arXiv subjects
Publications and source records attributed to Richard B. Larson.
Paramount among the processes that ended the cosmic `dark ages' must have been the formation of the first generation of stars (the so-called Population III). We summarize recent progress in constraining its nature, and we discuss the basic physical reasons for why the first stars might have been quite massive, possibly even very massive with M > 100 M_sun.
To constrain the nature of the very first stars, we investigate the collapse and fragmentation of primordial, metal-free gas clouds. We explore the physics of primordial star formation by means of three-dimensional simulations of the dark matter and gas components, using smoothed particle hydrodynamics, under a wide range of initial conditions, including the initial spin, the total mass of the halo, the redshift of virialization, the power spectrum of the DM fluctuations, the presence of HD cooling, and the number of particles employed in the simulation. We find characteristic values for the temperature, T ~ a few 100 K, and the density, n ~ 10^3-10^4 cm^-3, characterising the gas at the end of the initial free-fall phase. These values are rather insensitive to the initial conditions. The corresponding Jeans mass is M_J ~ 10^3 M_sun. The existence of these characteristic values has a robust explanation in the microphysics of H2 cooling, connected to the minimum temperature that can be reached with the H2 coolant, and to the critical density at which the transition takes place betweeb levels being populated according to NLTE, and according to LTE. In all cases, the gas dissipatively settles into an irregular, central configuration which has a filamentary and knotty appearance. The fluid regions with the highest densities are the first to undergo runaway collapse due to gravitational instability, and to form clumps with initial masses ~ 10^3 M_sun, close to the characteristic Jeans scale. These results suggest that the first stars might have been quite massive, possibly even very massive with M_star > 100 M_sun.
This contribution summarizes briefly the main topics covered at this wide-ranging conference. Much of the evidence presented indicates that star formation occurs in discrete episodes or bursts that produce stellar groupings of all sizes, the fossil remnants of which make up the present stellar populations of galaxies.
The overall frequency and other statistical properties of binary systems suggest that star formation is intrinsically a complex and chaotic process, and that most binaries and single stars actually originate from the decay of multiple systems. Interactions between stars forming in close proximity to each other may play an important role in the star formation process itself, for example via tidally induced accretion from disks. Some of the energetic activity of newly formed stars could be due to bursts of rapid accretion triggered by interactions with close companions.
The first bound star-forming systems in the universe are predicted to form at redshifts of about 30 and to have masses of the order of 10^6 M_sun. Although their sizes and masses are similar to those of present star-forming regions, their temperatures are expected to be much higher because cooling is provided only by trace amounts of molecular hydrogen. Several recent simulations of the collapse and fragmentation of primordial clouds have converged on a thermal regime where the density is about 10^3-10^4 cm^{-3} and the temperature is about 300 K; under these conditions the Jeans mass is of the order of 10^3 M_sun, and all of the simulations show the formation of clumps with masses of this order. The temperatures in these clumps subsequently rise slowly as they collapse, so little if any further fragmentation is expected. As a result, the formation of predominantly massive or very massive stars is expected, and star formation with a normal present-day IMF seems very unlikely. The most massive early stars are expected to collapse to black holes, and these in turn are predicted to end up concentrated near the centers of present-day large galaxies. Such black holes may play a role in the origin of AGNs, and the heavy elements produced by somewhat less massive stars also formed at early times may play an important role in chemically enriching the inner parts of large galaxies and quasars.
In order to constrain the initial mass function (IMF) of the first generation of stars (Population III), we investigate the fragmentation properties of metal-free gas in the context of a hierarchical model of structure formation. We investigate the evolution of an isolated 3-sigma peak of mass 2x10^6 M_solar which collapses at z_coll=30 using Smoothed Particle Hydrodynamics. We find that the gas dissipatively settles into a rotationally supported disk which has a very filamentary morphology. The gas in these filaments is Jeans unstable with M_J~10^3 M_solar. Fragmentation leads to the formation of high density (n>10^8 cm^-3) clumps which subsequently grow in mass by accreting surrounding gas and by merging with other clumps up to masses of ~10^4 M_solar. This suggests that the very first stars were rather massive. We explore the complex dynamics of the merging and tidal disruption of these clumps by following their evolution over a few dynamical times.
How did star formation begin in the universe? Some of the questions addressed at this first meeting on "The First Stars" are summarized here from a theoretical perspective, and some brief comments are made on what we may have learned so far.
The current status of both the observational evidence and the theory of the stellar initial mass function (IMF) is reviewed, with particular attention to the two basic, apparently universal features shown by all observations of nearby stellar systems: (1) a characteristic stellar mass of the order of one solar mass, and (2) a power-law decline of the IMF at large masses similar to the original Salpeter law. Considerable evidence and theoretical work supports the hypothesis that the characteristic stellar mass derives from a characteristic scale of fragmentation in star-forming clouds which is essentially the Jeans scale as calculated from the typical temperature and pressure in molecular clouds. The power-law decline of the IMF at large masses suggests that the most massive stars are built up by scale-free accretion or accumulation processes, and the observed formation of these stars in dense clusters and close multiple systems suggests that interactions between dense prestellar clumps or protostars in forming clusters will play a role. A simple model postulating successive mergers of subsystems in a forming cluster accompanied by the accretion of a fraction of the residual gas by the most massive protostar during each merger predicts an upper IMF of power-law form and reproduces the Salpeter law with a plausible assumed accretion efficiency.
We summarize recent observational and theoretical progress aimed at understanding the origin of the stellar initial mass function (IMF) with specific focus on galactic star-forming regions. We synthesize data from various efforts to determine the IMF in very young, partially-embedded stellar clusters and find: i) no significant variations in the low-mass IMF have been observed between different star-forming regions; and ii) the mass distributions of young stars just emerging from molecular clouds are consistent with having been drawn from the IMF derived from field stars in the solar neighborhood. These results apply only to gross characterizations of the IMF (e.g. the ratio of high to low mass stars); present observations do not rule out more subtle regional differences. Further studies are required in order to assess whether or not there is evidence for a universal turnover near the hydrogen--burning limit. We also provide a general framework for discussing theories of the IMF, and summarize recent work on several physical mechanisms which could play a role in determining the form of the stellar initial mass function.
It has frequently been suggested that the stellar IMF in galaxies was top-heavy at early times. This would be plausible physically if the IMF depends on a mass scale such as the Jeans mass that was higher at earlier times because of the generally higher temperatures that were present then. In this paper it is suggested, on the basis of current evidence and theory, that the IMF has a universal Salpeter-like form at the upper end but flattens below a characteristic stellar mass that may vary with time. Much of the evidence that has been attributed to a top-heavy early IMF, including the ubiquitous G-dwarf problem, the high abundance of heavy elements in clusters of galaxies, and the high rate of formation of massive stars in high-redshift galaxies, can be accounted for with such an IMF if the characteristic stellar mass was several times higher during the early stages of galaxy evolution. However, significant variations in the mass-to-light ratios of galaxies and large amounts of dark matter in stellar remnants are not as easily explained in this way because they require more extreme and less plausible assumptions about the form and variability of the IMF. Metal-free 'population III' stars are predicted to have an IMF that consists exclusively of massive stars, and they could help to account for some of the evidence that has been attributed to a top-heavy early IMF, as well as contributing importantly to the energetics and chemical enrichment of the early universe.