Until recently the dynamical evolution of the interstellar medium (ISM) was
simulated using collisional ionization equilibrium (CIE) conditions. However,
the ISM is a dynamical system, in which the plasma is naturally driven out of
equilibrium due to atomic and dynamic processes operating on different
timescales. A step forward in the field comprises a multi-fluid approach taking
into account the joint thermal and dynamical evolutions of the ISM gas.Comment: Overview paper (3 pages) presented by M. Avillez at the Special
  Session "Modern views of the interstellar medium", XXVIIIth IAU General
  Assembly, August 27-30, 2012, Beijing. Chin

Asgekar, A.

Breitschwerdt, D.

de Avillez, M. A.

Spitoni, E.

Proceedings of the International Astronomical Union

English

arXiv

AbstractUntil recently the dynamical evolution of the interstellar medium (ISM) was simulated using collisional ionization equilibrium (CIE) conditions. However, the ISM is a dynamical system, in which the plasma is naturally driven out of equilibrium due to atomic and dynamic processes operating on different timescales. A step forward in the field comprises a multi-fluid approach taking into account the joint thermal and dynamical evolutions of the ISM gas.</jats:p

M. A. de Avillez

D. Breitschwerdt

A. Asgekar

E. Spitoni

Crossref

ISM simulations: an overview of models

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Until recently the dynamical evolution of the interstellar medium (ISM) was simulated using collisional ionization equilibrium (CIE) conditions. However, the ISM is a dynamical system, in which the plasma is naturally driven out of equilibrium due to atomic and dynamic processes operating on different timescales. A step forward in the field comprises a multi-fluid approach taking into account the joint thermal and dynamical evolutions of the ISM gas

Avillez, M. A. de

Breitschwerdt, Dieter

DepositOnce (Techn. Univ. Berlin)

Highlights of Astronomy, Volume 16XXVIIIth IAU General Assembly, August 2012T. Montmerle, ed.c© International Astronomical Union 2015doi:10.1017/S1743921314012393ISM simulations: an overview of modelsM. A. de Avillez1,2, D. Breitschwerdt2, A. Asgekar3 and E. Spitoni11Dept. of Mathematics, University of E´vora, 7000 E´vora, Portugalemail: mavillez,spitoni@galaxy.lca.uevora.pt2Dept. of Astronomy & Astrophysics, Technical University Berlin, D-10623 Berlin, Germanyemail: breitschwerdt@astro.physik.tu-berlin.de3ASTRON, P.O. Box 2, 7990 AA Dwingeloo, The NetherlandsAbstract. Until recently the dynamical evolution of the interstellar medium (ISM) was simu-lated using collisional ionization equilibrium (CIE) conditions. However, the ISM is a dynamicalsystem, in which the plasma is naturally driven out of equilibrium due to atomic and dynamicprocesses operating on diﬀerent timescales. A step forward in the ﬁeld comprises a multi-ﬂuidapproach taking into account the joint thermal and dynamical evolutions of the ISM gas.Keywords. ISM: general, ISM: structure, atomic processes, turbulence; MHD1. IntroductionThe attempts to model the supernova-driven ISM can be traced to the seminal modelsof Cox & Smith (1974; CS74) and McKee & Ostriker (1977, MO77). In the formersupernovae (SNe) maintain an interconnected tunnel network ﬁlled with X-ray emittinggas, while in MO77 the gas is distributed into three phases in global pressure equilibrium.In both models the Galactic volume (50% in CS74 and 70-80% in MO77) is ﬁlled withhot (> 105 K) low-density gas. Further ramiﬁcations include the break-out of the hotintercloud medium, cooling and condensing into clouds (galactic fountain; Shapiro &Field 1976) or escaping as a wind (e.g., chimney model; Norman & Ikeuchi 1989).Although these early works capture some of the essential physics, more complex andsophisticated models were devised by taking advantage of numerical simulations. Thesecomprise the evolution of a patch of the Galactic disk in two dimensions (2D) (hydrody-namical (HD): Bania & Lyon 1980; Chiang & Prendergast 1985; Chiang & Bregman 1988;Rosen et al. 1993; Magnetohydrodynamical (MHD): Vazquez-Semadeni et al. 1995), andin three-dimensions (3D), e.g.. the MHD evolution of a 2003 pc3 region (Balsara et al.2004) and the cosmic-rays driven ampliﬁcation of the ﬁeld in a diﬀerentially rotated do-main (0.5×1× [−0.6, 0.6] kpc3; Hanasz et al. 2004). The ﬁrst disk-halo evolution models(2D HD) were developed by Rosen & Bregman (1995). With increasing of computerpower, 3D HD (de Avillez model in 2000 and upgrades - see Avillez & Breitschwerdt2007; Joung & Mac Low 2006) and MHD (Korpi et al. 1999; Avillez & Breitschwerdt2005; Gressel et al. 2008; Hill et al. 2012) models have been developed.In general the disk-halo models consider parameters according to observations (e.g.,initial matter distribution with height, SN rates, background UV radiation ﬁeld). Dif-ferences are found in the number of physical processes included (magnetic ﬁelds, cosmicrays, heat conduction, etc.), numerical techniques, type of grid (ﬁxed or diﬀerentiallyrotated using the shear box technique), and grid resolutions and sizes. Resolutions areﬁxed or beneﬁt from the use of the adaptive mesh reﬁnement (AMR) technique (Berger& Oliger 1984). The highest resolutions cover a wide range from 0.5 pc to 10 pc, passingthrough 2 and 8 pc. The grid sizes in the vertical direction range from 0.1 kpc to 15 kpc606https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921314012393Downloaded from https://www.cambridge.org/core. Universitaetsbibliothek, on 23 Nov 2017 at 11:18:54, subject to the Cambridge Core terms of use, available atISM simulations: an overview of models 607on either side of the Galactic midplane. However, grids extending up to 2 kpc imply thatthe disk-halo-disk cycle can neither be established nor tracked - the simulations are validfor a small period of time before the gas escapes from the top and bottom boundaries.These simulations showed that: (i) the ISM does not become saturated by SN activity,(ii) the disk expands and relaxes dynamically as SN rate ﬂuctuates in time and space,(iii) the turbulent ﬁeld builds up exponentially within 20 Myr of disk evolution, (iv) themagnetic ﬁeld does not strongly correlate with density, except for the densest regions,(v) the magnetic ﬁeld does not prevent the matter escape into the halo as it only brieﬂydelays the disk-halo cycle, (vi) the volume ﬁlling factor of hot gas in the Galactic disk isonly ∼20 %, (vii) there are large pressure variations in the disk in contrast to MO77 withthe thermal pressure dominating at high temperatures (T > 106 K), magnetic pressureat T < 200 K, and ram pressure elsewhere.2. Thermal and dynamical evolution of the ISMAll models referred previously assumed the ISM plasma to be in CIE, represented by aunique and general cooling function (CF) taken from diﬀerent sources (e.g., Dalgarno &McCray 1972 (DM72); Sutherland & Dopita 1993; Gnat & Sternberg 2007). CIE assumesthat the number of ionizations is balanced by recombinations from higher ionizationstages. However, CIE is only valid provided the cooling timescale (τcool) of the plasmais larger than the recombination times scales of the diﬀerent ions (τZ,zrec ), something thatoccurs at T > 106 K (see references above). For lower temperatures τcool < τZ,zrec , anddeviations from CIE are expected (see, e.g., DM72). These departures aﬀect the localcooling, which is a time-dependent process that controls the ﬂow dynamics, feeding backto the thermal evolution by a change in the density and internal energy distribution,which in turn modiﬁes the thermodynamic path of non-equilibrium cooling.A major improvement in ISM studies is therefore to carry out time-dependent multi-ﬂuid calculations of the joint thermal and dynamical evolution of the plasma, i.e. to followeach ﬂuid element’s thermal history by determining its ionization structure and CF ateach time-step. Radiative losses are folded into the energy equation with the internalenergy including also the potential energies associated to the diﬀerent ionization stages.Historically, there have been a number of simulations, which have included part ofthe ionization history into HD simulations, (Cox & Anderson 1982; Innes et al. 1987;Borkowski et al. 1994; Smith & Cox 2001; among others), misty tailored for speciﬁcastrophysical problems. The eﬀect of delayed recombination has been emphasized byBreitschwerdt & Schmutzler (1994), who have modelled the soft X-ray background. Me-lioli et al. (2009), following the formation and evolution of Hi clouds, only considered thetime evolution of selected ions (Hi , Hii , Cii -Civ , and Oi -Oiii ) for temperatures below106 K, using a ﬁt to the Sutherland & Dopita (1993) CF for T  106 K. This setuphas severe implications in the cooling of the gas as their calculation does not trace therelevant ions recombining to Civ , and Oiii .Recently, owing to the development of the Atomic+Molecular Plasma Emission Code(EA+MPEC) and its coupling to a PPM based AMR code, it has been possible to carryout multi-ﬂuid calculations of the ISM tracing both the thermal and dynamical evolutionsof the gas self-consistently. The ionization structure, cooling and emission spectra of H,He, C, N, O, Ne, Mg, Si, S, and Fe ions (with solar abundances; Asplund et al. 2009)are traced on the spot at each time step assuming an equal Maxwellian temperature forelectrons and ions (see details and references in Avillez & Breitschwerdt 2012).These simulations showed several interesting eﬀects: (i) in a dynamic ISM, the ion-ization structure and, therefore, the CF, varies with space and time, depending on thehttps://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921314012393Downloaded from https://www.cambridge.org/core. Universitaetsbibliothek, on 23 Nov 2017 at 11:18:54, subject to the Cambridge Core terms of use, available at608 M. A. de Avillez, et al.initial conditions and its history, (ii) the cooling paths in general do not follow the onepredicted by static plasma emission calculations, (iii) non-equilibrium ionization X-rayemission in the ∼ 0.25 keV band of gas with T < 105K can dominate the correspondingCIE emission at even T = 106.2 K as a result of delayed recombination, (iv) the presenceof Ovi ions at temperatures < 105 K corresponding to 70% of the total Ovimass, and(v) a large fraction of electrons are found in the thermally unstable regime and have alog normal distribution with similar properties (mean and dispersion) to those derivedfrom observations against pulsars with known distances.3. ConclusionsThe dynamical and thermal evolution of the ISM are strongly coupled, because theionization structure determines the CF, which in turn controls the dynamics and therebythe ionization structure, closing a feedback loop. Consequently, strong deviations fromCIE occur due to severe mismatches between the diﬀerent ionization/recombination anddynamical time scales of the plasma. Similar eﬀects due time-dependent cooling areexpected in other astrophysical contexts.AcknowledgementsM.A.A. thanks the IAU and speciﬁcally Y.-H. Chu and V. Reuter for ﬁnancial support.ReferencesAsplund, M., Grevesse, N., Sauval, A. J., & Scott, P. 2009, ARAA 47, 481Balsara, D. S., Kim, J., Mac Low, M.-M., & Mathews, G. J. 2004, ApJ 617, 339Bania, T. M. & Lyon, J. G. 1980, ApJ 239, 173Berger, M. J. & Oliger, J. 1984, JCP 484Borkowski, K. J., Sarazin, C. L., & Blondin, J. M. 1994, ApJ 429, 710Breitschwerdt, D., Schmutzler T. 1994, Nature 371, 774Chiang, W.-H. & Prendergast, K. H. 1985, ApJ 297, 507Chiang, W.-H. & Bregman, J. N. 1988, ApJ 328, 427Cox, D. P. & Anderson, P. R. 1982, ApJ 253, 268Cox, D. P. & Smith, B. W. 1974, ApJ (Letters) 189, L105de Avillez, M. A. & Breitschwerdt, D. 2005, A&A 436, 585de Avillez, M. A. & Breitschwerdt, D. 2007, ApJ (Letters) 665, L35de Avillez, M. A. & Breitschwerdt, D. 2012, ApJ (Letters) 761, L19Gnat, O. & Sternberg, A. 2007, ApJS 168, 213Gressel, O., Elstner, D., Ziegler, U., & Ro¨diger, G. 2008, A&A 486, 35Hanasz, M., Kowal, G, Otmianowska-Mazur, K., & Lesch, H. 2004, A&A (Letters) 605, L33Hill, A. S., Joung, M. R., Mac Low, M.-M., et al. 2012, ApJ 750, 104Innes, D. E., Giddings, J. R., & Falle, S. A. E. G. 1987, MNRAS 227, 1021Joung, M. K. R. & Mac Low, M.-M. 2006, ApJ 653, 1266Korpi, M. J., Brandenburg, A., Shukurov, A., et al. 1999, ApJ (Letters) 514, L99Melioli, C., Brighenti, F., D’Ercole, A., & Gouveia Dal Pino, E. M. 2009, MNRAS 399, 1089McKee, C. F. & Ostriker, J. P. 1977, ApJ 218, 148Norman, C. A. & Ikeuchi, S. 1989, ApJ 345, 372Rosen, A., Bregman, J. N., & Norman, M. L. 1993, ApJ 413, 137Rosen, A. & Bregman, J. N. 1995, ApJ 440, 634Shapiro, P. R. & Field, G. B. 1976, ApJ 205, 762Smith, R. K. & Cox, D. P. 2001, ApJS 134, 283Sutherland, R. S. & Dopita, M. A. 1993, ApJ 88, 253Vazquez-Semadeni, E., Passot, T., & Pouquet, A. 1995, ApJ 441, 702https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921314012393Downloaded from https://www.cambridge.org/core. Universitaetsbibliothek, on 23 Nov 2017 at 11:18:54, subject to the Cambridge Core terms of use, available at

Until recently the dynamical evolution of
the interstellar medium (ISM) was simu-
lated using collisional ionization equilibrium (CIE) conditions. However, the ISM is a dynamical
system, in which the plasma is naturally driven out of equilibrium due to atomic and dynamic
processes operating on different timescales. A step forward in the field comprises a multi-fluid
approach taking into account the joint thermal and dynamical evolutions of the ISM gas

ISM Simulations: An Overview of Models

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