Encyclopedia of Physical Science and Technology - Astronomy

353 Pages·2001·11.054 MB·English

by  Robert Allen Meyers (Editor)

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Astrochemistry
Steven N. Shore
IndianaUniversitySouthBend
I. Introduction
II. PhysicalProcesses
III. Dust
IV. MolecularEnvironments
V. ChemicalProcesses
VI. CosmologicalChemistry
VII. Conclusion
GLOSSARY medium. Clouds have large size (of order 1–10 pc)
andhighdensities(≥103cm−3).
Astrochemistry The theoretical study of chemical pro- Polycyclic aromatic hydrocarbons (PAHs) Hydrocar-
cesses in cosmic environments and the observational bonsincomplexchainsandagglomeratedringsthought
determinationofphysicalparametersthroughthestudy toberesponsibleforthediffuseemissionlinesobserved
ofabundancesofmolecularspecies.Thisreviewcon- in dust nebulae in the near-infrared. These molecules
centratesontherecentresultsconcerningcircumstellar formthelowest-massendofthedustdistributionand
envelopesandtheinterstellarmedium.Thefielddeals, are responsible for ubiquitous diffuse emission in the
however, with synthesis of molecules in cometary 1-to25-µmgalacticbackgroundradiation.
nuclei and planetary atmospheres, as well as stellar Units Parsec (pc), 3.1×1018 cm; solar mass (M(cid:5)), 2×
photospheres. 1033g;Jansky(Jy),10−23erg−1cm−2Hz−1.
Fractionation Processbywhichisotopesareincludedin Vibronic Transition Molecular transition involving
molecules,eitherbytheprocessofdirecttransferorby stateswhicharesplitbyrotationthatisinducedthrough
chargeexchange. rotation–vibrationalcoupling;thefine-structurestates
Large-velocity gradient approximation (also called ofelectronictransitions.
Sobolev approximation) the assumption that the line
width due to thermal broadening is small compared
with the large-scale velocity field in a medium. It is I. INTRODUCTION
basictotheassumptionthattheopticaldepthofaline
dependsonlyonthevelocitygradient. Astrochemistry is a field that spans virtually all cosmic
Molecular cloud The densest phase of the interstellar environments,fromcometsandplanetaryatmospheresto
665
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666 Astrochemistry
theinterstellarmedium.Assuch,itismoreconcernedwith into fine-structure levels, called (cid:4)-doubling. Interaction
processesandwhattheyrevealaboutthephysicalnature withnuclearspinsproduceshyperfinesplittingofthero-
ofthemediumthanwiththespecificarenainwhichthese tationallevels,withquantumnumber F.Thesehyperfine
processesoccur.Itisoneofthefewastrophysicalfieldsin transitionsinOHareresponsiblefortheobservedmaser
whichlaboratoryworkispossible,andinwhichconditions emission.
similartothosestudiedastronomicallycanbesimulated.
In this review, we shall concentrate on the most recent
work, concerned mainly with stellar mass outflows and 2. RotationalTransitions
the interstellar medium. For want of space, planets and
The nuclei, which are the massive components of mole-
stellarphotosphereshavebeenexcluded.
cules, are free to rotate and precess about the center of
Observationalastrochemistryisaccomplishedprimar-
mass.Thus,amoleculewithamomentofinertia, I,has
ilywithinfrared(IR)andmillimetertechniques,areasof
arotationalangularmomentum J whichisquantizedand
technologywhichhavebeenseenexplosiveexpansionin
thustakesononlydiscretevalues.Theenergiesofthese
thepastdecade.Withtheimprovementofsuperconducting
statescanbeshowntobe
detectors and the development of interferometric arrays,
h
tthhiesnfieexltddiescoandee.whichwillsurelychangesignificantlyin EJ = 4πI J(J +1)≡ BvJ(J +1), (1)
We will begin by examining the physical processes
sothattransitionsbetweenstatesofunequalJshowastep-
needed to diagnose the conditions in astrochemical en-
ladderpatternintheseparationoflinesofthesameseries.
vironmentsandthenexaminesomeofthechemicalprod-
Becauseofthelargevalueofthemomentsofinertia,due
ucts thus detected. It is important to keep in mind that,
tothemassofthenucleus,theseparationofthesestatesis
with the exception of solar system objects, astrochemi-
small,oforder0.01eV,increasingwithincreasing J.The
cal analyses are quintessentially remote sensing, studied
simplerepresentationthathasjustbeenused,however,is
by observations of spectral lines emanating from distant
onlyappropriatefordiatomicspecies,whichhaveonlyone
sourcesthroughtheapplicationsofradiativetransferthe-
rotationalaxisthatisdegenerateinthetwoaxesorthogonal
oryandmolecularlineformation.
totheinternuclearaxisaboutthecenterofmass.
If the molecule is more complex, for example, H O,
2
thentwoorthreeaxesareneededtofullydescribethero-
II. PHYSICAL PROCESSES
tation.Eachofthesehasanassociatedmomentofinertia,
depending on the details of the electronic states and the
A. BasicMolecularSpectroscopy
internuclear distances and masses. The projection of the
1. ElectronicTransitions rotationalongthebodyaxis,K,nowappearsintheterms
fortherotationalsplitting.Forinstance,forasymmetric
Theelectronicstatesinamolecule,analogsoftheatomic
topmoleculelineCH ,
states and characterized by total electronic angular mo- 3
(cid:3) (cid:4)
mentum and spin, are generally separated in energy by F|v| = B|v|J(J+1)+ B|v|−A|v| K2±2A|v|ζK, (2)
aboutthesameorderofmagnitudeasforisolatedatoms,
usually several electron volts (eV). Thus the transitions whereζ describesthecouplingofthevibrationsandthe
from molecular electronic states, which also correspond rotations(vibronicstates)andsplitstheotherwisedegen-
todifferentpotentials,arebestobservedintheultraviolet erate K levels. The constants A and B are the moments
(UV) region. Excitation depends on the presence of UV ofinertiaabouttheparallelandperpendicularaxesofro-
radiation,andelectro(cid:2)nictransitionsareusuallyseeninab- tation,relativetothebodyaxis.Therotationallineswill
sorption, as in the 1 + state of H . The strength of the be distributed in a way that depends on the ratio of the
g 2
transitiondependsonthedipole(heteronuclearmolecules momentsofinertiaoftheprincipalaxesofthemolecule.
andions)orthequadrupole(homonuclear)moments.Vi- Thus there will be multiplets for lines which are closely
brationalstatesdominatetheopticalandinfrared,andro- spaced in energy and which can be strongly radiatively
tationalstatesarebestobservedinthemillimeterandcen- coupled(seediscussionofmasers).
timeterportionsofthespectrum.
Fordiatomicmolecules,thesestatesareclassifiedbythe
3. VibrationalTransitions
projectionoftheelectronicangularmomentumalongt(cid:2)he
internuclearaxis,(cid:4),andtheprojectedcombinedspin Vibrationalstatesdistributeasthoseofaharmonicoscil-
andaregroupedintomultipletsaccordingtothecoupling lator,with
between these states and the rotational angular mome(cid:2)n- (cid:3) (cid:4)
tum J. For (cid:4)(cid:8)=0, the states are split by (cid:5) = (cid:4)+ Ev =hν0 v+ 12 , (3)
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Astrochemistry 667
whereν isthevibrationalfrequencyofthegroundstate. where A is the Einstein spontaneous transition prob-
0 21
Polyatomic molecules have additional vibrational states ability and φν is the line profile function that describes
duetothemultiplemodespresentedbydifferentconfig- thefrequencydependenceoftheline.Initsmostgeneral
urations.Forinstance,bendingmodesinwaterarestates form,φν istheconvolutionoftheintrinsiclineprofiledue
for which the O moves and the H remains fixed, while to radiative and collisional broadening of the upper and
othershaveboththeOandHmovingoppositely(theso- lowerstates(usuallyaLorentzianprofile)andtheextrinsic
calledν andν modes),inadditiontothefundamentalν broadeningduetotherandommotionsofthemolecules(a
2 3 1
mode which involves the O–H bond stretch. Vibrational Gaussianprofilewhosewidthdependsonthethermaland
couplingproducesanangularmomentumwhichsplitsro- turbulentvelocitiesaddedquadratically).Theequationof
tationalstates,asmentionedaboveundervibronictransi- radiativetransferis
tioCnso.upling of vibrational and electronic states—that is, dIν =−κνIν + jν, (6)
dl
between (cid:4) and v—produces an angular momentum K,
whereI istheintensityandlisthepathlengththroughthe
whichforpolyatomicmoleculesdependsontheaxisabout
medium.Collisionsdominatemostmolecularexcitation,
whichtherotationisexecuted.Forinstance,inH Othere
2 andiftheemissionrateislow,asusuallyoccursinmolec-
isaprolateandoblaterotationalaxisforthemolecule,so
ularclouds,thenthepopulationscanbeassumedtobein
that a state J is split by two values of K and labeled by
localthermalequilibrium,hencegivenbytheBoltzmann
J . More complex states are possible, depending on
K+K− distribution:
the complexity of the molecule. For example, inversion
N g
transitionsofmoleculessuchasNH3,whichoccuratcen- 2 = 2e−E12/kTex, (7)
timeter wavelengths, result from small perturbations of N1 g1
rotationalstatesbyvibrationaltransitionbetweenmirror whereT istheexcitationtemperature,whichisassumed
ex
molecularconformations. tobeoftheorderofthekinetictemperatureoftheexcit-
ingparticles,and g isthestatisticalweightofthestates,
which are separated by E . The optical depth for a line
12
B. RadiativeTransfer:Observational isproportionaltothepathlengththroughthemedium,so
Astrochemistry that
(cid:5)(cid:6) (cid:7)
1. LineRadiativeTransfer κndl =κndv dv = cκ n (cid:14)ν , (8)
dl ν 0 dv/dl
Molecularobservationsarealmostalwaysconcernedwith
specificdiscretetransitions.Thesearegenerallyobserved where (cid:14)ν is the line width in frequency, κ0 is the opac-
atmillimeterorcentimeterwavelengths.Theintensityof ity at line center, n is the number density, and dv/dl is
a source is determined by the rate of collisional versus thevelocitygradientalongthelineofsight.Theso-called
radiative transitions between levels. Because of the ex- large velocity gradient or Sobolev approximation (also
tremely low densities usually associated with molecular calledthe“onthespot”approximationbecausetheemis-
environments, whether in a circumstellar envelope or a sionand/orabsorptionisassumedtodependononlythe
molecularcloud,pressurebroadeningisunimportant.In- localconditions)assumesthatthisgradientislargerthan
stead, the molecule radiates at its local velocity into the thethermalspeedssothattheopticaldepthofthemedium
lineofsight.Thisdispersionofvelocitymaybeduestrictly is small. For molecular clouds, observed line widths are
tothethermalmotionsoftheparticles,oritmaybedueto usually a few kilometers per second, while the thermal
thepresenceofturbulenceorlarge-scalechaoticmotions speedisabout0.1kms−1,sothisapproximationseemsto
withinthemedium.Eitherway,thelocalprofile,φ(ν)isa bevalidforallbutthemostabundantspecies.
Gaussianwithafinitewidthinfrequency. Like their atomic counterparts, molecular lines satu-
Theabsorptioncoefficientforastatecanbewrittenas rate when the populations have reached the values asso-
ciated with strict equilibrium with the incoming radia-
κν =(n1B12−n2B21)φ(ν), (4) tion.Thisoccursfirstatthelinecenter.Anymotioninthe
medium,orderedorrandom,willbroadenthelineandthus
wheren isthepopulationoftheupper(n )orlower(n ) the molecules will “see” radiation at other wavelengths
j 2 1
state,andB andB aretheEinsteintransitionprobabil- against which they can absorb, or into which they can
12 21
ities for stimulated transitions. The emission coefficient, emit. If the medium is optically thick at line center but
duetospontaneoustransitions,isgivenby the velocity dispersion is large, the overall optical depth
canbeconsiderablyreducedbyspreadingoutthelinein
jν =n2A21φ(ν), (5) frequency.
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668 Astrochemistry
Themostabundantmolecules,becauseofthelowve- where Bv=Cv and Av are the moments of inertia for
locitiesobservedinthecloudsandhighcolumndensities, the rotational states and El=hc[BvJ(J+1)+(Av−
cannotbeinterpretedbysimpleopticallythinmodels.For Bv)K2].Intheopticallythin,LVGapproximation,thissuf-
CO(anyisotope),theratioofthe(2→1)to(1→0)tran- ficestodeterminetheabundanceofthespeciesofinterest.
sitions should be 3:1 in strength, if completely optically Itassumesthatallemissionisduetothermalequilibrium
thin,becauseoftheratioofthestatisticalweightsandtran- prevailingduetocollisionsamongthelevels.
sitionprobabilitiesandthetemperatureknowntoexistin
theclouds.However,12C16O(1→0)isoftenobservedto
2. Masers
show flat-topped profiles, not the Gaussian form which
would be typical of a randomly moving optically thin Because of the low densities, molecules in cosmic envi-
moleculargas.Also,theintensityratioofthetransitionsis ronmentscanshowpopulationsofmanylevelswhichare
oftenseentodepartfromthatexpectedforsuchamedium. inverted.Thatis,thehigherlevelssometimeshavehigher
The implication is that 12CO is optically thick, and that populationsthanthelowerones.Theprimaryreasonfor
thedensestpartsofthecloudmaynotbeobservableinthe thisisthattransitionstakeplacebetweenhighstateswhich
ground-statetransition.Lower-abundancespecies(forin- areonlyweaklycollisionallycoupledtolowerlevels,and
stance,theisotopes13COand12C18O,ormorehighlyex- forwhichradiativetransitionsarelong.Ifthereisastrong
citedstatesof12C16O(suchas3→2)may,however,probe backgroundradiationfieldatshorterwavelengththanthe
denser parts of the cloud. Further, the higher transitions transition of interest, upper states of the molecule may
requirehigherdensitiesforexcitation,sothereisadelicate beradiativelyexcitedwithsubsequentoverpopulationof
interplaybetweenchemistryandradiativetransferwhich someofthelowerstatesbyradiativeandcollisionaldeex-
entersintotheinterpretationofabundances.Thisiscrucial citation.Thus,formaserstooccur,morethantwostates
totheunderstandingoftheformationofthemolecules. mustbeinvolvedinstronglycoupledtransitions.
The abundance of a species is related to the observed Masers also serve as a warning that the intensity of
lineintensitybytheantennatemperature,thetemperature a spectral line is not necessarily a direct measure of its
which an equivalent blackbody radiator would have to abundance.Populationinversionsenhancethebrightness
have at the line frequency to equal the observed line temperatures, leading to overestimates of excitation and
intensity: abundanceinthosespeciesinwhichmasingoccurs.Be-
(cid:8)
∞ T causenotallmoleculesundergomaseramplification,the
Iline = νA2 dν, (9) assumptionofthermalequilibriumisusuallynotbad,but
0 shouldbeemployedwithcaution.
whichisintegratedoverthevelocitywidthoftheline.The Theemissionandabsorptioncoefficientsforthesystem
columndensity,bynumber,inthelowerlevelisdefined canbedefinedasbefore.Nowassumethatthereareatotal
as of n levels, and that they are coupled via collisions and
(cid:8)
4π3/2 kν (2J +1) 1 radiation to the levels 1 and 2. Then the time-dependent
N = l T dv, (10)
l (ln2)1/2hc2(2J +1) A B populationsof1and2aregivenby
u ul line
where k is the Boltzmann constant and v is the velocity dn1 =P (n−n −n )−(n B −n B ) (cid:5) I
width of the line. Then the total column density of the dt 1 1 2 1 12 2 21 4π
speciesisfoundusingtheBoltzmanndistributionforthe −n C −n (cid:15); (14)
1 12 1
levels:
(cid:6) (cid:7) dn (cid:5)
N = N Q(T) exp El , (11) dt2 =P2(n−n1−n2)−(n2B21−n1B12)4πI
tot l(2J +1) kT
l −n (A −C )−n (cid:15). (15)
2 21 21 2
where Q(T)isthemolecularpartitionfunction,thesum
HereP andP arethepumpratesfromthehigher-lying
over the population probabilities for all of the rotational 1 2
levels through radiation and collisions, (cid:5) is the solid
levels,
(cid:9) angle, and (cid:15) is the rate at which the masing levels are
Q(T)= gJ,Ke−EJK/kT, (12) depopulated.
J,K For molecular masers, it can be assumed that the two
whichcanoftenbefoundinclosedform.Forinstance,for masing levels have the same statistical weight. Thus,
asymmetrictopmolecule, B = B = B.Thenumberoflevelsinvolvedinthepar-
12 21
Q(T)=(cid:6) π (cid:7)1/2(cid:6)kT(cid:7)3/2exp(cid:6)Bvhc(cid:7), (13) triacduilaatrivpeopcouulaptliionnginbveetwrseioennisstastmesalwl.hTichhis,ifmorplsioesmsetrroenag-
AvBv2 hc 4kT son, are selectively pumped by the external sources of
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Astrochemistry 669
radiation. The OH molecule is an excellent example of whosestrengthcorrelateswellwiththeextinctionofvis-
thisbehavior. iblestarlight.TheUVfeature,near2175A˚,islikelydue
Iftheabsorptioncoefficientisnegative,inotherwords, tosomeformofsolidcarbon,somethinglikegraphiteor
if the populations are sufficiently inverted, the radiation anamorphousstateofcarbon.Itsstrengthandshapeare
in the 2 → 1 transition will amplify along its path until variable throughout the galactic plane, although not en-
the maser saturates, that is, until the populations do not tirelyabsentalongmostlinesofsightthroughtheplane,
change along the path length. The amplification selects and these also are variable from one galaxy to another.
out the line center, and the line gradually narrows as a Theidentificationoftheinfraredbandat10µmismore
resultofincreasingpathlength.Thisbehaviorisofgreat secure, being due to silicates and at 11 µm due to SiC.
importance,becausethebrightnesstemperatureincreases Dustisnormallyvirtuallytransparentatthiswavelength
asthelinegetsnarrower.Further,theradiationhasafinite because of the size of the grains, and in the diffuse in-
amplificationlength;themasercansaturate.Intheabsence terstellarmediumthecolumndensitiesareinsufficientto
of collisions and in steady state, (cid:15) can be replaced by produceappreciableabsorptionintheIRband;itisseen
(cid:15) = 2BI((cid:5)/4π),sothatthebrightnesstemperatureofa inthe atmospheresofhighlyevolved redsupergiants.In
saturatedmaserisgivenby thesestars,becauseofthelowemissionfromtheenvelope
hν (cid:15) (cid:5) asawholeandthelargespatialextentoftheouterstellar
TB,s = 2k A4π (16) layers,thefeatureisoftenseeninemission.Unlessthestar
happenstobesufficientlycoldthattheouteratmosphere
This makes masers very intense radiation sources, since isemittingsignificantlyatthesewavelengths,thefeature
thepumpradiationathigherfrequencyhasbeenconverted willalwaysbeseeninemission;afewverydenseshells
both to lower frequency and narrower bandwidth by the showabsorptionatthesamewavelength.
amplificationprocess.Theemissionisalsohighlypolar- Thepresenceofsilicatesinboththeinterstellarmedium
ized,sinceitiscoherent. (ISM)andstellarenvelopesiscertain,buttheprecisephys-
Masing depends on the presence of a strong radiation icalstateofthesilicateisnotwellknown.Asistypicalof
fieldforexcitationandmaintenance.Suchradiation,usu- solids,mostofthedetailedinformationabouttheinternal
ally infrared, is significant in several environments, no- structure of the radiating species is lost due to the com-
tablyincircumstellarenvelopes(CSEs)andinmolecular plexity of the lattice structure and the effects of nearest-
clouds.InCSEs,far-infraredradiationisconvertedtocen- neighbor perturbations to the energy states. These result
timeter radiation by OH, which has transitions centered inbroaddiffuseabsorptionoremissionbands.
around1665MHz.Ammonia,water,HCN,andSiO,are OtherIRfeaturesinthe3-to10-µmregionhavebeen
alsoimportantstellarmasersources.Watermasersarealso identified with both water ice (near 3.2 µm) and with
associatedwithregionsofactivestarformation,whereIR polycyclicaromatichydrocarbons(PAHs)(severalbands,
from the protostellar cores can excite the millimeter ra- especially near 3.3, 6.2, 7.7, 8.6, and 11.3 µm). They
diation in the densest parts of the cloud. Because they are identified with C–H and C–C bending and stretch-
are strongly amplifying, the masing sites are easily dis- ing modes of complex organic molecules, although spe-
tinguishedfromthebackgroundandtheirpropermotions cific identifications are insecure. The water is presumed
can be directly measured using VLBI techniques. Their to condense onto the grains in dense environments, like
time variability is also well observed, although it is still molecular clouds. The PAHs are more like small grains
notfullyunderstoodtheoretically. thanmolecules,butarelikelyassociatedwiththeforma-
tion and destruction of dust, forming the small particle
endofthesizespectrum.Becausetheyarenearlymolec-
III. DUST ular,theyarenotinequilibriumwiththeradiationfield—
thatis,theydonotradiatelikeblackbodies.Instead,they
A fundamental constituent of the atmospheres of the deexcite from UV radiative absorption from the diffuse
coolest stars (whether red giants or brown dwarfs) and interstellarradiationfield(DIRF)viavibronictransitions
oftheinterstellarmediumisthesolidmaterialthathasbe- in the near-IR. This means that their emission requires
comeknownasdust.Althoughdustwasrecognizedand someUVexcitation,whichissupportedbytheirpresence
characterized more than 50 years ago, many of its basic inphotodissociationregionsattheboundariesofmolec-
properties are still debated. In large measure, this is due ular clouds and planetary nebulae. The observed IR dif-
to the very indirect way in which information about the fusebands,whichhaveopticalanalogsseeninabsorption
compositionandstructureofthedustisobtained. against background sources, are likely due to the C–H
The spectral signature of dust is the presence of sev- bondstretch,theanalogoftheSi–Ovibrationresponsible
eralverybroadfeaturesintheinfraredandtheultraviolet, forthe10-µmfeature.
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Dustradiatesintheinfrared.ByKirchhoff’slaw,solid IV. MOLECULAR ENVIRONMENTS
materialinthermalequilibriumradiateslikeablackbody.
Most of the incident energy falling on the grain is scat- A. CircumstellarEnvelopes
tered, hence the blueness of reflection nebulae and the
Moleculesarefrequentlyobservedintheouterenvelopes
reddening of starlight. The absorbed photons are reradi-
ated at a rate approximated by jν=κνBν(T), where κν of red supergiants with surface temperatures less than
about 5000 K. These stars have strong stellar winds, of
iisstthheePmlaonncokchfuronmctiaotincaatbtseomrppteiroantucreoeTffi.cTiehneteaqnudiliBbrνi(uTm) order10−7to10−6M(cid:5)yr−1,withvelocitiestypicallyless
than50kms−1.Afewhotterstars,suchas89Herculisand
temperatureofthegraindepends,therefore,onitssizeand
HD161796,havealsobeenfoundtoshowCOemission;
composition.CarbongrainsabsorbeffectivelyintheUV
due to the diffuse 2175-A˚ band, but radiate inefficiently theseandrelatedstarsareproto-planetarynebulaobjects
in the process of becoming white dwarf stars. For some
in the IR, so they are hotter than silicates, for which the
veryhighlyevolveddustystars,strongfar-IRemission,in-
reverseholds.ThePAHsaredistinguishedbytwoeffects.
dicativeofdust,isaccompaniedbymaseremission.These
Theyshowamuchhighercolortemperaturethanthelarger
aretheso-calledOH/IRstars,whicharemostfrequently
grainsand,inadditiontotheirlines,theycannotbeinther-
Miravariables.SomeevolvedstarsalsoshowSiOmasers.
malequilibriumsincetheirspecificheatsaretemperature
Afewstars,suchastheextremesupergiantIRC+10216,
dependent(seebelow).
are veritable chemical factories, displaying almost all of
Grains provide a solid surface on which chemical re-
the molecular species observed in comets and in dense
actions take place. In fact, they are the primary site for
interstellarmolecularclouds.
H synthesis. In addition, metallic ions deplete onto the
2
grains. This is evident from the lower-than-stellar abun-
dances observed for the heavy metals, such as iron and
B. TheEnvironmentoftheInterstellarMedium
calcium,inthediffuseinterstellarmedium.Itappearsthat
most of the heavy metals in the diffuse and molecular Theinterstellarmediumisaveryinhomogeneous,disequi-
cloud phases of the ISM may be tied up in the grains, libratedplace.Thediffusemediumhasdensitiesof0.01to
which nonetheless constitute only about 10−6, by num- 1cm−3intheionizedphaseandabout1to103cm−3forthe
ber,oftheISM.COandH Omayalsosticktothegrain coolerneutralphase.Themediumisheatedbysupernova
2
surfaces, and models and laboratory simulations show a stellar wind shocks, and has a sufficiently long cooling
hostofcomplexorganicmoleculescanbesynthesizedin time that it never becomes any colder than about 106 K.
theresultantmantle.Itremainstobedeterminedwhether Thisisbecausetheatomsareveryinefficientcoolants.In
these simulations are relevant for interstellar conditions; denser regions, the temperature is reduced to about 106
theydoseemtomimicmanyofthereactionproductsob- K, and cooling due to neutral hydrogen recombination
servedinsituincometHalley. becomesefficient.Forlowertemperature,thecoolingin-
Animportantproblemindustchemistryistheprecise creasesdramatically,firstduetohydrogenlineemission,
determinationofboththeformationmechanismandsize which reduces the temperature to 104 K, and then from
spectrum of the grains. At the smallest-particle end, the atomic fine-structure transitions, which reduce the tem-
grains behave like large molecules. The PAHs are sta- peraturetoseveralhundredsofdegrees.Tolowerthistem-
bleagainstUVradiationandalsocanbecleavedfromthe peraturefurthertakestwoadditionstothemedium—dust
largergraphitegrains.Thesignatureofsmallgrainsisthat grainsandmolecules.
theycannotcomeintoequilibriumwiththeUVradiation Indiffuseregions,thedustwillalwaysbecolderthan
field, and do not radiate like blackbodies. Instead, their thegas,primarilybecauseofthelargernumberofmodes
specific heats depend explicitly on the number of avail- available for the redistribution of the energy. In fact, the
ablemodes, N,proportionaltothenumberofconstituent temperature,orequivalentlytheexcitation,ofthedustis
molecules. They radiate with an excitation temperature sensitivemoretothespectrumoftheradiationincidenton
dependingontheincidentphotonenergyasT =hν/NC. it than to its dilution. The cooling of the dust is strictly
Hence, they have color temperatures which are of order radiative, efficiently absorbing in the UV and radiating
1000KinthepresenceoftheDIRF,inspiteofthefactthat intheIR.ThehardertheUVradiationwhichisincident
theyareneverinequilibriumandsohavenotruekinetic on the grain, the warmer the grain will be, regardless of
temperature.Theresultingemissionbandsarevibrational the intensity of the radiation. The grains are the critical
transitionswhichredistributetheincidentUVradiationon shield for the cloud material from background radiative
shorttimescales. heating.Thepresenceofdustalsoeffectivelycoolsthegas
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because of surface atom interactions which promote the isobservedinmolecularclouds,weshallconcentrateon
formationofmolecules,especiallyH .Molecularcooling the second mechanism. The first type of mechanism has
2
isduetocollisionalexcitationofabundantspecies,which been implicated in star formation processes in the early
then reradiate their energy in the far-IR and millimeter universe.
wavelengths,atwhichthegrainsareopticallythin.Deep Assumethatthegrainconsistsofasimplelattice,like
inthecoresofmolecularclouds,thesituationisreversed. graphite. Should an atom of neutral hydrogen strike the
Herethegrainsarewarmerthanthegas,andactuallyheat surface,thereisastickingprobability,S,suchthattheatom
the gas through collisions of the particles with the grain willbecomeboundtothesurfaceratherthanbereflected
surface.Assuch,theyserveasthefuelforthechemistry. back into the diffuse medium. The rate of impact on the
Collisionsofmolecularhydrogenwith,forinstance,CO surfaceofhydrogenatomsisgivenbythemeancollision
excitethelatter,whichradiatesitsenergyfromthecloudat time of(cid:2)a H atom with a grain having a geometric cross
theexpenseofthegaskineticenergy.Thus,theIRwhich section .Thevelocitydispersioninthegasisv ∼T1/2,
th
penetratesthecloudandheatsthegrainscanbetransferred sothat
totheexcitationofthevariouschemicalconstituentsofthe
dn
mediumeffectively,servingtopowerthereactionswhich dt =nHngSπag2vth (21)
buildcomplexmolecularspecies.
andtherateofhopping,ormigration,amonglatticesitesis
t .Then,ifK isthereactionrate,therateofformation
mig HH
ofH isapproximatelygivenby
V. CHEMICAL PROCESSES 2
(cid:9)
n ≈ K n S σt β−1, (22)
A. SurfaceChemistry:Formation H2 HH Hg mig
ofMolecularHydrogen where β is the rate of release of H2 from trapping sites
backintothemedium.AnempiricalrateforH formation
2
Thebasicprobleminthestudyoftheinterstellarmedium
is
istheformationofmolecularhydrogen,H .Thisisofpri-
2
dn(H )
maryimportancebecause,atthelowtemperaturescharac- 2 ≈3×10−17cm3s−1n2. (23)
teristicofthecloudenvironments,thismoleculeisrespon- dt H
siblefortheexcitationofCO.Onemightinitiallyexpect Heren istheambientgasdensity,withthegrainsscal-
H
reactionsoftheform ing as a fixed fraction of n . There is, however, reason
H
to believe that at the lowest neutral hydrogen densities,
2H→H +γ, (17)
2 theformationprocessdependsontherandomrateofar-
whereγ isanemittedphoton.Thisprocess,theso-called rival of molecules on the surface and there is an expo-
radiativeassociationmechanism,isimportantforthefor- nential threshold for the molecular formation (see Caselli
mation process. The rate is, alas, many orders of mag- et al., 1998). Grains are a prerequisite for the formation of
nitudeshortofthatrequiredtoproducethemolecule.In molecularhydrogeninthepresentgalaxy,butgas-phase
fact,itappearsthattheformationofH canonlyproceed reactionsinthedenseregionsthataretypicalofmolecular
2
viaoneoftwopossibleavenues:(1)ifthereisasufficient cloudswillotherwiseproduceallofthespecieswhichare
abundanceoffreeelectrons, observed.Therefore,inwhatfollows,weshallconcentrate
on the work which done in the 1990s on the problem of
H+e →H− (18)
gas-phasechemistry.
H−+2H→H− (19)
3
→H∗+H−; (20) B. Gas-PhaseChemistry
2
or (2) via some form of surface interaction where radia- Thefirstobservationsofdiatomicspeciesinthediffusein-
tiveassociationisreplacedbyreactiononasolidsurface terstellarmediumseveraldecadesagoposedseriouschal-
of two neutral atoms in the presence of a UV radiation lengesfortheoristsbecauseoftheextremelylowdensities
fieldwhichiscapableofexceedingtheappropriatebind- whicharefoundthere.Radiativeassociationseemedun-
ingenergyofthemoleculetothegrainsurface.Thefirstof abletoproduceanyoftheobservedspecies,mostimpor-
theseisindependentoftheabundanceofmetals,whilethe tantlyCH,andthismeantthatexoticmechanismswereini-
latteriscriticallydependentontheexistenceofinterstel- tiallyheldresponsibleforthepresenceofsuchmolecules.
largrainsonwhichthereactionscantakeplace.Because WorkontheabundanceofH ,followingtheobservationof
2
it appears that the solid lattice is far more efficient, and themoleculeinthediffusemediumintheultravioletbythe
becauseitexistsintheinterstellarenvironmentthatnow Copernicussatelliteinthemid-1970s,andthediscoveryof
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672 Astrochemistry
elementaldepletionalongmanylinesofsightintheinter- measurements at room temperature suffice for the deter-
stellarmedium,ledtothesuggestionthatgrainswerealso minationoftheratecoefficients, K =k,whereusuallyk,
thefundamentalsiteforthechemistryrequiredtoproduce thereactionconstant,isoforder10−8 to10−13 cm3 s−1.
eventhesimplestdiatoms.TheFusemission,launchedin Mostionicreactionshavelittleornopotentialbarriers,be-
1999, covers the same spectral range (900–1200A˚) as inggenerallyexothermic.Hencethereisusuallyasimple
did Copernicus, but with significantly higher sensitivity constant volumetric rate which is assumed to be a con-
andresolution,andisnowbeingusedtostudymorethor- stant.Neutralreactionsaremostlikelytoinvolvesubstan-
oughlythemolecularhydrogencomponentofthegalaxy. tialactivationenergieswhichgreatlyinhibittheirratesof
Inaddition,theISOmissiondetectedlargeabundancesof formation.Ifaneutralchannelisimportantinanetwork
H inthefar-IRevenfromregionsthathaveverylowCO of reactions, it will likely be a bottleneck for the forma-
2
abundances. tion of the product species. These have strong tempera-
Thediscoveryoflarge,coldmolecularcloudcomplexes turedependencesbecauseoftheiractivationenergiesand
dramatically altered this view, providing the necessary usually are several orders of magnitude slower than the
conditions for low-temperature, high-density gas-phase ion-neutralchannels.Electronicrecombinationreactions
reactions to occur. The development of many computa- typicallyscaleasT−1/2.
tionalschemaforhandlingenormousreactionnetworks, Beforeproceedingwithadiscussionofspecificresults,
ofteninvolvingthousandsofreactionsandhundredsofre- onepointshouldbeemphasized.Inmanyofthereaction
actingspecies,alsospurredtheoreticalworkonthissub- networks,amongalloftherateswhichmustbetabulated
ject.Thissectionismeantonlytoserveasaguidetothese and all of the reactions which must be tracked, many of
calculations.Thebasicphysicalinputisreallyquitesim- the rates have to be approximated by guesses or simple
ple;itisthecomputationalcomplexitythatmakesforthe fitstolaboratorydata.Fewmeasurements(exceptonthe
differencesfoundamongvariousworkersinthefield. SpaceShuttle)atinterstellarconditionsareavailable,and
Thechemistryofgas-phasereactions,eitherinthein- this is a very significant challenge for future laboratory
terstellar medium or in stellar atmospheres, is mediated astrophysics.Inmanyofthenetworks,perhapsasfewas
bytheabundanceofions.Thesecanbeformedinseveral 10% of the rates are known to within a factor of 50%,
ways: by cosmic-ray ionization, or by the direct photo- andperhapsasmanyashalfarebaldguessesandmaybe
ionizationoftheatomsinvolvedinthereactionswithsub- uncertaintoafactorof10.Thisisafieldstillinitsinfancy,
sequentchargetransfertothemolecules.Ionicreactions whereonlythedominantchannelsarewellunderstood,but
are generally exothermic and so occur efficiently at low manyofthedetailsarestillextremelyimportantbecause
temperature.Inthepresenceofanion,aneutralmolecule of the physical conditions that can be probed by trace
or atom develops an induced dipole which increases its species.
capturecrosssection.Thusthereactionsoccurquicklyand
leadtostablestates,inadditiontoallowingthemolecule
2. Ionization
toforminaradiativelyunstableexcitedstatewhich,upon
decaying,radiatestheenergyofformationawayfromthe Ionization in the densest parts of molecular clouds de-
siteofthereaction. pendsonthepenetrationofcosmicraysandUVradiation,
aswellasthepresenceofshocksgeneratedbysuchpro-
cessesascloud–cloudcollisionsandinternalstarforma-
1. ReactionRates
tion.Inordertoprobetheelectrondensityintheclouds,it
Ingeneral,reactionsdependontwofactors,theactivation isimportanttobeabletoaccountforthepresenceofcom-
energy(cid:14)andthetemperature.Crosssectionscanbeob- plexpolyatomicmolecules,whoseformationrequiresion
serveddirectlyinthelaboratoryatsomecontrolledtem- gas-phasereactions.
perature, usually near room temperature (about 300 K), Therateforcosmic-ray(CR)ionization,ζ ,isabout
CR
andthenscaledtothetemperaturesfoundinclouds.They (3±1)×10−17s−1.Thisisanintegraloverthecollisional
canalsobededucedfromfirstprinciples.Generally,these ionization cross section for low (MeV)-energy CR pro-
reactions have the form K =(cid:18)σv(cid:19) where the average of tons, but it is approximately a constant for most of the
the cross section, σ, is taken over the velocity distribu- species of interest. An obstacle in our understanding of
tionoftheinteractingparticleswheretherelativevelocity the detailed structure of molecular clouds is our igno-
is v. For this reason, assuming the reacting particles are rance of the precise specification of this rate. The low-
thermalized, the rates depend on temperature. For ionic energyendofthecosmic-rayspectrumisdifficulttode-
reactions,wherethepotentialisthecoulombinteraction, termineempiricallyfromterrestrialobservation,because
theso-calledLangevinapproximationapplies,anditcan theseparticlespropagatediffusivelythroughtheinterplan-
beshownthattheratesareapproximatelyconstant.Thus etary medium, scattering off of turbulence in the solar
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Astrochemistry 673
wind; their spectrum cannot be observed directly , even An interesting aspect of stellar envelopes is that they
withinsitumeasurementsfromtheVoyagerandUlysses may have two different sources of UV radiation, inter-
spacecraft,andmustbeinferredfrommodelsfortheirmo- nal and external. Work on the envelope of two extreme,
tion through the heliosphere. The more easily observed low-temperature, evolved supergiants, IRC+10216 and
cosmicrayprotonsandelectrons,intheGeVandhigher α Ori, showed that the outer limit of the molecular en-
range,havelittleornoeffectontheionizationofthein- velope is determined by the DIRF, which destroys the
terstellar medium because of the small interaction cross outermostmolecularspeciesbyphotodissociation,while
sectionsforatomsatsuchhighenergies. theinnerboundaryissetbyboththetemperatureandUV
Inmolecularclouds,atomicspecieswithionizationen- emissionfromthestellarchromospheres.Inthisrespect,
ergiesgreaterthan13.6eVmustbepredominantlyneutral since the dynamics can be probed in exquisite detail for
becauseoftheshieldingeffectsofneutralhydrogen.Itis several of the nearer supergiants through molecular ob-
mainlytheheavierelements,suchasC,N,andO,which servations,andsincetheinputabundancesareknownand
areobservedintheperipheralportionsofthecloudstobe atomic in nature, it is possible to use these stars as very
inthepartiallyionizedstate.Forcircumstellarenvelopes, well-conditioned laboratories for the study of the same
cosmicraysloseouttophotoprocessesandthechemistry processeswhichmustbeinvolvedinatleastsomeaspects
ismediatedbytheinputofstellarphotosphericradiation ofmolecularcloudchemistry.Forthedensestenvelopes,
(inthehotterstarsandinnovaeandsupernovae)andfrom whicharecompletelyopticallythickandhenceverysim-
thediffuseinterstellarradiationfield. ilar to molecular clouds, cosmic rays are significant in
The basic equations for two body interactions can be governing the ion fractions but can be neglected in thin
writtenintheform envelopes(lowmasslossrates).
(cid:9) (cid:9)
dN
i = K N N − K(cid:20) N N , (24)
dt j,k(cid:8)=i ijk j k j ij i j 3. CoolingProcesses
where K is the formation rate for the ith molecular Chemistry also feeds back into the thermal balance of
ijk
species,whileK(cid:20) isthedestructionrateforthemolecule. the clouds. Molecules radiate in portions of the spec-
ij
TheinclusionofUVphotoprocessesisaccomplishedby trum where the medium is usually optically thin. Since
thephotodissociationrate: thisradiationcanescapefromthecloud,itistheprimary
(cid:8)
∞ dν means whereby the clouds cool. Star formation requires
Rpd = ν0 κνFνe−τν hν, (25) tuhnasttaobthlee,rwaipsreohceysdsrowsthaitcihccclaonudbsebaeffceocmteedgbryavtihtaetiroantealolyf
where Fν istheincidentphotonflux,τν istheopacityof energylossaswellasbyexternalperturbations.Thustime-
the ambient medium (presumed to be from dust), κν is dependentprocesses,thosewhichcausethestabilityofthe
thecontinuousabsorptioncoefficientforthedissociative clouds to alter with time, are extremely important, since
continuum,andthedissociationenergyishν0. thetimescaleformolecularformationisnottooshort(of
An aspect in which circumstellar environments dif- order106years)comparedwiththeestimatedlifetimesof
fer from interstellar is the net mass advection through theclouds(≤108years).Forexample,thecoolingratefor
the medium. Abundances become time dependent—and COdependsontheabundanceofbothdustandofH and
2
hencespacedependent—intheenvelope,duebothtothe COby
implicittimedependenceofthereactionsandtothetrans-
1.1×10−30n((cid:14)v/v )T1/2
portofmatterthroughdifferentradiiviastellarwindflow. (cid:4) = th ergcm−3s−1,
The atomic abundances are fixed at stellar photosphere, CO 1+1.4×10−4nT1/2(1+N/Nc)
(26)
rather than having to be assumed for some mixture of
physical parameters of temperature and pressure as they whereN =2×1018T cm−2andN isthecolumndensity,
c
mustformolecularclouds.Itisthenessentiallyaninitial- relatedtotheextinction.Molecularspeciesaretherefore
valueproblemtocomputetheabundanceswhichwillbe quite efficient in radiatively removing energy from the
a function of radius in the envelope. For a steady-state cloudsand,literally,refrigeratingthemedium.
wind, the abundances become strictly a function of ra-
dius. Also, unlike a molecular cloud, the density profile
4. ShockChemistry
oftheenvelopeisspecifiedfromtheassumptionofsteady
mass loss at the terminal velocity for the wind, so that Hydrodynamic and magnetohydrodynamic (MHD)
ρ(r) = M˙/(4πr2v∞),where M˙ isthemasslossrateand shocksareimportantinthetime-dependentchemistryof
v∞istheterminalvelocityofthewind. thediffuseinterstellarmedium.Thetimescalesarevery