1. Photodissociation Regions Amiel Sternberg School of Physics & Astronomy Tel Aviv University ISRAEL Dense ISM in Galaxies Zermatt Switzerland 22-26 September 2003

2. Operational Definition: Includes Dense PDRs Parameters density FUV field Habing/Draine grain scattering clumpiness H2 formation rates Operational definition: Neutral interstellar hydrogen gas where (6-13.6 eV) FUV radiation dominates the physical and chemical structure. Hence also called “photon-dominated regions” (PDRs). Molecular photodissociation a key process. Interstellar PDRs include: The diffuse WNM and CNM clouds. Translucent clouds: A V < 5 n H < 1000 cm -3 weak interstellar FUV fields. Dense molecular clouds: A V up to ~10 n H > 1000 cm -3 including intense FUV fields near OB stars ~90% of the Galactic molecular ISM may be “photon-dominated”.

3. Dense PDRs: Dense PDRs exposed to intense FUV fields in star-forming molecular clouds are particularly important as sources of luminous fine-structure and molecular line (cooling) radiation and far-IR thermal dust continuum emission. vib rot H 2

4. Clumpy, time-dependent, PDRs: Hubble Space TelescopeNIRIM camera @ WIYN 3.5m Speck et al. 2003 PASP 115, 170 Molecular Hydrogen emission in the Ring Nebula. 1-0 S(1) 2.12  m

5. Basic structure: 1-D steady-state model, escape probability method. FUV 6-13.6 eV H 2 self-shielding and dust attenuation vs. grain surface formation. Controlling parameters: 1) cloud density/pressure 2) FUV intensity 3) grain scattering properties 4) H 2 formation rate coefficient 5) geometry/clumpiness 6) gas phase abundances 7) magnetic field.

6. Local Far-Ultraviolet (FUV) Radiation Field (6 - 13.6 eV 912 - 2070 Å): “Draine Field” (1978). Empirically based for solar neighborhood. Mean energy densities: Draine / Habing = 1.7 Paravanno, Hollenbach & McKee 2003 ApJ 584, 797 Abinitio computations of the local FUV field, based on the Galactic birth-rate of OB associations. monte-carlo Key Results 1) At any point, dust opacity limits contributing associations to within 500 pc. 1)Large angle scattering produces a diffuse field at the 10% level. 1)Time-dependent due to statistical fluctuations in the OB-association formation rates. 1)Median predicted value within 6% of the Draine field!

7. Heating Mechanisms: Photoelectric heating beginning with Spitzer 1948 ApJ, 107, 6 FUV-pumping of H 2 in dense PDRs.

8. grain charging net photoelectron energy Draine & Weingartner 2001 ApJS, 234, 162 Grain photoelectric heating efficiency: Heating dominated by smallest grains/PAHs. Half the heating from grains with radii a < 15 Å. The rest from grains with 15 Å < a < 100 Å for a dn = a -3.5 da (MRN) grain size distribution. Bakes & Tielens 1994 Different assumed electron-grain sticking coefficients.

9. CII 158  m fine-structure line cooling: Expected theoretically - e.g. Dalgarno & McCray 1972. First far-IR (Lear jet) detections by Russell et al. 1980 in NGC 2024 and Orion. Widely detected since with KAO, ISO... Tielens & Hollenbach 1985 ApJ 291 722 CNM WNM Wolfire et al. 2003 dense PDRs

10. Accounting for CII 158  m OI 63  m and other “low-ionization” fine-structure emission lines observed in star-forming molecular clouds. Line strengths ~ 1% of IR continuum. FUV (6-13.6 eV) heating via photoelectric emission from dust grains: mean photon energy = 10 eV typical grain work function = 6 eV (for neutral grains). photoelectric yield = 0.1 Thus, heating efficiency = (4/10) x 0.1 = 0.04 (smaller for positively charged grains.) Low-ionization fine-structure emission lines in star-forming molecular clouds:

11. FUV intensity line intensity (erg cm -2 s -1 sr -1 ) [CII] 158  m [OI] 63  m Fine-Structure Line Emission: Hollenbach & Tielens 1997 ARAA, 35, 179 Hollenbach & Tielens 1999 Rev. Mod. Phys., 71, 173 FUV intensity log hydrogen density (cm -3 ) [OI] 63  m / [CII] 158  m Kaufman et al. 1999 ApJ, 527, 795 [OI] 63  m can become optically thick.

12. Gas-Phase Chemistry: Sternberg & Dalgarno 1995 ApJS 99, 565 Jansen et al. 1995

13. Gas-Phase Production of Hydroxyl and Water: Ion-molecule chemistry in cold gas OHH2OH2O e e H3O+H3O+ H2O+H2O+ OH + H3+H3+ H2H2 H2H2 O H2H2 H2H2 Neutral-neutral chemistry in warm gas regime of ion-molecule chemistry regime of neutral-neutral chemistry Neufeld & Kaufmann 1995

14. OH and H 2 O in PDRs: OHH2OH2O e e H3O+H3O+ H2O+H2O+ OH + H3+H3+ H2H2 H2H2 O   H2H2 H2H2 Removal by photodissociation. OH/H 2 O remains large in warm PDR gas.

15. OH C+C+ CO + HCO + H2H2 Si + SiO + S+S+ SO + O O2O2 N NO OH driven chemistry in warm H/H 2 transition zone: T = 300 – 1000 K Prediction: CO + /HCO + large in PDR CO + /HCO + small in dark core

16. For example, CO + / HCO + At interface: G 0 = 2.4 x 10 3 n = 10 4 cm -3 Plateau de Bure & 30m CO + /HCO + Molecular Peak < 0.001 PDR Peak ~ 0.1 NGC 7023 (reflection nebula) Fuente et al. 2003, AA, 899, 913 Molecular Peak PDR Peak illuminating star (Herbig B3Ve) HCO + CO + HCO + CO +

17. Neutral Atomic Carbon: C + /C/CO conversion S + C +  S + + C

18. Kamegai et al. 2003 ApJ 589, 378 L1688 in  Oph Mt Fuji submm-telescope 1.2m [CI] 609  m fine-structure line emission in  Oph: Extended CI may be due: 1) Scattered FUV in a clumpy medium. 2) Possible time-dependent effects. Recombining C + in shadowed PDR clumps where cooling-time is longer than C formation time. Stoerzer, Stutzki & Sternberg 1997

19. Polycyclic Aromatic Hydrocarbons (PAHs): NGC 7023 (reflection nebula) Optical  “internal conversion”  IR

20. PAHs in PDRs : Bakes & Tielens 1998 ApJ 499, 258 PAH+PAHPAH- dashed – without PAHs solid – with PAHs (2 x 10 -7 ) mutual neutralization X + + PAH-  X + PAH competes with or dominates radiative recombination X + + e  X + h Lepp & Dalgarno 1988, ApJ, 335, 769

21. Extragalactic PDRs: NGC 4038/4039 interacting galaxies CII emission from NGC 4038/4039 Nikola et al. 1998 ApJ 504, 749 (KAO) Dense PDRs dominate: G 0 = 500 n H = 10 5 cm -3 (CNM a small fraction.)

22. CII line deficit in Ultra-Luminous IR Galaxies (ULIRGs): ISO-LWS ULIRG sample. L IR > 10 12 L  Luhman et al. 2003 astro-ph 0305520 Possible Explanations 1)G 0 / n >> 1 cm 3 in ULIRGs inefficient photoelectric gas heating. 1)Underabundance of OB stars (unlikely). 1)Absorption of UV photons in dusty HII regions. Requires high ionization- parameters (U   dust ). Is this consistent with nebular emission-line excitation? TBD (basic assumption: FIR is reradiated FUV energy.)

23. The interacting galaxies NGC 4038/4039 Infrared Space Observatory Keck

24. H 2 lines UV-pumped Molecular Hydrogen in the Antennae: Mid-IR Cluster Gilbert et al. 2000, ApJ, 533, L57

25. FUV Pumping and Photodissociation of Molecular Hydrogen: Near IR FUV Pumping Dissociation Effective Potential Energy v=0 1 2 3 Internuclear Separation Energy Black & Dalgarno 1976 Black & van Dishoeck 1987 Sternberg 1988 Sternberg & Dalgarno 1989 Draine & Bertoldi 1996 Sternberg & Neufeld 1999

26. H 2 level populations in the mid-IR cluster: Indicating radiative excitation in PDRs. v = 1v = 2v = 3 T vib = 6000 K non-thermal ortho para o/p ratio for excited states  1.8 a signature of warm T > 300 K gas !

27. Curve of growth for absorption lines log column density in ground state log equivalent width LINEAR FLAT SQUARE ROOT thin ------------- thick -------------------- saturated log number of photons absorbed Doppler core radiative damping wings

28. The FUV H 2 absorption lines are highly saturated. Therefore, Sternberg & Neufeld 1999 ApJ, 516, 371 H 2 ortho-to-para ratio:

29. Magnetically Regulated Star-Formation:

30. Ionization Structure : Cosmic-ray ionized core. Rapid magnetic flux loss. Magnetic field “frozen in”. ambipolar diffusion time t AD = 10 14 x e yr

31. Cosmic ray core PDR Photoionization Regulated Star-Formation: McKee 1989 ApJ 345, 782 star formati on sterile envelope

32. Cosmic ray cores collapse via ambipolar diffusion increased star-formation rate Energy injection via protostellar outflows, cloud expansion, cosmic ray ionized mass fraction decreases. Photoionization regulated star-formation: Reduced star-formation rate, cloud contraction, enhanced shielding Predicts: 1) Galactic star-formation rate of 4 M  yr -1 2) Star-formation is restricted to regions of relatively high extinction, A V > 4. (consistent with observations)

33. A golden future! Space Infrared Telescope Facility (SIRTF) Stratospheric Observatory for Infrared Astronomy Atacama Large Millimeter Array (ALMA) Herschel Space Observatory (submillimeter)