Substellar fragmentation in self-gravitating fluids with a major phase transition

Abstract

Context. 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. At the extreme of low temperatures ( < ∼ 10 K), H 2 itself is subject to a phase transition crossing the entire cosmic gas density scale. Aims. 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. Methods. On the one hand, the equilibrium of general non-ideal fluids is studied using the virial theorem and linear stability analysis. On the other hand, the non-linear dynamics is studied using computer simulations to characterize the expected formation of solid bodies analogous to comets. The simulations are run with a state-of-the-art molecular dynamics code (LAMMPS) using the LennardJones inter-molecular potential. 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. Results. 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 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). Conclusions. 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. Fluids presenting a phase transition are gravitationally unstable, independent of the strength of the gravitational potential. Arbitrarily small H 2 clumps may form even at relatively high temperatures up to 400-600 K, according to virial analysis. The combination of phase transition and gravity may be relevant for a wider range of astrophysical situations, such as proto-planetary disks.

Cosmic Snow Clouds: Self-gravitating Gas Spheres Manifesting Hydrogen Condensation

“…Consequently the fluid temperature rises only slightly on compression, and the pressure response is therefore smaller than for condensate-free gas. 1 1 Füglistaler and Pfenniger (2015) suggested that the adiabatic sound-speed of H 2 in phase equilibrium is zero, corresponding to infinite adiabatic compressibility. Their result was obtained by scaling the isothermal sound speed, which is zero in phase equilibrium, by the ratio of specific heats, C P /C V , which they assumed to be finite.…”

“…However, at constant pressure the fluid releases/absorbs heat without any change in temperature, in response to condensation/sublimation of the solid. Consequently C P is infinite, whereas C V is not, so the method of sound-speed calculation suggested by Füglistaler and Pfenniger (2015) does not yield a well-defined result.…”

The Population of Interstellar Objects Detectable with the LSST and Accessible for In Situ Rendezvous with Various Mission Designs

“…The nongravitational acceleration has been postulated to be driven by radiation pressure (Micheli et al 2018), which could be explained if 'Oumuamua was an ultralow-density fractal aggregate (Moro-Martín 2019b; Sekanina 2019; Luu et al 2020) or a millimeter-thin artificial light sail (Bialy & Loeb 2018). If the nongravitational acceleration was powered by cometary outgassing (Micheli et al 2018;Seligman et al 2019), it has been demonstrated that the energetics are consistent with the sublimation of H 2 (Füglistaler & Pfenniger 2015;Seligman & Laughlin 2020;, N 2 , or CO (Seligman et al 2021). Recently, Flekkøy & Brodin (2022) calculated infrared and optical spectral signatures that would differentiate between the various proposed scenarios.…”

The Population of Interstellar Objects Detectable with the LSST and Accessible for $\textit{In Situ}$ Rendezvous with Various Mission Designs

“…The non-gravitational acceleration has been postulated to be driven by radiation pressure (Micheli et al 2018), which could be explained if 'Oumuamua was an ultra low-density fractal aggregate (Moro-Martín 2019; Luu et al 2020;Sekanina 2019), or a millimeter-thin artificial light sail (Bialy & Loeb 2018). If the non-gravitational acceleration was powered by cometary outgassing (Micheli et al 2018;Seligman et al 2019), it has been demonstrated that the energetics are consistent with the sublimation of H 2 (Füglistaler & Pfenniger 2015;Seligman & Laughlin 2020;, N 2 , or CO (Seligman et al 2021).…”