Interstellar Turbulence I Observations and Processes

a r X i v :a s t r o -p h /0404451v 1 22 A p r 2004INTERSTELLAR TURBULENCE I:OBSER V ATIONS

AND PROCESSES

Bruce G.Elmegreen

IBM Research Division,Yorktown Heights,New York 10598;email:

bge@467d4f65f5335a8102d22050

John Scalo

Department of Astronomy,University of Texas,Austin,Texas 78712;e-mail:

parrot@467d4f65f5335a8102d22050

ABSTRACT

Turbulence a?ects the structure and motions of nearly all temperature and density regimes in the interstellar gas.This two-part review summa-rizes the observations,theory,and simulations of interstellar turbulence and their implications for many ?elds of astrophysics.The ?rst part begins with diagnostics for turbulence that have been applied to the cool inter-stellar medium,and highlights their main results.The energy sources for interstellar turbulence are then summarized along with numerical esti-mates for their power input.Supernovae and superbubbles dominate the total power,but many other sources spanning a large range of scales,from swing ampli?ed gravitational instabilities to cosmic ray streaming,all con-tribute in some way.Turbulence theory is considered in detail,including the basic ?uid equations,solenoidal and compressible modes,global in-viscid quadratic invariants,scaling arguments for the power spectrum,phenomenological models for the scaling of higher order structure func-tions,the direction and locality of energy transfer and cascade,velocity probability distributions,and turbulent pressure.We emphasize expected di?erences between incompressible and compressible turbulence.Theo-ries of magnetic turbulence on scales smaller than the collision mean free path are included,as are theories of magnetohydrodynamic turbulence and their various proposals for power spectra.Numerical simulations of interstellar turbulence are reviewed.Models have reproduced the basic features of the observed scaling relations,predicted fast decay rates for supersonic MHD turbulence,and derived probability distribution func-tions for density.Thermal instabilities and thermal phases have a new interpretation in a supersonically turbulent 467d4f65f5335a8102d22050rge-scale models with various combinations of self-gravity,magnetic ?elds,supernovae,and

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star formation are beginning to resemble the observed interstellar medium

in morphology and statistical properties.The role of self-gravity in turbu-

lent gas evolution is clari?ed,leading to new paradigms for the formation

of star clusters,the stellar mass function,the origin of stellar rotation and

binary stars,and the e?ects of magnetic?elds.The review ends with a

re?ection on the progress that has been made in our understanding of the

interstellar medium,and o?ers a list of outstanding problems.

Subject headings:turbulence,interstellar medium,energy sources,mag-

netohydrodynamics,turbulence simulations

1.INTRODUCTION

In1951,von Weizs¨a cker(1951)outlined a theory for interstellar matter(ISM) that is similar to what we believe today:cloudy objects with a hierarchy of structures form in interacting shock waves by supersonic turbulence that is stirred on the largest scale by di?erential galactic rotation and dissipated on small scales by atomic viscos-ity.The“clouds”disperse quickly because of turbulent motions,and on the largest scales they produce the?occulent spiral structures observed in galaxies.In the same year,von Hoerner(1951)noticed that rms di?erences in emission-line velocities of the Orion nebula increased with projected separation as a power law with a power αbetween0.25and0.5,leading him to suggest that the gas was turbulent with a Kolmogorov energy cascade(for whichαwould be0.33;Section4.6).Wilson et al. (1959)later got a steeper function,α～0.66,using better data,and proposed it resulted from compressible turbulence.Correlated motions with a Kolmogorov struc-ture function(Section2)in optical absorption lines were observed by Kaplan(1958). One of the?rst statistical models of a continuous and correlated gas distribution was by Chandrasekhar&M¨u nch(1952),who applied it to extinction?uctuations in the Milky Way surface brightness.Minkowski(1955)called the ISM“an entirely chaotic mass...of all possible shapes and sizes...broken up into numerous irregular details.”

These early proposals regarding pervasive turbulence failed to catch on.Interstel-lar absorption and emission lines looked too smooth to come from an irregular network of structures–a problem that is still with us today(Section2).The extinction glob-ules studied by Bok&Reilly(1947)looked too uniform and round,suggesting force equilibrium.Oort&Spitzer(1955)did not believe von Weizs¨a cker’s model because they thought galactic rotational energy could not cascade down to the scale of cloud linewidths without severe dissipation in inpidual cloud collisions.Similar concerns about dissipation continue to be discussed(Sections3,5.3).Oort and Spitzer also noted that the ISM morphology appeared wrong for turbulence:“instead of more or less continuous vortices,we?nd concentrated clouds that are often separated by much larger spaces of negligible density.”They expected turbulence to resemble the model of the time,with space-?lling vortices in an incompressible?uid,rather than today’s model with most of the mass compressed to a small fraction of the volume in shocks fronts.When a reddening survey by Sche?er(1967)used structure functions to infer power-law correlated structures up to5?in the sky,the data were characterized by saying only that there were two basic cloud types,large(70pc)and small(3pc),the

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same categories popularized by Spitzer(1968)in his textbook.

Most of the interesting physical processes that could be studied theoretically at the time,such as the expansion of ionized nebulae and supernovae(SNe)and the collapse of gas into stars,could be modeled well enough with a uniform isothermal medium.Away from these sources,the ISM was viewed as mostly static,with discrete clouds moving ballistically.The discovery of broad emission lines and narrow absorp-tion lines in H I at21cm reinforced this picture by suggesting a warm intercloud medium in thermal pressure balance with the cool clouds(Clark1965).ISM models with approximate force equilibrium allowed an ease of calculation and conceptualiza-tion that was not present with turbulence.Supernovae were supposed to account for the energy,but mostly by heating and ionizing the di?use phases(McCray&Snow 1979).Even after the discoveries of the hot intercloud(Bunner et al.1971,Jenkins& Meloy1974)and cold molecular media(Wilson et al.1970),the observation of a con-tinuous distribution of neutral hydrogen temperature(Dickey,Salpeter,&Terzian 1977),and the attribution of gas motions to supernovae(e.g.,McKee&Ostriker 1977),there was no compelling reason to dismiss the basic cloud-intercloud model in favor of widespread turbulence.Instead,the list of ISM equilibrium“phases”was simply enlarged.Supersonic linewidths,long known from H I(e.g.,McGee,Milton, &Wolfe1966,Heiles1970)and optical(e.g.,Hobbs1974)studies and also discovered in molecular regions at this time(see Zuckerman&Palmer1974),were thought to represent magnetic waves in a uniform cloud(Arons&Max1975),even though tur-bulence was discussed as another possibility in spite of problems with the rapid decay rate(Goldreich&Kwan1974,Zuckerman&Evans1974).A lone study by Baker (1973)found large-scale correlations in H I emission and presented them the con-text of ISM turbulence,deriving the number of“turbulent cells in the line of sight,”instead of the number of“clouds.”Mebold,Hachenberg&Laury-Micoulaut(1974) followed this with another statistical analysis of the H I emission.However,there was no theoretical context in which the Baker and Mebold et al.papers could?ourish given the pervasive models about discrete clouds and two or three-phase equilibrium.

The presence of turbulence was more widely accepted for very small scales.Ob-servations of interstellar scintillation at radio wavelengths implied there were corre-lated structures(Rickett1970),possibly related to turbulence(Little&Matheson 1973),in the ionized gas at scales down to109cm or lower(Salpeter1969;Inter-stellar Turbulence II,next chapter,this volume).This is the same scale at which cosmic rays(Interstellar Turbulence II)were supposed to excite magnetic turbulence by streaming instabilities(Wentzel1968a).However,there was(and still is)little understanding of the physical connection between these small-scale?uctuations and the larger-scale motions in the cool neutral gas.

Dense structures on resolvable scales began to look more like turbulence af-ter Larson(1981)found power-law correlations between molecular cloud sizes and linewidths that were reminiscent of the Kolmogorov scaling 467d4f65f5335a8102d22050rson’s work was soon followed by more homogeneous observations that showed similar correlations (Myers1983,Dame et al.1986,Solomon et al.1987).These motions were believed to be turbulent because of their power-law nature,despite continued concern with decay times,but there was little recognition that turbulence on larger scales could also form the same structures in which the linewidths were measured.Several re-views during this time re?ect the pending transition(Dickey1985,Dickman1985, Scalo1987,Dickey&Lockman1990).

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Perhaps the most widespread change in perception came when the Infrared As-tronomical Satellite(IRAS)observed interstellar“cirrus”and other clouds in emission at100μ(Low et al.1984).The cirrus clouds are mostly transparent at optical wave-lengths,so they should be in the di?use cloud category,but they were seen to be ?lamentary and criss-crossed,with little resemblance to“standard”clouds.Equally complex structures were present even in IRAS maps of“dark clouds,”like Taurus,and they were observed in maps of molecular clouds,such as the Orion region(Bally et al. 1987).The wide?eld of view and good dynamic range of these new surveys?nally allowed the di?use and molecular clouds to reveal their full structural complexity,just as the optical nebulae and dark clouds did two decades earlier.Contributing to this change in perception was the surprising discovery by Crovisier&Dickey(1983)of a power spectrum for widespread H I emission that was comparable to the Kolmogorov power spectrum for velocity in incompressible turbulence.CO velocities were found to be correlated over a range of scales,too(Scalo1984,Stenholm1984).By the late 1980s,compression from interstellar turbulence was considered to be one of the main cloud-formation mechanisms(see review in Elmegreen1991).

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