Showing papers by "Marco Spaans published in 2004"
TL;DR: In this paper, the formation of molecular hydrogen on dust grain surfaces and apply their results to the high-redshift universe was studied and it was found that a range of physical parameters such as dust temperature and gas temperature, but not so much dust surface composition, influenced the formation rate of H2.
Abstract: We study the formation of molecular hydrogen on dust grain surfaces and apply our results to the high-redshift universe. We find that a range of physical parameters—in particular dust temperature and gas temperature, but not so much dust surface composition—influences the formation rate of H2. The H2 formation rate is found to be suppressed above gas kinetic temperatures of a few hundred K and for dust temperatures above 500 K and below 10 K. We highlight the differences between our treatment of the H2 formation process and other descriptions in the literature. We also study the relative importance of H2 formation on dust grains with respect to molecular hydrogen formation in the gas phase, through the H- route. The ratio of the formation rates of these two routes depends to a large part on the dust abundance, on the electron abundance, and on the relative strength of the far-ultraviolet (extra-)galactic radiation field. We find that for a cosmological evolution of the star formation rate and dust density consistent with the Madau plot, a positive feedback effect on the abundance of H2 due to the presence of dust grains can occur at redshifts z ≥ 3. This effect occurs for a dust-to-gas mass ratio as small as 10-3 of the galactic value.
74 citations
TL;DR: In this article, the polytropic equation of state (EOS) of primordial gas clouds with modest enrichment is computed, motivated by the recent observations of very Fe-deficient stars, such as HE 0107-5240 and CS 29498-043.
Abstract: The polytropic equation of state (EOS) of primordial gas clouds with modest enrichment is computed, motivated by the recent observations of very Fe-deficient stars, [Fe/H]~10^{-3.5}-10^{-5}, such as HE 0107-5240 and CS 29498-043. These stars are overabundant, relative to Fe, in C and O. We assume that the observed abundances of species like C, O, Si and Fe are representative of the gas from which the currently observed metal-deficient stars formed. Under this assumption, we find that this primordial metal abundance pattern has profound consequences for the thermal balance and chemical composition of the gas, and hence for the EOS of the parental cloud. The polytopic EOS is soft for low, [O/H] 10^{-2} due to the large opacity in the CO and H2O cooling lines. It is further found that a regulating role is played by the presence and temperature of the dust, even when the overall carbon abundance is only [C/H]~10^{-2}. When the dust is warmer than the gas, a region with gamma~1.2 results around a density of ~10^4 cm^{-3}. When the dust is colder than the gas, a region with gamma~0.8 is found for a density of ~10^6 cm^{-3}. Implications for the primordial initial mass function (IMF) as well as the IMF in starburst galaxies, where the metallicity is super-solar, are explored and related to processes that influence the temperature of the ambient dust.
11 citations
TL;DR: In this article, the formation of molecular hydrogen on dust grain surfaces and apply their results to the high redshift universe was studied and found that a range of physical parameters, in particular dust temperature and gas temperature, but not so much dust surface composition, influence the formation rate of H$_2$.
Abstract: We study the formation of molecular hydrogen on dust grain surfaces and apply our results to the high redshift universe. We find that a range of physical parameters, in particular dust temperature and gas temperature, but not so much dust surface composition, influence the formation rate of H$_2$. The H$_2$ formation rate is found to be suppressed above gas kinetic temperatures of a few hundred K and for dust temperatures above 200-300 K and below 10 K. We highlight the differences between our treatment of the H$_2$ formation process and other descriptions in the literature. We also study the relative importance of H$_2$ formation on dust grains with respect to molecular hydrogen formation in the gas phase, through the H$^-$ route. The ratio of formation rates of these two routes depends to a large part on the dust abundance, on the electron abundance, and also on the relative strength of the FUV (extra-)galactic radiation field. We find that for a cosmological evolution of the star formation rate and dust density consistent with the Madau plot, a positive feedback effect on the abundance of H$_2$ due to the presence of dust grains can occur at redshifts $z\ge 3$. This effect occurs for a dust-to-gas mass ratio as small as 10$^{-3}$ of the galactic value.
6 citations