Accretion-driven turbulence as universal process: galaxies, molecular clouds, and protostellar disks

Abstract

Context. Even though turbulent motions are found everywhere in astrophysical systems, the origin of this turbulence is poorly understood. When cosmic structures form, they grow in mass via accretion from their surrounding environment. Aims. We propose that accretion is able to drive internal turbulent motions in a wide range of astrophysical objects and study this process in the case of galaxies, molecular clouds, and protoplanetary disks. Methods. We use a combination of numerical simulations and analytical arguments to predict the level of turbulence as a function of the accretion rate, the dissipation scale, and the density contrast, and compare our models with observational data. Results. We find that in Milky Way type galaxies the observed level of turbulence in the interstellar medium can be explained by accretion, provided that the galaxies gain mass at a rate comparable to the rate at which they form stars. This process is particularly relevant in the extended outer disks beyond the star-forming radius. For it to drive turbulence in dwarf galaxies, the accretion rate needs to exceed the star formation rate by a large factor, so we expect other sources to dominate. We also calculate the rate at which molecular clouds grow in mass when they build up from the atomic component of the galactic gas and find that their internal turbulence is likely to be driven by accretion as well. It is the very process of cloud formation that excites turbulent motions on small scales by establishing the turbulent cascade. In the case of T Tauri disks, we show that accretion can drive subsonic turbulence if the rate at which gas falls onto the disk is comparable to the rate at which disk material accretes onto the central star. This also explains the observed relation of accretion rate and stellar mass,Ṁ ∝ M 1.8 . The efficiency required to convert infall motion into turbulence is a few percent in all three cases. Conclusions. We conclude that accretion-driven turbulence is a universal concept with far-reaching implications for a wide range of astrophysical objects.

The Link Between Turbulence, Magnetic Fields, Filaments, and Star Formation in the Central Molecular Zone Cloud G0.253+0.016

“…The fact that we see turbulence thus leads us to conclude that it must be driven by some physical stirring mechanism. In general, potential driving mechanisms include supernova explosions and expanding radiation fronts and shells induced by high-mass stellar feedback (McKee 1989;Balsara et al 2004;Krumholz et al 2006;Breitschwerdt et al 2009;Goldbaum et al 2011;Peters et al 2011;Lee et al 2012), winds , gravitational collapse and accretion of material (Vazquez-Semadeni et al 1998;Elmegreen & Burkert 2010;Klessen & Hennebelle 2010;Vázquez-Semadeni et al 2010;Federrath et al 2011b;Robertson & Goldreich 2012;Lee et al 2015), and Galactic spiral-arm compressions of H I clouds turning them into molecular clouds (Dobbs & Bonnell 2008;, as well as magnetorotational instability (MRI) and shear (Piontek & Ostriker 2007;Tamburro et al 2009). Jets and outflows from young stars and their accretion disks have also been suggested to drive turbulence (Norman & Silk 1980;Matzner & McKee 2000;Banerjee et al 2007;Nakamura & Li 2008;Cunningham et al 2009Cunningham et al , 2011Carroll et al 2010;Wang et al 2010;Plunkett et al 2013Plunkett et al , 2015Federrath et al 2014;Offner & Arce 2014).…”

The impact of turbulence and magnetic field orientation on star-forming filaments

“…An initial trans-to supersonic turbulent velocity field as well as the subsequent injection of turbulence by accretion (Klessen & Hennebelle 2010;Arzoumanian et al 2013;Heitsch 2013) are not sufficient to stabilize a filament at a width of 0.1 pc. To overcome this problem, turbulence would have to be replenished in a more efficient way, e.g.…”

“…As stated in Klessen & Hennebelle (2010), only a small fraction of the infalling energy can be converted into turbulent motions. Hence, in order to find out which amount of turbulence can be sustained by the observed infall rate, we have to solve the following equation for σ:…”

SHOCKFIND - an algorithm to identify magnetohydrodynamic shock waves in turbulent clouds

“…High resolution threedimensional simulations of molecular clouds have shown that the star formation rate and efficiency depend on whether the turbulence is solenoidally or compressively driven (Federrath & Klessen 2012, and that the stellar initial mass function is sensitive both to non-ideal MHD effects such as ambipolar diffusion (McKee et al 2010) and to the driving and Mach number of the turbulence (Hennebelle & Chabrier 2009Hopkins 2013). The importance of stellar feedback (Krumholz et al 2007;Cunningham et al 2011;Myers et al 2014;Nakamura & Li 2007;Wang et al 2010; Price & E-mail: andrew.lehmann@mq.edu.au Bate 2008Bate , 2009Offner & Arce 2014;Federrath et al 2014;Federrath 2015;Padoan et al 2015), whether gravity drives turbulent motions (Elmegreen & Burkert 2010;Klessen & Hennebelle 2010;Vzquez-Semadeni et al 2010;Federrath et al 2011;Robertson & Goldreich 2012) and the role that turbulence plays in producing the ubiquitously observed filaments (Arzoumanian et al 2011;André et al 2014;Smith et al 2014Smith et al , 2016Federrath 2016;Kainulainen et al 2016;Hacar et al 2016) are big questions that continue to be studied. Rigorous observational effects distinctly revealing the presence or dominance of the various physical processes are strongly sought after.…”