1. 6 th IRAM 30m Summer School Star formation near and far A. Fuente Observatorio Astronómico Nacional (OAN, Spain) Photon Dominated Regions II. Chemistry

2. Some history Before 1937: Scientists thought that the ISM was composed by atoms 1968: Charles Townes and collaborators detected H 2 O y NH 3. 1937-1941: The first three diatomic molecules (CH,CN,CH + ) are detected (optical lines). 1963: Weinreb and collaborators detected OH in the Galactic Center. - - - - THE BIRTH OF RADIOASTRONOMY - - - -

3. Gas molecular Because of the extinction of the UV radiation produced by interstellar grains, complex molecules can form and survive within molecular clouds H 2 is the most abundant molecule in the ISM, and it is basically 100 % of the molecular gas. The second most abundant molecule is CO, with a fractional abundance wrt H 2 of, [CO]/[H 2 ] = 10 -4 More than 123 molecular species have been detected hasta in the ISM. Some are common in the Earth such as NH 3, H 2 CO, HCOOH,.... Others can survive long enough only in extreme conditions (low density and temperatue), and in the Earth they are only found in laboratory (CO +, C 2 H, HCS +,...). Moleculas ¿Which is the composition of the molecular gas?

4. List of detected molecules

5. Orion KL Spectral survery at 3mm, 2mm and 1mm carried out with the 30m telescope by B. Tercero and collaborators (Tercero et al., 2010 A&A 517, 96;,2011, A&A 528, 26)

6. Orion KL Spectral survery at 3mm, 2mm and 1mm carried out with the 30m telescope by B. Tercero and collaborators (Tercero et al., 2010 A&A 517, 96;,2011, A&A 528, 26)

7. Charateristics of the ISM composition There are two important characteristics that defines the chemical composition of the ISM and are different from that in Earth: 1. There are many molecular ions and radicals in the list of ISM molcules. These species are very reactive and consequently unstable in the physical conditions prevailing in the Earth (e.g. C 3 H 2, C 3 H, o C 3 N ) 2. Organic compounds are usually un-saturated (few H atoms per molecule). Saturated compounds (CH 2 CHCN, CH 3 CH 2 CN) are only found in warm (T k >100 K) and dense (>10 6 cm -3 ) regions (“hot cores”).

8. ¿How are molecules formed in the ISM? Most molecules are formed in gas-phase by chemical reactions between more simple species : Earth Three-body reactions A+B →AB* AB*+C→ AB + C Reactions neutral-neutral with activation energies, kT a ~ 0.3 eV. ISM No three-body reactions The first important consequence is that H 2 cannot be formed in gas- phase. Reactions without activation energy, mainly exothermic (kT MI ~ 0.01 eV). The most important reactions are ion-molecule.

9. Chemical reactions in Space

10. Photochemistry The FUV photons permeating the diffuse ISM are a dominant destruction agent for small molecules. Typical bonding energies of molecules are in the range 5 – 10 eV. However, direct absorption into dissociating continuum is usually negligible. More frequently, the dissociation occurs through a transition to a electronic state followed by dissociation. The photodissociation rates are usually expressed as The rates are expressed in units of the Habind field. Therefore, the characteristic time is given by τ=1/(k pd G 0 ) Different at each layer of the PDR

11. Photochemistry

12. Grain-Surface reactions The most abundant molecule in space, H 2, cannot be formed in gas phase. Must be formed on grain surfaces. Grain-surface reactions are also key to explain the abundances of other molecules such as CH 3 OH, H 2 CO, NH 3 or H 2 O. Evaporation Photo-desorption Chemi-desorption Sputtering

13. Grain-Surface reactions The accretion rate is The depletion time for a cold core is (4 x10 9 )/n, i.e., less than 10 5 yr in a dense core. We can also express this lifetime as the time required for a species to arrive at a grain. In a gas with kinetic temperature of 10 K, for CO

14. Grain-Surface reactions The evaporation time is E b =binding energy The evaporation time is very sensitive to the dust temperature Td. In PDRs, the main mechanism to release molecules from the grain mantles is photo-desorption. Assuming the mean interstellar UV field, the grains will remain clean of ices for

15. Grain-Surface reactions Other important concept is migration time. Migration time is the time to move from one position to another on the grain surface. When the migration time is lower than the evaporation time, surface reactions occur.

16. PAHs

17. PDR Chemical Models

18. Stationary Chemical Models (Sternberg & Dalgarno 1995)

19. Stationary Chemical Models (Sternberg & Dalgarno 1995,ApJ SS 99, 565)

20. Reactive ions (CO +,HOC +,SO + ) Reactive ions are destroyed in every collision with e - and H 2. For this reason, their abundance are expected negligible in molecular clouds. Only in the HI/HII warm interface of PDRs, their rapid formation can overcome destruction and they reach measureable abundances. The first reactive ion, CO +, was detected toward the planetary nebular NG7027. Formation of CO + : Formation of HOC + : O + H 2 OH + H Endothermicity=3000K

21. Reactive ions (HOC + ) Branching ratio=0.8=α Branching ratio=0.6 Branching ratio=0.48 Usero et al. A&A 419, 897 Destruction: HOC + + H 2 HCO + + H 2 (1) HCO + + e - products (2) [HCO + ]/[HOC + ]=k 1 n H2 /α k 2 n e

22. Reactive ions (CO +,HOC +,SO + ) (Fuente et al. 2003, A&A 406, 899)

23. Reactive ions (CO +,HOC +,SO + ) (Fuente et al. 2003, A&A 406, 899)

24. Reactive ions Sternberg & Dalgarno, 1995, ApJSS 99, 565

25. Reactive ions Sternberg & Dalgarno, 1995, ApJSS 99, 565

26. Herschel (CH +,OH +,SH +...) Herschel has provided for the first time the opportunity of observing light reactive ions (CH +, OH +, SH +...) in the interstellar medium. Light molecules such as CH, OH, NH, NH 2,.. has been extensively observed Some light molecules like H 2 Cl + has been discovered for the first time. Others, like H 2 O + and CH + turned to be more abundant than expected.

27. HF, CH +,SH +,OH +,H 2 O + Falgarone et al., SF2A 2010 HF is a good tracer of H 2 The high abundances of CH + and SH + are only undersand in the context of turbulent models. OH + and H 2 O + proves that the absorbing layer is highly ionized.

28. Nitrogen hydrides (NH, NH 2,NH 3 ) Persson et al., 2010, A&A 521, L45 Although the [NH]/[NH 3 ] and [NH 2 ]/[NH 3 ] ratios in diffusse clouds are consistent with gas-phase PDR models, but their large abundances derived require of grain surface chemistry.

29. H 2 Cl + Lis et al. 2010, A&A 521, L9 Formation route: [HCl]/[H 2 Cl + ]~1 -10 in agreement with PDR chemical models H 2 Cl + column densities ~ 10 13 cm -2, in excess of model predictions

30. Sternberg & Dalgarno, 1995, ApJSS 99, 565 Failures

31. 1.- Millimeter observations provides chemical diagnostics of PDRs 1.1 CN/HCN 1.2 HCO/H 13 CO + 1.2 Small hydrocarbons (PAHs) Other chemical diagnostics

32. NGC 7023 CFHT

33. CN/HCN ratio (Fuente et al. 1993, A&A 276, 473) The CN/HCN ratio was first proposed as a tracer of PDRs by Fuente et al. (1993) based on their observations in NGC 7023. CN/HCN > 1 is considered a good evidence for PDR.

34. CN/HCN ratio in Orion (Fuente et al. 1996, A&A 312, 599; Rodríguez-Franco et al. 1998, A&A 329, 1097) C.R. O'Dell (Rice University) y NASA/ESA The CN/HCN ratio is ~3 across the Orion Bar

35. Ionized by the B0V star HD 211880 Draine field= 150 Ionized by IRS 1 IRS1: RA= 22:19:18.21, Dec=63:18:46.9 IF: RA= 22:19:11.53, Dec =63:17:46.9 S140: Two PDRs with different physcial conditions

36. HCO (Schilke et al. 2001, A&A 372, 291) In a pioneering work Schenewerk et al. (1988) proposed that HCO is easily detected in PDRs. Schilke et al. (2001) concluded that HCO is associated to PDRs and [HCO]/[H 13 CO + ] varies between 30 in the Orion Bar and 3 in NGC 7023

37. Horsehead Hily-Blant et al., 2005 A&A 440, 909

38. Horsehead (Gerin et al. 2009, A&A 494, 977) N (HCO)/N (H 13 CO + ) ∼ 50 in the FIR peak of the Horsehead!!!! They proposed a new gas-phase route to explain the high HCO abundance:

39. Small hydrocarbons (C 2 H, c-C 3 H 2, C 4 H...) (Teyssier et al. 2004, A&A 417, 135) Hily-Blant et al., 2005 A&A 440, 909

40. Horsehead (Pety et al. 2005, A&A 435,885) The abundance of small hydrocarbons in the PDR position is similar to that of these species in dark clouds. This cannot be understood in terms of gas -phase chemistry. The correlation between the abundances of these species and the PAHs suggests to propose an alternative path: C 2 H 2, CH 3,... are released from PAHs by photolysis and subsequent chemistry in gas phase produce enchanced abundance of small hydrocarbons.

41. Summary 1.- Millimeter observations provides chemical diagnostics of PDRs 1.1 CN/HCN (well explained gas-phase stationary models) 1.2 HCO/H 13 CO + (grain surface chemistry?) 1.2 Small hydrocarbons (PAHs) 1.3 Reactive ions 2.- Far-IR spectroscopy (Herschel) 2.1 Hydrides (turbulence is required to explain some species) 2.2 New molecules (H 2 Cl + ) I. Observations

42. References 1.- “The Physics and Chemistry of the Interstellar Medium”, A.G.G.M. Tielens, de. Cambridge University Press

43. Summary 1.- Include grain-surface chemistry (UCL and under developement in Meudon PDR code) 2.- Include PAH chemistry (under development in Meudon) 3.- 2-D chemistry (Leiden code) and 3-D (under developement in Meudon PDR code) 4.- Coupling dynamics and chemistry in a time dependent code (UCL) 5.- Include clumpyness (KOSMA) II. Models