Water: from clouds to planets

2.7 MB·

by  Ewine F. van Dishoeck

Water: from clouds to planets
EwineF. vanDishoeck
LeidenObservatory,LeidenUniversity,TheNetherlands;MaxPlanckInstituteforExtraterrestrialPhysics,Garching,Germany
EdwinA. Bergin
UniversityofMichigan,USA
DariuszC. Lis
4
1 CaliforniaInstituteofTechnology,USA
0
2
JonathanI. Lunine

b
CornellUniversity,USA
e
F
Results from recent space missions, in particular Spitzer and Herschel, have lead to
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significant progressinour understanding of theformationand transport of water fromclouds
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to disks, planetesimals, and planets. In this review, we provide the underpinnings for the

basic molecular physics and chemistry of water and outline these advances in the context of
]
A waterformation inspace, itstransport toaformingdisk, itsevolution inthedisk, andfinally
the delivery to forming terrestrial worlds and accretion by gas giants. Throughout, we pay
G
closeattentiontothedispositionofwaterasvapor orsolidandwhetheritmightbesubjectto
.
h processing at any stage. The context of the water in the solar system and the isotopic ratios
p (D/H) in various bodies are discussed as grounding data point for this evolution. Additional
- advances include growing knowledge of the composition of atmospheres of extra-solar gas
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giants, which may be influenced by the variable phases of water in the protoplanetary disk.
r
t Further, the architecture of extra-solar systems leaves strong hints of dynamical interactions,
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whichare important for the delivery of water and subsequent evolution of planetary systems.
a
[ Weconclude withanexplorationofwateronEarthandnotethatalloftheprocessesandkey

parametersidentifiedhereshouldalsoholdforexoplanetarysystems.
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v
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1. INTRODUCTION inspace,whatitsabundanceisinvariousphysicalenviron-
1
8 ments, andhowitistransportedfromcollapsingcloudsto
With nearly 1000 exoplanets discovered to date and
. formingplanetarysystems. At the same time, new results
1 statisticsindicatingthateverystarhostsatleastoneplanet
areemergingonthewatercontentofbodiesinourownsolar
0 (Batalhaetal., 2013), the next step in our search for life
4 system andin theatmospheresof knownexoplanets. This
elsewhere in the universe is to characterize these planets.
1 reviewattemptstosynthesizetheresultsfromthesediffer-
The presence of water on a planet is universally accepted
: entfieldsbysummarizingourcurrentunderstandingofthe
v
as essential for its potentialhabitability. Water in gaseous
i watertrailfromcloudstoplanets.
X form acts as a coolant that allows interstellar gas clouds
Speculations about the presence of water on Mars
to collapse to form stars, whereas water ice facilitates the
r and other planets in our solar system date back many
a sticking of small dust particles that eventually must grow
centuries. Water is firmly detected as gas in the atmo-
to planetesimals and planets. The developmentof life re-
spheres of all planets including Mercury and as ice on
quiresliquidwaterandeventhemostprimitivecellularlife
the surfaces of the terrestrial planets, the Moon, several
on Earth consists primarily of water. Water assists many
moons of giant planets, asteroids, comets and Kuiper
chemicalreactionsleadingtocomplexitybyactingasanef-
Belt Objects (see review by Encrenaz, 2008). Evidence
fectivesolvent. Itshapesthegeologyandclimateonrocky
for past liquid water on Mars has been strengthened
planets, and is a major or primary constituentof the solid
by recent data from the Curiosity rover (Williamsetal.,
bodiesoftheoutersolarsystem.
2013). Water hasalso been detected in spectra of the Sun
How common are planets that contain water, and how
(Wallaceetal., 1995) and those of other cool stars. In
doesthewatercontentdependontheplanet’sformationhis-
interstellar space, gaseous water was detected more than
toryandotherpropertiesofthestar-planetsystem? Thanks
40 years ago in the Orion nebula through its masing tran-
toanumberofrecentspacemissions,culminatingwiththe
sition at 22 GHz (1 cm; Cheungetal., 1969) and water
HerschelSpaceObservatory,anenormousstepforwardhas
ice was discovered a few years later through its infrared
been madein ourunderstandingof where water is formed
1
bands toward protostars (GillettandForrest, 1973). Wa- are1.17,0.94and0.92grcm−3, respectively,muchlower
ter vapor and ice have now been observed in many star- than those of rocks (3.2–4.4 gr cm−3 for magnesium-iron
and planet-forming regions throughout the galaxy (re- silicates).
views by CernicharoandCrovisier, 2005; Boogertetal., Clathrate hydrates are crystalline water-based solids in
2008; Melnick, 2009; BerginandvanDishoeck, 2012) which small non-polar molecules can be trapped inside
and even in external galaxies out to high redshifts (e.g., ‘cages’ofthehydrogen-bondedwatermolecules.Theycan
Shimonishietal.,2010;Lisetal.,2011;Weißetal.,2013). be formed when a gas of water mixed with other species
Waterisindeedubiquitousthroughouttheuniverse. condenses1 out at high pressure and has enough entropy
On their journey from clouds to cores, the water to forma stable clathrate structure(LunineandStevenson,
moleculesencounterawiderangeofconditions,withtem- 1985;Mousisetal.,2010). Clathratehydratesarefoundin
peratures ranging from <10 K in cold prestellar cores to large quantities on Earth, with methane clathrates on the
∼2000 K in shocks and the inner regions of protoplane- deepoceanfloorandinpermafrostasthebestknownexam-
tary disks. Densities vary from∼ 104 cm−3 in molecular ples. Theyhavebeenpostulatedtooccurinlargequantities
clouds to 1013 cm−3 in the midplanes of disks and 1019 onotherplanetsandicysolarsystembodies.
cm−3 in planetary atmospheres. The chemistry naturally
responds to these changing conditions. A major question 2.2. Waterspectroscopy
addressed here is to what extent the water molecules pro- Except for in-situ mass spectroscopy in planetary and
duced in interstellar clouds are preserved all the way to cometary atmospheres, all information about interstellar
exoplanetaryatmospheres,orwhetherwaterisproducedin andsolarsystemwatercomesfromspectroscopicdataob-
situ in planet-formingregions. Understandinghow, where tained with telescopes. Because of the high abundance of
andwhenwaterformsiscriticalforansweringthequestion waterintheEarth’satmosphere,thebulkofthedatacomes
whetherwater-containingplanetsarecommon. from space observatories. Like any molecule, water has
electronic,vibrationalandrotationalenergylevels. Dipole-
2. H OPHYSICSANDCHEMISTRY
2 allowedtransitionsbetweenelectronicstatesoccuratultra-
violet(UV)wavelengths,betweenvibrationalstatesatnear-
This section reviews the basic physical and chemical
to mid-infrared (IR) wavelengths, and between rotational
propertiesofwaterinitsvariousforms,asrelevantforinter-
statesfrommid-tofar-IRandsubmillimeterwavelengths.
stellar and planetary system conditions. More details, ex-
Interstellar water vapor observations target mostly the
amples and links to databases can be found in the recent
pure rotational transitions. H O is an asymmetric rotor
reviewbyvanDishoecketal.(2013). 2
with a highly irregular set of energy levels, characterized
2.1. Waterphases by quantum numbers J . Because water is a light
KAKC
molecule,thespacingofitsrotationalenergylevelsismuch
Water can exist as a gas (vapor or ‘steam’), as a solid
larger than that of heavy rotors, such as CO or CS, and
(ice), or as a liquid. At the low pressures of interstellar
the corresponding wavelengths much shorter (0.5 mm vs
space, only water vapor and ice occur, with the tempera-
3–7mmforthelowesttransitions).Thenuclearspinsofthe
tureatwhichthetransitionoccursdependingondensity.At
two hydrogenatomscan be either parallelor anti-parallel,
typicalclouddensities of 104 particles cm−3, water subli-
andthisresultsinagroupingoftheH Oenergylevelsinto
mates around 100 K (Fraseretal., 2001), but at densities 2
of 1013 cm−3, corresponding to the midplanes of proto- ortho(KA+KC =odd)andpara(KA+KC =even)lad-
ders,withastatisticalweightratioof3:1,respectively. Ra-
planetary disks, the sublimation temperature increases to
diativetransitionsbetweenthesetwoladdersareforbidden
∼160 K. According to the phase diagram of water, liq-
to high order, and only chemical reactions in which an H
uid water can exist above the triple point at 273 K and
atom of water is exchanged with an H-atom of a reactant
6.12 mbar (∼ 1017 cm−3). Such pressures and tempera-
cantransformortho-topara-H Oandviceversa.
turesaretypicallyachievedatthesurfacesofbodiesofthe 2
Infrared spectroscopy can reveal both water vapor and
sizeofMarsorlargerandatdistancesbetween0.7and1.7
ice. Water has three active vibrational modes: the fun-
AUforasolar-typestar.
damental v=1–0 bands of the ν and ν symmetric and
Watericecantakemanydifferentcrystallineandamor- 1 3
asymmetricstretches centeredat 2.7 µm and 2.65µm, re-
phousformsdependingontemperatureandpressure.Atin-
spectively, and the ν bendingmode at 6.2 µm. Overtone
terstellardensities,crystallizationofaninitiallyamorphous 2
(∆v = 2 or larger) and combination (e.g., ν +ν ) transi-
icetothecubicconfiguration,I ,occursaround90K.This 2 3
c
tionsoccurinhotgasatshorterwavelengths(seeFig.1for
phase change is irreversible: even when the ice is cooled
example). Gas-phase water thereforehas a rich vibration-
down again, the crystal structure remains and it therefore
rotationspectrumwithmanyindividuallinesdependingon
provides a record of the temperature history of the ice.
thetemperatureofthegas.Incontrast,thevibrationalbands
Below 90 K, interstellar ice is mostly in a compact high-
densityamorphous(HDA)phase,whichdoesnotnaturally
1Strictly speaking, thetermcondensation refers tothegastoliquid tran-
occur on planetary surfaces (JenniskensandBlake, 1994).
sition; weadoptheretheastronomical parlance whereitisalsousedto
ThedensitiesofwatericeintheHDA,LDAandIc phases denotethegas-to-solidtransition.
2
a dedicated chemical physics study (Danieletal., 2011).
Other collision partners such as H, He and electrons are
generally less important. In cometary atmospheres, water
itselfprovidesmostofthecollisionalexcitation.
Astronomers traditionally analyze molecular observa-
tionsthroughaBoltzmanndiagram,inwhichthelevelpop-
ulationsareplottedversustheenergyofthelevelinvolved.
Theslopeofthediagramgivestheinverseoftheexcitation
temperature. If collisional processes dominate over radia-
tiveprocesses,thepopulationsarein‘localthermodynamic
equilibrium’(LTE)andtheexcitationtemperatureisequal
tothekinetictemperatureofthegas,T =T . Generally
ex kin
Fig. 1.— The near-IR spectrum of the Earth showing the
level populations are far from LTE and molecules are ex-
many water vibrational bands together with CO . The
2 citedbycollisionsandde-excitedbyspontaneousemission,
bands below 3 µm are due to overtones and combination
leadingto T < T . The criticaldensityroughlydelin-
ex kin
bands and are often targeted in exoplanet searches. This
eatesthetransitionbetweentheseregimes:n =A /C
cr uℓ uℓ
spectrum was observed with the NIMS instrument on the
and therefore scales with µ2 ν3 , where A is the Einstein
GalileospacecraftduringitsEarthflybyinDecember1990. uℓ uℓ
spontaneousemissioncoefficient,µtheelectricdipolemo-
From Encrenaz (2008), with permission from Annual Re-
ment and ν the frequencyof the transition u → ℓ. In the
views,basedonDrossartetal.(1993).
case of water, the combination of a large dipole moment
(1.86 Debye) and high frequencies results in high critical
of water ice have no rotationalsubstructureand consist of densitiesof108–109cm−3forpurerotationaltransitions.
very broad profiles, with the much stronger ν band over- Analysisof water linesis muchmorecomplexthanfor
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whelming the weak ν band. The ice profile shapes de- simple molecules, such as CO, for a variety of reasons.
1
pend on the morphology, temperature and environmentof First,becauseofthelargedipolemomentandhighfrequen-
thewatermolecules(Hudginsetal.,1993). Crystallinewa- cies, the rotational transitions of water are usually highly
ter ice is readily distinguished by a sharp feature around opticallythick,evenforabundancesaslowas10−10. Sec-
3.1 µm that is lacking in amorphous water ice. Libration ond,thewatertransitionscoupleeffectivelywithmid-and
modes of crystalline water ice are foundat 45 and 63 µm far-infrared radiation from warm dust, which can pump
(MooreandHudson,1994). higher energy levels. Third, the fact that the ‘backbone’
Spectra of hydrous silicates (also known as phyllosil- levels with KA=0 or 1 have lower radiative decay rates
icates, layer-lattice silicates or ‘clays’) show sharp fea- than higher KA levels can lead to population ‘inversion’,
tures at 2.70–2.75 µm due to isolated OH groups and a in which the population in the upper state divided by its
broader absorption from 2.75–3.2 µm caused by interlay- statistical weightexceedsthat forthe lower state (i.e., Tex
ered (‘bound’) water molecules. At longer wavelengths, becomesnegative). Infraredpumpingcanalsoinitiate this
variouspeakscanoccurdependingonthecomposition;for inversion.Theresultisthewell-knownmaserphenomenon,
example,thehydroussilicatemontmorillonitehasbandsat whichiswidelyobservedinseveralwatertransitionsinstar-
49and100µm(Koikeetal.,1982). forming regions (e.g., Furuyaetal., 2003; Neufeldetal.,
Bound-boundelectronictransitionsofwateroccuratfar- 2013;Hollenbachetal.,2013). Thebottomlineisthatac-
UVwavelengthsaround1240A˚,buthavenotyetbeende- curate analysis of interstellar water spectra often requires
tectedinspace. additionalindependentconstraints,forexamplefromH128O
or H17O isotopologues,whoseabundancesare reducedby
2
2.3. Waterexcitation factorsofabout550and2500,respectively,andwhoselines
are moreopticallythin. Atinfraredwavelengths, linesare
Thestrengthof an emissionor absorptionline ofwater
oftenspectrallyunresolved,whichfurtherhinderstheinter-
dependsonthenumberofmoleculesinthetelescopebeam
pretation.
and,forgaseouswater,onthepopulationsoftheindividual
energylevels.Thesepopulations,inturn,aredeterminedby
2.4. Waterchemistry
thebalancebetweenthecollisionalandradiativeexcitation
andde-excitationofthelevels. Theradiativeprocessesin- 2.4.1. Elementalabundancesandequilibriumchemistry
volvebothspontaneousemissionandstimulatedabsorption Theoverallabundanceofelementaloxygenwithrespect
andemissionbyaradiationfieldproducedbyanearbystar, to total hydrogennuclei in the interstellar medium is esti-
bywarmdust,orbythemoleculesthemselves. matedtobe5.75×10−4 (Przybillaetal.,2008),ofwhich
ThemaincollisionalpartnerininterstellarcloudsisH2. 16–24% is locked up in refractory silicate material in the
Accurate state-to-state collisional rate coefficients, Cuℓ, diffuseinterstellarmedium(Whittet,2010). Theabundance
of H2O with both ortho- and para-H2 over a wide range ofvolatileoxygen(i.e.,nottiedupinsomerefractoryform)
of temperatures have recently become available thanks to ismeasuredtobe3.2×10−4indiffuseclouds(Meyeretal.,
3
Ion gen nuclei nH = n(H) + 2n(H2)). Equilibriumchemistry
Molecule High-T EA [K] isestablishedatdensitiesaboveroughly1013 cm−3,when
~10 000
three body processes become significant. Such conditions
O+ H+ O ~~32 000000 arefoundinplanetaryatmospheresandintheshieldedmid-
H2 H+ Surface planesoftheinnerfewAUofprotoplanetarydisks.
OH+ 3 H hv H2 Undermostconditionsininterstellarspace,however,the
s-O densities are too low for equilibrium chemistry to be es-
s-O
H2 OH s-O2 tablished. Also, strong UV irradiation drives the chem-
e- 3 istryoutofequilibrium,eveninhigh-densityenvironments,
HO+
2 e- H hv H2 hv hv HO,, HO2H suchastheupperatmospheresofplanetsanddisks. Under
H2 theseconditions,thefractionalabundancesaredetermined
T
HO+ HCO+ HO s-HO bythekineticsofthetwo-bodyreactionsbetweenthevari-
3 e- 2 hv 2 ousspeciesinthegas.Figure2summarizesthethreeroutes
towaterformationthathavebeenidentified. Eachofthese
Gas Phase Grain Surface
routesdominatesinaspecificenvironment.
Fig. 2.— Summary of the main gas-phase and solid-state 2.4.2. Lowtemperaturegas-phasechemistry
chemicalreactionsleadingtotheformationanddestruction
In diffuse and translucent interstellar clouds with den-
ofH2O undernon-equilibriumconditions. Threedifferent sities less than ∼ 104 cm−3 and temperatures below 100
chemical regimes can be distinguished: (i) ion-molecule
K, water is formed largely by a series of ion-moleculere-
chemistry, which dominates gas-phase chemistry at low
actions (e.g., HerbstandKlemperer, 1973). The network
T;(ii)high-temperatureneutral-neutralchemistry;and(iii) starts with the reactions O + H+ and O+ + H leading to
solid state chemistry. e stands for electron, ν for photon OH+. The H+ ion is produce3d by interactio2ns of ener-
ands−XindicatesthatspeciesXisonthegrains. Simpli- geticcosmic-ra3yparticleswiththe gas, producingH+ and
fiedversionoffigurebyvanDishoecketal.(2011). H+, with thesubsequentfastreactionofH+ + H le2ading
2 2
to H+. The cosmic ray ionization rate of atomic hydro-
3
gen denoted by ζ can be as high as 10−15 s−1 in some
1998), so this is the maximumamountof oxygenthat can H
diffuse clouds, but drops to 10−17 s−1 in denser regions
cyclebetweenwatervaporandiceindenseclouds. Count-
(IndrioloandMcCall,2012;Rimmeretal.,2012).Theion-
ing up all the forms of detected oxygen in diffuse clouds,
izationrateofH isζ ≈2ζ .
thesumislessthantheoverallelementaloxygenabudance. 2 H2 H
A series of rapidreactionsof OH+ and H O+ with H
Thus, a fraction of oxygenis postulatedto be in some yet 2 2
leadtoH O+,whichcandissociativelyrecombinetoform
unknownrefractoryform,calledUDO(‘unknowndepleted 3
H O andOH withbranchingratiosof∼0.17and0.83,re-
oxygen’),whosefractionmayincreasefrom20%indiffuse 2
spectively (Buhretal., 2010). H O is destroyed by pho-
clouds up to 50% in dense star-forming regions (Whittet, 2
todissociationandbyreactionswithC+,H+andotherions
2010). For comparison, the abundances of elemental car- 3
bonandnitrogenare3×10−4and1×10−4,respectively, suchasHCO+. PhotodissociationofH2Ostartstobeeffec-
tiveshortwardof1800A˚ andcontinuesdowntotheioniza-
withabout2/3ofthecarbonthoughttobelockedupinsolid
tionthresholdat983A˚ (12.61eV),includingLyαat1216
carbonaceousmaterial.
A˚. Its lifetime in the general interstellar radiation field, as
Foragasinthermodynamicequilibrium(TE),thefrac-
givenbyDraine(1978),isonly40yr.
tional abundanceof water is simply determined by the el-
emental composition of the gas and the stabilities of the
2.4.3. High-temperaturegas-phasechemistry
molecules and solids that can be produced from it. For
standard interstellar abundances2 with [O]/[C]> 1, there Attemperaturesabove230K,theenergybarriersforre-
aretwomoleculesinwhichoxygencanbelockedup: CO actionswith H2 canbeovercomeandthereactionO + H2
and H O. At high pressures in TE, the fraction of CO re- → OH + H becomes the dominant channel initiating wa-
2
sultsfromtheequilibriumbetweenCOandCH ,withCO terformation(ElitzurandWatson,1978). OHsubsequently
4
favored at higher temperatures. For the volatile elemen- reacts with H2 to form H2O, a reaction which is exother-
tal abundances quoted above, this results in an H O frac- mic,buthasanenergybarrierof∼2100K(Atkinsonetal.,
2
tional abundance of (2 − 3) × 10−4 with respect to to- 2004). Thisroutedrivesalltheavailablegas-phaseoxygen
tal hydrogen, if the CO fractional abundance ranges from intoH2O,unlessstrongUVorahighatomicHabundance
(0−1)×10−4. With respecttoH , thewater abundance convert some water back to OH and O. High-temperature
2
wouldthenbe(5−6)×10−4assumingthatthefractionof chemistry dominates the formation of water in shocks, in
hydrogeninatomicformisnegligible(thedensityofhydro- theinnerenvelopesaroundprotostars,andinthewarmsur-
facelayersofprotoplanetarydisks.
2Thenotation[X]indicatestheoverallabundanceofelementXinallforms,
beitatoms,moleculesorsolids.
4
2.4.4. Icechemistry phaseabundancesofH Oashighastheoriginaliceabun-
2
dances. These simulations use a binding energy of 5600
The timescale for an atom or molecule to collide with
K for amorphous ice and a slightly higher value of 5770
a grain and stick to it is t = 3 × 109/n yr for nor-
fo H2 K for crystalline ice, derivedfrom laboratoryexperiments
mal size grains and sticking probabilities close to unity
(Fraseretal., 2001). Thermal desorption of ices con-
(Hollenbachetal., 2009). Thus, for densities greater than
104cm−3,thetimescalesforfreeze-outarelessthanafew tributestothegas-phasewaterabundanceinthewarminner
protostellarenvelopes(‘hotcores’)andinsidethesnowline
×105yr,generallysmallerthanthelifetimeofdensecores
indisks.
(atleast105yr). Reactionsinvolvingdustgrainsarethere-
forean integralpartofthe chemistry. Evenweaklybound
2.4.5. Waterdeuteration
species, such as atomic H, have a long enough residence
time on the grains at temperatures of 10–20 K to react; Deuteratedwater,HDOandD2O,isformedthroughthe
H alsoparticipatesinsomesurfacereactions,butremains same processesas illustrated in Figure2. Thereare, how-
2
largelyinthegas.TielensandHagen(1982)postulatedthat ever,anumberofchemicalprocessesthatcanenhancethe
theformationofwaterfromOatomsproceedsthroughthree HDO/H2O and D2O/H2O ratios by orders of magnitude
routesinvolvinghydrogenationofs-O,s-O ands-O , re- compared with the overall [D]/[H] ratio of 2.0×10−5 in
2 3
spectively,wheres-Xindicatesaspeciesonthesurface.All the local interstellar medium (Prodanovic´etal., 2010). A
three routes have recently been verified and quantified in detaileddescriptionisgiveninthechapterbyCeccarelliet
thelaboratoryanddetailednetworkswithsimulationshave al.,hereonlyabriefsummaryisprovided.
been drawn up (see Cuppenetal., 2010; Obaetal., 2012; In terms of pure gas-phase chemistry, the direct ex-
Lambertsetal.,2013,forsummaries). changereactionH2O +HD↔HDO+H2 isoftenconsid-
Water ice formationisin competitionwithvariousdes- ered in solar system models (Richetetal., 1977). In ther-
orption processes, which limit the ice build-up. At dust mochemical equilibrium this reaction can provide at most
temperatures below the thermal sublimation limit, pho- a factor of 3 enhancement, and even that may be limited
todesorptionisaneffectivemechanismtogetspeciesback by kinetics (Le´cluseandRobert, 1994). The exchangere-
to thegasphase, althoughonlya smallfractionoftheUV actionD+OH→H+OD,whichhasabarrierof∼100K
absorptions results in desorption of intact H O molecules (SultanovandBalakrishnan,2004),isparticularlyeffective
2
(AnderssonandvanDishoeck, 2008). The efficiency is in high-temperature gas such as present in the inner disk
about 10−3 per incident photon, as determined through (Thietal., 2010b). Photodissociation of HDO enhances
laboratory experiments and theory (Westleyetal., 1995; ODcomparedwithOHbyafactorof2–3,whichcouldbe
O¨bergetal., 2009; Arasaetal., 2010). Only the top few aroutetofurtherfractionation.
monolayersof the ice contribute. The UV needed to trig- The bulk of the deuterium fractionation in cold clouds
ger photodesorption can come either from a nearby star, comesfromgas-grainprocesses.Tielens(1983)pointedout
or from the general interstellar radiation field. Deep in- thatthefractionofdeuteriumrelativetohydrogenatomsar-
side clouds, cosmic rays produce a low level of UV flux, rivingonagrainsurface,D/H,ismuchhigherthantheover-
∼ 104 photons cm−2 s−1, through interaction with H all[D]/[H]ratio,whichcanbeimplantedintomoleculesin
2
theice. ThisnaturallyleadstoenhancedformationofOD,
(PrasadandTarafdar, 1983). Photodesorptionvia X-rays
is judged to be inefficient, although there are large uncer- HDOandD2Oiceaccordingtothegrain-surfaceformation
routes.ThehighatomicD/Hratiointhegasarisesfromthe
tainties in the transfer of heat within a porous aggregate
enhancedgaseousH D+,HD+,andD+abundancesatlow
(Najitaetal.,2001). UVphotodesorptionoficeisthought 2 2 3
temperatures(≤25K),whentheortho-H abundancedrops
to dominate the production of gaseous water in cold pre- 2
and their main destroyer, CO, freezes out on the grains
stellarcores,thecoldouterenvelopesofprotostarsandthe
(Paganietal.,1992;Robertsetal.,2003). Dissociativere-
outerpartsofprotoplanetarydisks.
combinationwithelectronsthenproducesenhancedD.The
Othernon-thermalicedesorptionprocessesincludecos-
enhanced H D+ also leads to enhanced H DO+ and thus
mic ray inducedspot heating (which worksfor CO, but is 2 2
generally not efficient for strongly bound molecules like HDOincoldgas,butthisisusuallyaminorroutecompared
withgas-grainprocesses.
H O)anddesorptionduetotheenergyliberatedbythere-
2
Onthegrains,tunnelingreactionscanhavetheopposite
action (called ‘reactive’ or ‘chemical’ desorption). These
effect, reducingthe deuteriumfractionation. Forexample,
processesare less well exploredthanphotodesorption,but
arecentlaboratorystudyofs-D+s-OD→s-D Osuggests theOD+ H2 tunnelingreactionproducingHDOiceisex-
2
pected to occur slower than the OH + H reaction leading
thatasmuchas90%oftheproductcanbereleasedintothe 2
to H O ice. On the other hand, thermal exchange reac-
gasphase(Dulieuetal., 2013). Thedetailsofthismecha- 2
tions in the ice, such as H O + OD → HDO + OH have
nism, which has not yet been included in models, are not 2
beenshowntooccurrapidlyinicesathighertemperatures;
yetunderstoodandmaystronglydependonthesubstrate.
Once the dust temperature rises above ∼100 K (pre- thesecanbothenhanceanddecreasethefractionation.Both
cise value being pressure dependent), H O ice thermally thermaldesorptionathigh ice temperaturesand photodes-
2
sublimates on timescales of years, leading to initial gas- orptionatlowice temperatureshavea negligibleeffecton
5
thedeuteriumfractionation,i.e.,thegaseousHDO/H Oand
2
D O/H O ratiosreflecttheice ratiosifnoothergas-phase
2 2
processesareinvolved.
3. CLOUDS AND PRE-STELLAR CORES: ONSET
OFWATERFORMATION
Inthisandfollowingsections,ourknowledgeofthewa-
ter reservoirs during the various evolutionary stages from
clouds to planets will be discussed. The focus is on low-
mass protostars (<100 L⊙) and pre-main sequence stars
(spectral type A or later). Unless stated otherwise, frac-
tional abundances are quoted with respect to H and are
2
simply called ‘abundances’. Often the denominator, i.e.,
the(column)densityofH ,ismoreuncertainthanthenu-
2
merator.
Thebulkofthewaterinspaceisformedonthesurfaces
ofdustgrainsindensemolecularclouds. Althoughasmall
amount of water is produced in the gas in diffuse molec-
ularcloudsthroughion-moleculechemistry,itsabundance
of∼ 10−8 foundbyHerschel-HIFI(Flageyetal.,2013)is
negligible compared with that produced in the solid state.
In contrast, observations of the 3 µm water ice band to-
ward numerousinfrared sources behind molecular clouds,
from the ground and from space, show that water ice for-
mation starts at a threshold extinction of AV ≈ 3 mag Fig. 3.—Herschel-HIFIspectraoftheH2O110–101lineat
(Whittetetal.,2013).Thesecloudshavedensitiesofatleast 557 GHz in a pre-stellar core (top), protostellar envelope
1000 cm−3, but are not yet collapsing to form stars. The (middle) and two protoplanetary disks (bottom) (spectra
ice abundanceiss-H O/H ≈ 5×10−5, indicatingthata shiftedverticallyforclarity). Thereddashedlineindicates
2 2
significantfractionoftheavailableoxygenhasbeentrans- the rest velocity of the source. Note the different scales:
formedto water ice even at this early stage (Whittetetal., water vaporemission is strong toward protostars, but very
1988; Murakawaetal., 2000; Boogertetal., 2011). Such weakincoldcoresanddisks. Thefeatureat-15kms−1 in
highiceabundancesaretoolargetoresultfromfreeze-out the TW Hya spectrum is due to NH . Figure by L. Kris-
3
ofgas-phasewaterproducedbyion-moleculereactions. tensen,adaptedfromCasellietal.(2012),Kristensenetal.
The densest cold cores just prior to collapse have such (2012)andHogerheijdeetal.(2011,andinprep.).
highextinctionsthatdirectIRiceobservationsarenotpos-
sible. In contrast, the water reservoir (gas plus ice) can
ice,butwheretheyarenolongereffectiveinphotodissociat-
beinferredfromHerschel-HIFIobservationsofsuchcores.
ingthewatervapor.Inthecentralshieldedpartofthecore,
Fig.3presentsthedetectionoftheH O1 –1 557GHz
line toward L1544 (Casellietal., 20212). 1T0he01line shows cosmic ray induced UV photons keep a small, ∼ 10−9,
but measurable fraction of water in the gas (Casellietal.,
blue-shiftedemissionandred-shiftedabsorption,indicative
2012). Quantitatively,themodelsindicatethatthe bulkof
of inward motions in the core. Because of the high criti-
theavailableoxygenhasbeentransformedintowatericein
caldensityofwater,theemissionindicatesthatwatervapor
mustbepresentinthedensecentralpart.Theinfallingred- thecore,withaniceabundanceof∼ 10−4 withrespectto
H .
shiftedgasoriginatesonthenear-side. Becausethediffer- 2
entpartsofthelineprofileprobedifferentpartsofthecore,
4. PROTOSTARSANDOUTFLOWS
the line shape can be used to reconstruct the water vapor
abundance as a function of position throughout the entire 4.1. Outflows
core.
Herschel-HIFI and PACS data show strong and broad
The best-fit water abundance profile is obtained with a
water profilescharacteristicofshocksassociatedwith em-
simplegas-grainmodel,inwhichatomicOisconvertedinto
beddedprotostars,fromlowtohighmass. Infact,forlow-
watericeonthegrains,withonlyasmallfractionreturned
mass protostars this shocked water emission completely
back into the gas by photodesorption(Berginetal., 2000;
overwhelms the narrower lines from the bulk of the col-
RobertsandHerbst, 2002; Hollenbachetal., 2009). The
lapsingenvelope,eventhoughtheshockscontainlessthan
maximum gas-phase water abundance of ∼ 10−7 occurs
1%ofthemassofthesystem. Mapsofthewateremission
inaringattheedgeofthecorearoundA ≈4mag,where
V around solar-mass protostars such as L1157 reveal water
externalUVphotonscanstillpenetratetophotodesorbthe
notonlyattheprotostellarpositionbutalsoalongtheout-
6
gratedalongthelineofsightare(0.5–1)×10−4.
The water vapor abundance in protostellar envelopes
is probedthroughspectrally-resolvedHerschel-HIFIlines.
Because the gaseous water line profiles are dominated by
broad outflow emission (Fig. 3), this component needs to
besubtracted,oranopticallythinwaterisotopologueneeds
to be used to determine the quiescent water. Clues to the
water vapor abundance structure can be obtained through
narrow absorption and emission features in so-called (in-
verse)P-Cygniprofiles(seeNGC1333IRAS4AinFig.3).
The analysis of these data proceeds along the same lines
as for pre-stellar cores. The main difference is that the
dusttemperaturenowincreasesinwards, froma low value
of 10–20 K at the edge to a high value of several hun-
dred K in the center of the core (Fig. 4). In the simplest
Fig. 4.— Schematic representationof a protostellar enve- spherically symmetric case, the density follows a power-
lopeandembeddeddiskwithkeystepsinthewaterchem- lawn ∝ r−p withp=1–2. Asforpre-stellarcores,thedata
istry indicated. Water ice is formed in the parent cloud requirethepresenceofaphotodesorptionlayerattheedge
before collapse and stays mostly as ice until the ice sub-
of the core with a decreasing water abundance at smaller
limation temperature of ∼100 K close to the protostar is radii,wheregaseouswaterismaintainedbythecosmicray
reached.Hotwaterisformedinhighabundancesinshocks inducedphotodesorptionofwaterice(Coutensetal.,2012;
associated with the outflow, but this water is not incorpo-
Mottrametal., 2013). Analysis of the combined gaseous
rated into the planet-forming disk. Figure by R. Visser, water and water ice data for the same source shows that
adaptedfromHerbstandvanDishoeck(2009). theice/gasratioisatleast104(BoonmanandvanDishoeck,
2003). Thus, the bulk of the water stays in the ice in this
cold part, at a high abundance of ∼ 10−4 as indicated by
flow at ‘hot spots’ where the precessing jet interacts with
directmeasurementsofboththewatericeandgas.
the cloud (Nisinietal., 2010). Thus, water traces the cur-
rentlyshockedgasatpositions,whicharesomewhatoffset
4.3. Protostellarenvelopes: thewarminnerpart
fromthebulkofthecoolerentrainedoutflowgasseeninthe
When the infalling parcel enters the radius at which
red-andblue-shiftedlobesoflow-JCOlines(Tafallaetal.,
the dust temperature reaches ∼100 K, the gaseous wa-
2013;Leflochetal.,2010).
ter abundance jumps from a low value around 10−10
Determinations of the water abundance in shocks vary
from values as low as 10−7 to as high as 10−4 (see to values as high as 10−4 (e.g., Boonmanetal., 2003;
Herpinetal.,2012;Coutensetal.,2012).The100Kradius
vanDishoecketal.,2013,forsummary).Innon-dissociative
shocks,thetemperaturereachesvaluesofafewthousandK scalesroughlyas2.3×1014p(L/L⊙)cm(Bisschopetal.,
2007), and is small, <100 AU, for low-mass sources and
andallavailableoxygenisexpectedtobedrivenintowater
a few thousand AU for high-mass protostars. The pre-
(KaufmanandNeufeld,1996). Thelowvalueslikelypoint
cise abundance of water in the warm gas is still uncer-
to the importance of UV radiation in the shock chemistry
tain, however, and can range from 10−6–10−4 depend-
and shock structure. For the purposes of this chapter, the
ingonthesourceandanalysis(Emprechtingeretal.,2013;
mainpointisthateventhoughwaterisrapidlyproducedin
Visseretal.,2013).Ahighwaterabundancewouldindicate
shocksatpotentiallyhighabundances,theamountofwater
that all water sublimates from the grains in the ‘hot core’
containedintheshocksissmall,and,moreover,mostofit
before the material enters the disk; a low abundance the
islosttospacethroughoutflows.
opposite.
4.2. Protostellarenvelopes: thecoldouterreservoir The fate of water in protostellarenvelopeson scales of
the size of the embeddeddisk is currentlynot well under-
Asthecloudcollapsestoformaprotostarinthecenter,
stood,yetitisacrucialstepinthewatertrailfromcloudsto
the water-ice coated grains created in the natal molecular
disks.ToprobetheinnerfewhundredAU,ahighexcitation
cloud move inward, feeding the growing star and its sur-
lineofawaterisotopologlinenotdominatedbytheoutflow
rounding disk (Fig. 4). The water ice abundance can be
or high angular resolution is needed: ground-based mil-
measured directly through infrared spectroscopy of vari-
limeterinterferometryoftheH18O3 −2 (E =204K)
ouswater ice bandstowardthe protostaritself. Close to a 2 13 20 u
line at 203GHz (Perssonetal., 2012) and deep Herschel-
hundredsourceshavebeenobserved,fromverylow lumi-
HIFI spectra of excited H18O or H17O lines, such as the
nosity objects (‘proto-brown dwarfs’) to the highest mass 2 2
3 −3 (E =249K)lineat1095GHzhavebeenused.
protostars (Gibbetal., 2004; Pontoppidanetal., 2004; 12 03 u
Boogertetal., 2008; Zasowskietal., 2009; O¨bergetal., Twomainproblemsneedtobefacedintheanalysis. First,
comparison of ground-based and Herschel lines for the
2011). Inferred ice abundances with respect to H inte-
2
7
samesourceshowthatthehighfrequencyHIFIlinescanbe
opticallythickevenforH 18OandH17O,becauseoftheir
2 2
muchhigherEinsteinAcoefficients. Second,the physical
structure of the envelope and embedded disk on scales of
afewhundredAUisnotwellunderstood(Jørgensenetal.,
2005), so that abundances are difficult to determine since
the column of warm H is poorly constrained. Compact
2
flattenedduststructuresarenotnecessarilydisksinKeple-
rianrotation(Chiangetal.,2008)andonlyafractionofthis
materialmaybeathightemperatures.
JørgensenandvanDishoeck (2010b) and Perssonetal.
(2012)measurewatercolumnsanduseH columnsderived
2
from continuum interferometry data on the same scales
(∼1′′)todeterminewaterabundancesof∼10−8−10−5for Fig. 5.—SchematicviewofthehistoryofH Ogasandice
2
threelow-massprotostars,consistentwith the factthatthe throughouta youngdisk atthe endofthe accretionphase.
bulkofthegasonthese scalesiscoldandwaterisfrozen. Themainoxygenreservoirisindicatedforeachzone. The
Froma combinedanalysisof the interferometricand HIFI percentagesindicatethefractionofdiskmasscontainedin
data,usingC18O9–8and10–9datatodeterminethewarm each zone. Zone 1 contains pristine H O formed prior to
2
H column, Visseretal. (2013) infer water abundancesof starformationandneveralteredduringthetrajectoryfrom
2
2×10−5−2×10−4inthe≥100Kgas,asexpectedforthe cloudtodisk.InZone7,theicehassublimatedonceandre-
larger-scalehotcores. condensedagain.Thus,theiceinplanet-andplanetesimal-
The important implication of these results is that the forming zones of disks is a mix of pristine and processed
bulk of the water stays as ice in the inner few hundred ice. FromVisseretal.(2011).
AU and that only a few % of the dust may be at high
enough temperatures to thermally sublimate H O . This
2
dedphase,orevenbeingcreatedthroughhigh-temperature
smallfractionofgaspassingthroughhigh-temperaturecon-
chemistryinsuchashock(Watsonetal.,2007). Thisview
ditions for ice sublimation is consistent with 2D mod-
thataccretionshocksdonotplayarolealsocontrastswith
els of collapsing envelopeand disk formation, which give
the traditional view in the solar system community that
fractions of < 1 − 20% depending on initial conditions
all ices evaporate and recondense when entering the disk
(Visseretal., 2009, 2011; Ileeetal., 2011; Harsonoetal.,
(Lunineetal.,1991;OwenandBar-Nun,1993).
2013;Hincelinetal.,2013).
Figure5showsthehistoryofwatermoleculesindisksat
4.4. Enteringthedisk: theaccretionshockandhistory the end of the collapse phase at tacc = 2.5×105 yr fora
ofwaterindisks standardmodelwithaninitialcoremassof1M⊙,angular
momentum Ω = 10−14 s−1 and sound speed c = 0.26
Thefactthatonlyasmallfractionofthematerialwithin 0 s
km s−1 (Visseretal., 2011). The material ending up in
afewhundredAUradiusisat≥100K(§4.3)impliesthat
zone 1 is the only water that is completely ‘pristine’, i.e.,
most of the water is present as ice and is still moving in-
formedasiceinthecloudandneversublimated,endingup
wards(Fig.4). Atsomeradius,however,thehigh-velocity
intact in the disk. Material ending up in the other zones
infallingparcelsmustencounterthelow-velocityembedded
containswater thatsublimatedat some pointalong the in-
disk, resulting in a shock at the boundary. This shock re-
falling trajectory. In zones 2, 3 and 4, close to the out-
sultsinhigherdusttemperaturesbehindtheshockfrontthan
flow cavity, most of the oxygen is in atomic form due to
those achievedby stellar heating(NeufeldandHollenbach
photodissociation,with varyingdegreesofsubsequentref-
1994; see Visseretal. 2009 for a simple fitting formula)
ormation. Inzones5and6,mostoxygenisingaseouswa-
and can also sputter ices. At early times, accretion veloc-
ter. Materialinzone7entersthediskearlyandcomesclose
ities arehighandalliceswouldsublimateor experiencea
enoughtothestartosublimate. Thismaterialdoesnotend
shock strong enough to induce sputtering. However, this
up in the star, however, but is transported outward in the
material normally ends up in the star rather than in the
diskto conserveangularmomentum,re-freezingwhenthe
disk, so it isnotof interestforthe currentstory. The bulk
temperaturebecomeslow enough. The detailed chemistry
of the disk is thought to be made up through layered ac-
andfractionsofwaterineachofthesezonesdependonthe
cretion of parcels that fall in later in the collapse process,
adoptedphysicalmodeland on whetherverticalmixing is
and which enterthe disk at largeradii, where the shock is
included (SemenovandWiebe, 2011), but the overall pic-
muchweaker(Visseretal.,2009). Indeed,thenarrowline
tureisrobust.
widthsofH18Oofonly1kms−1 seenintheinterferomet-
2
ricdata(JørgensenandvanDishoeck,2010b)argueagainst
5. PROTOPLANETARYDISKS
earliersuggestions,basedonSpitzerdata,oflargeamounts
ofhotwatergoingthroughanaccretionshockintheembed- Once accretion stops and the envelope has dissipated,
a pre-main sequence star is left, surrounded by a disk of
8
gas and dust. These protoplanetarydisks form the crucial H2O OH OH H2O
VLT/CRIRES Herschel/PACS
link between material in clouds and that in planetary sys- et
s
tems. Thankstothenewobservationalfacilities,combined + off T Tauri T Tauri
with sophisticated disk chemistry models, the variouswa- x
u
ter reservoirs in disks are now starting to be mapped out. d fl
Throughoutthischapter,wewillcallthediskoutofwhich ze
ourownsolarsystemformedthe‘solarnebuladisk’.3 mali
or Herbig Ae
N
5.1. Hotandcoldwaterindisks: observations Herbig Ae
2.930 2.932 2.934 65 66 67
With increasing wavelengths, regions further out and
Wavelength (micron)
deeper into the disk can be probed. The surface layers of
the inner few AU of disks are probed by near- and mid- Fig.6.—Near-IR(left)andfar-IR(right)spectraofaTTau
IR observations. Spitzer-IRS detected a surprising wealth and a Herbig Ae disk, showingOH lines in both but H O
2
of highly-excited pure rotational lines of warm water at primarilyindisksaroundcoolerTTaustars. FigurebyD.
10–30µm(CarrandNajita,2008;Salyketal.,2008), and Fedele,basedonFedeleetal.(2011,2013).
theselineshavesincebeenshowntobeubiquitousindisks
aroundlow-mass T Tauristars (Pontoppidanetal., 2010a;
Salyketal.,2011),withlineprofilesconsistentwithadisk In principle, the patternof water lines with wavelength
origin(Pontoppidanetal.,2010b).Typicalwaterexcitation should allow the transition from the gaseous water-rich to
temperatures are T ≈450 K. Spectrally resolved ground- thewater-poor(thesnowline)to beprobed. Asshownby
ex
based near-IR vibration-rotation lines around 3 µm show LTE excitation disk models, the largest sensitivity to the
thatinsomesourcesthewateroriginatesinbothadiskand locationofthesnowlineisprovidedbylinesinthe40–60
aslowdiskwind(Salyketal.,2008;Mandelletal.,2012). µm region,whichis exactlythewavelengthrangewithout
Abundanceratiosare difficultto extractfromthe observa- observationalfacilitiesexceptforSOFIA (Meijerinketal.,
tions,becausethelinesarehighlysaturatedand,inthecase 2009). For one disk, that around TW Hya, the available
of Spitzer data, spectrally unresolved. Also, the IR lines shorter and longer wavelength water data have been used
only probe down to moderate height in the disk until the to puttogethera water abundanceprofile across the entire
dustbecomesopticallythick.Nevertheless,withinthemore disk(Zhangetal., 2013). Thisdiskhasa dustholewithin
thananorderofmagnitudeuncertainty,abundanceratiosof 4AU,withinwhichwaterisfoundtobedepleted.Thewater
H O/CO∼1–10havebeeninferredforemittingradiiuptoa abundance rises sharply to a high abundance at the inner
2
fewAU(Salyketal.,2011;Mandelletal.,2012).Thisindi- edgeoftheouterdiskat4AU,butthendropsagaintovery
catesthattheinnerdiskshavehighwaterabundancesofor- lowvaluesaswaterfreezesoutinthecoldouterdisk.
der∼10−4andarethusnotdry,atleastnotintheirsurface The cold gaseous water reservoir beyond 100 AU is
layers. TheIRdatashowacleardichotomyinH Odetec- uniquely probed by Herschel-HIFI data of the ground ro-
2
tionratebetweendisksaroundthelower-massTTauristars tationaltransitionsat557 and1113GHz. Weak, but clear
and higher-mass, hotter A-type stars (Pontoppidanetal., detections of both lines have been obtained in two disks,
2010a; Fedeleetal., 2011). Also, transitiondisks with in- aroundthe nearbyT Taustar TWHya(Hogerheijdeetal.,
ner dust holes show a lack of water line emission. This 2011) andthe HerbigAe star HD 100546(Hogerheijdeet
is likelydueto morerapidphotodissociationbystarswith al., in prep.) (Fig. 3). These are the deepest integrations
higherT∗,andthusstrongerUVradiation,inregionswhere obtained with the HIFI instrument, with integration times
themoleculesarenotshieldedbydust. upto25hrperline. Similarlydeepintegrationson5other
Moving to longer wavelengths, Herschel-PACS spectra disks do not show detections of water at the same level,
probegasatintermediateradiiofthe disk, outto100AU. nor do shallower observations of a dozen other disks of
Far-IRlinesfromwarmwaterhavebeendetectedina few different characteristics. One possible exception, DG Tau
disks (Rivie`re-Marichalaretal., 2012; Meeusetal., 2012; (Podioetal., 2013), is a late class I source with a well-
Fedeleetal.,2012,2013). Asfortheinnerdisk,theabun- known jet and a high X-ray flux. The TW Hya detection
dance ratios derived from these data are highly uncertain. implies abundances of gaseous water around 10−7 in the
SourcesinwhichbothH OandCOfar-infraredlineshave intermediate layer of the disk, with the bulk of the oxy-
2
been detected (only a few) indicate H O/CO columnden- gen in ice on grainsat lower layers. Quantitatively, 0.005
2
sity ratiosof10−1, suggestinga water abundanceoforder Earth oceans of gaseouswater and a few thousandoceans
10−5 atintermediatelayers,butupperlimitsinotherdisks ofwatericehavebeendetected(1Earthocean=1.4×1024
suggest values that may be significantly less. Again the gr=0.00023MEarth). While thisis plentyofwatertoseed
disksaroundTTauristarsappeartobericherinwaterthan anEarth-likeplanetwithwater,asingleJovian-typeplanet
thosearoundA-typestars(Fig.6). formedinthisice-richregioncouldlockupthebulkofthis
water.
3Alternative nomenclatures in the literature include ‘primordial disk’, Direct detections of water ice are complicated by the
‘presolardisk’,‘protosolarnebula’or‘primitivesolarnebula’. factthatIRabsorptionspectroscopyrequiresabackground
9
frameworkof thediskthermalstructure,potentiallymodi-
fied by motionsof the varioussolid or gaseousreservoirs.
Thisisbroadlyconsistentwiththeobservations.
Traditionally, the snow line plays a critical role in the
distributionofwater,representingthecondensationorsub-
limation front of water in the disk, where the gas tem-
peratures and pressures allow water to transition between
the solid and gaseous states (Fig. 7). For the solar nebula
disk, thereis a richliteratureonthe topic(Hayashi, 1981;
SasselovandLecar,2000;PodolakandZucker,2004;Lecaretal.,
2006;Davis,2007;Dodson-Robinsonetal.,2009). Within
ourmodernastrophysicalunderstanding,thisdividingline
in the midplane is altered when viewed within the frame-
Fig. 7.— Cartoon illustrating the snow line as a function workoftheentirediskphysicalstructure.Thereareanum-
ofradiusandheightinadiskandtransportoficyplanetes- ber of recent models of the water distribution that eluci-
imals across the snowline. Diffusion of water vapor from datethese keyissues (Glassgoldetal.,2009;Woitkeetal.,
inner to outer disk followed by freeze-out results in pile- 2009b;BethellandBergin,2009;WillacyandWoods,2009;
upof ice justbeyondthe snowline(the coldfingereffect). Gortietal.,2011;Najitaetal.,2011;Vasyuninetal.,2011;
Figure by M. Persson, based on Meijerinketal. (2009); Fogeletal.,2011;Walshetal.,2012;Kampetal.,2013).
CieslaandCuzzi(2006).
5.2.1. Generaldistributionofgaseouswater
Fig. 8 shows the distribution of water vapor in a typi-
lightsource, andthusa favorablenearedge-onorientation
cal kinetic chemical disk model with radius R and height
of the disk. In addition, care has to be taken that fore-
z. The disk gas temperature distribution is crucial for the
ground clouds do not contribute to the water ice absorp-
chemistry. Itiscommonlyrecognizedthatdustonthedisk
tion (Pontoppidanetal., 2005). The 3 µm water ice band
surfaceiswarmerthaninthemidplaneduetodirectstellar
hasbeendetectedinonlya fewdisks(Teradaetal., 2007;
photonheating(Calvetetal.,1992;ChiangandGoldreich,
Hondaetal., 2009). To measure the bulk of the ice, one
1997). Furthermorethegastemperatureisdecoupledfrom
needs to go to longer wavelengths, where the ice features
thedustin theupperlayersdueto directgasheating(e.g.,
can be seen in emission. Indeed, the crystalline H O fea-
2 KampandDullemond, 2004). There are roughly 3 areas
turesat45or60µmhavebeendetectedinseveralsources
where water vapor is predicted to be abundant and there-
with ISO-LWS (Malfaitetal., 1998, 1999; Chiangetal.,
forepossiblyemissive(seealsodiscussioninWoitkeetal.,
2001)andHerschel-PACS(McClureetal.,2012,Bouwman
2009a). These3areasor“regions”arelabelledwithcoor-
etal.,inprep). Quantitatively,thedataareconsistentwith
dinates(radialandvertical)thatarespecifictothephysical
25–50%oftheoxygeninwatericeongrainsintheemitting
structure(radiationfield,temperature,density,dustproper-
layer.
ties)ofthismodel.Differentmodels(withsimilardust-and
The ISO-LWS far-infrared spectra also suggested a
gas-richconditions)findthesamegeneralstructure,butnot
strong signature of hydrated silicates in at least one tar-
attheexactsamephysicallocation.
get(Malfaitetal.,1999).NewerHerschel-PACSdatashow
Region1(R=innerradiusto1.5AU;z/R<0.1): this
nosignofsuchafeatureinthesametarget(Bouwman,priv.
regioncoincideswiththecondensation/sublimationfrontin
comm.). An earlier claim of hydrated silicates at 2.7 µm
the midplane at the snow line. Inside the snow line wa-
in diffusecloudshasnowalso beenrefuted(Whittetetal.,
ter vapor will be abundant. Reaction timescales imposed
1998;Whittet,2010). Moreover,thereisnoconvincingde-
by chemical kinetics limit the overall abundance depend-
tection of anymid-infraredfeature of hydratedsilicates in
ing on the gas temperature. As seen in Fig. 8, if the gas
hundredsofSpitzerspectraofTTauri(e.g.,Olofssonetal.,
temperatureexceeds∼400Kthenthemidplanewaterwill
2009; Watsonetal., 2009), Herbig Ae (e.g., Juha´szetal.,
bequiteabundant,carryingallavailableoxygennotlocked
2010) and warm debris (e.g., Olofssonetal., 2012) disks.
in CO and refractory grains. If the gas temperatureis be-
Overall,thestrongobservationalconsensusisthatthesili-
low this value, but above the sublimation temperature of
catespriortoplanetformationare‘dry’.
∼160K,thenchemicalkineticscouldredistributetheoxy-
gentowardsotherspecies. Duringtheearlygas-anddust-
5.2. Chemicalmodelsofdisks
rich stages up to a few Myr, this water vapor dominated
Theobservationsofgaseouswaterdiscussedin§5.1in- regionwill persist and is seen in nearly all models. How-
dicate the presence of both rotationally hot Tex ≈ 450 K ever,assolidsgrow,thepenetratingpowerofUVradiation
and cold (Tex < 50 K) water vapor, with abundances of is increased. Since water vapor is sensitive to photodisso-
∼10−4andmuchlowervalues,respectively. Basedonthe ciation by far-UV, this could lead to gradualdecay of this
chemistry of water vapor discussed in § 2.4, we expect it layer,whichwouldbeconsistentwiththenon-detectionof
tohavearelativelywellunderstooddistributionwithinthe
10