Hideya Uehara

Does Deuterium Enable the Formation of Primordial Brown Dwarfs

We investigate thermal and dynamical evolution of a primordial gas cloud with an updated deuterium chemistry. We consider a fragment of a postshock-cooled sheet that is expected to form by collapse of a massive cloud ( greater, similar108 M middle dot in circle) and by blast waves due to supernova explosions. At first we investigate molecule formation in a primordial shock. We show that almost all deuterium can be converted to HD within the age of the universe at the collapsed redshift in the case of a cloud that has a virial temperature of approximately 106 K and collapses at z>1. When the postshock sheet fragments owing to gravitational instability, the fractional H2 and HD abundances become approximately 10-2 and approximately 10-5, respectively, which are 103-104 times higher than the result of molecule formation in the expanding universe after recombination. To study the subsequent evolution of a fragment, we performed one-dimensional simulations of a spherical/cylindrical cloud, of which initial conditions (e.g., fractional abundances of chemical composition, temperature) are derived from the result of the shock. It is found that, in case of a cylindrical collapse, the cooling by HD molecules keeps the temperature of the cloud less than 100 K and the cloud evolves almost isothermally. When the cloud becomes optically thick to the HD line emission ( approximately 1010 cm-3) and the gravitational fragmentation of the cylindrical cloud becomes effective, the Jeans mass becomes comparable to 0.1 M middle dot in circle. This series of processes enables the formation of primordial low-mass stars, and possibly brown dwarfs, in primordial gas clouds.

Fragmentation of the Primordial Gas Clouds and the Lower Limit on the Mass of the First Stars

We discuss the fragmentation of primordial gas clouds in the universe after decoupling. Comparing the timescale of collapse with that of fragmentation, we obtain the typical mass of a fragment both numerically and analytically. We show that the estimated mass gives the minimum mass of a fragment that is formed from the primordial gas cloud and essentially determined by the Chandrasekhar mass.

H2 Emission Spectra with New Formation Pumping Models

It has been predicted that H2 newly formed on the surface of dust grains should be in highly vibrationally and rotationally excited states in the ground electronic state via the formation pumping mechanism. This suggests that the rovibrational emission spectrum of H2 may be detectable even in regions without a source of UV pumping or dynamical excitation. Moreover, the infrared H2 emission arising from formation pumping can be a new probe that will directly show us the spot where the evolution from H I clouds to H2 clouds occurs. In the present work, we investigate the pure effects of formation pumping in infrared H2 emission spectra with our new formation pumping models. We build up our formation pumping models for H2 newly formed on icy mantles, carbonaceous dust, and silicate dust, on the basis of the recent progress of basic studies about the H2 formation process on the surface of dust grains. Then, we calculate the H2 emission spectra with our new formation pumping models, considering the early stage of an evolving region from H I clouds to H2 clouds dominated by the H2 formation process on the surface of dust grains, of uniform H I density nH = 103 cm-3 and the gas temperature T = 10 K, in the cold interior of interstellar clouds under a weak ultraviolet (UV) radiation field. We find that the resulting infrared H2 emission spectra dominated by formation pumping can be discriminated from those dominated by UV pumping as well as have information about the property of dust. We also find that the fluxes of the most intense emission lines arising from formation pumping should be strong enough to be detected. In the wavelength range λ 1.6 μm, if the line intensities of many transitions from higher rotational states such as 1-0 S(3), 1-0 S(5), and 1-0 S(7) become stronger than that of the 1-0 S(1) transition, it should be the evidence of H2 emission arising from formation pumping. In the wavelength range λ 1.6 μm, if many transitions from higher vibrational states are detected, the emission should be originated from H2 newly formed on icy mantles or silicate dust. On the contrary, if not, the emission should be from H2 formed on carbonaceous dust.

Thermal and Dynamical Evolution of Primordial Gas Clouds On the Formation of First Luminous Objects

We investigate the thermal and dynamical evolution of primordial gas clouds in the universe after decoupling. Comparing the time scale of dynamical evolution with that of fragmentation, we can estimate the typical fragmentation scale. We propose the following scenario of the formation process of first luminous objects consisting of large number stars. First, by pancake collapse of the overdensity regions in the expanding universe or collision between clouds in potential wells, quasi-plane shocks form. If the shock-heated temperature is higher than about 10 4 K, the post-shock gas cools down to several hundred K by H2 line cooling, and the shock-compressed layer fragments into filamentary clouds. The filamentary cloud collapses dynamically once more and fragments into cloud cores, Finally, a primordial star forms in a cloud core. We show that the minimum mass of the first star is essentially determined by the Chandmsekhar mass. Also, we investigate the dynamical collapse of cloud cores by numerical simulation and show that the evolution paths of the central regions of the cores depend only very weakly on the total core mass. After mass accretion, a massive star may be formed in a core, since the estimated mass accretion rate is very large. In such a case, it may be possible for many massive stars form almost simultaneously. Then the clouds can be luminous objects. On the other hand, if the shock-heated temperature is lower, effective star formation is delayed significantly.

Evolution of Primordial Protostellar Clouds —Quasi-Static Analysis—

The contraction processes of metal-free molecular clouds of starlike mass (or cloud cores) are investigated. We calculate radiative transfer of the H2 lines and examine quasi-static contraction with radiative cooling. Comparing two time-scales, the free-fall time tff and the time-scale of quasi-static contraction tqsc (� tcool, the cooling time) of these cores, we find that the ratio of the two time-scales tff/tqsc, i.e., the efficiency of cooling, becomes larger with contraction even under the existence of cold and opaque envelopes. In particular, for � @� � .

The Minimum Mass of the First Stars and the Anthropic Pinciple

The lower limit of the mass of the first stars suggested recently may imply the formation of massive stars of mass greater than 8 solar mass irrespective of the details of the initial mass function. The production of heavy metals from the first stars will ensure a requisite for the existence of life without the anthropic principle.