Dust modeling and evolutionary implications

Dottorato di Ri er ain Fisi a

Ci loXXVI

Dust Modeling

and

Evolutionary Impli ations

Settore s ientifi odis iplinare di afferenza

FIS/05-Astronomiae Astrofisi a

Presentatada: AlbertoZon a

Coordinatoredottorato: Prof. PaoloRuggerone

From the beginning of the 20th entury astronomers started to move their studies

towardthat parti ular omponentwhi h ouldberesponsibleofthereddeningandthe

extin tion of stars radiation. They agreedthat this omponent must be represented

byapopulationofmi ros opi parti lesnamedinterstellardust,thatitmustbemixed

withtheinterstellargas. Lateron,inthese ondhalfofthe entury,dustwasre ognized

toplayverymanyimportantrolesintheMilkyWayandothergalaxies. Thesendings

hanged the astronomer's viewof the interstellar spa e, that is now fully onsidered

asan a tiveelementin gala ti evolution. For this reasonto a hievea morereliable

understanding of the nature of interstellar dust is ne essaryto takeinto a ount the

response of dust to the physi al onditions of the regions in whi h dust grains are

embedded.

Overtheyearsastronomersusedthe variationofextin tionrespe t towavelength,

the interstellar extin tion urve, to study osmi dust. In this thesis I elaborate a

physi al and hemi almodelof dust, to investigatetheremarkablevarietyof gala ti

interstellarextin tion urves. InChapter1Iintrodu etherolesoftheinterstellardust,

its pro essing in the interstellarmedium, and thetoolsto addressthe problem of its

nature. In Chapter 2 I present the exploited model in a simplied version, and an

appli ation to somepe uliar lines of sight in our galaxy. In Chapter 3the omplete

versionof the model isput forward,and, together with itssimplied version,applied

to a large sample of Milky Way interstellar extin tion urves. All dierent avours

of observed extin tion urves, ranging from the average gala ti extin tion urve to

very pe uliar proles, an be des ribed by su h a model. I show that a mixture of

oremantlesili ate/ arbongrainstogetherwitharelativelysmallnumber(54spe ies

infour hargestates)ofpoly y li aromati hydro arbons anreprodu ethefeaturesof

theextin tion urveintheultraviolet,dismissinganoldobje tiontothe ontribution

of poly y li aromati hydro arbonsto the interstellar extin tion urve. In Chapter

4 I ompare an evolutionary model of the physi al properties of arbona eousgrain

mantleswith theirdeterminationthroughthettingpro edure outlinedin Chapter3.

The results of su h omparison demonstrates, that in the framework of the adopted

dust model, the whole sample of gala ti extin tion urvesare in striking agreement

withtheproposed evolutionarys enario,requiringphysi al onditionsfully onsistent

withstandardvaluesofdensity,temperature,radiationeldintensity,andaverageage

ofdiuse interstellar louds. The resultsofthis thesisshowthat thepresent modelis

ableto re on ilethe great varietyof observedinterstellar extin tion urves within an

uniedevolutionarys enario,withoutmakingdrasti hangesto stru turalproperties

1 Interstellar dustfrom the beginning 1

1.1 Introdu tion. . . 1

1.2 Themanyrolesofdustin theinterstellarmedium . . . 3

1.3 Interstellardustlife y le . . . 5

1.4 Theproblem of osmi abundan es . . . 11

1.5 Dustin ourgalaxy . . . 13

1.6 Dustin externalgalaxies . . . 16

1.7 Dustmodels. . . 20

1.7.1 Draine'smodel . . . 20

1.7.2 TheMessina/Cagliarimodel . . . 21

2 The model and its rst appli ation 23 2.1 Themodelinitssimpliedversion . . . 23

2.1.1 Classi omponent . . . 24

2.1.2 PAHs . . . 28

2.2 Thesamplesele tion . . . 32

2.3 Results. . . 33

2.4 Dis ussion . . . 39

3 Modeling gala ti extin tion 41 3.1 Introdu tion. . . 41

3.2 Thenewsimplied versionofthe

[CM]

2

model . . . 42

3.3 Thedetailed versionofthe

[CM]

2

model . . . 42

3.4 Fittingdetails . . . 44

3.5 Results. . . 46

3.5.1 Classi aldustdistribution parameters . . . 48

3.5.2 Comparisonbetweensimplied anddetailedmodel results . . . . 49

3.5.3 The hargeofPAH mixture . . . 52

3.5.4 DetailedPAH omposition: un onstrained. . . 58

3.6 Dis ussion . . . 60

4 The evolvingdust 67 4.1 Introdu tion. . . 67

4.2 Themodelofinterstellardustevolution . . . 69

4.2.1 Outline . . . 69

4.2.2 Propertiesofthesolutions . . . 70

4.3 Observationaldata . . . 73

4.5 Con lusionsandfutureworks . . . 78

A The inverse problem 83

B Analyti approximations 87

Interstellar dust from the beginning

1.1 Introdu tion

Intheeighteenth enturyastronomershadnoti edwideregionsin theskywherethey

ouldnotobservestars. Therstastronomerwhodis ussedthiseviden ewasWilliam

Hers hel: he alled lö her in den himmel (holes in the sky)these regions apparently

withoutstars. Manyyearslater,WilhelmStruverealizedthatstarlightsuers

absorp-tion in proportion to the distan e travelled. In 1930, studying open lusters, Robert

J.Trumplerdis overedananomalyintheopen lusterssize;sohededu edthat

some-thingwaspresentbetweentheobservingastronomerandtheopen lusters,mi ros opi

solids,lateron alledinterstellardust, ouldae t themeasure ofstarsdistan es.

Astronomersmeasurestellardistan es omparingtheapparentmagnitudewiththe

absolutemagnitude,obtainedwiththespe tral lassi ation

m

V

− M

V

= 5 log d

′

− 5

(1.1)

where

m

V

and

M

V

areapparentandabsolutevisualmagnitudes,respe tivelyand

d

′

is

theapparentmean lusterdistan einparse s 1

. Knowing

d

′

,Trumpler oulddedu ethe

lineardiameterof ea h lustergeometri allyfrom themeasuredangular diameter. He

derivedaremarkabletrend: lustersdiametersappearedtoin reasewithdistan efrom

theSolarSystem. Tomakesurethatthe lusterdiameterswereindependentofdistan e,

heintrodu edadistan edependent orre tioninthelefthandsideofequation(1.1)

m

V

− M

V

− A

V

= 5 log d

− 5

(1.2)

where

d

is now the real distan e of the luster and

A

V

is the orre tion. To shed light on the nature of the orre tion, Trumpler studied the olours of stars involved

in the measurements, nding that their spe tra appeared to be shifted towards the

red,orreddened: learly

A

V

shouldbemoree ientforwavelengthsshorterthanin thevisible. Inparti ular, having omparedthe apparentluminosities of similarstars,

Trumplerinferred that

A

V

wasalinearfun tion ofthewavenumber,

A

V

∝ λ

−1

. This

resultindi atedthatinterstellarextin tionmustbeduetothepresen eofsolidparti les

1

This equation is derived by the denition of magnitude m from the brightness F:

m(λ) =

−2.5 log F (λ) + C

inwhi hC isa onstantdependentbythewavelength. Theabsolutemagnitudeis denedliketheapparentbrightnesswhi hastarwouldhaveifitwerepla edatastandarddistan e

of10p away;thenexploitingtherelationbetweenluminosityandbrightness:

L = 4πd

2

_·

_F

,inwhi h

d isthetruedistan eofthesour efromtheobserver,Iobtain

M − m = −2.5 log(4πd

2

_F/4π10

2

_{F ) =}

withdimensions omparabletovisualwavelengths. Thiswastherstdeterminationof

a toolthat astronomersare now routinely using to des ribe the intera tion of stellar

radiationwithdustalongaparti ularlineofsight: theextin tion urve,arelationthat

links theextin tion ofstellarradiationto thewavenumber.

Theknowledgeoftheexisten eofdispersedsolidsinspa e ouldhavehelpedthose

astronomerswho, in 1920, had dis overedthat many stars loseto thegala ti plane

appeared redderthen expe ted onthe basis oftheir spe tral types. There wasa

dis- repan y in stellar temperature derived by spe tros opy and photometry. By means

ofspe tral lassi ationstartemperaturesmaybeevaluatedthroughthepresen eand

relativeintensities ofspe trallines in thestellar photosphere. Instead olour indi es,

like (

m

B

− m

V

) 2

, are indi ators of temperature based on the ontinuum slope and

its equivalent bla kbody temperature. Many stars, that showed spe tral features of

early-typestars, had olour indi es moreappropriate to late-type stars. Su h results

are easilyexplainedbythereddening ausedbyforegroundinterstellardustalongthe

line of sight (Whittet, 2002): stellar spe tra in a limited range do not hange with

reddening, instead olourindi esdependontemperatureandreddening.

Usually the extin tion is reliably determined using the pair method, omparing

spe trophotometry of two stars of the same spe tral lass: if one star has negligible

foregrounddust,whilethese ondstarisreddened, omparisonoftwospe tra,withthe

assumption that extin tion goesto zeroat verylongwavelength, allowsto determine

theextin tionas afun tion ofwavelength(Draine,2003).

Thedegreeofreddeningorsele tiveextin tionisquantiedthroughthe olorex ess

E

B−V

= (m

B

− m

V

)

− (m

B

− m

V

)

0

,

(1.3)

where

m

B

− m

V

and

(m

B

− m

V

)

0

are observed and intrinsi valuesof olour index, respe tively. Sin e the extin tion is greater in the B band than in V one,

E

B−V

is a positive quantity for reddened stars and zero for unreddenedstars. An important

relationshipbetweentotalextin tionand olorex essis

R

V

=

A

V

E

B−V

.

(1.4)

Inequation(1.4)

E

B−V

isdire tlymeasurable,whereas

A

V

isevaluatedonlyif

R

V

an bedetermined. Asextin tionde reasesrapidlyaswavelengthin reases,Iobtain

R

V

=

− lim

λ→∞

E

λ−V

E

B−V

.

(1.5)

Theoreti ally,

R

V

is expe ted to depend on the omposition and size distribution of dust grains. Inthelowdensityinterstellarmedium,

R

V

isvirtually onstant

R

V

≈ 3.05

(1.6)

(Fitzpatri kandMassa,2007).

Priortothemid-1960's,interstellarextin tionwasa essibletostudymainlyinthe

visible part of the spe trum, where it was known to in rease linearly for de reasing

wavelength,andwasthereforebelievedto beessentiallydue tos atteringand

absorp-tion bysomeform of small dustparti les. With the openingof theUltraViolet (UV)

2

m

B

istheapparentluminosityinbluerangeofele tromagneti spe trum;therelativeextin tion atageneri wavelengthisexpressedas

m

λ

.

and InfraRed (IR) portions of the spe trum to astronomi al observations, Whitford

(1948)produ edanInterStellarExtin tionCurve(ISEC)thatshowed urvatureatthe

near-UV and IR regions. As pointed out remarkablyby Greenberg and Shen (1999)

"things were beginning to make somephysi al sense from the point of viewof small

parti les attering". Indeed,atthattimevandeHulst(1949)putforwardtherst

om-prehensivemodel of interstellar dust, and having arealisti dust model he developed

thes atteringtoolstohandletheproblem(vandeHulst,1957). Sin etheninterstellar

dusthasbe omeoneofthesubje tsin theforefrontofastrophysi s.

1.2 The many roles of dust in the interstellar medium

Sin eitsdis overy,theastronomersregardedthedustasadisturbfortheobservationsof

stars,anirritatingfogwhi hpre ludeda learviewofstellarspe trum. Forthisreason

therststudiesofinterstellardustweremotivatedbythedesireto orre tphotometri

data for its presen e. Nevertheless, as it was later understood, dust has very many

importantrolesto playwithintheMilkyWayGalaxy(MWG)andothergalaxies,and

itisa ru ial omponentintheevolutionofgalaxies.

An important role of the dust is the shielding of nebular regions from starlight,

favouring the build-up of a omplex hemistry. Mole ules formed in this hemistry

are important be ause they anbe used by astronomersto tra e the presen eof gas

anditsphysi al onditionthroughIRand radioemissionswi hareunae tedbydust

extin tion. DustgrainsaretheprimaryrepositoryintheInterStellarMedium(ISM)for

many hemi alelements,and atalysetheformationof

H

2

,themostabundantmole ule intheISM.Hydrogenexistspredominantlyinits

1s

groundstate,and ollisionsbetween hydrogenatoms anpro eedalongtwopotentialenergy urves,inwhi htheele troni

spins are either parallel (triplet state) or anti-parallel (singlet state). As the atoms

are initially unbound, their total energy is positive (the zero of the total energy is

takenat the mole ulardisso iationlimit). Inorder to stabilize, the systemmustlose

energy and its total energy be ome negative. In the gaseous phase this may o ur

eitherbymeansofthree-body ollisions,thethirdbodytakingawaytheex essenergy,

orbymeansofradiativepro esses. Three-body ollisionsareextremelyimprobableat

interstellardensities,and sotheonlywayin whi hthesystem anstabilizeisthrough

the emission of a photon. Unfortunately, transitions between the triplet and singlet

ele troni potential energy urves are forbidden to ele tri dipole radiation as they

involve a hange in the total spin quantum number. Radiative transitions involving

the nu leardegrees offreedom (rotation and vibration) are also forbidden, asthe H

2

mole uleishomonu learanddoesnotpossessapermanentdipolemoment. Itiswidely

a eptedthatH

2

formsongrainsurfa es: Hatomssti ktograinsandbe ometrappedat

surfa edefe tsinthegrainstru ture. Duringthere ombinationtheH

2

bindingenergy,

4.476 eV, must be partitioned between the rovibrational ex itation and translational

energyofthenas entmole uleandheatingofthedustgrain(ParneixandBre higna ,

1998). Iftheinternalandtranslationalenergiesofnewlyformedmole ulesarerelatively

small,thensigni antgrainheatingmusttakepla e,whi hmayleadtothedesorption

of volatile mole ules from the dust grain surfa e(Duley and Williams 1993, Roberts

et al. 2007). The H

2

internal energy distribution ould have asigni ant impa t on

the hemistryo urring in theISM be ausevibrationally ex ited

H

2

will in reasethe overall energy budgetof gas-phase pro esses. There havebeen many theoreti aland

(e.g., Sizun etal. 2010,Martinazzoand Tantardini2006 andreferen estherein). It is

possiblethatthisformationpumpingmaybeobservableintheIRspe traofparti ular

astronomi alregions(Islamet al.,2010).

Surfa erea tionsmayleadtotheformationofspe iesheavierthanH

2

. Atthelow

temperatures of dense mole ular louds gas-phase spe ies ondense onto dust grains

forming i y mantles. During the pro ess, when spe ies su h as e.g., OH and CO

freeze-out on dust, they be ome hydrogenated via surfa e rea tions and form H

2

O

andmethanol. Therateatwhi htheserea tionso urin reaseswiththedensityofthe

loud. I y mantlesexposed to sho ksorheated nearbyanas entstarbe omesour es

of omplexmole ules. Thus, at alllevelof density,the ex hange ofmaterial between

interstellar gas andgrains isimportant forthe hemi al omposition and evolutionof

theISM asawhole.

Dust is involved in the formation and evolution of stars and planetary systems.

Indeed stars are born within very dense loud of dust and gas and the presen e of

dust, beingane ientradiatorofex essheatenergy,favoursgravitational ollapseto

form protostars. As protostar ontra ts, rotation attens the ir umstellar envelope

in adisk-likestru ture. Proto-planetarydiskshavebeendete tedaround youngstars

throughextin tion and emissionarisingfromthe dustthey ontain. Theee t ofthe

radiationpressureongrainsmayinuen ethedynami sofstellarenvelopesurrounding

bothyoungand evolved stars. Grains ondensing in theenvelopesof late-typegiants

maydrivetheoutowsofevolvedstars,and ontributesigni antlytotheenri hment

oftheISM withheavyelements.

Sin e dust obs ures stellar light, itmustre-emit at longer wavelengths (Witt and

Gordon, 2000). Thus, dust manifests itself through ontinue emission at

λ > 1 µm

. Su h emission beyond afew

µm

and up to 100

µm

is mainly due to dis rete photon heating of verysmall dust grains;dust heated bymassivestars to temperature larger

than50Kalso ontributetotheuxinthewavelengthregionbelow

∼ 50 µm

. Beyond

40

− 70 µ

m,theemissionisduetodustgrainsinnearlysteadybalan ewiththeaverage heating by starlight. Dust shows also emission and absorption features, that, in the

near/mid-IR, in lude absorption bands su h ase.g., those of sili ate materials at 9.7

and 18

µm

, frozen

H

2

O

and

CO

2

at 3.0 and 4.3

µm

(Öberg et al., 2011), and the emissionbands at3.3,6.2,7.7, 8.6and11.3

µm

(Tielens,2013).

Inadditiontoextin tion,dustparti lesprodu epolarization,mainlylinear,ofstellar

light. To explain interstellar polarization, dust grains are represented as spheroids,

des ribingarotationaboutthemajoraxisofanellipse(prolatespheroid)ortheminor

axis(oblatespheroid);grainshapesare hara terisedbytheelongation,dened asthe

ratiobetweenmajorandminorsemiaxes. Itisgenerallyfoundthatstellarradiationis

polarizedbypartially alignedspheroidaldust grains whi h u tuateand rotateabout

the axis of greatestmoment of inertia. Thedegree of linear polarization depends on

thetypeofthespheroid,prolateoroblate,itselongationandthealignmente ien y.

When dete table, ir ular polarization an also provide informations on grain shape

andelongation.

Finally,dustindu esthereddeningofextragala ti ba kgroundlight,the umulative

radiativeoutputfromallenergysour esintheuniversesin etheepo hofre ombination.

ThesubsequentemissionatIRwavelengthsprodu esthe osmi IRba kground(Dwek,

2001).

The properties of the dust are quite well known in general terms for the MWG

Sin e thespe traldistribution depends on the omposition and geometry of grains,I

an on ludethat dust grains evolvein response to regionalphysi al onditions, su h

ase.g., hemi alenri hment,dynami s,irradiation, andenergy ontent.

1.3 Interstellar dust life y le

Fromspe tralfeaturesinextin tion,s atteringandemissionastronomers aninferthe

hemi al ompositionofdust. Basedonsu hstudies arbonandsili onare onsidered

asprin ipal omponentsforinterstellardustgrains.

Thestrongest spe tralfeature observablein theextin tion urveis entered at4.6

µm

−1

and it is named bump (see gure 1.2). It suggests the presen e of arbon in

aromati form, sin ethis feature is attributed to

π

∗

← π

absorption. Other features distin tiveofaromati arbonaretheemissionbandsat3.3,6.2,7.7,8.6,and11.3

µ

m, alled Aromati InfraredBands(AIBs). These emission features are asso iated to H

atoms atta hed to the edge of a arbon ring skeleton giving rise to the following

vi-brational modes (Allamandola et al., 1989): i) 3.3

µm

CH stret hing mode, ii) 6.2

µm

CC stret hing mode, iii) 7.7

µm

CCstret hing mode, iv) 8.6

µm

CHin plane bendingmode,v)11.3

µm

CHoutofplanebendingmodeforthe asein whi hthere arenotH atomsadja ent.

Indeed,theonlyse ureidenti ationof arbonindustgrainsisthebroadfeatureat

3.4

µ

mobservableindiuseatomi regions,andindi ativeoftheCHstret hingmode inaliphati hydro arbons.

Sili ate mineralsgenerallyhavestrongabsorptionresonan esaround10

µm

dueto SiO stret hing. The observational eviden e of an emission feature near su h

wave-lengthobservedinoutowsfrom ooloxygenri hstars(wheresili atematerialswould

ondense) but not in outows from arbonri h stars(where oxygenshould form CO

and notsili ates)supports theidenti ation of sili atesin spa e. Moreover,aweaker

featureat18

µ

misalsoobserved,suggestingthepresen eoftheOSiObendingmode insili ates.

Interstellardust is thought to form around evolvedstars (i.e. giant bran h stars,

asymptoti giant bran h stars and supernovae), and then eje ted into the

surround-ing ISM by stellar winds. Here dust grains are subje t to violent pro essing in fast

sho kwavesgeneratedbysupernova. Su hstarsare believed toberesponsibleforthe

formation of dust in high redshift galaxies too. Amounts of dust largerthan

10

8

_M

⊙

aredete tedin submillimetregalaxies andquasarsat highredshift fromanumberof

instrumentssu hasSCUBA,MAMBO,MAMBO-2andVLA.TheageoftheUniverse

at z> 6 wasless than 1 Gyr, and early star formation took pla e at redshift

∼

10, approximately500Myrafter theBig Bang(Greifand Bromm,2006). The maximum

time available to build a similar large dust mass was at most

400

− 500

Myr. Con-sideringthat dust is produ ed bystars towardsthe ends oftheir life,in high redshift

galaxiesonlymassivestarshavesu ientlyshortlifetimewithintheageoftheUniverse

atthis redshift,su hashighmassasymptoti giantbran hstarswithmasses between

3

− 8 M

⊙

havelifetimes of a few

10

7

_{− 10}

8

years and they an be potential

ontrib-utors to dust produ tionin high redshift galaxies. Somefeatures in extin tion urves

ofvarious obje tsat highredshifthavebeenattributedto dust ofsupernovaoriginas

theplateauobservedataround

170

− 300

nm,possiblyarisingfromamorphous arbon and magnetite supernova dust (Maiolino et al., 2004). However, the orresponden e

of

3

− 8 M

⊙

evolvetoAGB starsand letoutdust throughintensemassloss; ii)stars moremassivethan8

M

⊙

explodeassupernova,buttodatethe orresponden eamong thestarmass,thetypeofthesupernovaandthedustmassisnot learyet(Galletal.,

2011). Exploiting theHers hel satelliteat far-IRwavelengthsMatsuura et al.(2011)

present the rst dire t eviden e that substantial amounts of dust an be reated in

supernovae. Matsuuraet al.(2011)observations demonstratethe presen e ofdust in

supernova 1987A in a mu h largeramount than previouslyknown, and inferred that

indeedmostoftherefra torymaterialintheeje taofthesupernovahas ondensedinto

dust. Although substantial, the dust produ tion rate appears to be not su ient to

eliminatetheneedforgraingrowthintheISMin theLargeMagellani Cloud(LMC),

aswellas,for omparison, inourowngalaxy(M Kee,2011).

InISMsili atedustappearstobeamorphousduebydustpro essingofnewlyformed

rystallinesili ategrains. Thedierentiala elerationofthegasanddustinsupernova

sho kwavesprodu eshighvelo ityionimpa tsongrainsthat ansputteratomsofthe

grainsand/oramorphise rystallinematerials(Jones,2004). PriortotheInfraredSpa e

Observatory(ISO)missionallsili atesinspa ewerethoughttobeamorphousasaresult

of their rapid gas phase formation in lowdensity ir umstellar shells. Observations

withISO,atwavelengths overingthe

2.5

− 200 µm

range,revolutionisedourthinking ofdust inspa ewiththedis overyofthermalemissionfrom rystallinesili atesinthe

dustshellsaroundstarsintheirlatestages(Jones,2009). However, rystallinesili ates

wereidentiedindustextra tedfromthe omet81P/Wild2andtheirpresen einother

omets has been revealed by IR spe tralfeatures (Wooden, 2008). Other samples of

rystalline sili ates with anomaloussignatures that indi ate that theyformed around

evolvedstarsin presolarepo hhavebeenextra tedfrom primitivemeteorites. Their

isotopi ompositionsshowvalues veryfar from thesolar systemones. Su h isotopi

anomalies are hara teristi s of the nu leosyntheti pro esses o urring within some

stars at parti ular phasesin their evolution(Jones, 2009). The formationpro ess for

rystalline sili ate is still un ertain,but somespe ulation wasmade. Ábrahám et al.

(2009),observingthemid-IRspe traofthestarEXLupi(ayoungSunlikeeruptivestar

whi hshowsrepetitiveoutburst)intwoseparateepo hs,wereabletore ognizetypi al

feature foramorphoussili atein therstepo h spe trum (thestar wasinaquies ent

phase), and peaks hara teristi of rystalline sili ates superimposed on amorphous

sili atefeature inthese ond epo hspe trum. Thesefeaturesareverysimilarto those

observed in ometary spe tra. A possible interpretation onsiders rystallinizationas

produ edbythermal annealingin thestar's inner diskby outburst. Aproblem ould

be represented by the transport of these rystalline sili ates to be in orporated into

ometsasobserved,sin ethe ometformingzoneisthe old,outerregionofthestar's

disk. A possibleanswer ould begiven by theXwind model, in whi h dust grains

are ballisti ally laun hed above the disk's midplane and transported outward (Shu

etal.,1996). Thismodel ouldhavesomeproblemsinexplainingtheobservedlevelsof

transport. However,itisalsopossiblethatsome rystallinesili atesaremadeinsituin

ometary omae(Li,2009). IntheISM

∼

95%ofsili atematerialisamorphous(Liand Draine,2002),insome ir umstellardisksthe rystallinefra tion appearstobehigher

but the bulk of the sili ates remains amorphous. On the other hand arbona eous

materialsare rystallineintheformofgraphiteordiamond,oramorphousintheform

ofHydrogenatedAmorphousCarbon(HAC).

Dust evolvesin theISM through onstru tivepro esses, and, duringits life y le,

o urinavarietyofinterstellarenvironments,in ludingregionsofintensestellar

radia-tionelds(HIIregions,

γ

raybursts,...),inthehotgasbehindsupernovaremnantsand in interstellar sho k waves. Ingeneraltermadust destru tion pro ess isany

intera -tionbetweenagrainandanotherparti lethatleadstoanetbaryonlossfromthegrain

(Jones,2004),withtheout omeoftheintera tiondependingbythetotalenergyofthe

ollisionandthe ouplingbetweentheparti leandthegrain. Theimportan eofthese

pro essesare evidentfrom thelargevariationsseenin theISEC fromonesightlineto

another: thesedieren esareoriginatedbyinterstellarpro essesbe auseea hsightline

isexpe tedtoaverageoverthestardustprodu edbymanyindividualstars. Themajor

parti legrainintera tionsarethefollowing:

ˆ Photongrain intera tions; low energy photons (radio and IR wavelengths) are

weaklyabsorbedbyrefra tory dust materials. Ontheother hand higherenergy

photons anbeabsorbedands atteredleadingtotheheatofgrainsand ausing

thermalemission. Su hkindofintera tions arenotdestru tivepro esses. When

similarpro esseso urformorevolatilematerials,su h asi emantles,the

absorptionofUVphotonsbe omesanimportantdestru tionme hanism(Draine

and Salpeter1979a,Westleyet al. 1995a,b). Muñoz Caro et al.(2010), in their

experiments,measuredCOphotodesorptionfromUVirradiationat7,8and15K.

TheyfoundthephotodesorptionyieldofCOi eis onstantduringirradiationand

independentofthei ethi kness. Theyfoundalsothatthephotodesorptionyield

at15K(

∼

3.5

±

0.5

×10

−2

COmole ulesphoton(

7.3

− 10.5eV

)

−1

)isaboutone

order ofmagnitudehigherthanthepreviousestimate. Onlyhigh energyphoton

intera tions anresultin graindestru tionviadire t heating(Guhathakurtaand

Draine,1989)andviaextremegrain hargingee ts(DraineandSalpeter1979b,

Waxman and Draine 2000, Fru hter et al. 2001). The intera tion of grain with

UV photonsand thegrain heating are responsible for thelossof H atoms from

the arbona eousmaterial in the polymeri form (

sp

3

)and its transition to the

graphiti form (

sp

3

)(Jones et al., 1990). Be ause of irradiation, in arbon

ma-terialsexposed toabove andnearbandgapradiationtheabsorption oe ient

overabroadrangeoffrequen iesin reases. Theamountof in reasedepends on

the wavelength and the intensityof the indu ingradiation, and the durationof

the exposure, leading to a shift of the opti al absorption edge to lower energy

and an in rease in the band tail absorption. The physi al and hemi al

trans-formation is omplex, involving sele tive photodisso iation of hemi al bonds,

sele tivesputteringwhi hredu estherelativeH,N,andO ontent,restru turing

of hemi al bonds to form in reasingly largearomati platelets (see e.g. Jones

2012a,b). However,thenetapparentee t isa hange in olourofthepro essed

arbona eous material, with a progressive darkening, i. e. from transparent to

yellow stu and to brown stu (Greenberg, 1984). Su h a pro ess is alled

photo-darkening.

ˆ Ele trongrainintera tions;generallythis kindofintera tionsarenotimportant

destru tive pro esses. These intera tions produ e ele tron sti king leading to

negativegrain harging. A ordingtoDraineandSalpeter(1979b)positivegrain

hargingis also possiblewhen energeti ele tronsimpa ts ausethe emissionof

se ondaryele trons.

Whenthe parti le energy is

≥

MeV it is lassiedas osmi ray and the inter-a tionregardsessentiallyCoulombintera tionsbetween osmi raysand harged

dust grains. The ee t de reaseswith thein reasingrelativespeedbetweenthe

grainandthe hargedparti le. Thes atteringfrequen y

ν

0

ofthisintera tion an beapproximatedastheprodu toftheRutherford rossse tion,proportionalto

v

−4

cr

(

v

cr

is the speed of the osmi ray) with the density of s atterers and the speed of osmi ray; the s attering frequen yresults

ν

0

∝ v

−3

cr

. It is learthat these intera tions have a relevant role if there is alarge energy density in low

energy(

≤

100MeV) osmi rays(Byleveld et al.,1993). These intera tions an produ e grain heating, harging and ionization ee ts; regardingthis last ee t

osmi rays whi h pass through the dust grain may be more important than

Coulomb intera tions, due to the net ee t of all osmi rays that passwithin

aDebyelength 3

(oforder hundredsofmeters) ofthedust grain. Unfortunately,

low energy osmi rays annot be observed into the solar system be ause solar

osmi raysdominatethe osmi rayspe trumat energieslowerthan 100MeV,

so urrentlywehave s ar e information about theabundan e and energy

spe -trum ofgala ti osmi -rays. Insteadat lowerenergies intera tions anprodu e

dierentee ts. Forenergies>10eVpernu leontheseintera tions anprodu e

sputteringofatomsfromthegrain. Thispro esso ursinsho kwavesgenerated

bysupernovaeexplosions;it learlydependsbythein identatom/ionenergyand

dierent sputtering yields are obtained with dierent energies. The sputtering

yield, in addition to the kineti energyof the in ident atom/ion,depends upon

theangleofin iden ea ordingtherategivenbyDraineandSalpeter(1979b). If

thesputteringe ien yislessthanunity(it anhappenforrefra torymaterials)

atom/ionsputteringalsoprodu esan implantationinto thegrain in reasingthe

grainmass. Hen e,thereisadestru tionpro essthat isa ompaniedbyagrain

growthpro ess.

ˆ Graingrain intera tions; low energygraingrain ollisions withvelo ities lower

than0.02km/s,produ egrainsti kingand oagulationofgrainsintoaggregates,

resulting in anet in rease in the average grain size (Chokshi et al., 1993). At

velo itiesoftheorderof1km/sgrainfragmentationordestru tion ano ur: i)

forsolidhomogeneousparti lesthispro ess onsistsintheshatteringofthesolid

parti le in a multitude of smaller fragments, ii) for aggregates of parti les the

pro ess resultsin the disaggregationofthe grain into its omponent subgrains

(DominikandTielens,1997). Finallyathighvelo ities(

v

≥ 20

km/s)graingrain ollisionprodu esthepartialortotalvaporizationofthe ollidinggrains.

Supernovasho kwaveshaveafundamentalroleindustpro essinganddust

destru -tion,throughsputtering,vaporization,shatteringanddisaggregation,ifthesho kwave

velo ityatthepointofintera tionisfastenough(>50km/s). Followingthedis ussion

of M Kee (1989), the times ale for supernova sho k waves to destroy all dust in the

gala ti ISM is

t

d

=

9.7

_{× 10}

7

R ǫ(v

s7

)v

−3

s7

dv

s7

yr,

(1.7)

where

v

s7

isthesho kvelo ityinunitsof

10

7

m/sand

ǫ(v

s7

)

isthegraindestru tion e ien y for a sho k wave with velo ity

v

s7

. Jones et al. (1996), onsidering grain

3

ele -shatteringee tsongraingrain ollisions(thispro essallowstheredistributionofgrain

massthroughthefragmentationoflargegrainsintosmallgrains),foundalifetimeofthe

order

4

× 10

8

yearsforsili ategrainsand

6

× 10

8

yearsfor arbongrains. Thislifetime

maybe shorter by aboutan order of magnitude in the ase of large grains (

a

≥ 100

nm). ComparingthislifetimewiththeformationrateIobtain

t

f

≈ 3×10

9

yr,assuming

adusttogasratiois0.01,typi alsupernovamassinje tionofheavyelementsintoISM

∼ 4M

⊙

,dustformatione ien yof50%inwhi hitisin orporated25%oftheavailable Ointosili atesoroxides,andasupernovarateof1/(30yr)(JonesandTielens,1994).

Clearly dustdestru tion seemsto befaster thandust formationbyaboutan orderof

magnitude. Thus, to explainthe presen e of dust in the ISM,I needto assumethat

dust anberea retedandre ondensedintheISMassuggestedbyDwek(1998)and

WeingartnerandDraine(1999). Consideringtheaverageresiden e timeofanatomin

agrain,thetotalmassofISM andthestar formationratefor theMWG, oneexpe ts

that only 20% of Si atoms would be found in the original stardust parti le. On the

otherside, astronomersinferthat the90%of Siismissing fromgasphase. Therefore,

mostoftheSiatomsininterstellargrainsshould bein orporatedintodustintheISM:

mostofinterstellardustisnotofstellaroriginbutitisformedfrom physi alpro esses

intheISM (Draine,2003).

For galaxies like the MWG a possible interstellar formation pro ess is a retion.

Elaboratingaparti ulargalaxyevolutionmodeltakingintoa ountthemetalli ityand

agedependen eonthevariousdustsour es,Asanoetal.(2013)foundthatthea retion

time-s ale depends bythe gasmetalli ity. If metalli ityex eedsa given riti al value

thedustmassgrowthbe omesa tive,withdustgrainsrapidly in reasingtheirmasses

untilmetals aredepletedfrom theISM.This riti almetalli ity hangeswith thestar

formationtimes ale: itislargerforafaststarformationrate(seegure1.1). Thenthe

dustmassgrowthin theISM ouldbethedominantsour eofdustingalaxieslikethe

MWG.

Animportantformofdustmassa retionistheformationof arbona eousmantles

onto sili ate ores. Sili ate materials are refra tory and then more long lived than

arbona eousmaterials. Inhydrogen/ arbonri h,lowtemperatureenvironmentssolid

arbon an be deposited on the surfa es of sili ate ores with the following rate of

in reaseforthedustgrainmass

dm

d

dt

= ξπa

2

_n

C

(2.5kT

k

m

C

)

1

2

(1.8)

in whi h

m

d

isthemassofthegrain,

ξ

isthesti king oe ient,aisthegrainsize,n

C

is thenumeri aldensityfor a reting arbonparti les, m

C

isthe atomi arbonmass

and

T

k

is thekineti temperatureof the gas. Thenif the mantle hasadensity

ρ

the rateofgrowthofgrain isgivenby

da

dt

=

1

4πa

2

_ρ

dm

d

dt

= 0.4ξn

C

ρ

−1

_(kT

k

m

C

)

1

2

.

(1.9)

The equation 1.9 shows that the mantle growth is independent by the value of

the grain size;in atime interval

∆t

mantles with thesamewidth are formed overall grains. Other fa tors aninuen e themantle formation: i) mantle andesorb more

rapidlyfromsmallestgrainsifasigni antUVradiationeldispresent;ii) oagulation

may operate together with the mantlegrowth. The ondition and the me hanism of

deposition suggestthat arbonwhi h form themantle isin theHAC polymeri form

(Jonesetal.,1990). From onsiderationsbasedontheRandomCovalentNetworkmodel

andtheConstraintCountingMethod Jonesetal.(1990)obtainedthe

sp

3

_/sp

2

ratiofor

arbona eousmaterials omposed by arbonwith this bonds andhydrogen in atomi

fra tion

f

H

n

sp

3

n

sp

2

=

6f

H−1

8

_{− 13f}

H

.

(1.10)

where

n

sp

2

and

n

sp

3

arethedensitiesof

sp

2

and

sp

3

bondingsites,respe tively.

The hydrogenfra tional abundan e is thus in the range

1/6 < f

H

< 8/13

, whi h in per entage results between 17% and 62%. From equation 1.10 I an infer that

de reasingthehydrogenfra tionalabundan ein reasethequantityof arbonhybridized

sp

2

andin reasingthehydrogenabundan ein reasethequantityof arbonhybridized

sp

3

. It is known that in the hydrogen/ arbonri h and lowtemperature environment

of the ISM solid arbon will be deposited on grains ore. At low temperatures, the

intera tions of H atoms with solid arbon produ e

H

2

and the me hanism by means the arbon is deposited is the hemi al me hanism of arbon insertion, parti ularly

e ient at low temperature for both C atoms and

C

+

ions fa ilitating the grow of

hydrogenated arbon hains. ThereforewhenHAC materialis deposited onthe grain

surfa eit ishydrogenri hand, from theequation (1.10), itisessentially

sp

3

bonded.

Due to thermal annealing orexposure to UV radiation hydrogen atoms maydesorbe

from themantle,leadingto anin reaseinthe

sp

2

bonddensity. Thenetresultisthe

formation ofadouble layerof arbona eousmaterials: an outerfreshly deposited

sp

3

layer embedding an internal

sp

2

zone. Clearly, during the grain lifetime, the inverse

pro essisalsopossible,inwhi hthegraphitizedmaterialabsorbs(hot)hydrogenatoms

in itsmatrixand

sp

3

bondsin rease(Mennellaetal.,2001).

Themodel exploited in thisthesisis inspiredbythiss enarioin whi hthe arbon

is deposited in the aliphati (

sp

3

bonded) form and it is pro essed by annealing an

radiationUV inthearomati (

sp

2

1.4 The problem of osmi abundan es

An important and open question in our understanding of the ISM is the amount of

metals in orporatedinto dust grains,aquantity that annotbederiveddire tly from

observations. The hemi al omposition of thedust is found by omparing the

abun-dan es of the various elements in the gas phase alone derived from highly sensitive,

high-resolutionUVabsorptionmeasurementswithanISreferen eabundan eset(e.g.,

Milleret al. 2007). Additional information omesfrom X-rayabsorption and

s atter-ingmeasurements(Valen i andSmith,2013),analysisof omponentsofinterplanetary

dustparti les(Bradleyetal.,1999),and,indense louds,fromIRobservationsof

solid-stateabsorption bands (Öberg et al.,2011). Thederiveddust metalabundan esare,

however,veryun ertain, be ause su h inferen erelies onthe assumed ompositionof

thetotal,gas+dust,abundan esintheISM.

Hydrogen and helium onstitute about 98% of the matter in the Universe. The

elements that are mostly lo ked in dust grains are C, O, N, Mg, Si and Fe. The

determinationoftheirreferen eabundan eshasalonghistory. Be ausetheSunisthe

loseststar,its hemi alabundan es anbedeterminedwithhigha ura y. Moreover,

the hemi al omposition of the Sun wasthought to be typi al for our Galaxy, and,

indeed, solar omposition represents a good referen e for several astrophysi s studies

asGala ti hemi alevolutionofolderstarpopulationsandsolar-typestars. About30

yearsago,Greenberg(1974)publishedaremarkablepaperentitledin partwhere have

thoseatomsgone?: thiswastherst riti allookattheproblemof hemi alabundan es

intheISM.Greenbergpointedoutthat,onthebaseoftheCoperni ussatellitedata,the

abundan esofelementssu h as arbon,oxygen,andnitrogen,mu hmorewasmissing

fromthegasthat ouldbein orporatedintodustunderanyreasonablemodels. Twenty

years later Snow and Witt (1996) gave an answer to the Greenberg's question: the

una ountableatomswerenevermissingafterall. Inotherwords,solarabundan esare

notaproxiefortheISM.Basi ally,the ru ialproblemistounderstandtowhatextent

thelo al ISMis hemi allyhomogeneous,andthetypesofstarsto beusedasproxies

forISM abundan es: theSun,younghotstarsoryoung oolstars? All of these stars

havebeenexploitedbutthederivedabundan esoftendisagree,andmoreover,theyare

rather errati (Soa, 2004). From the paper put forward by Snow and Witt (1995),

osmi abundan es posed severe onstraints on dust models: the wasno longer any

needtondhiddenreservoirsofinterstellarC,N,andO.Moreover,theformersurplus

of rawmaterials for theinterstellardust wasturned into ashortage. Snow andWitt

(1995)determineda arbonabundan ein theISMof225

±

50ppM,againsta urrent value(atthat time)of335ppM for arbonabundan ein solarenvironment(Grevesse

etal.,1994). Thisproblemopenedtheso alled arbon risis,inwhi hexistingmodels

of osmi dustneededto bemodiedtaking intoa ounttheratherlimitedelemental

budgetavailable.

Now, theelemental abundan esrepresent anadditional onstraint to introdu e in

modelsreprodu inginterstellarextin tion. Theverytightlimitsimposed bytheSnow

and Witt (1995) observations were fortunately soondismissed. From newer

observa-tions,itwasdis overedthat itwasquitehardtodene astandardreferen efor

inter-stellarabundan es,withobservationstowardsstarsofdierenttypesshowingdierent

amountsofmetals in orporatedinto dust (Zubkoet al.,2004). The on entrationsof

theelements hangesigni antlyalongdierent linesof sight,with awidedispersion.

phaseistheindire tpro edure,thatis, tomeasurethegasphaseabundan eand

sub-tra tthis fromthetotal(gas+dust)abundan e. Inthesolarneighbourhood, hemi al

abundan es anbeinferredfrom absorptionlinestudiesof oldandwarmISMorfrom

emissionline spe tros opyalongtheOrionnebula. Overlargedistan es,evenin other

galaxies, region HII an be onsidered good sites for the determination of hemi al

abundan es; but the omposition ofionized gaspresentsimportant ompli ations due

to the dependen e of the derived abundan es on the indi ators used in the analysis

(Simón-Díaz andStasi«ska,2011),totheionization orre torfa torsandtolarge

sys-temati errors from temperature u tuations in the nebula (Mathis, 1995). A good

alternativeto nd areferen e for the hemi al omposition of ISM is representedby

unenvolvedearlyBtypestars. Thesestarshaveamassabout

8

− 18 M

⊙

andthey an beobservedinthesolarneighbourhoodandalsoatlargerdistan esintheMWGandin

othergalaxies. Theirlifeisveryshort,sotheyhavenotthene essarytimetotravelfar

awayfromtheirbirthsiteandspe tros opi studiesofearlyBtypestarsaresimple

be- ausetheirphotosphereisnotae tedbystellarwinds. Butdespitethissimplephysi s,

spe tralanalysesofmainsequen eBtypestarsprodu edun learresults,summarized

in a tenden y towards a metal poor omposition in omparison with olderstars like

theSunandalargerangeinelementalabundan es. A ordingto somestudiesBstars

annotbe onsideredana urateindi atorofISMabundan es,be auseelement

strati- ationduetodiusionis ommoninthesestars(HempelandHolweger,2003). Thus,

theabundan esmeasuredattheirsurfa esdonotne essarilyree ttheabundan esof

the loudsfrom whi h theyare formed. Thelatestworkson Bstarabundan esseem

to easedieren esbetweenthemand solarvalues(NievaandSimón-Díaz 2011,Nieva

and Przybilla 2012). Re ently, studying nearby B stars, Nieva and Przybilla (2012)

foundanhighdegreeof hemi alhomogeneityofthestarsin thesample,sothey ould

deneaCosmi Abundan eStandard(CAS)forthea tual hemi al ompositionofthe

osmi matterinthesolarneighbourhood. IntheTable1.1itishighlightedtheredu ed

CASmetalli ity,

Z = 0.014

obtainedwithrespe ttothe anoni alsolarone,

Z

⊙

= 0.02

(AndersandGrevesse,1989).

Hydrogen Helium Metals

Massfra tion 0.710 0.276 0.014

Table1.1: Massfra tionsforhydrogen,heliumandmetalsa ordingtoNievaandPrzybilla(2012).

In thetable (1.2)abundan es, derivedobserving along dierent type of stars, are

displayed-Thegasphase omponentofthelo alISMiswellknownbe ausedetermined

bymanyUVopti alabsorptionstudies. Su h omponentappearsindependentbythe

lineofsighttowardstheassumedreferen estaranditsuggeststhatthereisastandard

lo al ISM abundan e. Parti ularlyyoungstars should provide agood referen esin e

little hemi alevolutionaso urredsin etheirformation. ForthisreasonBstarsand

younglatertypestars(asFandGstars)arepopularproxiesfortheISM omposition.

In thetable (1.2)to inferthe abundan esavailable for elements lo kedin grains it is

su ienttomakethedieren ebetweenthetypi al hemi al ompositionofthestarand

theISMgasphaseabundan es. NievaandPrzybilla(2012)obtainedtheirabundan es

analysing 29 Bstars withsophisti ated models, inferring asili ate ri h and relatively

poor arbonnatureofthelo alISMdust. Otherstudiesseemto onsiderthat Bstars

Carbon Nitrogen Oxygen Magnesium Sili on Iron

Gas − phase

a

91

±

6 62

±

4 389

±

9 1.5

±

0.1 2.2

±

0.3 0.2

±

0.0 B stars

Total

b

214

±

20 62

±

6 575

±

66 36.3

±

4.2 31.6

±

3.6 33.1

±

2.3 Dust 123

±

23 0

±

7 186

±

67 34.8

±

4.2 29.4

±

3.6 32.9

±

2.3 F

&

Gstars

Total

c

358

±

82  445

±

156 42.7

±

17.2 39.9

±

13.1 27.9

±

7.7 Dust 267  56 41.2 37.7 27.7 Sun

Total

d

269

±

31 68

±

8 490

±

56 39.8

±

3.7 32.4

±

2.2 31.6

±

2.9 Dust 178 6 101 38.3 30.2 31.4

Table1.2: Observed,gas-phase,andinferred,dust-phase,abundan esinthediuseISMpermillions

ofhydrogennu lei.

(a)

Soaetal.(2011)forC,Meyer etal.(1997)forN,Cartledgeetal.(2004)forO,Cartledgeetal.

(2006)forMg,Si,Fe.

(b)

NievaandPrzybilla(2012).

(c)

Soaand Meyer (2001) obtainedas averages fromthe surveys byEdvardssonet al.(1993) and

Tomkinetal.(1995).

(d)

Asplundetal.(2009).

Fand G starswere onsideredgood andidate asanproxiesforthe ISM abundan es

(Soa andMeyer,2001). Abundan es for thesestars provide reasonablevaluesfor all

elements,but thenitrogen(seetab. 1.2).

Aparti ularproblemisrepresentedbytheoxygendepletionintotheISM.Are ent

study ofJenkins (2009)presentedanextensivereanalysisof ar hivaldatafor 17

ele-ments. Inthisworkisunderlinedthatoxygenisbeingdepletedfromdiuseinterstellar

gas at a rate that ex eeds by far that at whi h it an be in orporated into sili ates

and metalli oxides parti les. Studying a wide range of environments, from tenuous

inter loudgasanddiuse loudstodense loudswherei emantlesandgaseousCOare

importantreservoirsofO,Whittet(2010)triedtounderstandwhat ouldbethe

poten-tial arrier. He on ludesthat themostplausiblereservoir anbeaformofObearing

arbona eousmattersimilartotheorgani matterfoundin ometaryparti lesreturned

bytheStardust mission(vanDishoe k,2008). Furtherstudiesarene essaryto assess

if this lass of materials is present in quantities su ientto a ount for asigni ant

fra tionoftheuna ounteddepletedoxygen.

1.5 Dust in our galaxy

In spiral galaxies like the MWG, dust resides mainly in the gala ti dis , suggesting

itsasso iationwiththerelativelystellardis population. Within thedis , mostofthe

material is onnedto the spiralarms. In ourgalaxydust representsonly1% of the

ISMmass(Williams,2000).

Thedistributionofinterstellarmatterinthedis ofourgalaxyisextremelyuneven

with inhomogeneitieson allsize s alesfrom

10

−4

to

10

3

p . Thegeneraltenden y of

the extin tion to in reasewith the distan e, due to the in reaseof traversed olumn

density, is not linearwith the distan e. Usually lumps witha density of dust above

ˆ diuse louds  are themosttransparent louds, theyare omposed mainly by

atomi gas, although astrikingly mole ular inventory has been dete ted (Liszt

etal.,2008);thedensityis intherange

n

H

∼ 100 − 500 cm

−3

,whilegas

temper-atures are

30

− 100

K (Snow and M Call, 2006). Usually these louds havean extin tion

A

V

≤ 2

mag;

ˆ translu ent louds  these obje tsare denser than diuse louds; they havean

extin tionof

2

≤ A

V

≤ 5

andthey anbeinthephaseofgravitational ontra tion whi h istheprelude tostarformation;

ˆ dense louds  theyare thedensestgas ondensations(

10

3

_{− 10}

6

_cm

−3

);kineti

temperaturesrangesbetween10and50K;theyaremainly omposedbymole ular

gasand,duetotheirhighextin tionsobs ureba kgroundstars.

OurSolarSystemresidesinarelativelytransparentinter loudregionneartheedge

of theOrion Armof theMWG with little reddening,

E

B−V

≤ 0.03

, for stars within a distan e of 100p . Onaverage,in a distan e of1 kp areddening of

E

B−V

≈ 0.6

o urs. Then,exploiting theequations(1.4)and(1.6)Iobtaintheaverageratioofthe

visualextin tionto pathlength

h

A

_L

V

i ≈ 1.8 mag kpc

−1

.

(1.11)

Thisestimateis validonlyfordistan es uptoafewkiloparse sfromthe Sun,and

forlinesofsightwhi hare losetotheplaneoftheMWG.Atgreaterdistan esitisvery

hard to determinethis ratio be auseeven the most luminousstars be ome too faint;

e.g., themagnitudeof asupergiant,6.5 kp faraway,is about20 magforanaverage

reddening. Theextin tion towardtheIR lusterat thegala ti enter isestimated to

be

A

V

≈ 30

magfor apathof

∼

8kp (Ro he,1988). Forsimilar largedistan es IR photometry ouldbeusefulandinformationsaboutvisualextin tion ouldbeinferred

fromassumptionsregardingthewavelengthdependen eoftheinterstellarextin tion. In

generalobservationsthatextendbeyond3kp arebasedonlongwavelengthastronomy.

For somelines of sight extin tion is oddlylow, making possible to extend studies at

visible wavelengthsuptodistan es of

∼

10kp .

Correlationofdustwithgaswas al ulatedusingUVabsorptionlinespe tros opy

ofreddenedstarswithin1kp oftheSun. (SnowandM Call,2006)foundthefollowing

relation

N

H

E

B−V

= 5.9

_{× 10}

21

cm

−2

mag

−1

(1.12)

to be representativeofdust in diuse regions(Gudennavaret al. 2012, olle ting

ab-sorption linedatatoward3008stars,updatethis valueto

6.12

× 10

21

_cm

−2

_mag

−1

). In

this relation

N

H

is onsideredasthesumofmole ularandatomi hydrogen

N

H

= N (HI) + 2N (H

2

)

(1.13)

with thefa tor twoallows for the fa t that

H

2

ontainstwoprotons. Exploiting the equations (1.4) and (1.6) I an onvert reddening

E

B−V

in extin tion in the visual wavelength

A

V

N

H

A

V

= 1.93

_{× 10}

21

_cm

−2

_mag

−1

_.

This equation is representative of dust in diuse louds. Combining theequation

(1.14) with the equation (1.11), I aneliminate

A

V

and write the average hydrogen numberdensity

hn

H

i = h

N

H

L

i ≈ 1.04 cm

−3

(1.15)

ThisistheaveragedensityofISMinthedis ofourgalaxy,butsubstantialdeviations

(alsoofordersofmagnitude) anbefoundforindividualregions ausethemattertends

tobedistributedinto lumpswith

n

H

>>

hn

H

i

andinter lump gaswith

n

H

<<

hn

H

i

. Usingtheequation(1.15)todeterminethemassdensityandanheights aleof100p

fortheISMI analsondthesurfa emassdensity:

hσ

H

i = 5.3 M

⊙

pc

−2

.

Awayfromthegala ti dis theextin tionisgenerallylow,butitsentityisimportant

to study intrinsi properties of external galaxies. Be ause the low densityof dust in

the halo of our galaxy, the reddening of stars whi h are observed in high latitude (

|b| > 20

◦

)isindependentfromthedistan e. Consideringthedis asaat,uniformslab

withthesolar systemin the entralplane I aninferadependen e of extin tionfrom

thelatitudeb

A

V

(b) = A

P

cosec

|b|.

(1.16) Thisdependen efollowsa ose antlawand

A

P

isthevisualextin tionatthegala ti poles

4

. Clearly thisformulationmustbe onsidered justasanapproximationbe ause

the dust distributionis very inhomogeneous, and it annot be representedon small

s ales by asmoothlyvarying fun tion; indeed some louds situated at high latitudes

are denseenough to ontain amole ular phaseand to produ esigni antextin tion:

A

V

∼ 1

(Penprase, 1992). Many of these dense louds are extensions of lo al dark loud omplexes,but other loudsappeartobeisolated. Thenforlinesofsightwhi h

passthroughthese loudstheequation (1.16)will predi talowerextin tionthan the

ee tivelyobservableone.

In theMWGISECsshowagreat varietyof features and shapesfor dierentlines

ofsight. Ingure(1.2)areplotted threegala ti ISECsobservedalongthree dierent

linesofsight. Broadly hara teristi sofatypi alISEC anbesummarizedinthisway:

i) alinear trend in the visible portion of ele tromagneti spe trum; ii) the bump at

4.6

µm

−1

(toexplainthepresen eofthisprominentfeaturemanyhypothesisaboutits

naturewerebeenelaboratedovertheyears);iii)anonlinearriseintheFarUltraViolet

(FUV)rangeof theele tromagneti spe trum.

Cardelliet al.(1989)elaboratedan analyti formulawhi hallowsto derivean

ex-tin tionlawdepending only from oneparameter:

R

V

. This work startedby the on-sideration that dust grains must suered the same hemi al pro esses for the whole

dimensionaldistribution,thatis hemi alandphysi alpro essesmustnotbesele tive

andtheya tedwithoutdistin tiononthewholedustgrainspopulation.

Fitzpatri kandMassa(2007)presentedanextensiveset of328Gala ti extin tion

urvesthatshowaremarkablevarietyofshapesandfeatures. These urveshavebeen

des ribed in the UV (

λ

−1

_{≥ 3.7 µ}

m

−1

) by means of a 7parameterdes ription. The

ommon features in the Fitzpatri k and Massa (2007) sample are a Drude prole, a

nonlinearriseintheFUV,andalinearbaseline

E(λ

− V )

E(B

_{− V )}

=



c

1

+ c

2

x + c

3

D(x, x

0

, x

1

),

x

≤ c

5

,

c

1

+ c

2

x + c

3

D(x, x

0

, x

1

) + c

4

(x

− c

5

)

2

,

x > c

5

,

(1.17) 4

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 1 2 3 4 5 6 7 8 9

A

λ

A

V

λ

−1

(µm

−1

)

ALS908 CD-424120 COLLINDER46318

Figure 1.2: ISECs observed alongthreegala ti dierentlinesof sigth: ALS908, CD-424120and

COLLINDER46318. where

x

≡ λ

−1

in

µm

−1

and

D(x, x

0

, x

1

) =

x

2

(x

2

_{− x}

2

0

)

2

+ x

2

x

2

1

.

(1.18)

These authors produ ed an estimate of the typi al or average wavelength

depen-den eof interstellar extin tion. Whilethe derivation isstraightforward,theextent to

whi hthepropertiesoftheaverageISECree ttheaveragepropertiesoftheGala ti

extin tion is problemati to dene. As a onsequen e, it is di ult to derivea

stan-dard for Gala ti dust. In turn this means that, in thesame way, it is very hard to

quantifyhowpe uliarthenatureofdustisalongalineofsightwhoseextin tiondiers

signi antlyfrom theaverage ISEC.Indeed, the shapes ofthe ISECsmay simply

re-e t manifestationsofdustgrainsinwhi hparti ularphysi al/ hemi alpropertiesare

emphasised, leavingrelativelyunmodiedtheunderlyingglobalpi ture.

1.6 Dust in external galaxies

An important dis overyin extragala ti astronomywasthe identi ationof luminous

IR galaxies: galaxies whi h emit more energy in the mid and far IR than all other

wavelengths. The sour e of su h emission is heated dust. The presen e of dust in

galaxies removeshalf ormoreof thestellarenergyfrom theUVopti alrange oming

galaxies in the nearby Universe ontain dust. Morethan onethird ofthe bolometri

luminosityoflo algalaxiesistransferredbydustinIR.Thefra tionofstellarradiation

repro essedintotheIRAS8-120

µm

windowisaround25%30%;in ludingbolometri orre tionsfromtheIRASwindowtothefullIRrange,theIRenergyfra tionbe omes

35%40%ofthetotalbolometri energy within

∼ 100

Mp . This average value ould besubje ttolargevariationsbetweengalaxies,dependingonluminosity,morphologi al

typeanda tivitylevel(Calzetti,2001).

Dustlimitsastronomersabilitytoknowthelo alanddistantUniverseposingserious

problems problems in the interpretation of the spe tral energy distribution. Any

a - uratedeterminationsoffundamental parameters,su hasage,stellarpopulationmix,

starformationrates,and stellarInitialMass Fun tionare prevented. Moreoveryoung

stellarpopulationsaremoredeeplyembeddedindusty loudsthanolderstars,implying

thatapoorknowledgeofthestarformationhistoryingalaxies. Finally,measuringthe

fra tion of metals lo ked into dust at dierentepo hs is is possibleto tra e hemi al

evolutionof galaxies. The dust ontentof galaxies isproportionalto both metalli ity

andgas ontent;asstellarpopulationevolvesintimeandmoregasislo kedintostars,

the metalli ityof agalaxy in reaseswhile itsgas ontentsde reases. Galaxiesin the

lo alUniverseareonlymoderatelyopaque,andextremevaluesoftheopa ityareonly

foundinthestatisti allynondominantmorea tivesystems.

Frommultiwavelengthopti al andnearIRimagingofedgeonornearlyedgeon

systemsindi ate thatthes aleheightofthedustisabouthalfthatofthestars,whi h

meansthatthedustismore on entratedneartheplaneofthedisk,anditappearsmore

radiallyextended thanthestellar distribution;thedust massis foundto beaboutan

order of magnitudemorethan previouslymeasured using theIRAS uxes, indi ating

the existen e of a old dust omponent. The gas to dust mass ratio is lose to the

valuederivedforourGalaxy(Xilourisetal.,1999). IRAS100

µm

(Nelsonetal.,1998) andISOlongwavelengthmaps(Radovi hetal.,2001) onrmthat olddustemission

extends beyond the limits of the opti al disks along the radialdire tion, with s ales

thatare

∼

40%largerthanthoseofBbandemittingstars,butstillwellwithintheHI disks.

Irregulargalaxieshavealowermetalli itythanspiralgalaxies:in irregulargalaxies

the massin interstellarmetals perunit

L

B

5

is roughly3times smaller thanin spiral

galaxies.

Among lo al galaxies there is aspe ial groupof galaxies: starburst galaxies. The

average UV and IR luminosities perunit omoving volume in rease with redshift by

roughlyanorder ofmagnitudeuptoz

∼

1-2. Despitetheheterogeneousset that UV-sele tedstarburstsform,theirdustreddening andobs urationpropertiesareuniform.

Inthesesystems,thegeometryofdustthatbestdes ribesthereddeningoftheionized

gas emission is that of a foregroundlike distribution. In starbursts region, internal

dustdoesn'tappeartobepresentinlargeamounts,andthelittlepresentislikelytobe

in ompa t lumps(Calzetti et al., 1995)and the prin ipal sour eof opa ityappears

to be given bydust that is external, or mostlyexternal, to the starburst region, but

still internal to thehost galaxy, similar to a lumpy dust shell surrounding a entral

starburst.

Wehavesomeinformationsaboutthenatureofdustinthenearestgalaxiesbelonging

to the lo al luster. Thelo al luster in ludes

∼

70 galaxies. The twomost massive membersofthe luster,theMWGandtheAndromedaGalaxy(M31)haveasystemof

satellitegalaxies. AmongtheothersarefoundtheSmallandLargeMagellani Clouds

5

(MCs), satellites of our own galaxy. The MCs and the M31 are the only external

galaxieswheresofarithasbeenpossibletostudytheextin tion alonglinesofsightto

individual stars.

ˆ LargeMagellani Cloud(LMC):thisgalaxyislo atedatadistan eof50kp and

ithasanalmostfa eonviewingangle. Ithasmetalli ityatlevel0.5oftheMilky

Way ISM (Welty et al.,2001), anda dust to gas ratio is redu ed bya fa tor4

(Fitzpatri k,1986). Thetotalaveragestarformationrateisrelativelylowas0.26

M

⊙

yr

−1

(Kenni uttetal.,1995),whiletheoneoftheMWGis

0.68

−1.45 M

⊙

yr

−1

(Robitaille and Whitney, 2010). These physi al onditions are very similar to

thoseoftheearlyevolutionofhighredshift andmetalpoorgalaxies. Stars more

than

∼

500p awayfromthe30Doradusregion,astarburstregionofLMC,show

R

V

≈ 3.2

with anextin tion law resembling the MWG diuse loud extin tion law(Misseltetal.,1999). Inthe30Doradusregiontheextin tionlawshowsvery

dierent properties ompared to the gala ti average one. Indeed, asthegure

(1.3)shows,thefeature at4.6

µm

−1

isweakenedandin theFUVastrongerrise

thanthat ommonintheMWGisobservable(Cartledgeetal.,2005).

ˆ Small Magellani Cloud (SMC): thisgalaxy islo ated at 61kp awayfrom the

MWG andit is seenalmost fa eonfrom ourgalaxy. It has adust to gasratio

redu ed by a fa tor 10 with respe t to the MWG (Bou het et al., 1985). Its

metalli ityis

0.2

×

theMWGmetalli ity(Bernardetal.,2008). ObservedISECs forthisgalaxyareverydierentfromtheaverage urvefortheMWG(seegure

1.3), indeedGordon et al. (1997)dis overedthat UV extin tion in SMCis very

similartothatobservedinstarburst galaxieswhi hhavebeendete tedfromlow

tohigh (

z > 2.5

)redshifts. Thebump strengthisfurther diminishedthanISECs forLMCor absent,asin AzV18,andtheriseintheFUVisfurther steep.

ˆ Andromeda Galaxy: this isaspiralgalaxylo ated atadistan eof750kp from

theMWGand ithas ametalli ityverysimilarto that of ourgalaxy(Blair and

Kirshner,1985). Forthisgalaxywehavepoorinformations butobservedISECs

are more similar to the Gala ti average one than typi al interstellar urves of

SMCand LMC; animportant dieren e isthat thebump strengthis weakerin

M31thanin MWG(Bian hiet al.,1996).

A uratemeasurementsof extin tion urvesare almost ex lusively limited to the

Galaxy,theLMC,andtheSMC,be auseatgreaterdistan esitbe omesimpossibleto

obtainthephotometryorspe tros opyofindividualstarsneededforextin tion

deter-minations. Inthelast de adeobservershavedis overedhugequantities of interstellar

dust near themostdistantquasarsin theveryyounguniverse, only700million years

afterprimordialnu leosynthesis.Su hstudies(e.g.,Jiangetal.2006)indi atethathigh

redshift quasarsystemshavebeenalreadyenri hedwithdust uptoalevel omparable

to nearbydustygalaxies. Thereisavigorousdebateandtwodistin ts enariosforthe

originof dustat high redshifts,in whi h supernovaeand lower-massasymptoti giant

bran hstarsareinvolved(seeSe tion 1.3). Whateverthe asemaybe,dustexists at

thehighestredshiftsprobedsofar(

z

∼ 6

).

Abouttwode adesago,quasarswithdampedLy

α

systemsintheforegroundhave beenstudiedbyPeietal.(1991)andwerefoundtobeonaverageredderthanthose

MilkyWayaverage urve AzV18 Sk-68 155

λ

−1

_(µm

−1

₎

A

λ

A

V

10 9 8 7 6 5 4 3 2 1 0 10 8 6 4 2 0

Figure1.3: AverageISECfortheMWG(Fitzpatri kandMassa,2007) omparedtoanISECobserved

inLMCnearthe30Doradusregion(Sk-68155)(Cartledgeet al.,2005)andaninterstellarextintion

urveobservedinSMC(AzV18)(Cartledgeetal.,2005).

extin tion, and fully ompatible withSMC extin tion. Su h ndingswere questioned

byMurphyandLiske(2004)thatfoundnotra eofextin tion. Yorketal.(2006)found

no eviden e of the 217.5 nm bump, and extin tion urves similar to SMC extin tion

in asample of quasars of theSloan DigitalSky Survey. Extin tion urves that dier

from the SMC, LMC, and MWG have been obtained in the ase of A tive Gala ti

Nu lei(AGNs),although mildlyreddenedquasarsappeartofollowanSMC-like

dust-reddeninglaw(e.g.,Hopkinsetal.2004). Inthe aseofstar-forminggalaxiesextin tion

urvesaredi ulttoinferduetothe rowdingofdustwithemittingsour es. Starburst

galaxies anbe hara terisedbythela kingofthe217.5nmabsorptionfeature,likethe

SMC,andasteepfar-UV rise,intermediatebetweenthose intheSMCandtheMWG

extin tion urves. However,forUV luminousgalaxies,theextin tion urvesrange

be-tweenthosetypi aloftheSMCandLMC,withsomeofthemexhibitingweakbumpsas

inLMC(NollandPierini,2005). Ingeneral,asigni ant217.5nmbumpisobservedin

thespe traof star-forminggalaxies at

z

∼ 2

,indi atinganLMC-likeextin tion urve (Nolletal.,2007). Chenet al.(2006)analysedtheextin tion urvefor10GammaRay

Burst(GRB)hostgalaxies,ndingrelativelyatandgrayproles,togetherwithothers

1.7 Dust models

Afterrstobservationswhi hhighlightedthepresen eofinterstellardustinourgalaxy,

astronomersbegintodevelopinterstellardustmodelstakingthe uefromtheobserved

interstellar extin tion and theelemental depletions in the ISM. Current dust models

tra etheiran estrytothemodelofMathisetal.(1977)(MRNfrominitialsofauthors'

surname). Those authors assumed the existen e of two distin t types of interstellar

dust, graphitegrains andsili ate grains. All grains were assumedto bespheri aland

to havea ontinuousdistribution ofradii,

a

, from 5nmto 250nm, with thenumber ofgrainswithradiiin therange

a

→ a + da

beingproportionalto

a

−3.5

; i.e.,thereare

verymany moresmall grains than large. The al ulations were performedusing Mie

theory(1908)andthismodelwassu essfulinreprodu ingverywelltheaverageISEC

in theopti alandUV. Manyofthedistin tivefeatures oftheMathiset al. modelare

retained in moresophisti ated models. There are too many models dis ussed in the

re entliteratureforustobe omprehensive,soIshallfo usontwoexamplesofmodels

that havesomesimilaritiesandsomedieren es.

1.7.1 Draine's model

Draineandhis many ollaboratorshavemadethemost omprehensive, onsistent,and

impressive developments of the MRN pi ture, and have applied their model to the

widest range of observational tests with onsiderablesu ess. Draine and Lee(1984)

introdu ed opti al fun tions for "astronomi alsili ate",derivedfor the olivinefamily

of sili ates from laboratory data in dierent wavelength regimes, pie ed together in

a manner onsistentwith the Kramers-Kronigrelations but omitting any feature not

observedininterstellardustspe tra. Thisapproa hhasbeenwidelyadoptedbyothers.

TheworkofDraineandLee(1984)wasrevisedandalsoextendedintotheX-rayregime

for both graphiteand astronomi alsili ate byDraine (2003). Wherethe model isfor

spheri algrainsofuniformmaterialthe al ulationswere arriedoutbyMietheory.

Asigni ant hangetotheMRNpi turehasbeentheassumptionbymanymodellers

thatPAHmole ulesformanimportantadditional omponentintheISM,equivalentin

somerespe tstoverysmallgrains. Thepotentialimportan eofthePAH ontribution

to dust-related phenomenahas beenexplored by LiandDraine (2001b),Weingartner

and Draine (2001), Zubko et al. (2004), and more re ently byDraine and Li(2007).

The eviden e supporting the in lusion of aPAH omponent is thewidespread

obser-vation (espe iallybytheSpitzer IR spa emission)of theUnidentied InfraredBands

(so- alledUIBs)between3.3and12

µ

m. Manydierentmodelsnowin orporateaPAH omponent(usinglaboratoryandtheoreti alopa ities)whoseabundan eisadjustedto

reprodu e the observed strength of the UIBs. If they are present at this level, then

PAHs will also ontribute signi antly to the "bump" and to the far UV extin tion.

Weingartnerand Draine(2001)nd that atrimodal arbona eousgrainsize

distribu-tion isneeded to mat hextin tion urve,theUIBs, and IR emission -thus departing

signi antlyfrom thesimplepowerlawdistributionoftheMRNmodel.

ThemodelbyDraineand ollaboratorshasbeenappliedtoextin tionintheMWG

and othergalaxies(DraineandLi,2007),toIR emissionin ludingthedust- orrelated

mi rowaveemissionobservedbyCOBE(DraineandLazarian,1998),andtopolarization

inthefar-IRandsubmillimetreDraineandFraisse(2009),andX-rays attering(Draine

Travis 1994, Mish henko et al. 1997). The model does, however, appear to require

ex essiveamountsofMg,Si,andFe,andalsostrainstheabundan eof arbon,leaving

littleavailableforthegas-phase arbon-bearingspe iesC

+

,C,andCO.

1.7.2 The Messina/Cagliari model

A model with signi ant dieren es from that of Draine and ollaborators has been

developedbyresear hersatMessinaandCagliari. The omponentsofthismodelin lude

"astronomi alsili ates",solid arbon,andPAHs, asintheDrainemodel;however,the

arbonisassumedtobehydrogenatedamorphous arbon ontainingbothsp

2

(H-poor

graphiti )andsp

3

(H-ri hpolymeri ). A avityis allowedtobepresentin thesili ate

ores,tomimi possibleporosityandtorelieveabundan e onstraintsonsili on. Fifty

typesof PAHs are alsoin luded, and these are in neutral, ation, di ation and anion

forms,withopa ities omputedab initiobyMallo ietal.(2007). Themostsigni ant

dieren e is that the arbon (other than the PAHs) is assumed to be deposited on

thesili ate ores,ratherthanasseparate arbongrains. Thisdes riptionissuggested

by the work of Jones et al. (1990)who argued that there is a y le of arbonin the

ISM; arbonis deposited ongrains asH-ri h sp

3

(polymeri ) arbon,annealed to sp

2

(graphiti ) arbonbytheinterstellarradiationeld,andultimatelyremovedinsho ks.

A further dieren efrom other models isthat small andlarge grainsare distin tsets

ratherthan ontinuous;however,the

a

−3.5

distributionappliestobothsmallandlarge

grains. This model an t a wide variety of extreme MWG extin tion urves, and

also some unusual extin tion urves from external galaxies. The dieren es between

dierent lines of sight require hanges to arbonlayer, rather thanto the underlying

(refra tory)sili ates; thus theymay beregardedasevolutionary hanges. Themodel