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ohp_meeting_september_2018 [2018/09/22 22:04] admin-mist |
ohp_meeting_september_2018 [2018/09/22 22:17] (current) admin-mist |
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| + | ==== Presentations ==== | ||
| + | |||
| + | {{ :ohp_lehmann.pdf |Andrew}} | ||
| + | {{ :ohp-lesaffre.pdf |Pierre L}} | ||
| + | {{ :mist-ohp.pdf |François L}} | ||
| + | |||
| + | ==== Notes taken by FL ==== | ||
| + | |||
| **EDITH FALGARONE | **EDITH FALGARONE | ||
| ** | ** | ||
| Line 72: | Line 80: | ||
| * LVG approximation not valid because the postshock region is not showing large gradients. | * LVG approximation not valid because the postshock region is not showing large gradients. | ||
| * Implemented ALI+Ng for radiative transfer. | * Implemented ALI+Ng for radiative transfer. | ||
| + | |||
| + | **PIERRE LESAFFRE | ||
| + | ** | ||
| + | * MISTy questions : Origin of molecules in dilute and violent media (CO in diffuse irradiated media, warm H2, CH+) ? Origin of the clumpy structure of the cold ISM (fragmentation) ? Origin and structure of the B field (link with matter) ? | ||
| + | * Are these questions related to MHD turbulence dissipation ? | ||
| + | * Large-scale energy transferred to small scales by turbulent cascade | ||
| + | * Also cooling cascade leading to phase separation : how is kinetic energy shared between resulting phases ? What are the sizes of clumps ? How long does it take to form ? | ||
| + | * Dissipation at small scales occurs in bursts : intermittency | ||
| + | * from diffuse to stars : 30 orders of magnitude in density, but only two in velocity dispersion… Unclear what is meant here by the velocity dispersion (thermal ? macroscopic ?) | ||
| + | * Numerical simulations by PL solve isothermal MHD equations. | ||
| + | * Three parameters : Mach, viscosity, resistivity | ||
| + | * Numerical resolution on a grid leads to extra dissipation. Need to increase a lot the physical ones to make them stand out. | ||
| + | * Types of dissipation : Ohmic, viscous and ambipolar | ||
| + | * Initial conditions such that RMS comparable to Alfven speed | ||
| + | * Dissipation distribution log-normal | ||
| + | * Dissipative structures appear as sheets | ||
| + | * Have not looked at how these evolve in time. | ||
| + | * Presumably, where there is dissipation, there is heating and this would be where molecules are produced and observed. | ||
| + | * One dissipative structure is quite clearly either viscous or Ohmic in incompressible simulations, but more mixed in compressible simulations | ||
| + | * B field preferentially parallel to dissipative structure in 3D | ||
| + | * Distribution of dissipation vs convergence shows two branches, maybe more. | ||
| + | * Computation of the norm of the gradient of the various quantities combined (rho, u, B). Local diagonalization, find the principal axes. Where gradient is large, one direction dominates the others, so isocontours close to plane-parallel. Can work in 1D. Looking for ways to decompose gradients as a propagating waves. | ||
| + | * Paint the simulations with chemistry ? If shocks are identified, use the results of Andrew and BG to put chemistry. | ||
| + | |||
| + | **PIERRE HILY-BLANT | ||
| + | ** | ||
| + | * CO factories in molecular clouds | ||
| + | * Gould Belt clouds in the Solar neighborhood | ||
| + | * 12CO(1-0) more diffuse than 13CO(1-0), which is closer to the morphology of dust emission (SPIRE 500µ) | ||
| + | * If you stack spectra, even where you do not « see » CO, there appears CO after stacking (only was below noise level) | ||
| + | * Below 1e15 or a bit above, there is CO, but not seen in individual pixels | ||
| + | * Some chemical aspects : appreciable fraction (2/3) of hydrogen is atomic / Ortho-to-Para ratio / 1/3 of the oxygen not accounted for. | ||
| + | * Formation of MC ? Through thermal instabilities ? Lifetime 10-20 Myr ? | ||
| + | * Regulation of star formation : turbulence, magnetic field support and ambipolar diffusion | ||
| + | * Herchel revealed the existence of very diffuse filaments | ||
| + | * Differences between Taurus and Polaris : Polaris modre diffuse, less massive, more turbulent, not forming stars. | ||
| + | * Observations of 12CO(2-1) : variations will depend on chemistry (existence of CO), excitation (to levels >0), and radiative transfer (observation of photons) | ||
| + | * Idea of explaining the large velocity gradient : two large-scale HI+H2 flows counter-rotaing, where they meet, CO formation, then downstream CO-rich gas. | ||
| + | * Structure function (computed on the centroid velocity) scaling very close to SL91 (incompressible turbulence) | ||
| + | * Intermittency (large deviations) show up at high p, but careful that the higher p, the higher weight is given to the noise. | ||
| + | * Taurus structure function scaling in between K41 and SL91, but this cannot be true (not incompressible, and there is intermittency) | ||
| + | * Maps of centroid velocity increment extrema highly non-Gaussian (filamentary) | ||
| + | |||
| + | {{ ::a45bd86b38b18112daee0f079839d325.jpg?600 |}} | ||
| + | |||
| + | **ERWAN ALLYS | ||
| + | ** | ||
| + | * Power spectrum does not encode all the information in a given field, when it has non-Gaussian properties. | ||
| + | * WST : scattering coefficients S0, S1, S2 : about 1000 coefficients. Based on translation and small deformations. | ||
| + | * A second reduction is related to physics (isotropy, rotations) : ended up with about 70 coefficients | ||
| + | * Power spectrum does not care whether a scale appears in conjunction with another or not. | ||
| + | |||
| + | **ELENA BELLOMI | ||
| + | ** | ||
| + | * Dynamical evolution of matter in the ISM is accompanied by a chemical evolution, and chemistry has in turn an impact on the dynamics (cooling) | ||
| + | * Interest in the diffuse ISM : partly ionized, partly molecular, turbulent | ||
| + | * Equipartition of energies | ||
| + | * Theoretical approaches : dynamical simulations / PDR models and TDR models for chemical evolution. Scales to model : from large scales 50-100 pc to dissipation scales (not resolvable) | ||
| + | * Numerical simulations : dissipation is numerical | ||
| + | * Astrochemical models | ||
| + | * Out-of-equilibrium chemically or thermally. Timescales for equilibria can be very different. Usually assuming chemical steady-state, driven by kinetics, not by thermodynamics. | ||
| + | * Focus on H2 chemistry : formation time is particularly long, influence on dynamics through heating (exothermic formation) and cooling (lines, also from C+, O, Lyman alpha) | ||
| + | * Photodissociation of H2 0.4 eV | ||
| + | * Heating and cooling curve : net loss L=n^2\lambda(T)-n\Gamma —> thermal instability curve, depending on mostly G0 and abundances of PAHs | ||
| + | * Simulations : perdiodic boundary conditions, isotropic turbulent forcing in Fourier space, no thermal conduction, no gravity. H2 formation and destruction computed on the fly. | ||
| + | * Two fluids (H and H2) plus a prescription for C+, O, for cooling. | ||
| + | * H2 treated out of equilibrium because timescale for reaching steady-state for H2 is long and determines the timescales for other species. | ||
| + | * Large fraction of the gas is out of equilibrium thermally | ||
| + | * N(H2) vs N_H shows bimodal distribution : transition H -> H2 with self-shielding of H2 leas to phase transition | ||
| + | * Different setups in density, G0, forcing amplitude, B field, resolution and box size. | ||
| + | * Separation of voxels in <300K, 300K-3000K,>3000K | ||
| + | * Influence of initial density : the larger, the more CNM in the « final » state. | ||
| + | * G0 increase leads to decrease of f(H2) by increased photodissociation | ||
| + | * Turbulence increase pushes more gas away from the thermal equilibrium curve. | ||
| + | * Increase of B prevents the formation of dense structures. | ||
| + | |||
| + | |||
| + | ==== Afterthoughts by BG ==== | ||
| + | |||
| + | Starting with the idea that the ISM is a multiphase turbulent medium, several observations | ||
| + | may help us to understand | ||
| + | * Pierre's shear | ||
| + | * Edith's starburst | ||
| + | |||
| + | |||
| + | |||
| + | I'm no observer so I'll just throw ideas even if they are ridiculous or not completely correct | ||
| + | |||
| + | • To trace the multiphase ISM | ||
| + | - - HI maps (mass of WNM+CNM) | ||
| + | - - ArH+ (tracer of purely atomic gas -> even better tracer than HI) | ||
| + | - - CII and OI fine structure (set the global thermal balance of CNM -> mass of CNM) | ||
| + | - - CII and OI metastable lines (trace the mass of WNM) | ||
| + | - - CI fine structure lines | ||
| + | - -> gives a measure of the gas thermal pressure | ||
| + | - -> may give the mass of unstable gas | ||
| + | - -> its relation with CO gives strong constrain on chemical models | ||
| + | - - CS, C2H, OH, and H2O (tracers of PDR in diffuse ISM) | ||
| + | - - HF (tracer of H2) | ||
| + | - - CH (tracer of PDR, except maybe at small column densities) | ||
| + | - - OH+ and H2O+ (tracers of CR ionization and molecular fraction) | ||
| + | |||
| + | |||
| + | • To trace the dissipation of turbulent energy at small scales and / or the turbulent mixing of phases at all scales | ||
| + | - - CH+ and SH+ | ||
| + | - - excited H2 | ||
| + | - - HCO+ and CO | ||
| + | - - CH (anomaly with PDR predictions at small column densities) | ||
| + | - - SH (not sure) | ||
| + | |||
| + | |||
| + | • For the DENSE GAS in starburst galaxies, we could try | ||
| + | - - SH+ | ||
| + | - - the rotational diagram of CH+ (as high as possible) -> this would be new by the way | ||
| + | - - CO, OH or H2O which behave oppositely to CH+ in irradiated shocks. | ||