Fragmentation of Shocked Flows: Gravity, Turbulence, and Cooling

Heitsch, Fabian; Hartmann, L.; Burkert, Andreas

doi:10.1086/589919

Cited by 78 publications

(81 citation statements)

References 48 publications

(103 reference statements)

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“…The streamlines show that the velocity field is nearly laminar in the WNM and becomes very turbulent inside the compressed layer and therefore also around the CNM structures. This is similar to what is reported by Heitsch et al (2006Heitsch et al ( , 2008b who point out the influence of the Kelvin-Helmholtz instability as a plausible source of turbulence within the forming cloud. Figure 3 displays density maps of a slice of the 2-phase run (left) and of the isothermal one (right).…”

Section: Global Statisticssupporting

confidence: 91%

On the structure of the turbulent interstellar clouds

Audit¹,

Hennebelle

2010

A&A

102

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Context. It is well established that the atomic interstellar hydrogen is filling the galaxies and constitutes the building blocks of molecular clouds. Aims. To understand the formation and the evolution of molecular clouds, it is necessary to investigate the dynamics of turbulent and thermally bistable as well as barotropic flows. Methods. We perform high resolution 3-dimensional hydrodynamical simulations of 2-phase, isothermal and polytropic flows. Results. We compare the density probability distribution function (PDF) and Mach number density relation in the various simulations and conclude that 2-phase flows behave rather differently than polytropic flows. We also extract the clumps and study their statistical properties such as the mass spectrum, mass-size relation and internal velocity dispersion. In each case, it is found that the behavior is well represented by a simple power law. While the various exponents inferred are very similar for the 2-phase, isothermal and polytropic flows, we nevertheless find significant differences, as for example the internal velocity dispersion, which is smaller for 2-phase flows. Conclusions. The structure statistics are very similar to what has been inferred from observations, in particular the mass spectrum, the mass-size relation and the velocity dispersion-size relation are all power laws whose indices well agree with the observed values. Our results suggest that in spite of various statistics being similar for 2-phase and polytropic flows, they nevertheless present significant differences, stressing the necessity to consider the proper thermal structure of the interstellar atomic hydrogen for computing its dynamics as well as the formation of molecular clouds.

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Section: Global Statisticssupporting

confidence: 91%

On the structure of the turbulent interstellar clouds

Audit¹,

Hennebelle

2010

A&A

102

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“…While usually assumed to be intrinsically single supersonic lines, these broad profiles might reveal a superposition of multiple transonic line velocity components separated by supersonic velocity differences. In contrast to the classical interpretation in the framework of the unresolved, microscopic turbulence, these findings would suggest that the internal dynamics of molecular clouds would be described by a macroscopic turbulence, for example, as generated in molecular clouds that form in a cooling converging gas flows (e.g., Heitsch et al 2008a). These numerical simulations indeed lead to a highly clumpy cold component with intrinsically subsonic turbulence, embedded in a more diffuse and warm inter-clump medium.…”

Section: Discussionmentioning

confidence: 69%

“…Molecular clouds that consist of large-scale fiber-like, elongated structures arise naturally in numerical simulations of diffuse, warm HI gas flows that undergo a cooling instability coupled with various hydrodynamical instabilities (Ballesteros-Paredes et al 1999;Heitsch et al 2008a;Ntormousi et al 2011). Another possibility might be gravitational modes, sweeping up material at the edges of collapsing structures (Burkert & Hartmann 2004;Hartmann & Burkert 2007;Vázquez-Semadeni et al 2007;Heitsch et al 2008b).…”

Section: Discussion: Macroscopic Turbulence and Tangled Molecular Cloudsmentioning

confidence: 99%

Opacity broadening and interpretation of suprathermal CO linewidths: Macroscopic turbulence and tangled molecular clouds

Hacar

Alves

Burkert

et al. 2016

A&A

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Context. Since their first detection in the interestellar medium, (sub-)millimeter line observations of different CO isotopic variants have routinely been employed to characterize the kinematic properties of the gas in molecular clouds. Many of these lines exhibit broad linewidths that greatly exceed the thermal broadening expected for the low temperatures found within these objects. These observed suprathermal CO linewidths are assumed to originate from unresolved supersonic motions inside clouds. Aims. The lowest rotational J transitions of some of the most abundant CO isotopologues, 12 CO and 13 CO, are found to present large optical depths. In addition to well-known line saturation effects, these large opacities present a non-negligible contribution to their observed linewidths. Typically overlooked in the literature, in this paper we aim to quantify the impact of these opacity broadening effects on the current interpretation of the CO suprathermal line profiles. Methods. Combining large-scale observations and LTE modeling of the ground J = 1−0 transitions of the main 12 CO, 13 CO, C 18 O isotopologues, we have investigated the correlation of the observed linewidths as a function of the line opacity in different regions of the Taurus molecular cloud. Results. Without any additional contributions to the gas velocity field, a large fraction of the apparently supersonic (M ∼ 2-3) linewidths measured in both 12 CO and 13 CO (J = 1−0) lines can be explained by the saturation of their corresponding sonic-like, optically thin C 18 O counterparts assuming standard isotopic fractionation. Combined with the presence of multiple components detected in some of our C 18 O spectra, these opacity effects also seem to be responsible for most of the highly supersonic linewidths (M > 8-10) detected in some of the broadest 12 CO and 13 CO spectra in Taurus.Conclusions. Our results demonstrate that most of the suprathermal 12 CO and 13 CO linewidths reported in nearby clouds like Taurus could be primarily created by a combination of opacity broadening effects and multiple gas velocity components blended in these saturated emission lines. Once corrected by their corresponding optical depth, each of these gas components present transonic intrinsic linewidths consistently traced by the three isotopologues, 12 CO, 13 CO, and C 18 O, with differences within a factor of 2. Highly correlated and velocity-coherent at large scales, the largest and highly supersonic velocity differences inside clouds are generated by the relative motions between individual gas components. In contrast to the classical interpretation within the framework of microscopic turbulence, this highly discretized structure of the molecular gas traced in CO suggest that the gas dynamics inside molecular clouds could be better described by the properties of a fully resolved macroscopic turbulence.

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“…Filamentary structures which resemble observed clouds are seen in simulations of supersonic turbulence (Padoan et al 2001), of turbulence and self-gravity (Heitsch et al 2008;Gomez & Vazquez-Semadeni 2014), and of turbulence, self-gravity, and magnetic fields (Li et al 2010;Chen & Ostriker 2014). Filaments which are self-gravitating become more concentrated and persist longer than unbound filaments.…”

Section: Introductionmentioning

confidence: 84%

Characteristic Structure of Star-Forming Clouds

Myers

2015

ApJ

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This paper presents a new method to diagnose the star-forming potential of a molecular cloud region from the probability density function of its column density (N-pdf). This method provides expressions for the column density and mass profiles of a symmetric filament having the same N-pdf as a filamentary region. The central concentration of this characteristic filament can distinguish regions and can quantify their fertility for star formation. Profiles are calculated for N-pdfs which are pure lognormal, pure power law, or a combination. In relation to models of singular polytropic cylinders, characteristic filaments can be unbound, bound, or collapsing depending on their central concentration. Such filamentary models of the dynamical state of N-pdf gas are more relevant to star-forming regions than are spherical collapse models. The star formation fertility of a bound or collapsing filament is quantified by its mean mass accretion rate when in radial free fall. For a given mass per length, the fertility increases with the filament mean column density and with its initial concentration. In selected regions the fertility of their characteristic filaments increases with the level of star formation.

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Fragmentation of Shocked Flows: Gravity, Turbulence, and Cooling

Cited by 78 publications

References 48 publications

On the structure of the turbulent interstellar clouds

On the structure of the turbulent interstellar clouds

Opacity broadening and interpretation of suprathermal CO linewidths: Macroscopic turbulence and tangled molecular clouds

Characteristic Structure of Star-Forming Clouds

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