Cooling of Pre-Galactic Gas Clouds by Hydrogen Molecule (original) (raw)
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The cooling of astrophysical media by H2
Monthly Notices of the Royal Astronomical Society, 1999
The results of recent quantum mechanical calculations of cross-sections for rotational transitions within the vibrational ground state of HD are used to evaluate the rate of radiative energy loss from gas containing HD, in addition to H, He and H 2. The cooling function for HD (i.e. the rate of cooling per HD molecule) is evaluated in steady state on a grid of values of the relevant parameters of the gas, namely the gas density and temperature, the atomic to molecular hydrogen abundance ratio and the ortho:para-H 2 density ratio. The corresponding cooling function for H 2 , previously computed by Le Bourlot et al., is slightly revised to take account of transitions induced by collisions with ground-state ortho-H 2 J 1X The cooling functions and the data required for their calculation are available from http://ccp7.dur.ac.uk/. We then make a study of the rate of cooling of the primordial gas through collisions with H 2 and HD molecules. In this case, radiative transitions induced by the cosmic background radiation field and, in the case of H 2 , collisional transitions induced by H 1 ions should additionally be included.
On the Mechanism of H2 Formation in the Interstellar Medium
Astrochemistry, 1987
The problem of the formation of molecular hydrogen in interstellar clouds is revisited. The role played by cosmic ray bombardment under certain circumstances is considered mainly in the light of the low formation rate of H2 on grains due to the reduced mobility of adsorbed H atoms on their amorphous surfaces at interstellar temperatures. 1. H 2 PRODUCTION RATE AND SURFACE MIGRATION The direct formation of molecular hydrogen by a radiative association of two H atoms in gas phase do not occur because the formed molecule cannot release the excess formation energy by radiative transitions. The mechanism which is now accepted for the formation of ^uses interstellar grains as a catalyst (Hollenback and Salpeter, 1971). In this model H atoms, which stick onto grains migrate on their surface till they encounter another H atom. The energy of the reaction which forms tnen released to the grain avoiding the problem of the two body reaction in the gas phase. Hollenback and Salpeter (1971) in their works assumed a cubic crystal structure and that hydrogen atoms could be adsorbed in regularly spaced sites. At interstellar températures, where thermal hopping occurs at very low rates, the mobility (which is fundamental for the mechanism to work) of hydrogen was assured by tunneling. The diffusion timescale of 10" 12 sec is so short that adsorbed H atoms will encounter before evaporating, forming hydrogen molecules. Under this assumption one can estimate the production rate of H£ for diffuse and dense clouds. Assume a density n^ ~ 10 cm and a kinetic temperature Τ ~ 80K for the former and for the latter n H ~ lO^cm and Τ * I0 h cm and Τ ~ 10K, then for n g ~ 4x10" 13 n H one obtains: R(H 2)-7.2xlO" 16 cm" 2 s" 1 for diffuse clouds R(H 2)-4.8xlO~1 3 cm" 2 s" 1 for dense clouds Recently Smoluchowski (1983) has quantitatively shown that the orig-167
Molecular Hydrogen in Star‐forming Regions: Implementation of its Microphysics in CLOUDY
Astrophysical Journal, 2005
Much of the baryonic matter in the Universe is in the form of H 2 which includes most of the gas in galactic and extragalactic interstellar clouds. Molecular hydrogen plays a significant role in establishing the thermal balance in many astrophysical environments and can be important as a spectral diagnostic of the gas. Modeling and interpretation of observations of such environments requires a quantitatively complete and accurate treatment of H 2 . Using this micro-physical model of H 2 , illustrative calculations of prototypical astrophysical environments are presented. This work forms the foundation for future investigations of these and other environments where H 2 is an important constituent. 1 0, 2 J ∆ = ± 0, 1 J ∆ = ± 1 J ∆ = ± . For C + , the
Possible flakes of molecular hydrogen in the early Universe
Astronomy and Astrophysics, 2003
The thermochemistry of H2 and HD in non-collapsed, non-reionized primordial gas up to the end of the dark age is investigated with recent radiation-matter and chemical reaction rates taking into account the efficient coolant HD, and the possibility of a gas-solid phase transition of H2. In the standard big-bang model we find that these molecules can freeze out and lead to the growth of flakes of solid molecular hydrogen at redshifts z ≈ 6 − 12 in the unperturbed medium and under-dense regions. While this freezing caused by the mere adiabatic cooling of the expanding matter is less likely to occur in collapsed regions due to their higher than radiation background temperature, on the other hand the super-adiabatic expansion in voids strongly favors it. Later reionization (at z ≈ 5 − 6) eventually destroys all these H2 flakes. The possible occurrence of H2 flakes is important for the degree of coupling between matter and radiation, as well as for the existence of a gas-grain chemistry at the end of the dark age.
HD/H 2 molecular clouds in the early Universe: The problem of primordial deuterium
Astronomy Letters-a Journal of Astronomy and Space Astrophysics, 2010
We have detected new HD absorption systems at high redshifts, z abs = 2.626 and z abs = 1.777, identified in the spectra of the quasars J0812+3208 and Q1331+170, respectively. Each of these systems consists of two subsystems. The HD column densities have been determined: log N HDA = 15.70 ± 0.07 for z A = 2.626443(2) and log N HDB = 12.98 ± 0.22 for z B = 2.626276(2) in the spectrum of J0812+3208 and log N HDC = 14.83 ± 0.15 for z C = 1.77637(2) and log N HDD = 14.61 ± 0.20 for z D = 1.77670(3) in the spectrum of Q1331+170. The measured HD/H2 ratio for three of these subsystems has been found to be considerably higher than its values typical of clouds in our Galaxy.We discuss the problem of determining the primordial deuterium abundance, which is most sensitive to the baryon density of the Universe Ωb. Using a well-known model for the chemistry of a molecular cloud, we have estimated the isotopic ratio D/H=HD/2H2 = (2.97 ± 0.55) × 10−5 and the corresponding baryon density Ωbh 2 = 0.0205 −0.0020+0.0025. This value is in good agreement with Ωbh 2 = 0.0226 −0.00060.0006 obtained by analyzing the cosmic microwave background radiation anisotropy. However, in high-redshift clouds, under conditions of low metallicity and low dust content, hydrogen may be incompletely molecularized even in the case of self-shielding. In this situation, the HD/2H2 ratio may not correspond to the actual D/H isotopic ratio. We have estimated the cloud molecularization dynamics and the influence of cosmological evolutionary effects on it.
The Astrophysical Journal, 2007
We have investigated the time scale for formation of molecular clouds by examining the conversion of HI to H 2 using a time-dependent model which includes H 2 photodissociation with rate dependent on dust extinction and self shielding. H 2 formation on dust grains and cosmic ray destruction are also included in one-dimensional model slab clouds which incorporate time-independent density and temperature distributions. We calculate 21cm spectral line profiles seen in absorption against a background provided by general Galactic HI emission, and compare the model spectra with HI Narrow Self-Absorption, or HINSA, profiles absorbed in a number of nearby molecular clouds. The time evolution of the HI and H 2 densities is dramatic, with the atomic hydrogen disappearing in a wave propagating from the central, denser regions which have a shorter H 2 formation time scale, to the edges, where the density is lower and the time scale for H 2 formation longer. The model 21cm spectra are characterized by very strong absorption at early times, when the HI column density through the model clouds is extremely large. Excess emission produced by the warm edges of the cloud when the background temperature is relatively low can be highly confusing in terms of separating the effect of the foreground cloud from variations in the background spectrum. The minimum time required for a cloud to have evolved to its observed configuration, based on the model spectra, is set by the requirement that most of the HI in the outer portions of the cloud, which otherwise overwhelms the narrow absorption, be removed. The characteristic time that has elapsed since cloud compression and initiation of the HI → H 2 conversion is a few × 10 14 s or ≃ 10 7 yr. This sets a minimum time for the age of these molecular clouds and thus for star formation that may take place within them.
Dynamical Formation of Dark Molecular Hydrogen Clouds around Diffuse H ii Regions
The Astrophysical Journal, 2007
We examine the triggering process of molecular cloud formation around diffuse H II regions. We calculate the time evolution of the shell as well as of the H II region in a two-phase neutral medium, solving the UV and FUV radiative transfer, the thermal and chemical processes in the time-dependent hydrodynamics code. In the cold neutral medium, the ambient gas is swept up in the cold (T ∼ 30 − 40 K) and dense (n ∼ 10 3 cm −3 ) shell around the HII region. In the shell, H 2 molecules are formed from the swept-up H I gas, but CO molecules are hardly formed. This is due to the different efficiencies of the self-shielding effects between H 2 and CO molecules. The reformation of H 2 molecules is more efficient with a higher-mass central star. The physical and chemical properties of gas in the shell are just intermediate between those of the neutral medium and molecular clouds observed by CO emissions. The dense shell with cold HI/H 2 gas easily becomes gravitationally unstable, and breaks up into small clouds. The cooling layer just behind the shock front also suffers from thermal instability, and will fragment into cloudlets with some translational motions. We suggest that the predicted cold "dark" HI/H 2 gas should be detected as the H I self-absorption (HISA) feature. We have sought such features in recent observational data, and found shell-like HISA features around the giant H II regions, W4 and W5. The shell-like HISA feature shows good spatial correlation with dust emission, but poor correlation with CO emission. Our quantitative analysis shows that the HISA cloud can be as cold as T ∼ a few × 10 K. In the warm neutral medium, on the other hand, the expanding diffuse H II region is much simpler owing to the small pressure excess. The UV photons only ionize the neutral medium and produce a warm ionized medium.
Molecular Hydrogen in Space: Evolution of Primordial H2 for Different Cosmological Models
2000
Primordial chemistry began in the recombination epoch when the adiabatic expansion caused the temperature of the radiation to fall below 4000 K. The chemistry of the early Universe involves the elements hydrogen, its isotope deuterium, helium with its isotopic forms and lithium. In this talk I will present results on the evolution of the primordial H2 abundance for different cosmological models and the influence on the thermal decoupling.
Simulations of Early Structure Formation: Primordial Gas Clouds
The Astrophysical Journal, 2003
We use cosmological simulations to study the origin of primordial star-forming clouds in a ΛCDM universe, by following the formation of dark matter halos and the cooling of gas within them. To model the physics of chemically pristine gas, we employ a non-equilibrium treatment of the chemistry of 9 species (e − , H, H + , He, He + , He ++ , H 2 , H + 2 , H −) and include cooling by molecular hydrogen. By considering cosmological volumes, we are able to study the statistical properties of primordial halos and the high resolution of our simulations enables us to examine these objects in detail. In particular, we explore the hierarchical growth of bound structures forming at redshifts z ≈ 25 − 30 with total masses in the range ≈ 10 5 − 10 6 M ⊙. We find that when the amount of molecular hydrogen in these objects reaches a critical level, cooling by rotational line emission is efficient, and dense clumps of cold gas form. We identify these "gas clouds" as sites for primordial star formation. In our simulations, the threshold for gas cloud formation by molecular cooling corresponds to a critical halo mass of ≈ 5 × 10 5 h −1 M ⊙ , in agreement with earlier estimates, but with a weak dependence on redshift in the range z > 16. The complex interplay between the gravitational formation of dark halos and the thermodynamic and chemical evolution of the gas clouds compromises analytic estimates of the critical H 2 fraction. Dynamical heating from mass accretion and mergers opposes relatively inefficient cooling by molecular hydrogen, delaying the production of star-forming clouds in rapidly growing halos. We also investigate the impact of photo-dissociating ultraviolet (UV) radiation on the formation of primordial gas clouds. We consider two extreme cases by first including a uniform radiation field in the optically thin limit and secondly by accounting for the maximum effect of gas self-shielding in virialized regions. For radiation with Lyman-Werner band flux J > 10 −23 erg s −1 cm −2 Hz −1 str −1 , hydrogen molecules are rapidly dissociated, rendering gas cooling inefficient. In both the cases we consider, the overall impact can be described by computing an equilibrium H 2 abundance for the radiation flux and defining an effective shielding factor. Based on our numerical results, we develop a semi-analytic model of the formation of the first stars, and demonstrate how it can be coupled with large N-body simulations to predict the star formation rate in the early universe.
Warm and Cold Molecular Gas in Galaxies
The Astronomical …, 2005
New and archival interferometric 12 CO(1→0) datasets from six nearby galaxies are combined with H 2 2.122 µm and Hα maps to explore in detail the interstellar medium in different star-forming galaxies. We investigate the relation between warm (H 2 at T ∼ 2000 K) and cold (CO at T ∼ 50 K) molecular gas from 100 pc to 2 kpc scales. On these scales, the ratio of warm-to-cold molecular hydrogen correlates with the fν (60µm) fν (100µm) ratio, a ratio that tracks the star formation activity level. This result also holds for the global properties of galaxies from a much larger sample drawn from the literature. The trend persists for over three orders of magnitude in the mass ratio, regardless of source nuclear activity.