2- the chemistry in the ism. - institute for astronomy · · 2005-01-12c.ceccarelli : water in...

C.Ceccarelli : Water in circumstellar disks and interstellar mediumAstrobiology Winter School , Hawaii 2005

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2- The chemistry in theISM.

The formation of water :gas phase and grain surface formation.

The present models. Observations of molecules in the ISM.


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Why studying the ISM chemistry?2- The chemistry in the ISM.

1- The thermal balance, and therefore the physical structure of theISM depends on the chemical composition of the gas, because thecooling of the gas is dominated by the line emission, which depends onthe particular molecule emitting the line, and therefore on itsabundance: in other words, the physical structure of the ISM dependson the chemical composition of the gas.

2- Since the chemical structure itself is function of the evolution andphysical structure of the gas, it is a powerful probe of the latters.

3- Last but not least, chemistry is particularly important in starforming regions, because the chemical complexity in those regions canhave consequences on the terrestrial life, either directly, or, muchmore likely, indirectly.


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THE ABUNDANCE OF THE ELEMENTS2- The chemistry in the ISM.

~3x10-5S, Mg, Fe, Si

~3x10-6Na, Al, Ca

1.1x10-4Nitrogen

3.7x10-4Carbon

6.7x10-4Oxygen

0.069He

Abundance/HElement Molecules are formed inthe interiors of the ISMclouds, where FUV photons-which photodissociate themolecules- do notpenetrate (becauseabsorbed by the dust).Because of the elementabundance, the mostabundant species are: H2,CO, O, O2 and H2OIn practice, in MC, all gaseous carbon is in CO, whereas

the oxygen is shared between O, H2O and O2.


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The molecules observed in the ISM

2- The chemistry in the ISM.

2 atoms: AlF AlCl C2 CH CH+ CN CO CO+ CP CS CSi HCl H2 KCl NH NO NS NaClOH PN SO S0+ SiN SiO SiS HF SH FeO 3 atoms: C3 C2H C20 C2S CH2 HCN HCO HCO+ HCS+ HOC+ H20 H2S HNC HNOMgCN MgNC N2H+ N20 NaCN OCS S02 c-SiC2 CO2 NH2 H3

+ 4 atoms: c-C3H l-C3H C3N C30 C3S C2H2 CH2D+? HCCN HCNH+ HNCO HNCSHOCO+ H2CO H2CN H2CS H30+ NH3 SiC35 atoms: C5 C4H C4Si l-C3H2 c-C3H2 CH2CN CH4 HC3N HC2NC HCOOH H2CHNH2C20 H2NCN HNC3 SiH4 H2COH+ 6 atoms: C5H C50 C2H4 CH3CN CH3NC CH30H CH3SH HC3NH+ HC2CHO HCONH2 l-H2C4 C5N 7 atoms: C6H CH2CHCN CH3C2H HC5N HCOCH3 NH2CH3 c-2H4O CH2CHOH8 atoms: CH3C3N HCOOCH3 CH3COOH C7H CH2OHCHO 9 atoms: CH3C4H CH3CH2CN (CH3)20 CH3CH20H HC7N C8H +10 atoms: CH3C5N (CH3)2CO NH2CH2COOH? HC11N

The most recent census (2003) counts 123 molecules, containing from2 to 13 atoms, withouth taking into account the forms in the leastabundant isoptopes (D, 13C, 18O, 17O, 15N, etc ….).


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How molecules form in the ISM2- The chemistry in the ISM.

There is a variety of processes that lead to the formation ofmolecules in the ISM. These can be separated into two broad classes:1- gas phase reactions, which occur in the gas phase,2- grain surface reactions, which occur on the surfaces of the grains.Both processes are important, even though not for the same moleculesand/or in the same regions. Indeed, there is a strong interplay ofthese two mechanisms in the process of forming simple and complexmolecules. Molecules formed in the gas phase can be frozen onto thegrain surfaces, where they can undergo further chemical reactions,and eventually they can be realeased back into the gas phase indifferent, usually more complex molecules. Once in the gas phase intheir « new » form they can start new chemical reactions and undergofurther transformations or promote new reactions. An outstandingexample is H2 , formed on the grain surfaces, and a key molecule forthe formation of all other molecules in the gas phase.


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GAS PHASE REACTIONSGas phase reactions can be divided into three main different categories:

1- bond formation reactions, which link atoms into simple or complex molecules;2- bond destruction reactions, which breakdown molecules in smaller molecules;3- bond re-arrangement reactions, which transfer parts of one co-reactant toanother one.

10-7A+ + e C + DDissociative recombinationA + B AB + hνRadiative association

10-9A+ + B A + B+Charge transfer2x10-9A+ + B C + + DIon-Molecule4x10-11A + B C + DNeutral-Neutral10-9 (s-1 )AB +hν A + BPhoto-dissociation

10-9A- + B AB + eAssociative detachment

Rate(cm3 s-1 )

Reaction


Genericexamplesaresummarizedin the rightTable.


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GRAIN SURFACE CHEMISTRYInterstellar grains provide asurface on which accretedspecies can meet and react.Grain surface chemistry istherefore governed by theaccretion rate –which setsthe overall timescale for theprocess- and the surfacemigration rate –which governsthe reaction network.


In practice, all molecules stay where they freeze outonto, whereas the H and O atoms scan the grain surfaceand can therefore hydrogenate and oxydize theencountered species


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The accretion rate of a species on grains is given by

kac = S π agrain2ngrain vx s-1

S = sticking coefficient, which depends on the accreting species; except for the Hatom, S is expected to be close to unity at low temperatures. For H, S can be as

low as 0.3 on a clean H2O ice, but likely on interstellar ice surface is also close tounity.

vx = thermal velocity of the gas =(2kT/mx)1/2

agrain= average grain radius

ngrain= dust grain density.

Thus, the characteristic timescale to deplete a species x on the grain surfaces is:

τac ≈ 4x105 (S/1)-1 (n/104cm-3)-1 (T/10K)-1/2 (agrain/0.1µm)-2 (mx/mCO)1/2 yr (Eq. 2.1)

In a molecular cloud with density about 104cm-3, the CO depletion timescale is oforder of 4x105 yr. In denser regions the timescale becomes even shorter.


GRAIN SURFACE CHEMISTRY : mantle formation


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WATER FORMATION IN THE ISM2- The chemistry in the ISM.

There are three main routes of water formationin the ISM, depending on the physical conditionsof the region:

1- IN THE COLD GAS

2- IN THE HOT (>220K) GAS

3- ON THE GRAIN SURFACES


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H2O FORMATION IN COLD GAS


Water is formed in the interiors of molecular clouds,where FUV photons cannot penetrate. The mainformation route is the H3O+ dissociative recombinationwith electrons (H3O+ is formed by ion-neutral reactions).O + H3

+ OH+ + H2 ; OH+ + H2 H2O+ + H ;

H2O+ + H2 H3O+ + H ; H3O+ + e H2O + H


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H2O FORMATION IN COLD GAS2- The chemistry in the ISM.

Water is formed in theinteriors of molecular clouds,where FUV photons cannotpenetrate. Molecular cloudsform following the contractionof diffuse clouds (permeatedby FUV photons).

The figure above shows typical theoretical predictions:after 104yr water abundance is expected to be around3x10-7/H2.


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H2O FORMATION IN HOT GAS2- The chemistry in the ISM.

In warm gas (T>220K), a chain of endothermic reactions(O+H2->OH+H2->H2O) convert all the gaseous oxygen -not locked into CO molecules- into water : x(H2O) ~ 10-4.

Or at the interfacebetween the outflows andthe surroundings (shocks).

This occurs around massiveprotostars, or even in lowmass protostars, close to thecentral star (hot cores).


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H2O FORMATION ON THE GRAINS2- The chemistry in the ISM.

Water is thought to form by hydrogenation of Oxygenatoms that stick on to the grains. Before beingevaporated they meet Hydrogen atoms and form H2O.The left Figure shows the Tielens &Hagen (1982) theoretical predictionsof the water-ice molecular fractionas function of the gas density. Atrelatively low (<104cm-3) densities allOxygen not in CO is eventuallyconverted into iced-water, whereasat large densities Oxygen atomscombine into O2.


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Summary2- The chemistry in the ISM.

• The most abundant elements after Hydrogen and Heliumare Oxygen and Carbon. The most abundant species inmolecular Clouds are expected to be H2, CO, O, O2 andH2O.

• Molecules can be formed either in the gas phase

• or on the grain surfaces.

•Water in the gas phase is formed by ion-neutralreactions in cold (<250K) gas and by endothermicreactions in warm gas.

• Water ices can be formed on the grain surfaces.

2- the chemistry in the ism. - institute for astronomy · · 2005-01-12c.ceccarelli : water in...

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