Uncertainties in H2 and HD chemistry and cooling and their role in early structure formation

By S. Glover, Tom Abel. 2008

220 citations recorded by the source corpus.

At low temperatures, the main coolant in primordial gas is molecular hydrogen, H 2 . Recent work has shown that primordial gas that is not collapsing gravitationally but is cooling from an initially ionized state forms hydrogen deuteride, HD, in sufficient amounts to cool the gas to the temperature of the cosmic microwave background. This extra cooling can reduce the characteristic mass for gravitational fragmentation and may cause a shift in the characteristic masses of Population III stars. Motivated by the importance of the atomic and molecular data for the cosmological question, we assess several chemical and radiative processes that have hitherto been neglected: the sensitivity of the low-temperature H 2 cooling rate to the ratio of ortho-H 2 to para-H 2 , the uncertainty in the low-temperature cooling rate of H 2 excited by collisions with atomic hydrogen, the effects of cooling from H 2 excited by collisions with protons and electrons, and the large uncertainties in the rates of several of the reactions responsible for determining the H 2 fraction in the gas. It is shown that the most important of neglected processes is the excitation of H 2 by collisions with protons and electrons. Their effect is to cool the gas more rapidly at early times, and consequently to form less H 2 and HD at late times. This fact, as well as several of the chemical uncertainties presented here, significantly affects the thermal evolution of the gas. We anticipate that this may lead to clear differences in future detailed three-dimensional studies of first structure formation. In such calculations it has previously been shown that the details of the timing between cooling and merger events decide between immediate runaway gravitational collapse and a slower collapse delayed by turbulent heating. Finally, we show that although the thermal evolution of the gas is in principle sensitive to the ortho-para ratio, in practice the standard assumption of a 3:1 ratio produces results that are almost indistinguishable from those produced by a more detailed treatment.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/bdfc59588e1b18a7c5b7eb8fb277b97ded6add63

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

Citations in the corpus

Implementing molecular hydrogen in hydrodynamic simulations of galaxy formation (2012, background): “This assumption is justified in Glover & Abel (2008).”
The First Stars: Mass Growth Under Protostellar Feedback (2011, background): “We finally note that H2 cooling through collisions with protons and electrons is included, as they play an important role within H ii regions (Glover & Abel 2008).”
The birth of a galaxy – II. The role of radiation pressure (2012, methodology): “We use the nine-species (H i, H ii, He i, He ii, He iii, e−, H2, H2 +, H−) non-equilibrium chemistry model in enzo (Abel et al. 1997; Anninos et al. 1997) and the H2 cooling rates from Glover & Abel (2008).”
Formation and evolution of primordial protostellar systems (2012, background): “The resulting rate is intermediate in terms of the large uncertainty discussed in Glover & Abel (2008), which is reflected by the substantial variation of the thermal and morphological evolution of primordial gas clouds found in Turk et al. (2011).”
The First Stars (2012, background): “Collisions of H2 with a number of different species contribute to Λ0, as explored in Glover & Abel (2008), but in the earliest minihalos, the dominant contributions come from collisions with H and He.”
Building up the Population III initial mass function from cosmological initial conditions (2016, background): “At each timestep, after gravitational and hydrodynamic forces are updated, abundances and temperatures are evolved according to a network of chemical reaction rates and heating/cooling rates (Glover & Abel 2008).”
H$_2$ formation on interstellar dust grains: the viewpoints of theory, experiments, models and observations (2017, background): “…and MHD simulations have shown that the foration of H 2 begins in the initial phases of the molecular cloud ormation ( Clark et al., 2012; Glover and Abel, 2008 ), and that ompressive motions within the gas (either driven by the galaxy ynamics or the turbulence) accelerate the formation…”
ENVIRONMENTAL DEPENDENCE OF THE KENNICUTT–SCHMIDT RELATION IN GALAXIES (2010, methodology): “(A5) The rate coefficients k1-k32 are taken from Glover & Abel (2008); we do not list here all these reactions for brevity.”
THE BIRTH OF A GALAXY: PRIMORDIAL METAL ENRICHMENT AND STELLAR POPULATIONS (2010, methodology): “We use the nine-species (H I, H II, He I, He II, He III, e−, H2, H2 +, H−) non-equilibrium chemistry model in Enzo (Abel et al. 1997; Anninos et al. 1997) and the H2 cooling rates from Glover & Abel (2008).”
MODELING MOLECULAR HYDROGEN AND STAR FORMATION IN COSMOLOGICAL SIMULATIONS (2008, background): “The latter reactions are comprehensively summarized elsewhere (Shapiro & Kang 1987; Abel et al. 1997; Galli & Palla 1998; Glover & Jappsen 2007; Glover & Abel 2008) and are not important for our purposes; we keep them in the equations to ensure that our model also works in the metal-free regime.”
GRACKLE: a chemistry and cooling library for astrophysics (2016, methodology): “The formation and destruction of simple molecules can be energetically important in some circumstances, such as Population III star formation (Omukai & Nishi 1998; Abel et al. 2002; Glover & Abel 2008; Turk et al. 2009).”
ENZO: AN ADAPTIVE MESH REFINEMENT CODE FOR ASTROPHYSICS (2013, background): “If the H2 chemistry network is enabled, the following H2 cooling processes are also considered: ro-vibrational transitions (Glover & Abel 2008; Galli & Palla 1998), heating and cooling from molecular formation and destruction (Turk et al. 2009), and collisioninduced emission (Ripamonti & Abel 2004).”
Physical Processes in the Interstellar Medium (2014, background): “…one have fewer H2 molecules to provide the cooling, but their collisional excitation rates also decrease, since collisions with H atoms are much less effective at exciting the rotational levels of H2 than collisions with other H2 molecules (see e.g. Glover & Abel, 2008, and references therein).”
SIMULATIONS ON A MOVING MESH: THE CLUSTERED FORMATION OF POPULATION III PROTOSTARS (2011, background): “…optically thin regime is modeled with the low-density cooling rates for collisions between H2 molecules and H and He atoms, H2 molecules, protons and electrons (Glover & Abel 2008), accounting for the transition to local thermodynamic equilibrium level populations at gas densities nH ≫ 10 4 cm−3.”
The birth of a galaxy – III. Propelling reionization with the faintest galaxies (2014, methodology): “For each halo, we calculate the total cooling rate tot within the virial radius from hydrogen and helium processes X using the rates of Sutherland & Dopita (1993), metal cooling Z using the method of Smith et al. (2008), and molecular hydrogen cooling H2 using the rates of Glover & Abel (2008).”
Galaxy formation theory (2010, background): “…understanding the properties of Population III stars, their various feedback processes (both positive andnegative) and initial mass function (Glover and Abel 2008; Trenti and Stiavelli 2009; Ohkubo et al. 2009; Machida et al.2008; Lawlor et al. 2008; O’Shea and Norman 2007, 2008; Harris et…”