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The Turbulent Interstellar Medium

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arXiv:astro-ph/0605088v1 3 May 2006Theturbulentinterstellarmedium

A.Burkert

UniversityObservatoryMunich,Scheinerstr.1,D-81679Munich,Germany

1Introduction

Steady-statemulti-phasemodelshavedominatedourpictureoftheinterstellar

medium(ISM)intheMilkyWayforalongtime.Accordingtotheearlymodel

ofFieldetal.(1969)andsubsequentmodificationstheISMrepresentsan

ensembleoftwostablegasphasesthatareinthermalpressureequilibriumwithameanpressureofn×T≈1000Kcm−3.Coldmolecularcloudswith

meandensitiesofn≈100cm−3andtemperaturesT≈10Kareembedded

inawarm,diffuseandpartlyionizedintercloudcomponentwithdensityn≈

0.1cm−3andtemperaturesoforder104K.Thegascloudscontainalarge

fractionofthetotalmassandmoveasstable,densespheroidalobjectsinthe

widespreadintercloudmediumwhichontheotherhandhasthedominant

volumefillingfactor.Starformationwouldeventuallyheatanddispersethe

massiveclouds.Newgenerationsofsmallcloudsformfromtheintercloud

mediumbycoolinginstabilities(Field1965)andsubsequentlygrowbyrandom

inelasticcloud-cloudcollisions(e.g.Elmegreen1989).Alreadyin1977McKee

&Ostrikernoticedthatthetwo-phasemodelcouldnotbevalidassupernova

explosionsshouldleadtoathird,hotandtenuousgasphase.Howeverthey

stillfocussedontheimportanceofthermalpressureequilibriumandasteady

statedescriptionasthemainphysicalconstrainttoevaluatethestateofthe

variousgasphases.

Thissituationhaschangeddrasticallyinthelastdecade.High-resolutionob-

servationse.g.withtheInfraredAstronomicalSatellite(IRAS)andmorere-

centlywiththeSpitzersatelliterevealacomplexkinematicalstateandspatial

structureoftheinterstellargas.TheISMappearstobefarfromhydrostatic

equilibriumandturbulent.Infact,itisthekineticturbulentpressure,not

thermalpressure,thatprobablydominatesthegasdynamicsandthatchar-

acteristizesitsdensitystructureandthedynamicsofthevariousgasphases.

Turbulencecouplesstructuresonverydifferentscales.Molecularcloudsmight

justrepresentthehigh-densitytailofthishierarchy,formingincollidinggas

flowsthatleadtotransientlocalcompressionsthatsubsequentlycool.We

willdiscussbelowthatthecomplexinternalstructureofmolecularclouds

thatdeterminestheirevolutionandtheircondensationintostarsandstellar

clustersisalsoaresultofturbulence,generatedbyvarioushydrodynamical

instabilitiesduringtheprocessofmolecularcloudformation.

WithturbulencebecomingthedominantsourceofstructureintheISM,the-

oreticalmodelslosttheirsimplicityandequilibriumdescriptionshadtobe

replacedbydynamicalmodelswherelargeandsmallscalesaresimultane-

ouslyconsidered.Wearejuststartingtoexploreandunderstandthisrichand

enormouslycomplexnewfieldofastrophysics.InthisshortreviewIcanonly

focusonafewinterestingtopicsandunsolvedquestions.Anexcellentand

comprehensivereviewofourcurrentunderstandingispresentedbyElmegreen

&Scalo(2004)andScalo&Elmegreen(2004).Reviewsthatfocusespecially

onnumericalsimulationsofISMturbulencearee.g.V´azquez-Semadenietal.

(2000)andBallesteros-Paredesetal.(2006).Asummaryofourunderstanding

ofstarformationinturbulentcloudsisgivenbyMacLow&Klessen(2004).

2

Fig.1.Characteristicvelocitydispersionσofasampleofdiskgalaxiesasfunctionoftheiraveragesurfacestarformationrateinunitsof(M⊙yr−1kpc−2)(Dibetal.2006).σisalmostindependentofthestarformationratefortypicalvaluesfoundinslowlyevolvingdiskgalaxiesliketheMilkyWay.Itrisessteeplyinstarburstregionswithhighstarformationrates.

2TurbulenceinthediffuseISM

ItisnowawellestablishedfactthattheISMingalacticdisksisdominatedby

irregularandoftensupersonicgasmotions(Larson1981;Scalo1987;Dickey

&Lockman1990).InmostspiralgalaxiesHIemissionlinesexceedthevalues,

expectedfromthermalbroadening,indicatingturbulentvelocitydispersionsσ

oforder10km/s.Thevelocitydispersioningalacticdisksingeneraldecreases

outwardsfromσ≈12-15km/sintheinnerregionstoσ≈4-6km/sin

theouterparts.Figure1showsthecharacteristicvelocitydispersionfora

sampleofgalaxiesasfunctionoftheirsurfaceaveragedstarformationrate

(Dibetal.2006).Notethatself-regulatedstarformationratesthataretypical

forMilkyWay-typediskgalaxiesleadtovelocitydispersionsoforderσ≈

6-8kms−1,independentofthestarformationrate.ThediffuseISMacts

likeathermostate.Thesituationchangesdrasticallyinsituationswherestar

formationisgettingoutofcontrol,leadingtostarburststhatlastashorttime,

oforder108yrs.Asshowninfigure1,thetransitionintothestarburstregime

ismarkedbyasteepincreaseofthevelocitydispersion.

32.1DrivingturbulenceinthediffuseISM

Severalphysicalprocesses,actingondifferentscalesandinjectingdifferent

amountsofkineticenergycontributetothedrivingofISMturbulence.How-

ever,despitealargeamountofnumericalworkinthisfield,thedominanten-

ergeticsourcesandthephysicalprocessesthatconvertthekineticenergyinto

turbulencearenotwellunderstood.Starsareobviouscandidates.Large-scale

expandinggasflowscoulde.g.begeneratedbyhigh-pressureHIIregions,re-

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star的结构 -回复

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star的结构 -回复

Title: The Structure and Evolution of Stars

Introduction:

Stars, the celestial bodies that have fascinated and inspired

humanity for centuries, are marvels of nature. They are born, evolve,

and eventually die in a complex and intricately structured manner.

In this article, we will explore the fascinating structure of stars, from

their birth in stellar nurseries to their ultimate demise as white

dwarfs, neutron stars, or black holes.

1. Stellar Nurseries:

Stellar nurseries, also known as molecular clouds, are vast regions

of interstellar gas and dust that serve as the birthplace of stars.

Gravitational forces act upon these clouds, causing them to

collapse under their own weight. As the cloud contracts, it rotates

and flattens into a disk-like structure known as a protoplanetary

disk. Within this disk, a dense core forms, attracting surrounding

关于太空的信息英文作文

关于太空的信息英文作文

关于太空的信息英文作文

Space: A Tapestry of Limitless Wonders.

Space, the vast and enigmatic expanse beyond our

planet's atmosphere, has captivated human imagination for

centuries. It holds countless mysteries and infinite

possibilities, inviting us to explore its uncharted

territories and unravel its profound secrets.

A Symphony of Celestial Bodies.

Space is a symphony of celestial bodies, each playing a

distinct part in the intricate cosmic dance. The Sun, the

glowing heart of our solar system, showers Earth with its

life-sustaining rays. Planets, spherical orbs of varying

sizes and compositions, orbit the Sun, tracing graceful

paths through the void.

Earth, our home and the cradle of life, is a vibrant

and dynamic planet. Its swirling oceans, verdant continents, and atmospheric envelope create an environment teeming with

闪闪发光的他四百字作文

闪闪发光的他四百字作文

闪闪发光的他四百字作文

英文回答：

In the vast expanse of the universe, a celestial body

of unparalleled beauty and allure captures the imagination:

the star. Its radiant glow, mesmerizing in its intensity,

has inspired awe and wonder throughout human history. But

what gives the star its captivating brilliance?

At the heart of a star's luminosity lies its nuclear

fusion, a process in which hydrogen atoms combine under

extreme heat and pressure to form helium, releasing vast

amounts of energy in the form of photons. These photons,

propelled outward from the star's core, carry with them the

energy that illuminates the star and reaches our eyes as

starlight.

The color of the star's light provides clues about its

temperature. Blue stars, with their scorching temperatures,

emit a shorter wavelength of light, while red stars, cooler in comparison, emit a longer wavelength of light. The

终点又是起点英语作文

终点又是起点英语作文

终点又是起点英语作文

英文回答：

The end is also the beginning. This paradoxical

statement holds profound significance in the cyclical

nature of life, the recursive patterns that govern

existence, and the interconnectedness of all things.

In the realm of nature, the end of one season marks the

genesis of another. Winter's icy grip gives way to the

vibrant hues of spring, which in turn transforms into the

warm embrace of summer. As the days grow shorter, autumn's

vibrant foliage heralds the return of winter, completing

the cycle. Thus, the end of each season is the起点 of the

next, ensuring an eternal dance of life and renewal.

Similarly, in human experience, the end of one chapter

often ushers in the beginning of another. The completion of

a degree signifies the beginning of a career. The end of a

relationship may lead to the blossoming of a new love. The loss of a loved one can ignite a profound spiritual journey.