. The mass functions for the isothermal simulations and simulations that include dust heating are drastically different. In the isothermal simulation, we do not form any objects with masses above 1 M(circle dot). However, the simulation with dust heating, while missing some of the low-mass objects, forms high-mass objects (similar to 20 M(circle dot)) which have a distribution similar to the Salpeter initial mass function. The envelope density profiles around the stars formed in our simulation match observed values around isolated, low-mass star-forming cores. We find the accretion rates to be highly variable and, on average, increasing with final stellar mass. By including radiative feedback from stars in a cluster-scale simulation, we have determined that it is a very important effect which drastically affects the mass function and yields important insights into the formation of massive stars.
Fragmentation And Evolution Of Molecular Clouds. III. The Effect Of Dust And Gas Energetics
(2012-09) Martel, Hugo; Urban, Andrea; Evans, Neal J.; Martel, Hugo; Evans, Neal J.
Dust and gas energetics are incorporated into a cluster-scale simulation of star formation in order to study the effect of heating and cooling on the star formation process. We build on our previous work by calculating separately the dust and gas temperatures. The dust temperature is set by radiative equilibrium between heating by embedded stars and radiation from dust. The gas temperature is determined using an energy-rate balance algorithm which includes molecular cooling, dust-gas collisional energy transfer, and cosmic-ray ionization. The fragmentation proceeds roughly similarly to simulations in which the gas temperature is set to the dust temperature, but there are differences. The structure of regions around sink particles has properties similar to those of Class 0 objects, but the infall speeds and mass accretion rates are, on average, higher than those seen for regions forming only low-mass stars. The gas and dust temperature have complex distributions not well modeled by approximations that ignore the detailed thermal physics. There is no simple relationship between density and kinetic temperature. In particular, high-density regions have a large range of temperatures, determined by their location relative to heating sources. The total luminosity underestimates the star formation rate at these early stages, before ionizing sources are included, by an order of magnitude. As predicted in our previous work, a larger number of intermediate-mass objects form when improved thermal physics is included, but the resulting initial mass function (IMF) still has too few low-mass stars. However, if we consider recent evidence on core-to-star efficiencies, the match to the IMF is improved.
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.