Radiative Interaction of Shocks With Small Interstellar Clouds as a Pre-Stage to Star Formation

Abstract

Cloud compression by external shocks is believed to be an important triggering mechanism for gravitational collapse and star formation in the interstellar medium. We have performed MHD simulations to investigate whether the radiative interaction between a shock wave and a small interstellar cloud can induce the conditions for Jeans instability and how the interaction is influenced by magnetic fields of different strengths and orientation. The simulations use the NIRVANA code in three dimensions with anisotropic heat conduction and radiative heating/cooling at an effective resolution of 100 cells per cloud radius. Our cloud has radius 1.5 pc, has density 17 cm −3 , is embedded in a medium of density 0.17 cm −3 , and is struck by a planar Mach 30 shock wave. The simulations produce dense, cold fragments similar to those of Mellema et al. and Fragile et al. We do not find any regions that are Jeans unstable but do record transient cloud density enhancements of factors ∼10 3 -10 5 for the bulk of the cloud mass, which then decline and converge toward seemingly stable net density enhancement factors ∼10 2 -10 4 . Our run with a weak, initial magnetic field (β = 10 3 ) perpendicular to the shock normal stands out as producing the most lasting density enhancements. We interpret this field strength as being the compromise between weak internal magnetic pressure preventing compression and sufficiently strong magnetic field to thermally insulate the condensations, thus helping them cool radiatively.

The Launching of Cold Clouds by Galaxy Outflows. Ii. The Role of Thermal Conduction

“…While the outer layers of the clouds evaporate due to thermal conduction, the cores are stretched into dense and cold filaments, as was also found by Mellema et al (2002), Fragile et al (2004), Orlando et al (2005), and Johansson & Ziegler (2013). The simulations show how a conductive interface forms, leading to an increased density in the core that manages to radiate the energy conducted into the cloud, while stabilizing the cloud against hydrodynamical instabilities.…”

It's Cloud's Illusions I Recall: Mixing Drives the Acceleration of Clouds from Ram Pressure Stripped Galaxies

“…Bruggen & Scannapieco (2016) include both radiative cooling and heat conduction in simulations of clouds being ejected in galactic outflows. They find that while the outer envelope is evaporated, the central region cools and stretches into dense filaments (in agreement with simulations that only include radiative cooling: Mellema et al 2002;Fragile et al 2004;Orlando et al 2005;Johansson & Ziegler 2013). These clouds are accelerated only at early times because their cross-section decreases dramatically as the outer layers evaporate, so they move more slowly than simulated clouds without heat conduction.…”

Momentum Injection by Supernovae in the Interstellar Medium

“…In addition, hydrodynamic instabilities are strongly suppressed with nearequipartition magnetic fields β ∼ 1 (e.g., Mac Low et al 1994;Fragile et al 2005;Shin et al 2008). Calculations of shock-cloud interactions including magnetic fields, anisotropic thermal conduction, and radiative cooling by Orlando et al (2008) have shown that magnetic fields with moderate strength (β = 4) indeed suppress heat transfer and hydrodynamic instabilities (see also Johansson & Ziegler 2013). In future work, it will be interesting to extend the SNR expansion simulations of this paper to include magnetic fields and anisotropic conduction, in order to quantify possible changes in the momentum injection, maximum mass of hot gas, and other basic properties.…”