2- The chemistry in the. The formation of water : gas phase and grain surface formation. The present models. Observations of molecules in the ISM.

Transcription

1 2- The chemistry in the ISM. The formation of water : gas phase and grain surface formation. The present models. Observations of molecules in the ISM. 1

2 Why studying the ISM chemistry? 1- The thermal balance, and therefore the physical structure of the ISM depends on the chemical composition of the gas, because the cooling of the gas is dominated by the line emission, which depends on the particular molecule emitting the line, and therefore on its abundance: in other words, the physical structure of the ISM depends on the chemical composition of the gas. 2- Since the chemical structure itself is function of the evolution and physical structure of the gas, it is a powerful probe of the latters. 3- Last but not least, chemistry is particularly important in star forming regions, because the chemical complexity in those regions can have consequences on the terrestrial life, either directly, or, much more likely, indirectly. 2

3 THE ABUNDANCE OF THE ELEMENTS Element Abundance/H Molecules are formed in the interiors of the ISM He clouds, where FUV photons Oxygen 6.7x10-4 -which photodissociate the Carbon 3.7x10-4 molecules- do not Nitrogen 1.1x10-4 penetrate (because absorbed by the dust). S, Mg, Fe, Si ~3x10-5 Because of the element Na, Al, Ca ~3x10-6 abundance, the most abundant species are: H 2, In practice, in MC, all gaseous CO, carbon O, is O 2 in and CO, Hwhereas 2 O the oxygen is shared between O, H 2 O and O 2. 3

4 The molecules observed in the ISM The most recent census (2003) counts 123 molecules, containing from 2 to 13 atoms, withouth taking into account the forms in the least abundant isoptopes (D, 13 C, 18 O, 17 O, 15 N, etc.). 2 atoms: AlF AlCl C 2 CH CH + CN CO CO + CP CS CSi HCl H 2 KCl NH NO NS NaCl OH PN SO S0 + SiN SiO SiS HF SH FeO 3 atoms: C 3 C 2 H C 20 C 2 S CH 2 HCN HCO HCO + HCS + HOC + H 2 0 H 2 S HNC HNO MgCN MgNC N 2 H + N 20 NaCN OCS S0 2 c-sic 2 CO 2 NH 2 H atoms: c-c 3 H l-c 3 H C 3 N C 30 C 3 S C 2 H 2 CH 2 D +? HCCN HCNH + HNCO HNCS HOCO + H 2 CO H 2 CN H 2 CS H NH 3 SiC 3 5 atoms: C 5 C 4 H C 4 Si l-c 3 H 2 c-c 3 H 2 CH 2 CN CH 4 HC 3 N HC 2 NC HCOOH H 2 CHN H 2 C 20 H 2 NCN HNC 3 SiH 4 H 2 COH + 6 atoms: C 5 H C 50 C 2 H 4 CH 3 CN CH 3 NC CH 30 H CH 3 SH HC 3 NH + HC 2 CHO HCONH 2 l- H 2 C 4 C5N 7 atoms: C 6 H CH 2 CHCN CH 3 C 2 H HC 5 N HCOCH 3 NH 2 CH 3 c- 2 H 4 O CH 2 CHOH 8 atoms: CH 3 C 3 N HCOOCH 3 CH 3 COOH C 7 H CH 2 OHCHO 9 atoms: CH 3 C 4 H CH 3 CH 2 CN (CH 3 ) 20 CH 3 CH 20 H HC 7 N C 8 H +10 atoms: CH 3 C5N (CH 3 ) 2 CO NH 2 CH 2 COOH? HC 11 N 4

5 How molecules form in the ISM There is a variety of processes that lead to the formation of molecules 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 molecules and/or in the same regions. Indeed, there is a strong interplay of these two mechanisms in the process of forming simple and complex molecules. Molecules formed in the gas phase can be frozen onto the grain surfaces, where they can undergo further chemical reactions, and eventually they can be realeased back into the gas phase in different, usually more complex molecules. Once in the gas phase in their «new» form they can start new chemical reactions and undergo further transformations or promote new reactions. An outstanding example is H 2, formed on the grain surfaces, and a key molecule for the formation of all other molecules in the gas phase. 5

6 GAS PHASE REACTIONS Gas 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 to another one. Generic examples are summarized in the right Table. Photo-dissociation Neutral-Neutral Ion-Molecule Charge transfer Radiative association Dissociative recombination Associative detachment Reaction AB +hν A + B A + + B A + + B A + B + A + B A + + e A - + B A + B C + D C + + D AB + hν C + D AB + e Rate (cm 3 s -1 ) 10-9 (s -1 ) 4x x

7 GRAIN SURFACE CHEMISTRY Interstellar grains provide a surface on which accreted species can meet and react. Grain surface chemistry is therefore governed by the accretion rate which sets the overall timescale for the process- and the surface migration rate which governs the reaction network. In practice, all molecules stay where they freeze out onto, whereas the H and O atoms scan the grain surface and can therefore hydrogenate and oxydize the encountered species 7

8 GRAIN SURFACE CHEMISTRY : mantle formation The accretion rate of a species on grains is given by k ac = S π a grain2 n grain v x s -1 S = sticking coefficient, which depends on the accreting species; except for the H atom, S is expected to be close to unity at low temperatures. For H, S can be as low as 0.3 on a clean H 2 O ice, but likely on interstellar ice surface is also close to unity. v x = thermal velocity of the gas =(2kT/m x ) 1/2 a grain = average grain radius n grain = dust grain density. Thus, the characteristic timescale to deplete a species x on the grain surfaces is: τ ac 4x10 5 (S/1) -1 (n/10 4 cm -3 ) -1 (T/10K) -1/2 (a grain /0.1µm) -2 (m x /m CO ) 1/2 yr (Eq. 2.1) In a molecular cloud with density about 10 4 cm -3, the CO depletion timescale is of order of 4x10 5 yr. In denser regions the timescale becomes even shorter. 8

9 WATER FORMATION IN THE ISM There are three main routes of water formation in the ISM, depending on the physical conditions of the region: 1- IN THE COLD GAS 2- IN THE HOT (>220K) GAS 3- ON THE GRAIN SURFACES 9

10 H 2 O FORMATION IN COLD GAS Water is formed in the interiors of molecular clouds, where FUV photons cannot penetrate. The main formation route is the H 3 O + dissociative recombination with electrons (H 3 O + is formed by ion-neutral reactions). O + H 3 + OH + + H 2 ; OH + + H 2 H 2 O + + H ; H 2 O + + H 2 H 3 O + + H ; H 3 O + + e H 2 O + H 10

11 H 2 O FORMATION IN COLD GAS Water is formed in the interiors of molecular clouds, where FUV photons cannot penetrate. Molecular clouds form following the contraction of diffuse clouds (permeated by FUV photons). The figure above shows typical theoretical predictions: after 10 4 yr water abundance is expected to be around 3x10-7 /H 2. 11

12 H 2 O FORMATION IN HOT GAS In warm gas (T>220K), a chain of endothermic reactions (O+H 2 ->OH+H 2 ->H 2 O) convert all the gaseous oxygen - not locked into CO molecules- into water : x(h 2 O) ~ This occurs around massive protostars, or even in low mass protostars, close to the central star (hot cores). Or at the interface between the outflows and the surroundings (shocks). 12

13 H 2 O FORMATION ON THE GRAINS Water is thought to form by hydrogenation of Oxygen atoms that stick on to the grains. Before being evaporated they meet Hydrogen atoms and form H 2 O. The left Figure shows the Tielens & Hagen (1982) theoretical predictions of the water-ice molecular fraction as function of the gas density. At relatively low (<10 4 cm -3 ) densities all Oxygen not in CO is eventually converted into iced-water, whereas at large densities Oxygen atoms combine into O 2. 13

14 Summary The most abundant elements after Hydrogen and Helium are Oxygen and Carbon. The most abundant species in molecular Clouds are expected to be H 2, CO, O, O 2 and H 2 O. Molecules can be formed either in the gas phase or on the grain surfaces. Water in the gas phase is formed by ion-neutral reactions in cold (<250K) gas and by endothermic reactions in warm gas. Water ices can be formed on the grain surfaces. 14