Polycyclic aromatic hydrocarbon processing in a hot gas

0.4 MB·English

by  E. R. Micelotta

Astronomy&Astrophysicsmanuscriptno.11683˙v2 c ESO2011
(cid:13)
January12,2011
Polycyclic aromatic hydrocarbon processing in a hot gas
E.R.Micelotta1,2,A.P.Jones2,A.G.G.M.Tielens1,3
1 SterrewachtLeiden,LeidenUniversity,P.O.Box9513,2300RALeiden,TheNetherlands
e-mail:[email protected]
2 Institutd’AstrophysiqueSpatiale,Universite´ParisSudandCNRS(UMR8617),91405Orsay,France
3 NASAAmesResearchCenter,MS245-3,MoffettField,CA94035,USA
Received20January2009;accepted16October2009
1
1
ABSTRACT
0
2
Context.PAHsarethoughttobeaubiquitousandimportantdustcomponentoftheinterstellarmedium.However,theeffectsoftheir

n immersioninahot(post-shock)gashaveneverbeforebeenfullyinvestigated.
a Aims.WestudytheeffectsofenergeticionandelectroncollisionsonPAHsinthehotpost-shockgasbehindinterstellarshockwaves.
J Methods.Wecalculatetheion-PAHandelectron-PAHnuclearandelectronicinteractions,abovethecarbonatomlossthreshold,in
0 Hiiregionsandinthehotpost-shockgasfortemperaturesrangingfrom103 108K.
1 Results.PAH destruction isdominated by Hecollisions at low temperature−s (T <3 104 K), and by electron collisionsat higher
temperatures.SmallerPAHsaredestroyedfasterforT <106K,butthedestructionrate×sareroughlythesameforallPAHsathigher
A] fteomunpdertaotubreesa.bTouhtetPhAouHsalnifdetyimeaersi,noardteernsuoofusmhaogtngitausde(nsHho≈rt0e.r0t1hacnmt−h3e,tTyp≈ic1a0l7lifKe)ti,mtyepoicfasluocfhtohbejceocrtso.nalgasingalacticoutflows,is
G Conclusions.Inahotgas,PAHsareprincipallydestroyedbyelectroncollisionsandnotbytheabsorptionofX-rayphotonsfromthe
hotgas.TheresultingerosionofPAHsoccursviaC lossfromtheperipheryofthemolecule,thuspreservingthearomaticstructure.
2
h. TheobservationofPAHemissionfromamilliondegree,ormore,gasisonlypossibleiftheemittingPAHsareablatedfromdense,
p entrainedclumpsthathavenotyetbeenexposedtothefulleffectofthehotgas.
-
o Keywords.shockwaves–dust,extinction–ISM:jetsandoutflows
r
t
s
a 1. Introduction in the lifecycle of such species (Dweketal. 1996; Jonesetal.
[
1996).
Themid-infraredspectralenergydistributionofthe generalin-
2
Observationally, there is little direct evidence for PAH
v terstellarmediumofgalaxiesisdominatedbystrongandbroad
emission unequivocally connected to the hot gas in super-
5 emission features at 3.3, 6.2, 7.7 and 11.3 µm. These fea-
nova remnants. Reachetal. (2006) have identified four super-
9 tures are now univocally attributed to vibrational fluorescence
nova remnants with IR colors that may indicate PAH emis-
5 of UV pumped, large ( 50 C-atoms) Polycyclic Aromatic
1 Hydrocarbon(PAHs)mol≃ecules.Theselargemoleculesarevery sion. Tappeetal. (2006) have detected spectral structure in the
2. abundant(3 10−7 bynumberrelativetoH-nuclei)andubiqui- emissioncharacteristicsofthesupernovaremnantN132Dinthe
× Large Magellanic Cloud that they attribute to spectral features
1 tousintheISM(forarecentreviewseeTielens2008).Besides
of PAHs with sizes of 4000 C-atoms. Bright 8 µm emission
9 largePAH molecules,thespectraalso revealevidenceforclus- ≃
has been observed by IRAC/Spitzer associated with the X-ray
0 tersofPAHs–containingsomehundredsofcarbonatoms–and
: verysmallgrains( 30Å).Indeed,PAHsseemtorepresentthe emission from the stellar winds of the ionizing stars in the M
v ≃ 17Hii region(Povichetal.2007).Likely,thisemissionisdue
i extensionoftheinterstellardustsizedistributionintothemolec-
X to PAHs – probably,in entrainedgasablated fromthe molecu-
ulardomain(e.g.De´sertetal.1990;Draine&Li2001).
lar clouds to the North and West of the stellar cluster. Finally,
ar PAHmoleculesareanimportantcomponentoftheISM,for brightPAH emission hasbeen detectedassociated with the hot
example,dominatingthephotoelectricheatingofneutralatomic
gas of the galactic wind driven by the starburst in the nucleus
gasand the ionizationbalance of molecularclouds. Small dust of the nearby irregular galaxy, M 82 (Engelbrachtetal. 2006;
grainsand PAHs can also be importantagentsin cooling a hot Beira˜oetal.2008;Gallianoetal.2008).
gas,at temperaturesabove 106 K (e.g.Dwek1987), through
their interactions with therm∼al electrons and ions. The energy Electronandioninteractionswithdustandtheimplications
transferredinelectronandioncollisionswiththedustisradiated of those interactions for the dust evolution and emission have
asinfraredphotons.Theevolutionofdustinsuchhotgas(T > already been discussed in the literature (e.g. Draine&Salpeter
106K),e.g.,withinsupernovaremnantsandgalacticoutflows,i∼s 1979; Dwek 1987; Jonesetal. 1994; Dweketal. 1996;
criticalindeterminingthedustemissionfromtheseregionsand Jonesetal.1996).Inthisworkweextendthisearlierworktothe
thereforethecoolingofthehotgas.ThedestructionofPAHsand caseforPAHs,usingourstudyofPAHevolutionduetoionand
smalldustgrainsinahotgasmayalsobeanimportantprocess electroninteractionsinshockwavesintheISM(Micelottaetal.
2009, hereafter MJT). Here we consider the fate of PAHs in
thehotgasbehindfastnon-radiativeshocksandina hotgasin
Sendoffprintrequeststo:E.R.Micelotta general.
2 E.R.Micelottaetal.:PAHprocessinginahotgas
The aim of this paper is to study the PAH stability against
ϑ
electronandioncollisions(H,HeandC)inathermalgaswith
α
temperatureT intherange103–108K.
The paper is organized as follows: Sect. 2 and Sect. 3 de-
scribethetreatmentofionandelectroninteractionswithPAHs, R
Sect.4illustratestheapplicationtoPAHprocessinginahotgas
andSect.5presentsourresultsonPAHdestructionandlifetime.
Theastrophysicalimplicationsarediscussedin Sect. 6 andour d
conclusionssummarizedinSect.7.
2. IoninteractionwithPAHs
2.1.Electronicinteraction
The ion – PAH collision can be described in terms of
two simultaneous processes which can be treated separately
(Lindhardetal. 1963): nuclear stopping or elastic energy loss
Fig.1. The coordinate system adopted to calculate the energy
andelectronicstoppingorinelasticenergyloss.
transferredto a PAH via electronic excitation by ion collisions
Thenuclearstoppingconsistsofa binarycollisionbetween
andbyimpactingelectrons.Themoleculeismodeledasa disk
theincomingion(projectile)andasingleatominthetargetma-
with radius R and thickness d. The trajectory of the incoming
terial.Acertainamountofenergywillbetransferreddirectlyto
particle is identified by the angle ϑ, while the angle α corre-
the targetatom, which will be ejected if the energytransferred
spondstothediagonalofthedisk.
is sufficient to overcome the threshold for atom removal. The
physicsofthenuclearinteractionforaPAHtargetwaspresented
inthecompanionpaper(MJT).Asummaryofthetheoryispro-
is assumedto be thesphericallysymmetricjelliumshell calcu-
videdinSect.2.2andweherepresenttheresultsofourcalcula-
latedbyPuska&Nieminen(1993),whichcanbeapproximated
tion.
bythefollowingexpression(Hadjaretal.2001):
In this paper we focuse on the electronic stopping, which
consists of the interaction between the projectile and the elec-
n =0.15 exp[ (r 6.6)2/2.7] (3)
tronsofthetargetPAH,withasubsequentenergytransfertothe 0 − −
wholemolecule.Theresultingelectronicexcitationenergywill
wherer(inatomicunits,a.u.1)isthedistancefromthefullerene
betransferredtothemolecularvibrationsofthePAHthroughra-
center.Theelectrondensitydecaysoutsidetheshellandtoward
diationlessprocesses(eg.,interconversion& intramolecularvi-
thecenterinsuchawaythatthemajorcontributioncomesfrom
brationalredistribution).Thevibrationallyexcitedmoleculewill
theregionwith4<r<9.
decaythrougheither(IR)photonemissionorthroughfragmen-
Thesimilarityinπelectronicstructureandbondingallowsus
tation(i.e.,H-atomorC H loss,wheren=0,1,2).
2 n toapplythisjelliummodelalsotoPAHs.However,thespherical
NospecifictheorydescribestheenergytransfertoaPAHvia
geometryis clearlynotappropriateforPAHs, which we model
electronicexcitation,soweadoptthesameapproachdeveloped
insteadasathickdiskanalogouslytothedistributionfromEq.3
by e.g. Schlatho¨lteretal. (1999) and Hadjaretal. (2001) who
modelled electronic interactions in fullerene, C60. To calculate n =0.15 exp[ (x)2/2.7] (4)
the energy transferred to a PAH, we treat the large number of 0 −
delocalizedvalenceelectronsinthemoleculeasanelectrongas,
where x is the coordinate along the thickness of the disk. This
where the inelastic energy loss of traveling ions is due to long
electrondensitypeaksatthecenterofthemoleculeandvanishes
rangecouplingtoelectron-holepairs(Ferrell1979).
outside,leadingtoathicknessd 4.31Å.TheradiusRofthe
In the energy range we consider for this study, the energy ∼
disk is given by the usual expression for the radius of a PAH:
transferredscaleslinearlywiththevelocityvoftheincidention
a = R = 0.9 √(N ) Å, where N is the number of carbon
andcanbedescribedintermofthestoppingpowerS,whichis PAH C C
atoms in the molecule (Omont 1986). For a 50 C-atoms PAH,
widelyusedinthetreatmentofion-solidcollisions.Thestopping
R=6.36Å.
powerrepresentstheenergylossperunitlengthandisdefinedas
TocalculatetheenergytransferredfromEq.2,weadoptthe
dT coordinatesystemshowninFig.1,wherethepathlengththrough
S = = γ(rs)v (1) thePAH sisexpressedasafunctionofthecoordinate xandof
ds −
the angleϑ betweentheaxesofthe moleculeand thedirection
where dT is the energy loss over the pathlength ds (Sigmund of the incomingion.In thisway foreach trajectorygivenby ϑ
1981). The total energy loss can then be obtained integrating thecorrespondingenergytransferredcanbecomputed.Wehave
Eq.1 dx=ds cos ϑandtheelectrondensityisthengivenby
n (s,ϑ)=0.15 exp[ (s cos ϑ)2/2.7] (5)
Te = γ(rs)vds (2) 0 −
Z
The density parameter can be rewritten as r (s,ϑ) =
s
Thefrictioncoefficientγ(Puska&Nieminen1983)dependson 4πn (s,ϑ) −1/3 The friction coefficient γ has been calculated
1/3 3 0
thedensityparameterrs = 43πn0 − whichisafunctionofthe (cid:16) (cid:17)
valenceelectron density n0(cid:16). The(cid:17)choice of the valence electron 1 Length(a.u.)=a0=0.529Å,Velocity(a.u.)=0.2√E(keV/amu),
densityisadelicateissueforPAHs.Inthecaseoffullerene,this Energy(a.u.)=27.2116eV
E.R.Micelottaetal.:PAHprocessinginahotgas 3
by Puska&Nieminen (1983) for variousprojectile ions and r Table 1. Analytical fit to the electron stopping power in solid
s
values.Itcanbeinterpolatedbytheexponentialfunction carbon.
(r (s,ϑ) 1.5)
γ(r )=Γ exp − s − (6)
s 0 " R # S(E)=h log(1+aE)/f Eg + bEd + cEe
2
a b c d
where Γ is the value of γ when r = 1.5. For hydrogen, he-
0 s
liumandcarbonthefitparameterR2equals2.28,0.88and0.90, -0.000423375 -3.57429×10−11 -3.37861×10−7 -3.18688
respectively.
e f g h
Theenergytransferredisthengivenbythefollowingequa-
tion -0.587928 -0.000232675 1.53851 1.41476
R/sinϑ
T (ϑ)=27.2116 vγ(r )ds (7)
e s
Z
R/sinϑ thermal electron energy of 10 keV, well below the relativis-
−
∼
withγ(r )fromEq.6.Theconstant27.2116convertsthedimen- tic limitof 500keV. Undertheselow-energyconditions,with
s ∼
sionsofT fromatomicunitstoeV.Foragivenvelocityofthe respecttotherelativisticregime,elasticcollisionsbetweenelec-
e
incidentparticle,thevalueofT willbemaximumforϑ = π/2 tronsandtargetnucleiarenoteffective.Theenergytransferoc-
e
(s = 2R, the longest pathlength) and minimum for ϑ = 0,π curs through inelastic interactions with target electrons (as for
(s = d, the shortest pathlength).The deposited energyalso in- electronicexcitationbyimpactingions),whichleadtoacollec-
creases with the pathlength across the molecule, and then will tiveexcitationofthemolecule,followedeventuallybydissocia-
behigherforlargerPAHsimpactedunderaglancingcollision. tionorrelaxationthroughIRemission.
The energy transferred determines the PAH dissociation Thecalculationoftheenergytransferredbysuch‘slow’elec-
probabilityuponelectronicexcitation,whichisrequiredtoquan- tronsisinfactadelicatematter.Thetheorydevelopedunderthe
tifythedestructioninducedbyinelasticenergylossinthehotgas firstBornapproximation(Bethe1930)canbeappliedonlytothe
(seeSect.4). mostenergeticelectrons(aroundfewkeV)butisunsuitablefor
therestofourrange.Atlowenergies(<10keV),wherethefirst
Born approximation is no longer valid, the Mott elastic cross
2.2.Nuclearinteractionabovethreshold
section must be used instead of the conventional Rutherford
Inthepresentstudywehavetoconsidernotonlytheelectronic cross section (Mott&Massey 1949; Czyzewskietal. 1990).
interactiondescribedabove,butthenuclearpartoftheioniccol- The Monte Carlo program CASINO (Hovingtonetal. 1997)
lisionaswell.Thetheoryofnuclearinteractionabovethreshold computes the Mott cross sections in the simulation of electron
hasbeendescribedindetailinMJT.Wesummarizehereforclar- interactions with various materials. Unfortunately the stopping
ity the essential conceptsandthe equationswhichwill be used power dE/dx is not included in the program output. An em-
inthefollowing. pirical expression for dE/dx has been proposed by Joy&Luo
Forthenuclearinteractionweconsideronlycollisionsabove (1989),whichneverthelessisreliableonlydownto50eV,while
threshold,i.e.collisionsabletotransfermorethantheminimum weareinterestedintheregionbetween10and50eVaswell.
energyT requiredtoremoveaC-atomfromthePAH.Tocalcu- Wedecidedthustoderivetheelectronstoppingpowerfrom
0
late the PAH destructiondue to nuclearinteraction,we use the experimentalresults. Measurementsof the electronenergyloss
rate of collisions above threshold between PAHs and ions in a inPAHsarenotavailableinourenergyrangeofinterest,sowe
thermalgas,asgivenbyEq.30inMJT: usethemeasurementofdE/dxinsolidcarbonforelectronswith
energybetween10eVand2keV(Joy1995).Thedatapointsare
R = N 0.5χ n ∞F vσ(v) f(v,T)dv (8) wellfitted(towithinfew%)bythefollowingfunction:
n,T C i H C
Z
v0 h log(1+aE)
where f(v,T) is the Maxwellian velocity distribution function S(E)= (9)
f Eg + bEd + cEe
forion,i,inthegas,F istheCoulombiancorrectionfactor,σ(v)
C
isthenuclearinteractioncrosssectionperatomabovethreshold whereEistheelectronenergy(inkeV).Thevaluesofthefitting
(Eq.22inMJT where E isthekineticenergycorrespondingto parametersarereportedinTable1.S(E)hasthesamefunctional
v),andthefactor1/2takestheaveragingovertheorientationof formastheZBLreducedstoppingcrosssectionfornuclearin-
thediskintoaccount. teraction (cf. Sect. 2.1 in MJT). The datapoints and the fitting
For T0 we adopt a value of 7.5 eV (see discussion in MJT, functionareshowninthetoppanelofFig.2.
Sect. 2.2.1). The kinetic energy required for the incoming ion The stoppingpower increases sharply at low electron ener-
to transfer T0 is the critical energy E0. The lower integration gies,reachesitsmaximumat 0.1keV,anddecreasessmoothly
limit v0 in Eq. 8 is the critical velocity corresponding to E0n, afterwards. Between 0.01 and∼0.1 keV a small variation of the
which is the minimumkinetic energyfor the projectile to have energyoftheincidentelectronwilltranslateintoalargechange
thenuclearinteractioncrosssectiondifferentfromzero(cf.Sect. inthetransferredenergyperunitlength.TheshapeofS(E)im-
2.2inMJT). plies that only those electrons with energies that fall in a well
definedwindowwillefficientlytransferenergy,while electrons
below 0.02 keV and above 2 keV are not expected to con-
3. ElectroncollisionswithPAHs ∼ ∼
tributesignificantlytoPAHexcitation.
Fast electrons are abundant in a hot gas. Because of their low OncethestoppingpowerdE/dxisknown,wecancalculate
mass, they can reach very high velocities with respect to the theenergytransferredbyanelectronofgivenenergywhentrav-
ions, and hence high rates of potentially destructive collisions. elling throughthe PAH. We adoptthe same configurationused
We consider gas temperaturesup to 108 K, correspondingto a forelectronicinteraction,showninFig.1.Thetrajectoryofthe
4 E.R.Micelottaetal.:PAHprocessinginahotgas
9 concernedwiththosecollisionsforwhichtheenergydeposition
Joy 1995 isinexcessofthethresholdenergy(> 10eV)overapathlength
8 Fit of 10 20 Å. Or to phrase it diff∼erently, at low energies a
ev / Å) 67 Carbon: S(E) mPAuH≃chs)lairng−eorrdpearthtolentrgatnhsfhearsstuoffibceietrnatveenrseerdgy(tthoancaiussreelfervaagnmtefonr-
er ( 5 tation. Then, since we know E0, we can calculate F(E0), and
w F(E ) isthengivenby F(E ) l(ϑ).We alsocalculated E asa
g po 4 func1tion of F(E). We can t0he−n determine E1 from F(E1) and,
pin 3 finally,TelecfromEq.11.
p
o 2
St
1 4. PAHdestruction
0
0.01 0.1 1 10 4.1.Dissociationprobability
Electron energy (keV) Ion or electronic collisions (or UV photon absorption) can
90 leavethemoleculeinternally(electronically)excitedwithanen-
ergyT .Internalconversiontransfersthisenergytovibrational
80 F(E) = integral of dE/S(E) E
modes,andthemoleculecanthenrelaxthroughdissociationor
70 IRemission.Thesetwo processesarein competitionwitheach
60 other. To quantifythe PAH destructiondue to ion and electron
Å) 50 collisions we need to determine the probability of dissociation
F(E) ( 40 rathIenrtthhaenmIiRcreomcaisnsoinoinc.aldescriptionofaPAH,theinternalen-
30 ergy,T ,is(approximately)relatedtotheeffectivetemperature
E
ofthesystem,T ,bythefollowingequation
20 eff
10 T (eV) 0.4 E (eV)
T 2000 e 1 0.2 0 (12)
0 eff ≃ N ! − T (eV)!
0 50 100 150 200 C e
where E isthebindingenergyofthefragment(Tielens2005).
Electron energy (eV) 0
This effective temperature includes a correction for the finite
Fig.2.Toppanel-ExperimentalmeasurementofdE/dxinsolid
size of the heat bath in the PAH. The temperature T is de-
eff
carbonforelectronswithenergybetween10eVand2keV,from
finedthroughtheunimoleculardissociationrate,k ,writtenin
diss
Joy (1995), overlaid with the fitting function S(E) (solid line).
Arrheniusform
Bottom panel- Integral F calculated numericallyas a function
oftheenergyoftheincidentelectron,E. k =k (T ) exp[ E /kT ] (13)
diss 0 eff 0 eff
−
where k = 8.617 10 5 eV/K is the Boltzmann’s constant (cf.
−
incoming electron is defined by its impact angle ϑ and by the ×
Tielens2005).
geometryofthemolecule.
Considerthecompetitionbetweenphotonemissionatarate
BecausethethicknessofthePAHisnon-negligiblewithre- k (photons s 1) and dissociation at a rate of k (fragments
IR − diss
specttoitsradius,thestoppingpowerisnotconstantalongthe s 1). For simplicity, we will assume that all photons have the
−
electron path. To calculate the energy loss we thus follow the
same energy, ∆ε. The probability that the PAH will fragment
proceduredescribedbelow.WehavedE/dx= S(E),then
betweenthenthand(n+1)thphotonemissionisgivenby
−
dE
dx= = F(E) (10) n
Z −Z S(E) ϕ = p (1 p) (14)
n n+1 i
−
Thus, x x = [F(E ) F(E )] where x x = l(ϑ) is Yi=0
1 0 1 0 1 o
− − − −
the maximumpathlengththroughthe PAH. Frominspectionof The (un-normalized) probability per step p is given by p =
i i
Fig. 1, one can see that, if tan(ϑ) < tan(α), l(ϑ) = d/ cosϑ, k (E)/k (E)andE =(T i ∆ε),withT theinitialinternal
| | | | diss i IR i i E E
otherwise l(ϑ) = 2R/ sinϑ. E is the initial energy of the im- − ×
0 energy,coincidentwiththeenergytransferredintothePAH.The
| |
pactingelectron,E istheelectronenergyafterhavingtravelled
1 totaldissociationprobabilityisthengivenby
the distance l(ϑ), which needs to be calculated so we can then
determinetheenergytransferredtothePAH nmax
P(n )= ϕ (15)
max n
T (ϑ)= E E (11)
elec 0− 1 Xn=0
The integral F, calculated numerically as a function of E, is
If we ignore the dependence of k on E, the temperatures
IR i
showninthebottompanelofFig.2.Werecognizethatforlow
(Eq.12)dropapproximatelybyT/T = (1 0.4∆ε/T ).The
i i 1 E
energies, F rises sharply, reflecting the small energy stopping probabilityratioisapproximately: − −
powerinthisenergyrange(cf.,toppanelinFig.2).Forhigher
energies,F(E)risesslowly(andlinearly)withincreasingenergy p E ∆ε
i =exp 0 0.4 (16)
overthe relevantenergyrange. We note that the initialrise de-
p " kT ! ε #
i 1 i 1
pendsstronglyonthe (uncertain)details ofthe stoppingpower − −
at low energies.However,it is of no consequencein ourdeter- These equationsbecome very difficult to solve in closed form.
mination of the amount of energy deposited since we are only However, let us just assume that p does not vary and is given
i
E.R.Micelottaetal.:PAHprocessinginahotgas 5
by p .Thenwehavethetotalun-normalizeddissociationprob-
av
ability
k exp[ E /kT ]
P(n )=(n +1)p = 0 − 0 av (17)
max max av k /(n +1)
IR max
whereweadoptk =100photonss 1(Jochimsetal.1994),and
IR −
theaveragetemperatureischosenasthegeometricmean
T = T T (18)
av 0× nmax
p
whereT andT aretheeffectivetemperaturescorresponding
0 nmax
toT (initialinternalenergyequaltotheenergytransferred)and
E
(T n ∆ε)(internalenergyaftertheemissionofn pho-
E max max
− ×
tons).FortheenergyoftheemittedIRphotonweadoptthevalue
∆ε=0.16eV,correspondingtoatypicalCCmode.
If the p’s were truly constant, then n would be n =
i max max
(T E )/∆ε.However,theydodecrease.So,ratherwetakeit
E 0
−
tobewhentheprobabilityperstephasdroppedbyafactor10.
Adirectcomparisonbetweenthefullevaluationandthissimple
Fig.3.TheappearanceenergyasafunctionofthenumberofC-
approximationyieldsnmax=10,20and40forNC=50,100and atomsinthePAH.Theredlineprovidesafittotheexperimental
200respectively.The quantitynmax scales with NC because for data using Eq. 13 for an assumed pre-exponential factor k0 =
a constant temperature (eg., required to get the dissociation to 1.4 1016 and a (fitted) Arrhenius energy, E = 3.65 eV. The
0
occur),theinternalenergyhastoscalewithNC.Asaresult,the data×pointsaretheexperimentalresultsofJochimsetal.(1994).
numberofphotonstobeemittedalsohastoscalewithNC. ThebluelineistheappearanceenergyforISMconditions(eg.,
The choice of the values to adopt for k0 and E0 is a deli- atarateof1s−1).
cate matter. In the laboratory the dissociation of highly vibra-
tionallyexcitedPAHsistypicallymeasuredontimescalesof1-
100µsbecauseeitherthemoleculesarecollisionallyde-excited
apparent.Indeed,thiskineticshiftcanamounttotensofeVfor
byambientgasorthemoleculeshaveleftthemeasurementzone
relevantPAHs.
of the apparatus. In contrast, in the ISM, the competing relax-
The derived Arrhenius energy of 3.65 eV is small com-
ation channelis throughIR emission and occurstypicallyon a
paredtothebindingenergyofaC H groupinaPAH (4.2eV,
timescaleof1s.Asisalwaysthecaseforreactionscharacterized 2 2
Ling&Lifshitz 1998). This is a well known problem in statis-
byanArrheniuslaw,alongertimescaleimpliesthattheinternal
tical unimolecular dissociation theories (cf. Tielens 2008). We
excitationenergycan be lower. This kinetic shiftis well estab-
emphasizethattheseresultsshowthatatypicalinterstellarPAH
lished experimentally and can amount to many eV. Moreover,
with a size of 50 C-atoms would have a dissociation probabil-
only small PAHs (up to 24 C-atoms) have been measured in
ity of 1/2afterabsorptionofanFUV photonof 12eV(cf.
the laboratory and the derived rates have to be extrapolated to
∼ ∼
Fig.3).Hence,PAHswouldberapidlylostintheISMthrough
muchlarger( 50C-atoms)PAHsthatareastrophysicallyrele-
∼ photolysis. It seems that the experiments on small PAHs can-
vant.Inan astrophysicalcontext,theunimoleculardissociation
not be readily extrapolated to larger, astrophysically relevant
ofhighlyvibrationallyexcitedPAHs–pumpedbyFUVphotons
PAHs. Possibly, this is because experimentally C H loss has
–hasbeenstudiedexperimentallybyJochimsetal.(1994)and 2 2
only been observedfor verysmall catacondensedPAHs with a
furtheranalyzedbyLePageetal.(2001).Here,wewillmodify
very open carbon skeleton (e.g., naphthalene, anthracene, and
theanalysisofTielens(2005)forH-lossbyUVpumpedPAHsto
phenanthrene) which are likely much more prone to dissocia-
determinetheparametersforcarbonloss.Thedissociationrate–
tionthantheastrophysicallymorerelevantpericondensedPAHs.
givenbyEq.13–isgovernedbytwofactors,thepre-exponential
Indeed, the small pericondensed PAHs, pyrene and coronene
factork andtheenergyE .Thepre-exponentialfactorisgiven
0 0
did not show any dissociation on the experimental timescales
by
(Jochimsetal.1994;Ling&Lifshitz1998).
kT ∆S Turningtheproblemaround,wecandeterminetheArrhenius
k = eff exp 1 + (19)
0 energy,E ,asafunctionofthedissociationprobabilitybyadopt-
h " R # 0
ingan IRrelaxationrate of1 s 1 andaninternalexcitationen-
−
where ∆S is the entropy change which we set equal to 10 cal ergyequaltoatypicalFUVphotonenergy(12eV).Theresults
mole 1 (Ling&Lifshitz 1998). We will ignore the weak tem- fora50C-atomPAHareshowninFig4.Ifweadoptalifetime,
−
peraturedependenceofk inthefollowingandadopt1.4 1016 τ , of 100millionyears,a PAH in thediffuseISM will have
0 PAH
s 1. The parameter E can now be determinedfroma fit×to the typicallysurvivedsomeσ N τ = 2 106N UVphoton
− 0 uv uv PAH C
experimentsbyJochimsetal.(1994)onsmallPAHs.Theseex- absorptions(withσ =7 10 18cm2,N ×=108photonscm 2
uv − uv −
perimentsmeasuredtheappearanceenergyE –theinternalen- s 1inaHabingfield).Henc×e,ifthelifetimeofthesmallestPAHs
ap −
ergy at which noticable dissociation of the PAH first occurred. intheISM(eg.,with N 50C-atom)issetbyphotodissocia-
C
The rate at which this happened was assumed to be 104 s 1. tionoftheC-skeleton,the≃probabilityfordissociationhastobe
−
Adopting this value, the appearance energy can be estimated 5 10 7 correspondingto an Arrheniusenergyof 4.6eV (Fig.
−
fromEq.13.TheresultsofouranalysisareshowninFig.3.The 4)×.We notethatinaPDRthephotonfluxishigher(G 105)
0
internalenergyrequiredtodissociateaPAHdependsstronglyon whilethelifetime (ofthePDR) issmaller (τ 3 1∼04 yr),
PDR
thePAHsize.Likewisethekineticshiftassociatedwiththerel- resultingin6 107UVphotonsabsorbedoverthe≃PDR×lifetime.
×
evanttimescaleatwhichtheexperimentwasperformedisquite SurvivalofPAHsinaPDRenvironmentwouldthereforerequire
6 E.R.Micelottaetal.:PAHprocessinginahotgas
both target and projectiles are charged, and thus the collision
crosssectioncouldbeenhancedordiminisheddependingonthe
chargesign. The electron coulombianfactor is always equalto
1 (to within 1%) because electrons have low mass. For a fixed
temperaturetheyreachhighervelocities,withrespecttotheions,
andarethuslesssensitivetotheeffectsoftheCoulombianfield.
ThequantityϑistheanglebetweentheverticalaxesofthePAH
andthedirectionoftheprojectile.Thetermσ isthegeometrical
g
crosssectionseenbyanincidentparticlewithdirectiondefined
by ϑ. The PAH is modelled as a thick disk with radius R and
thicknessd.Thecrosssectionisthengivenby
σ =πR2 cosϑ + 2Rd sinϑ (22)
g
which reduces to σ = πR2 for ϑ = 0 (face-on impact) and to
g
σ = 2Rd forϑ = π/2(edge-onimpact).Theterm P(v,ϑ)rep-
g
resentsthetotalprobabilityfordissociationuponelectroncolli-
sionsandelectronicexcitation,foraparticlewithvelocityv
PAH
and incoming direction ϑ. It was calculated from Eq. 17 using
Fig.4.Theprobabilityfordissociationofa50C-atomPAHex-
the appropriatedvalue of the energytransferred:T for elec-
citedby10eVasafunctionoftheArrheniusenergy,E . elec
0 trons(Eq.11)andT forelectronicexcitation(Eq.7)
e
Weareconsideringahotgasandthereforeweareinterested
inthethermalcollisionrate,givenby
asomewhatlargerE (oralternatively,onlyslightlylargerPAHs
0
couldsurviveinsuchanenvironment).
We note that the binding energy of a C2H2 group to small Re,T = ∞Re(v) f(v,T)dv (23)
Z
PAHsisestimatedtobe4.2eVandisprobablysomewhatlarger v0,e
for a 50 C-atom condensed PAH. Loss of pure carbon, on the wherev istheelectron/ionvelocitycorrespondingtotheelec-
0,e
other hand, requires an energy of 7.5 eV, close to the bind- tronicdissociationenergyE .ThetemperatureT isthesamefor
0
ing energy of C to graphite. Loss of C2 from fullerenes has a both ions and electrons, but these latter will reach much larger
measuredE0 of9.5 0.1eV(Tomitaetal.2001).Theselatter velocities. From Eq. 21 we then expect to find a significantly
±
twounimoleculardissociationchannelsareforallpracticalpur- highercollisionratewithrespecttotheioncase.
posesclosedunderinterstellarconditions.Finally,likely,H-loss
will be the dominant destruction loss channel for large PAHs
(E = 3.3 eV), leading to rapid loss of all H’s (LePageetal. 5. Results
0
2001; Tielens 2005). The resulting pure C-skeleton may then
5.1.PAHdestructioninahotgas
isomerize to much more stable carbon clusters, in particular
fullerenes,andthismaybethedominant‘loss’channelforinter- To describe the destructive effects on PAHs of collisions with
stellarPAHs(cf.Tielens2008, andreferencestherein).Itisclear ionsi(i= H,He, C) andelectronsin a thermalgaswe haveto
that there are many uncertainties in the chemical destruction evaluatetherateconstantforcarbonatomloss.
routesofinterstellarPAHsandthatthesecanonlybeaddressed For all processes the rate constant J is defined by the ratio
by dedicated experimental studies. For now, in our analysis of J = R /n , where R is the thermal collision rate appropriate
T H T
the unimoleculardissociationof PAHs –excitedbyelectronor fornuclearandelectronicexcitationandelectroncollisions(cf.
ioncollisions–wewilladoptE0 = 4.6eVasastandardvalue. Eq. 8 and 23), and nH is the density of hydrogen nuclei. For
WewillhoweveralsoexaminetheeffectsofadoptingE0 =3.65 electronicandelectroninteractionstheratemustbemultiplyied
eV(eg., p=1/2)andE0 =5.6eV(eg., p=3 10−12). byafactorof2,totakeintoaccountthefactthateachcollision
×
leadstothelossoftwocarbonatoms.
Theelectron,nuclearandelectronicrateconstantsforthree
4.2.Collisionrate
PAH sizes (Nc = 50, 100 and 200 C-atom) are shown in Fig.
Once the dissociation probability is determined, we can calcu- 5 as a function of the gas temperature. We assume for the nu-
late the destructionratethroughelectronicexcitationfollowing clearthresholdenergyT thevalueof7.5eV(MJT),andforthe
0
electron or ion collision. Adopting the configuration shown in electronicdissociationenergythevalue4.58eV(Sect.4.1).
Fig.1,thedestructionrateisgivenby Forthenuclearinteraction,thethresholdenergyT isinde-
0
pendentfromthePAHsize,sothethreecurvesstartatthesame
1
R =vχn F dΩσ (ϑ)P(v,ϑ) (20) temperature (not shown in the plot). The small separation be-
e H C 2πZ g tweenthecurvesisduetothefactthatthePAH‘surfacearea’–
andhencetherateconstant–scaleslinearlywith N , therefore
with Ω = sinϑdϑdϕ, ϕ running from 0 to 2π and ϑ from 0 to C
J ishigherforbiggerPAHs.
π/2.Eq.20canbere-writtenas
For nuclear (and electronic) interactions, the rate constants
π/2 decrease from hydrogen to carbon because of the lower abun-
R =vχn F σ (ϑ)P(v,ϑ) sinϑdϑ (21) danceoftheheavierprojectileswithrespecttohydrogen(H:He
e H C g
Zϑ=0 :C=1:0.1:10 4).Ontheotherhand,thenuclearcurvesshift
−
The term v is the velocity of the incident particle, χ is the toward lower temperatures from lighter to heavier projectiles.
ion/electron relative abundance in the gas, and F is the This is the reflection of the fact that the critical energy of the
C
Coulombian correction factor, taking into account the fact that particle,requiredtotransferthethresholdenergyT0,decreases
E.R.Micelottaetal.:PAHprocessinginahotgas 7
10−2
10−4 Nc = 50 Electronic
) Nc = 100
−1s 10−6 NNcc == 2 0500 Nuclear

3m 10−8 Nc = 100
Nc = 200
c Nc = 50
( 10−10 Nc = 100
nt Nc = 200
a 10−12
st ELECTRONS
on 10−14
C
e 10−16 Electrons −−> PAH
t
a
R 10−18 Hydrogen −−> PAH
10−20
103 104 105 106 107 108
Temperature (K)
10−2
10−4 Nc = 50 Electronic
) Nc = 100
−1s 10−6 NNcc == 2 0500 Nuclear
3m 10−8 NNcc == 120000
c Nc = 50
( 10−10 Nc = 100
nt Nc = 200
a 10−12
st ELECTRONS
on 10−14
C
e 10−16 Electrons −−> PAH
t
a
R 10−18 Helium −−> PAH
10−20
103 104 105 106 107 108
Temperature (K)
10−2
10−4 Nc = 50 Electronic
) Nc = 100
−1s 10−6 NNcc == 2 0500 Nuclear
3m 10−8 NNcc == 120000
c Nc = 50
( 10−10 Nc = 100
nt Nc = 200
a 10−12
st ELECTRONS
on 10−14
C
e 10−16 Electrons −−> PAH
t
a
R 10−18 Carbon −−> PAH
10−20
103 104 105 106 107 108
Temperature (K)
Fig.5.Nuclear(dashedlines),electronic(solidlines)andelectron(dashed-dottedlines)rateconstantforPAHcarbonatomlossdue
to collisions with H, He, C andelectronsin a thermalgas. The rate constantsare calculated as a functionof the gastemperature
forthreePAHsizesN =50,100,200C-atoms,assumingthenuclearthresholdenergyT =7.5eVandtheelectronicdissociation
C 0
energyE =4.58eV.
0
8 E.R.Micelottaetal.:PAHprocessinginahotgas
10−2
10−4 Nc = 50 Ionic
) Nc = 100
−1 10−6 Nc = 200
s Nc = 50 ELECTRONS

3m 10−8 Nc = 100
Nc = 200
c
( 10−10

t
n
a 10−12
t
s
on 10−14
C
e 10−16 Electrons −−> PAH
t
a
R 10−18 Hydrogen −−> PAH
10−20
103 104 105 106 107 108
Temperature (K)
10−2
10−4 Nc = 50 Ionic
) Nc = 100
−1 10−6 Nc = 200
s Nc = 50 ELECTRONS
3m 10−8 NNcc == 120000
c
( 10−10

t
n
a 10−12
t
s
on 10−14
C
e 10−16 Electrons −−> PAH
t
a
R 10−18 Helium −−> PAH
10−20
103 104 105 106 107 108
Temperature (K)
10−2
10−4 Nc = 50 Ionic
) Nc = 100
−1 10−6 Nc = 200
s Nc = 50 ELECTRONS
3m 10−8 NNcc == 120000
c
( 10−10

t
n
a 10−12
t
s
on 10−14
C
e 10−16 Electrons −−> PAH
t
a
R 10−18 Carbon −−> PAH
10−20
103 104 105 106 107 108
Temperature (K)
Fig.6. Analytical fits to the calculated rate constants shown in Fig. 5. The ‘Ionic’ curve is the fit to the sum of the nuclear and
electronicrateconstants,thusrepresentsthetotalcontributionfromioncollisionstoPAHdestruction.
E.R.Micelottaetal.:PAHprocessinginahotgas 9
withincreasingmassoftheprojectileitself.Then,acarbonatom thefittingparametersarereportedinTable2.Toprovideanex-
withatemperatureof 4 103Kishotenoughtotransfertheen- ampleoftheaccuracyofourfittingprocedure,Fig.7showsthe
∼ ×
ergyrequiredforatomremovalvianuclearinteraction,whereas comparisonbetweenthe calculatedrate constantandthecorre-
for hydrogena temperatureof at least 104 K is necessary. The spondinganalyticalfit,forelectronsandheliumimpactingona
almost-constantbehaviouraftertheinitialrisereflectsthelarge 50 C-atom PAH. The He fit is overthe sum of the nuclear and
maximum observed in the nuclear cross section (cf. Fig. 2 in electronicrateconstants.Theaveragefittingdiscrepancyis 15
∼
MJT). %andthefitsarethereforewellwithinanyuncertaintiesinthe
ThedissociationprobabilityP(n )(Eq.17)dependsonthe method.
max
bindingenergyofthefragmentE ,onthePAHsize N andon Fig. 8 shows the comparison between the carbon loss rate
0 C
theenergytransferred(throughT ),whichinturnsdependson constants for a very big PAH, N = 1000, and a 50 C-atom
av C
theinitialenergy(velocity)ofthe projectile.Forathermaldis- molecule.WeassumeT =7.5eVandE =4.6eV.Becauseof
0 0
tribution,thislatterwillbedeterminedbythegastemperatureT. the decrease of the dissociationprobabilitywhen the PAH size
The electronic rate constant curves for the different PAH increases,asexpected,theelectronandelectronicrateconstants
sizesarewellseparatedatthelowesttemperatures.Thisreflects arestronglysuppressed,andbothcurvesshifttowardhighertem-
the fact that, for a fixed value of the transferred energy and of peratures.Indeed,forsuchabigPAH,amuchhigherinternalen-
theelectronicdissociationenergyE ,thedissociationprobabil- ergyisrequiredtoreachtheinternaltemperaturewheredissocia-
0
ity decreases for increasing N because either more energy is tionsetsin.Ontheotherhand,thenuclearrateconstantincreases
C
requiredinthebondthathastobebrokenorbecausetheenergy linearlywiththePAHsize.Asaresult,fora1000carbonatoms
is spread over more vibrational modes and hence the internal PAH, the nuclearinteractionisthe dominant(andefficient)de-
excitationtemperatureislower.Ontheotherhand,themoreen- struction mechanism up to T 2 107 K. In conclusion, elec-
∼ ×
ergythatisdepositedintothePAH,thehigheristhedissociation trons are responsible for the destruction of small/medium size
probability.Theenergytransferredviaelectronicexcitation(and PAH,whileforbigmoleculesthisroleistakenbyions.
thenT )increaseswiththeenergyoftheprojectileuptoamax- AsmentionedattheendofSect.4.1,weexaminedtheeffects
av
imumvalue,correspondingtoanincidentenergyof 100keVfor ofadoptingdifferentvaluesfortheArrheniusenergy,E =3.65
0
H(andhigherformoremassiveparticles),anddecreasesbeyond and 5.6 eV, lower and higher, respectively, than our canonical
thatforhigherenergies.Theenergycontentofathermalgasat value 4.6 eV. The results are shown in Fig. 9. The dissociation
T =108 Kis 9keV,thus,inthetemperaturerangeconsidered probabilitydecreasesforincreasingE ,becausemoreenergyis
0
∼
in this study,the energytransferredincreaseswith temperature requiredinthebondthathastobebroken.Hence,asexpected,
(energy)andhencethedissociationprobabilityincreasesaswell. both the electron and electronic rate constants decrease in ab-
Thisisthebasisforthemonotonicriseoftheelectronicratecon- solutevalueandshifttowardhighesttemperatures.Inparticular
stant.Aftertheinitialseparation,thethreecurvesconverge,be- the electronicthermalshiftis verypronounced,indicatinghow
causetheriseinthetransferredenergycompensatestheeffectof sensitivethisprocessiswithrespecttotheassumedE .Avaria-
0
increasingN . tionintheadoptedelectronicexcitationenergytranslatesintoa
C
AsdiscussedinSect.3,theenergytransferredbyimpacting significantvariationoftherateconstant,reemphasizingtheim-
electronsrisessharplyforenergiesinexcessof10eV,peaksat portanceofexperimentalstudiesofthiscriticalenergy.
∼
100eVanddecreasesmoreslowlydownto10keV.Thisresults
inadissociationprobabilityshapedasastepfunction:for10eV
< v < 4 keV, P(n ) jumpsfromvaluesclose to zero up to 1. 5.2.PAHlifetime
max
∼Thes∼elimitingenergiesapplytoa50C-atomPAH;forNC=200 Underthe effectof electronand ion collisionsin a hotgas, the
thewidthofthestepissmaller(100eV<v<1keV),duetothe number of carbon atoms in a PAH molecule varies with time.
factthatforabiggerPAHmoreenergy∼has∼tobetransferredfor Afteratimet,thisnumberis
dissociation.Thisbehaviourisreflectedintheshapeoftheelec-
tronrateconstant,whereasteepriseisfollowedbyamaximum, NC(t)= NC(0)exp[ t/τ0] (25)
−
whichisemphasisedbythelogarithmicscale usedfortheplot.
andthenumberofcarbonatomsejectedfromthisPAHis
Asexpected,theelectronrateconstantovercomestheelectronic
one, exceptfor the highestgas temperatures.This results from F (t)= 1 exp[ t/τ ] (26)
L 0
thefactthat,foragiventemperature,electronscanreachhigher − −
(cid:0) (cid:1)
velocitieswithrespecttotheions(cf.Eq.21and23). Thequantityτ0isthetimeconstantappropriateforelectron,nu-
Tosummarize,fromFig.5wecaninferthat,accordingtoour clearandelectronicinteraction,givenby
model,thedestructionprocessisdominatedbynuclearinterac-
N N
tionwithheliumatlowtemperatures(below 3 104K),andby τ = C = C (27)
0
electron collisions abovethis value. Small P∼AH×s are easier de- (2)RT JnH/e
stroyedthanbigonesfortemperaturesbelow 106K,whilethe whereR and J are thethermalcollision rate andthe rate con-
∼ T
differenceinthedestructionlevelreducessignificantlyforhotter stant for electrons, nuclear and electronic interactions respec-
gas. tively, and n is the hydrogen/electrondensity. For electrons
H/e
Thecalculatedrateconstants,showninFig.5,arewellfitted andelectronicexcitation,theratemustbemultipliyedbyafactor
bythefunctiong(T)=10f(log(T)),where f(T)isapolynomialof of 2, because each interaction leads to the removalof two car-
order5 bon atomsfrom the PAH. The nuclearrate scales linearly with
f(T)=a+bT +cT2+dT3+eT4+ f T5 (24) NC, hence the correspondingfractionalcarbon atom loss FL is
independentofPAHsize.
ForeachPAHsizeweprovidethefittotheelectronrateconstant; For any given incoming ion and fixed PAH size N , F is
C L
fortheions,thefitisoverthesumofthenuclearandelectronic univocallydeterminedbythehydrogen/electrondensityn and
H/e
rateconstants,inordertoprovideanestimateoftheglobalcon- thegastemperatureT.WeassumethataPAHisdestroyedafter
tributionfromioniccollisions.ThefitsareshowninFig.6,and the ejection of 2/3 of the carbon atoms initially present in the
10 E.R.Micelottaetal.:PAHprocessinginahotgas
10−2
10−4 He (Data, Nuclear)
) He (Data, Electronic)
−1 10−6 He (Fit)
s ELECTRONS (Data)

3m 10−8 ELECTRONS (Fit)
c
( 10−10

t
n
a 10−12
t
s
on 10−14 Nc = 50
C
e 10−16 Electrons −−> PAH
t
a
R 10−18 Helium −−> PAH
10−20
103 104 105 106 107 108
Temperature (K)
Fig.7.Calculatedrateconstantsforelectronsandheliumimpactingona50C-atomPAH,overlaidarethecorrespondinganalytical
fits.TheHefitisforthesumofthenuclearandelectronicrateconstants.Theaveragediscrepancyis 15%.
∼
Table2.AnalyticalfitparametersforthePAHcarbonatomlossrateconstant,calculatedforelectronandioncollisions.
Fittingfunction f(T)=a+bT +cT2+dT3+eT4+ fT5
a b c d e f
Electrons 2136.83 1632.17 -499.822 76.4347 -5.82964 0.177174
H -1896.69 1480.8 -462.733 71.8957 -5.54719 0.169996
N =50 He -971.448 770.259 -245.561 38.8995 -3.05787 0.0954303
C
C -704.392 551.313 -175.063 27.6506 -2.16871 0.0675643
Electrons -2255.38 1681.45 -503.451 75.4339 -5.64956 0.168959
H -1645.64 1257.06 -384.309 58.4103 -4.41007 0.132356
N =100 He -945.901 747.921 -237.984 37.6808 -2.96613 0.0928852
C
C -711.244 558.48 -177.983 28.2547 -2.23145 0.0701374
Electrons -2234.37 1597.71 -459.647 66.332 -4.79841 0.139019
H -1473.64 1109.01 -334.292 50.1417 -3.74133 0.111164
N =200 He -963.188 765.639 -245.054 39.0745 -3.10123 0.0980047
C
C -738.791 584.928 -187.884 30.0819 -2.39748 0.0760639
molecule.Thisoccursafterthetimeτ (Eq.27)whichweadopt InM82,PAHsarecompletelydestroyedbyelectrons,even
0
asthePAH lifetimeagainstelectronandionbombardmentina for the largerPAH, before electronic and nuclear contributions
gaswithgivendensityandtemperature. starttoberelevant.Theelectronandelectronicfractionallosses
Table 3 summarizesrelevantdata andPAH lifetime, τ , for decrease with PAH size, thus bigger molecules can survive
0
fourobjectscharacterizedbywarm-to-hotgas,X-rayemission, longer,whilethenuclearlossisindependentofNC.Thedestruc-
and(bright)IRemission features.The lifetime hasbeen calcu- tiontimescale isveryshort:afteronethousandyearsthe PAHs
latedforPAHswithN =200.τ (ref)hasbeenevaluatedadopt- shouldhavecompletelydisappeared.
C 0
ingourreferencevaluesfortheinteractionparameters,E =4.6 In Orion the situation is reversed. At the temperature con-
0
eVandT =7.5eV,τ (min)correspondstotheminimumvalues sideredforthisobject,electronsandelectronicexcitationdonot
0 0
E = 3.65 eV and T = 4.6 eV, and τ (max) to the maximum contributetoPAHerosion(cf.Fig.5).Thedamageiscausedby
0 0 0
valuesE =5.6eVandT =15eV.τ isthelifetimeofthe nuclearinteractionduetoHecollisions,withamarginalcontri-
0 0 object
object. butionfromcarbonbecauseoflowabundance,andthetimescale
Clearly, PAHs or relatedlargerspeciescan survivein these ismuchlarger:onlyafter10millionyearsthePAHdestruction
environments.Fig. 10 showsthe fractionalC-atom loss, due to becomesrelevant.Ofcourse,wehavenotevaluatedthedestruc-
electronandion(H+He+C)collisions,fortwowidelydiffer- tion of PAHs by H-ionizing photons in the Orion Hii region,
entobjects:the OrionNebula (M 42)andthe M 82 galaxy(cf. whichisexpectedtobeveryimportant.
Table 3). The famousOrion Nebula is an Hii region with high The young ( 2500 yr) supernova remnant, N132D, in
density(n =104cm 3)andlowtemperature(T =7000K)gas, the Large Magel≃lanic Clouds has been studied in detail at
H −
whileM82isastarburstgalaxy,whichshowsoutsidethegalac- IR, optical, UV, and X-ray wavelengths (Morseetal. 1995;
tic plane, a spectacular bipolar outflow of hot and tenuous gas Tappeetal.2006).ASpitzer/IRSspectrumoftheSouthernrim
(n =0.013cm 3,T =5.8 106K). shows evidence for the 15-20 µm plateau – often attributed
H −
×