The density variance-Mach number relation in isothermal and non-isothermal adiabatic turbulence

C. A. Nolan, C. Federrath, R. S. Sutherland

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    Abstract

    The density variance-Mach number relation of the turbulent interstellar medium is relevant for theoretical models of the star formation rate, efficiency, and the initial mass function of stars. Here we use high-resolution hydrodynamical simulations with grid resolutions of up to 10243 cells to model compressible turbulence in a regime similar to the observed interstellar medium. We use FYRIS ALPHA, a shock-capturing code employing a high-order Godunov scheme to track large density variations induced by shocks. We investigate the robustness of the standard relation between the logarithmic density variance (σ2 s) and the sonic Mach number (M) of isothermal interstellar turbulence, in the non-isothermal regime. Specifically, we test ideal gases with diatomic molecular (γ = 7/5) and monatomic (γ = 5/3) adiabatic indices. A periodic cube of gas is stirred with purely solenoidal forcing at low wavenumbers, leading to a fully developed turbulent medium. We find that as the gas heats in adiabatic compressions, it evolves along the relationship in the density variance-Mach number plane, but deviates significantly from the standard expression for isothermal gases. Our main result is a new density variance-Mach number relation that takes the adiabatic index into account: σ2 s = ln (1 + b2M(5γ+1)/3) and provides good fits for bM≲ 1. A theoretical model based on the Rankine-Hugoniot shock jump conditions is derived, σ2 s = ln{1 + (γ + 1)b2M2/[(γ - 1)b2M2 + 2]}, and provides good fits also for bM> 1. We conclude that this new relation for adiabatic turbulence may introduce important corrections to the standard relation, if the gas is not isothermal (γ ≠ 1).

    Original languageEnglish
    Pages (from-to)1380-1389
    Number of pages10
    JournalMonthly Notices of the Royal Astronomical Society
    Volume451
    Issue number2
    DOIs
    Publication statusPublished - 1 May 2015

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