ROBO: a model and a code for studying the interstellar medium

Abstract

We present robo, a model and its companion code for the study of the interstellar medium (ISM). The aim is to provide an accurate description of the physical evolution of the ISM and to set the ground for an ancillary tool to be inserted in NBody-Tree-SPH (NB-TSPH) simulations of large-scale structures in the cosmological context or of the formation and evolution of individual galaxies. The ISM model consists of gas and dust. The gas chemical composition is regulated by a network of reactions that includes a large number of species (hydrogen and deuterium-based molecules, helium, and metals). New reaction rates for the charge transfer in H + and H 2 collisions are presented. The dust contains the standard mixture of carbonaceous grains (graphite grains and PAHs) and silicates. In our model dust are formed and destroyed by several processes. The model accurately treats the cooling process, based on several physical mechanisms, and cooling functions recently reported in the literature. The model is applied to a wide range of the input parameters, and the results for important quantities describing the physical state of the gas and dust are presented. The results are organized in a database suited to the artificial neural networks (ANNs). Once trained, the ANNs yield the same results obtained by ROBO with great accuracy. We plan to develop ANNs suitably tailored for applications to NB-TSPH simulations of cosmological structures and/or galaxies.

Modeling Dust Evolution in Galaxies With a Multiphase, Inhomogeneous Ism

“…To this end, we investigate various choices of the sticking coefficient: (1) a fixed value a = 1 for  T 300 gas K and a = 0 for > T 300 gas K,(2) a fixed value a = 1 for all temperatures,(3) anexperimentally measured α for chemisorption of H 2 molecules on silicate surfaces by Chaabouni et al (2012),and (4) a T T , gas dust ( ) from theoretical calculations of physisorption performed by Leitch-Devlin & Williams (1985). In theabsence of estimates of α for physisorption of Si on silicate grain surfaces, we provisionally adopt the data for carbon atoms arriving on a graphite lattice from the work by Leitch-Devlin & Williams (1985), in the functional form derived by Grassi et al (2011). The dust temperature is fixed to 20K.…”

Effects of grain size distribution on the interstellar dust mass growth

“…By using , the typical time‐scale τ () can be estimated as where a 0.1 ≡ a 0 /0.1 μ m, n 3 ≡ n H /10 3 cm −3 , T 50 ≡ T gas /50 K and S 0.3 = S /0.3. Unless otherwise stated, we adopt n H = 10 3 cm −3 and T = 50 K for the typical values derived from observational properties of Galactic molecular clouds (Hirashita 2000a), and S = 0.3 (Leitch‐Devlin & Williams 1985; Grassi et al 2011). Although S may be almost 1 in such low temperature environments as molecular clouds (Zhukovska et al 2008), the chemical factor (i.e.…”

Central free-free dominated 880-μm emission in II Zw 40

“…Hirashita & Kuo (2011) derive the dust‐growth time‐scale for silicate (a similar time‐scale is obtained for carbonaceous dust): where 〈 a 3 〉 and 〈 a 2 〉 are the averages of a 3 and a 2 ( a is the grain radius) for grain‐size distribution, Z is the metallicity, n H is the hydrogen number density, T gas is the gas temperature and S is the sticking efficiency of the relevant metal species on to the dust surface. We assume Z ⊙ (see Introduction), n H = 10 5 cm −3 (Table 3), T gas = 50 K (Wilson, Walker & Thornley 1997) and S = 0.3 (Leitch‐Devlin & Williams 1985; Grassi et al 2011). Then, we obtain τ grow ∼ 8.4 × 10 5 yr, which is comparable to the star‐formation time‐scale.…”