ISM & Astrochemistry Lecture 4

Nitrogen Chemistry (dark clouds) H N  NH + + H 2 Endothermic by ~ 100K N + + H 2  NH + + HEndothermic So, at low temperatures N-H bonds are very difficult to make Use the reactive OH radical to make N 2 as an intermediate: N + OH  NO + H NO + N  N 2 + O

Nitrogen Chemistry (dark clouds) In dark clouds, N 2 is destroyed by He + : He + + N 2  N + + N + HeExothermic, fast The excess of this reaction, since it produces atomic particles, results in species with excess kinetic energy, much greater than the thermal energy at 10K Kinetically excited N + can now react with H 2 to form NH +. Subsequent reactions with H 2 form NH 4 + which undergoes DR to produce NH 3. So, NH 3 needs N 2 to be formed first – it is a late time molecule – and its formation is inefficient. H N 2  N 2 H + + H 2 N 2 H + observed in IS clouds

Sulphur Chemistry S + + H 2  SH + + HEndothermic by ~ 0.4eV H S  SH + + H 2 Exothermic SH + + H 2  H 2 S + + HEndothermic H 2 S + + H 2  H 3 S + + H Endothermic So, S-H bonds are very difficult to make in interstellar clouds unless the appreciable exothermicities can be overcome. Again can use OH to make oxides: S + OH  SO + H And CH to make carbides: S + CH  CS + H

Deuterium and the Big Bang For temperatures larger than 10 9 K, a neutron and a proton can fuse to produce a deuteron p + n  2 H(D) The amount of Deuterium formed is sensitive to the baryon density - if large, then D fuses with another D to produce Helium and D is destroyed quickly - if small, then D does not get destroyed and He doe not form efficiently D/H ratio is a sensitive measure of the amount of mass in the Universe

Nucleosynthesis in the Big Bang

D/H Abundance If the Universe has enough mass the expansion we see can be reversed by gravity. At ‘critical mass’ the Universe will expand to a maximum size and stop. D/H is a probe of this density.

D/H Abundance Measurements Earth’s Oceans – Neptune, Uranus, Titan – Comet Halley – Jupiter, Saturn – Nearby Interstellar Medium –

ND 3 in Interstellar Clouds Submillimetre detection of ND 3 by Lis et al., Astrophysical Journal, 571, L55 (2002) ND 3 /NH 3 = , compared with (D/H) 3 ~

Deuterium in Interstellar Clouds HDH2D+H2D+ D2H+D2H+ DCNDNC N2D+N2D+ CCDC 3 HDDC 3 NDC 5 N HDOHDCSHDCOD 2 CODCO + CH 3 ODCH 2 DOHCHD 2 OHCD 3 OHCH 2 DCN NH 2 DNHD 2 ND 3 C4DC4DD 2 CS CH 2 DCCHCH 3 CCDHDSD2SD2S

Consider, D/H exchange reaction: A-H + + B-D A-D + + B-H +  E 0 kfkf krkr trace reservoir K(T) = k f /k r >> 1 when kT <<  E 0 k f = k L ; k r << k f HD, D Thermodynamic Effect (Gerlich, Roueff)

Criteria: neutral abundant, ion reasonably abundant; forward rate coefficient large (i) H HD H 2 D + + H K (ii) CH HD CH 2 D + + H K (iii) C 2 H HD C 2 HD + + H K (iv) H D H 2 D + + H K (v) OH + D OD + H K Important Fractionation Reactions

Consider reaction (i): (i) H HD H 2 D + + H K Enhancement Factors At steady state: k 1f n(H 3 + )n(HD) = k 1r n(H 2 D+)n(H 2 ) = k 1f exp(-220/T)n(H 2 D + )n(H 2 ) n(H 3 D + )/n(H 3 + ) = [n(HD)/n(H 2 )] exp(220/T) = 2(D/H) cosmic exp(220/T) = S 1 (T)2(D/H) cosmic So, potential for D to be enhanced exponentially in molecules at low T (<< 220K)

Once formed H 2 D + can transfer its deuteron to other species H 2 D + + CO(N 2,..)  DCO + (N 2 D+,..) + H 2 H 2 D + + CO(N 2,..)  HCO + (N 2 H +,..) + HD Statistically, DCO + (N 2 D +,..) formed in 1/3 of reactions, leads to the enhancement of D in DCO + (N 2 D +,..) at one-third the level of that in H 2 D +. Important because these molecular ions are easy to observe Secondary Fractionation

Enhancement Factors At low temperatures, k 1r tends to zero and we can use this to derive a constraint on f(e), the fractional abundance of electrons Since H 2 D + can react with other species – most importantly electrons and other neutral molecules, CO, N 2, H 2 O, … So, we can write the enhancement factor more completely as (M = neutral molecule)

Electron Fraction f(e) ~ – in dark clouds

Enhancement Factors – Depleted Cores

D 2 CO and N 2 D + in IRAS 16293

Multiple Deuteration (i)H 2 D + + HD D 2 H + + H K (ii)H 2 D + + HD D H K (iii)CH 2 D + + HD CHD H K (iv)CHD HD CD H K Model: 340 species – 125 singly deuterated, 30 doubly deuterated, 17 triply deuterated, 5 with 4 or more D atoms ~ 10,500 reactions linking these species

High density, low T drives multiple deuteration H3+H3+ HCO+,N 2 H+, OH+ H2D+H2D+ e- HD DCO+,HCO+,N 2 D+, N 2 H+,OD+,OH+ CO,N 2,O H2H2 H 2,HHD,H 2 D,H HD 2 + D3+D3+ HD,D 2,D,HD 2,D DCO+,N 2 D+,OD+

Fractionation in D 2 CO

Results from a pseudo-time dependent model with T=10K, n(H 2 )=10 6 cm -3 Fractional abundances varying over timeMolecular D/H ratios At late times the abundance of H 2 D + is similar to HD 2 + : this prediction was confirmed by Vastel et al. (2004) H 2 D + ~ HD 2 + ~ H 3 +, as seen by Caselli et al. (2003) towards the prestellar core L1544. D 3 + becomes the most abundant deuterated molecule (after HD). The atomic D/H ratio rises to ~0.8: important for surface chemistry

Observation of H 2 D +

Observation of D 2 H +