Shock interactions, turbulence, and the origin of the stellar mass spectrum
Ralph E. Pudritz, N. K. -R. Kevlahan
备注：
20 pages, 5 figures, to appear in theme issue "Turbulent Mixing" of the Philosophical Transactions of the Royal Society A, Snezhana I. Abarzhi, Serge Gauthier, and Katepalli R. Sreenivasan (Guest Editors), accepted for publication
Supersonic turbulence is an essential element in understanding how structure within interstellar gas is created and shaped. In the context of star formation, many computational studies show that the mass spectrum of density and velocity fluctuations within dense clouds, as well as the distribution of their angular momenta, trace their origin to the statistical and physical properties of gas that is lashed with shock waves. In this article, we review the observations, simulations, and theories of how turbulent-like processes can account for structures we see in molecular clouds. We then compare traditional ideas of supersonic turbulence with a simpler physical model involving the effects of multiple shock waves and their interaction in the interstellar medium. Planar intersecting shock waves produce dense filaments, and generate vortex sheets that are essential to create the broad range of density and velocity structure in clouds. As an example, the lower mass behaviour of the stellar initial mass function can be traced to the tendency of a collection of shock waves to build-up a log-normal density distribution (or column density). Vorticity - which is essential to produce velocity structure over a very broad range of length scales in shocked clouds - can also be generated by the passage of curved shocks or intersecting planar shocks through such media. Two major additional physical forces affect the structure of star forming gas - gravity and feedback processes from young stars. Both of these can produce power-law tails at the high mass end of the IMF.
Radiative shock waves in the Cygnus Loop and other supernova remnants show different morphologies in [O III] and Hα emission. We use HST spectra and narrowband images to study the development of turbulence in the cooling region behind a shock on the west limb of the Cygnus Loop. We refine our earlier estimates of shock parameters that were based upon ground-based spectra, including ram pressure, vorticity and magnetic field strength. We apply several techniques, including Fourier power spectra and the Rolling Hough Transform, to quantify the shape of the rippled shock front as viewed in differ…
Observations indicate that turbulence in the interstellar medium (ISM) is supersonic ($M_{\rm turb}\gg1$) and strongly magnetized ($β\sim0.01-1$), while in the intracluster medium (ICM) it is subsonic ($M_{\rm turb}\lesssim1$) and weakly magnetized ($β\sim100$). Here, $M_{\rm turb}$ is the turbulent Mach number and $β$ is the plasma beta. We study the properties of shocks induced in these disparate environments, including the distribution of the shock Mach number, $M_s$, and the dissipation of the turbulent energy at shocks, through numerical simulations using a high-order accurate code based …
The probability distribution of density in isothermal, supersonic, turbulent gas is approximately lognormal. This behaviour can be traced back to the shock waves travelling through the medium, which randomly adjust the density by a random factor of the local sonic Mach number squared. Provided a certain parcel of gas experiences a large number of shocks, due to the central limit theorem, the resulting distribution for density is lognormal. We explore a model in which parcels of gas undergo finite number of shocks before relaxing to the ambient density, causing the distribution for density to d…
Evidence for Decay of Turbulence by MHD Shocks in Molecular Clouds via CO Emission
我们利用观测的亚毫米旋转转换的CO从赫歇尔周期2开放时间程序（“COPS”，PI：J.绿色），以确定以前预测的湍流耗散磁流体动力学（MHD）冲击分子云。我们发现证据的冲击预期耗散的MHD湍流的材料不与任何原恒星。两个模型的拟合程度相当：模型1的密度为10 $^{3}$ cm $^{-3}$，激波速度为3 km s $^{-1}$，磁场强度为4 $μ$ G;模型2的密度为10 $^{3.5}$ cm $^{-3}$，激波速度为$2$ km s，磁场强度为8 $μ$ G。在这一区域中，湍流衰减的时间尺度与穿越时间相当。转换的CO高达$J$的8，观察到接近恒星形成的活跃网站，但不是在外流，可以跟踪湍流耗散的冲击搅拌形成过程。虽然转换是很难在个别位置检测，我们的赫歇尔-SPIRE调查的原恒星提供了一个网格的分子云内的空间分布的光谱。我们平均了七颗原恒星附近已知流出物的所有空间位置。我们发现显着的协议与预测模型的湍流耗散在稍密（10 $^{3.5}$厘米$^{-3}$）材料具有较强的磁场（24 $μ$ G）比一般的分子云。
Statistics of the magnetic field disturbances in the supernova remnants (SNRs) can be accessed using the second-order correlation function of the synchrotron intensities. Here we measure the magnetic energy spectra in supernova remnant Cassiopeia-A by two-point correlation of the synchrotron intensities, using a recently developed unbiased method. The measured magnetic energy spectra in the vicinity of supernova remnant shocks are found to be 2/3 power law over the decade of range scales, showing the developed trans-Alfvenic magnetohydrodynamic turbulence. Our results are globally consistent w…
Recent observations of molecular clouds show that dense filaments are the sites of present-day star formation. Thus, it is necessary to understand the filament formation process because these filaments provide the initial condition for star formation. Theoretical research suggests that shock waves in molecular clouds trigger filament formation. Since several different mechanisms have been proposed for filament formation, the formation mechanism of the observed star-forming filaments requires clarification. In the present study, we perform a series of isothermal magnetohydrodynamics simulations…
Turbulence in the interstellar medium (ISM) is crucial in the process of star formation. Shocks produced by supernova explosions, jets, radiation from massive stars, or galactic spiral-arm dynamics are amongst the most common drivers of turbulence in the ISM. However, it is not fully understood how shocks drive turbulence, in particular whether shock driving is a more solenoidal(rotational, divergence-free) or a more compressive (potential, curl-free) mode of driving turbulence. The mode of turbulence driving has profound consequences for star formation, with compressive driving producing thre…
We report three-dimensional hydrodynamical simulations of shocks (${\cal M_{\rm shock}}\geq 4$) interacting with fractal multicloud layers. The evolution of shock-multicloud systems consists of four stages: a shock-splitting phase in which reflected and refracted shocks are generated, a compression phase in which the forward shock compresses cloud material, an expansion phase triggered by internal heating and shock re-acceleration, and a mixing phase in which shear instabilities generate turbulence. We compare multicloud layers with narrow ($σ_ρ=1.9\barρ$) and wide ($σ_ρ=5.9\barρ$) log-normal …
Small-scale fluctuating magnetic fields of order $n$G are observed in supernova shocks and galaxy clusters, where its amplification is likely caused by the Biermann battery mechanism. However, these fields cannot be amplified further without the turbulent dynamo, which generates magnetic energy through the stretch-twist-fold (STF) mechanism. Thus, we present here novel three-dimensional magnetohydrodynamic (MHD) simulations of a laser-driven shock propagating into a stratified, multiphase medium, to investigate the post-shock turbulent magnetic field amplification via the turbulent dynamo. The…
Stellar bow shocks are observed in a variety of interstellar environments and are shaped by the conditions of gas in the interstellar medium (ISM). In situ measurements of turbulent density fluctuations near stellar bow shocks are only achievable with a few observational probes, including H$α$ emitting bow shocks and the Voyager Interstellar Mission (VIM). In this paper, we examine density variations around the Guitar Nebula, an H$α$ bow shock associated with PSR B2224$+$65, in tandem with density variations probed by VIM near the boundary of the solar wind and ISM. High-resolution Hubble Spac…
Shocks form the basis of our understanding for the density and velocity statistics of supersonic turbulent flows, such as those found in the cool interstellar medium (ISM). The variance of the density field, $σ^2_{ρ/ρ_0}$, is of particular interest for molecular clouds (MCs), the birthplaces of stars in the Universe. The density variance may be used to infer underlying physical processes in an MC, and parameterises the star formation (SF) rate of a cloud. However, models for $σ^2_{ρ/ρ_0}$ all share a common feature -- the variance is assumed to be isotropic. This assumption does not hold when …
