![]() ![]() 1) shows the domain of pressure conventionally attributed to molecular clouds. The condensation properties of H 2 relevant for molecular cloud conditions are well known from laboratory data ( Air Liquide 1976). Because of these detection difficulties, the real quantity of H 2 (as well as He, which shares similar properties of discreteness with H 2) in molecular clouds is still rather unknown, especially when the gas temperature is below ≲ 8 K down to the cosmic background temperature of 2.76 K. See Combes & Pfenniger (1997) for a review of several possible methods for detecting cold H 2. ![]() For example, H 2 only starts emitting at temperatures > 512 K. ![]() It is commonly admitted that the CO-related molecular hydrogen is present in relatively dense regions of the interstellar medium, molecular clouds with number density > 10 10 m -3 and temperatures of 7– 30 K ( Draine 2011).Įven though H 2 is by far the most abundant molecule ( ~ 90%), molecular clouds are mainly detected by CO emissions because of all the difficulties in detecting cold H 2 ( Bolatto et al. 2012), the estimated quantity of H 2 in the Milky Way has essentially been doubled, effectively revealing the nature of some of the dark baryons. 2010 Planck Collaboration XIX 2011 Paradis et al. Since the discovery of dark molecular hydrogen ( Grenier et al. Typically around 50% or more of the gas in spiral galaxies consists of H 2, inferred indirectly by CO or dust emission. The combination of phase transition and gravity may be relevant for a wider range of astrophysical situations, such as proto-planetary disks.įigures 33 −44 are available in electronic form at Arbitrarily small H 2 clumps may form even at relatively high temperatures up to 400– 600 K, according to virial analysis. Fluids presenting a phase transition are gravitationally unstable, independent of the strength of the gravitational potential. Observations, formal analysis, and computer simulations suggest the possibility of the formation of substellar H 2 clumps in cold molecular clouds due to the combination of phase transition and gravity. The simulations show that fluids presenting a phase transition are gravitationally unstable as well, independent of the strength of the gravitational potential, producing two distinct kinds of substellar bodies, those dominated by gravity (planetoids) and those dominated by molecular attractive force (comets).Ĭonclusions. The concept of super-molecule, where the phase transition conditions are preserved by the proper choice of the particle parameters, is tested with computer simulations, allowing us to correctly satisfy the Jeans instability criterion for one-phase fluids. The long-range gravitational forces can be taken into account together with short-range molecular forces with finite limited computational resources, using super-molecules, provided the right scaling is followed. The simulations are run with a state-of-the-art molecular dynamics code (LAMMPS) using the Lennard-Jones inter-molecular potential. On the other hand, the non-linear dynamics is studied using computer simulations to characterize the expected formation of solid bodies analogous to comets. On the one hand, the equilibrium of general non-ideal fluids is studied using the virial theorem and linear stability analysis. This well-known, laboratory-based fact motivates us to study the ideal case of a cold neutral gaseous medium in interstellar conditions for which the bulk of the mass, instead of trace elements, is subject to a gas-liquid or gas-solid phase transition. At the extreme of low temperatures ( ≲10 K), H 2 itself is subject to a phase transition crossing the entire cosmic gas density scale.Īims. The observation of various ices in cold molecular clouds, the existence of ubiquitous substellar, cold H 2 globules in planetary nebulae and supernova remnants, or the mere existence of comets suggest that the physics of very cold interstellar gas might be much richer than usually envisioned. Geneva Observatory, University of Geneva, 1290 Sauverny, SwitzerlandĬontext. Astronomical objects: linking to databases.Including author names using non-Roman alphabets.Suggested resources for more tips on language editing in the sciences Punctuation and style concerns regarding equations, figures, tables, and footnotes
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