Molecular Cloud Formation behind Shock Waves

By E. Bergin, L. Hartmann, J. C. Raymond, J. Ballesteros-Paredes. 2004

166 citations recorded by the source corpus.

Motivated by our previous paper, in which we argued for the formation of molecular clouds from large-scale flows in the diffuse Galactic interstellar medium, we examine the formation of molecular gas behind shocks in atomic gas using a one-dimensional chemical/dynamical model. In our analysis we place particular emphasis on constraints placed on the dynamical evolution by the chemistry. The most important result of this study is to stress the importance of shielding the molecular gas from the destructive effects of UV radiation. For shock ram pressures comparable to or exceeding typical local interstellar medium pressures, self-shielding controls the formation time of molecular hydrogen, but CO formation requires shielding of the interstellar radiation field by dust grains. We find that for typical parameters the molecular hydrogen fractional abundance can become significant well before CO forms. The timescale for (CO) molecular cloud formation is not set by the H2 formation rate on grains, but rather by the timescale for accumulating a sufficient column density or extinction, AV ≳ 0.7. The local ratio of atomic to molecular gas (4 : 1), coupled with short estimates for the lifetimes of molecular clouds (3-5 Myr), suggests that the timescales for accumulating molecular clouds from atomic material typically must be no longer than about 12-20 Myr. Based on the shielding requirement, this implies that the typical product of preshock density and velocity must be nv ≳ 20 cm-3 km s-1. In turn, depending on the shock velocity, this implies shock ram pressures that are a few times the typical estimated local turbulent gas pressure and comparable to the total pressures (gas plus magnetic plus cosmic rays). Coupled with the rapid formation of CO once shielding is sufficient, flow-driven formation of molecular clouds in the local interstellar medium can occur sufficiently rapidly to account for observations. We also provide detailed predictions of atomic and molecular emission and absorption that track the formation of a molecular cloud from a purely atomic medium, with a view toward helping to verify cloud formation by shock waves. However, our predictions suggest that the detection of the pre-CO stages will be challenging. Finally, we provide an analytic solution for time-dependent H2 formation that may be of use in numerical hydrodynamic calculations.

Subfields: Astronomy and Astrophysics, Atmospheric Science

View Paper: https://www.semanticscholar.org/paper/86a89a85dd1125733a1300f09bf6878bb754bcc4

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Citations in the corpus

Cores, filaments, and bundles: hierarchical core formation in the L1495/B213 Taurus region (2013, background): “For example, perturbations in the gas velocity on the order of 10% can easily produce over the expected 10-20 Myr accumulation time of the cloud (e.g., Bergin et al. 2004) the 1-2 Myr differences in the convergence time required to explain the L1495/B213 data (Sect.”
Phantom: A Smoothed Particle Hydrodynamics and Magnetohydrodynamics Code for Astrophysics (2017, methodology): “H2 chemistry is based on the model of Bergin et al. (2004), with H2 created on grain surfaces and destroyed by photo-dissociation and cosmicrays (see Dobbs et al. 2008 for computational details).”
Cold Dark Clouds: The Initial Conditions for Star Formation (2007, background): “Factors such as ram pressure, molecule and dust shielding, and gas-grain physics influence the timescales for both H2 and CO formation (Koyama & Inutsuka 2000; Bergin et al. 2004).”
STAR FORMATION IN DISK GALAXIES. I. FORMATION AND EVOLUTION OF GIANT MOLECULAR CLOUDS VIA GRAVITATIONAL INSTABILITY AND CLOUD COLLISIONS (2008, background): “…(e.g. Kim, Ostriker, & Stone 2003; Shetty & Ostriker 2006; Glover & Mac Low 2007), whereas ‘bottom-up’ processes have GMCs forming via agglomeration from inelastic collisions of clouds (e.g. Kwan 1979) or from turbulent or colliding flows (e.g. Bergin et al. 2004; Heitsch et al. 2008).”
Simulating the Formation of Molecular Clouds. II. Rapid Formation from Turbulent Initial Conditions (2006, methodology): “…two dimensions and with only highly ordered large-scale flows considered (see, e.g., Hennebelle & Pérault 1999, 2000; Koyama & Inutsuka 2000, 2002; Bergin et al. 2004 ), or insufficient chemistry has been included to address the question at hand (e.g., Korpi et al. 1999; de Avillez 2000; Wada…”
Cooling, Gravity, and Geometry: Flow-driven Massive Core Formation (2007, background): “…field lines in the diffuse ISM, arguing that if the field lines were oriented perpendicularly, the stiffening of the equation of state due to the increased magnetic pressure would counter the lowering of the effective adiabatic index due to (atomic line) cooling (see also Bergin et al. 2004).”
Gravity or turbulence? II. Evolving column density PDFs in molecular clouds (2011, background): “…of a giant molecular cloud involves timescales of a few tens of Myr, of which the “molecular stage” may represent a small final fraction (Hartmann, Ballesteros-Paredes & Bergin 2001; Bergin et al. 2004; Vázquez-Semadeni et al. 2007, 2010) as well as the so-called core-to-core velocity dispersion.”
Theory of Star Formation (2007, background): “…along field lines only), the postshock gas would have high enough density for significant amounts of H2 to form within the overall accumulation time, and the shielding from the diffuse UV is sufficient for CO to begin to form (Hartmann, Ballesteros-Paredes, & Bergin, 2001; Bergin et al., 2004).”
Is molecular gas necessary for star formation (2011, methodology): “We take our expressions for these quantities from Bergin et al. (2004).”
The Supernova Rate-Velocity Dispersion Relation in the Interstellar Medium (2005, background): “Molecular hydrogen starts to form when densities of the order of 103 cm 3 are reached and become shielded against UV radiation (Bergin et al. 2004).”
Modelling CO formation in the turbulent interstellar medium (2009, background): “…of cloud chemistry within the framework of a highly simplified dynamical model (e.g. Hennebelle & Pérault 1999, 2000; Koyama & Inutsuka 2000, 2002; Bergin et al. 2004) or on a detailed treatment of the dynamics, but without any treatment of the chemistry (e.g. Balsara et al. 2004; Kritsuk &…”
KINEMATIC DISTANCES TO MOLECULAR CLOUDS IDENTIFIED IN THE GALACTIC RING SURVEY (2009, background): “The compression and energy dissipation inherent to the shock, radiative cooling in the clumps, and orbit crowding then lead to the formation of molecular clouds within a few Myr (Bergin et al. 2004).”
The SILCC (SImulating the LifeCycle of molecular Clouds) project – I. Chemical evolution of the supernova-driven ISM (2014, methodology): “Finally, we model the effects of photoelectric heating using a heating rate given by (Bakes & Tielens 1994; Bergin et al. 2004) pe = 1.3 × 10−24 Geffn erg s−1 cm−3, (22) where Geff = G0exp (−2.5AV), and where the photoelectric heating efficiency is given by (Bakes & Tielens 1994; Wolfire et al.…”
Physical Processes in the Interstellar Medium (2014, background): “This sensitivity can be approximately represented by a scaling factor fthick = exp(−2.5AV ) (Bergin et al., 2004).”
The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function (2014, methodology): “The general approach in these models is to add a time-dependent chemical network to a hydrodynamic or MHD simulation (Koyama & Inutsuka 2000; Bergin et al. 2004; Glover & Mac Low 2007a,b; Pelupessy & Papadopoulos 2009; Gnedin et al. 2009; Christensen et al. 2012; Mac Low & Glover 2012; Clark et al.…”
Formation of Structure in Molecular Clouds: A Case Study (2005, background): “…waves propagating into the warm ISM will fragment in the presence of thermal instability and linear perturbations (Koyama & Inutsuka 2000, 2002; Inutsuka & Koyama 2004; Hennebelle & Pe´rault 1999, 2000) and allow H2 formation within a few megayears (Bergin et al. 2004) in a planeparallel geometry.”
The exciting lives of giant molecular clouds (2013, background): “Each particle is assigned a fraction of H2 (from 0 to 1) and the rate of change of n(H2) is calculated according to the prescription given in the appendix of Bergin et al. (2004), and Dobbs et al. (2006).”
Simulating the Formation of Molecular Clouds. I. Slow Formation by Gravitational Collapse from Static Initial Conditions (2006, background): “…ψ is given by ψ = χeffT 1/2 neφpah , (40) where χeff = e −2.5AVχ represents the strength of the UV background in the gas after dust shielding is taken into account (see Bergin et al. 2004), and where φpah is an adjustable parameter introduced by Wolfire et al. (2003) to make the heating rate…”
The formation of molecular clouds in spiral galaxies (2006, methodology): “Several models (Hollenbach & McKee 1979; Koyama & Inutsuka 2000; Bergin et al. 2004) propose formation of molecular hydrogen through shock compression of the ISM.”