Is multiphase gas cloudy or misty?

Grönke, Max; Oh, S. Peng

doi:10.1093/mnrasl/slaa033

Cited by 80 publications

(73 citation statements)

References 35 publications

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“…Using this model, a cold cloud in typical CGM conditions can have a characteristic size somewhere in the range of ℓ 1 pc 1 kpc cloudlet , depending on the gas pressure, metallicity, and UV background radiation. Additionally, depending on the conditions of the surrounding medium, the true size of cold gas clouds may be significantly larger, as clouds can coagulate to form larger structures (Gronke & Oh 2020).…”

Section: Cold Cloud Sizesmentioning

confidence: 99%

The Impact of Cosmic Rays on Thermal Instability in the Circumgalactic Medium

Butsky

Fielding

Hayward

et al. 2020

ApJ

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Large reservoirs of cold (∼10 4 K) gas exist out to and beyond the virial radius in the circumgalactic medium (CGM) of all types of galaxies. Photoionization modeling suggests that cold CGM gas has significantly lower densities than expected by theoretical predictions based on thermal pressure equilibrium with hot CGM gas. In this work, we investigate the impact of cosmic-ray physics on the formation of cold gas via thermal instability. We use idealized three-dimensional magnetohydrodynamic simulations to follow the evolution of thermally unstable gas in a gravitationally stratified medium. We find that cosmic-ray pressure lowers the density and increases the size of cold gas clouds formed through thermal instability. We develop a simple model for how the cold cloud sizes and the relative densities of cold and hot gas depend on cosmic-ray pressure. Cosmic-ray pressure can help counteract gravity to keep cold gas in the CGM for longer, thereby increasing the predicted cold mass fraction and decreasing the predicted cold gas inflow rates. Efficient cosmic-ray transport, by streaming or diffusion, redistributes cosmicray pressure from the cold gas to the background medium, resulting in cold gas properties that are in between those predicted by simulations with inefficient transport and simulations without cosmic rays. We show that cosmic rays can significantly reduce galactic accretion rates and resolve the tension between theoretical models and observational constraints on the properties of cold CGM gas.

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Section: Cold Cloud Sizesmentioning

confidence: 99%

The Impact of Cosmic Rays on Thermal Instability in the Circumgalactic Medium

Butsky

Fielding

Hayward

et al. 2020

ApJ

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“…They indicate two processes for which the condensation is shredded apart through high vorticity, and they are thus explained differently from the 'shattering' mechanism of McCourt et al (2018). In the first process, vorticity is generated in a rapid expansion phase by a Richtmyer-Meshkov (Richtmyer 1960;Meshkov 1972) process before the condensation fragments into smaller pieces (Gronke & Oh 2020). In the second process, smaller condensations are fragmented by pressure gradients between merging large clouds (Waters & Proga 2019a).…”

Section: Conclusion and Discussionmentioning

confidence: 99%

“…There is no consensus about the mechanism for fragmentation of high beta hydrodynamic gas clouds (see e.g. Waters & Proga 2019a,b;Gronke & Oh 2020;Jennings & Li 2021;Das et al 2021). One of those processes is 'shattering' (McCourt et al 2018), where a cloud fragments into small pieces that each cool isobarically, independently of each other.…”

Section: Conclusion and Discussionmentioning

confidence: 99%

Effect of optically thin cooling curves on condensation formation: Case study using thermal instability

Hermans,

Keppens

2021

Preprint

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Context. Non-gravitationally induced condensations are observed in many astrophysical environments. In solar physics, common phenomena are coronal rain and prominences. These structures are formed due to energy loss by optically thin radiative emission. Instead of solving the full radiative transfer equations, precomputed cooling curves are typically used in numerical simulations. In the literature, a wide variety of cooling curves exist, and they are quite often used as unquestionable ingredients. Aims. We here determine the effect of the optically thin cooling curves on the formation and evolution of condensations. We also investigate the effect of numerical settings. This includes the resolution and the low-temperature treatment of the cooling curves, for which the optically thin approximation is not valid. Methods. We performed a case study using thermal instability as a mechanism to form in situ condensations. We compared 2D numerical simulations with different cooling curves using interacting slow magnetohydrodynamic (MHD) waves as trigger for the thermal instability. Furthermore, we discuss a bootstrap measure to investigate the far non-linear regime of thermal instability. In the appendix, we include the details of all cooling curves implemented in MPI-AMRVAC and briefly discuss a hydrodynamic variant of the slow MHD waves setup for thermal instability.Results. For all tested cooling curves, condensations are formed. The differences due to the change in cooling curve are twofold. First, the growth rate of the thermal instability is different, leading to condensations that form at different times. Second, the morphology of the formed condensation varies widely. After the condensation forms, we find fragmentation that is affected by the low-temperature treatment of the cooling curves. Condensations formed using cooling curves that vanish for temperatures lower than 20 000 K appear to be more stable against dynamical instabilities. We also show the need for high-resolution simulations. The bootstrap procedure allows us to continue the simulation into the far non-linear regime, where the condensation fragments dynamically align with the background magnetic field. The non-linear regime and fragmentation in the hydrodynamic case differ greatly from the low-beta MHD case. Conclusions. We advocate the use of modern cooling curves, based on accurate computations and current atomic parameters and solar abundances. Our bootstrap procedure can be used in future multi-dimensional simulations to study fine-structure dynamics in solar prominences.

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“…• Multiple clouds; Kinematic Structure. To account for the observations, a line of sight has to pass through ∼ 100 − 1000 mixing layers; which is conceivable if the cold gas has a 'fog-like' structure (McCourt et al 2018;Gronke & Oh 2020b). Cold gas in a turbulent medium acquires a wide-ranging, almost scale-free range of sizes, but the covering fraction is dominated by small cloudlets (Gronke et al 2021, in preparation).…”

Section: Discussionmentioning

confidence: 99%

A Model for Line Absorption and Emission from Turbulent Mixing Layers

Tan,

2021

Preprint

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Turbulent mixing layers (TMLs) are ubiquitous in multiphase gas. They can potentially explain observations of high ions such as O , which have significant observed column densities despite short cooling times. Previously, we showed that global mass, momentum and energy transfer between phases mediated by TMLs is not sensitive to details of thermal conduction or numerical resolution (Tan et al. 2021). By contrast, we show here that observables such as temperature distributions, column densities and line ratios are sensitive to such considerations. We explain the reason for this difference. We develop a prescription for applying a simple 1D conductive-cooling front model which quantitatively reproduces 3D hydrodynamic simulation results for column densities and line ratios, even when the TML has a complex fractal structure. This enables sub-grid absorption and emission line predictions in large scale simulations. The predicted line ratios are in good agreement with observations, while observed column densities require numerous mixing layers to be pierced along a line of sight.

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Is multiphase gas cloudy or misty?

Cited by 80 publications

References 35 publications

The Impact of Cosmic Rays on Thermal Instability in the Circumgalactic Medium

The Impact of Cosmic Rays on Thermal Instability in the Circumgalactic Medium

Effect of optically thin cooling curves on condensation formation: Case study using thermal instability

A Model for Line Absorption and Emission from Turbulent Mixing Layers

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