20 Applications of Interstellar Chemistry

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1 20 Applications of Interstellar Chemistry 1. General Gas Phase Chemistry 2. Ionization Theory for Clouds 3. Molecular Ions 4. The Discovery of Interstellar H 3 Appendix A. Experiment on Dissociative Recombination of H 3 Appendix B. Deuterium Fractionation General References van Dishoeck, UCB Lectures Notes (2000) van Dishoeck, ARAA,

2 1. Review of CO Gas Phase Chemistry From Lec.19: Chemically recall that chemically active ions are generated from neutral atomic O: O (by charge exchange of H ) OH (by proton transfer of H 3 ). These ions are then hydrogenated by H 2, producing the series OH n with n=1,2,3, but mainly OH 3 (if the abundance of H 2 is large). Recombination of OH 3 leads to O, OH & H 2 O; CO is made from the neutral reaction: C OH CO H or by ionic reactions: C OH CO H CO H 2 HCO H followed by: HCO e CO H C H 2 O HCO H Once CO is formed, HCO can be produced directly from CO with H 3 : H 3 CO HCO H 2. Since H. 3 and H 3 O are spherical rotors & HCO is linear (with a large dipole moment), HCO is considered a strong signature of ionmolecule interstellar chemistry.

3 Extension of the CO Chemistry Questions immediately arise from the discussion of CO chemistry, e.g., Can other heavy reactive hydrides analogous to OH be generated? Can other heavy diatomics analogous to CO be generated from OH? A. More Complex Oxygen Chemistry - OH is the precursor of many molecules via moderately fast radical reactions e.g., O 2 and CO 2 : O OH O 2 H CO OH CO 2 H Chemical models had predicted full association of oxygen into H 2 O & O 2 in shielded regions, a conclusion not supported by SWAS & ISO. SWAS (4' beam) did not detect O 2 at 487 GHz (616 microns), giving upper abundance limits of It did detect the lowest rotational transition of H 2 O at 547 GHz (538 microns), giving H 2 O/H 2 ~10-8, & 10 higher for Orion & the Sgr B2 cloud (galactic center). ISO (1.3 beam) detected a variety of molecules in emission & absorption, giving a large range of H 2 O abundances: H 2 O/H 2 ~

4 Chemical Spectroscopy with ISO Van Dishoeck, ARAA CO 2 H 2 O There is much less CO 2 in the gas than in the solid phase.

5 Conclusions on Oxygen Chemistry Observations of O 2 and H 2 O suggest that the dominant gas phase oxygen species is often atomic oxygen, consistent with limited observations of the OI fine-structure emission lines at 63 & 145 µm. SWAS may have emphasized cool regions where H 2 O is frozen out onto grains or where the OH radical chemistry is inoperative due to activation energies (T a ~ 3000 K). ISO detections of H 2 O in warm regions probably arise from thermally desorbed H 2 O or OH radical chemistry, with heating by outflow shocks Grains are important: In addition to incorporating O (as oxides of Si etc.), volatile gases freeze out on grains (as shown by ISO); grains may also catalyze chemical reactions. Grain chemistry is poorly understood, but it is generally accepted that grains can hydrogenate the abundant heavy elements and synthesize H 2 O, CH 4, NH 3, and CH 3 OH (see van Dishoeck 2000 notes for further discussion.

6 B. More Complex Carbon Chemistry 1. Gas-phase Chemistry of Mono-carbon Species (CH n n=1-4, CH ) Problem #1: C H 2 CH H is endothermic by 0.4 ev. One Possibility is to invoke radiative association C H CH hν k ~ cm 3 s -1 C H 2 CH 2 hν k ~ 7x10-16 cm 3 s -1 Problem #2: The sequence of abstraction reactions is broken at CH 3. CH n H 2 CH n1 H Again radiative association has been invoked: CH 3 H 2 CH 5 hν N.B. Radiative association is difficult to measure in lab (de-excitation of initial complex by 3rd body); theory is unreliable by 1-2 dex. CH 4 is probably formed by hydrogenation on grains.

7 2. Gas-phase Chemistry of Complex Carbon Species More than half of the interstellar molecules are polyatomic hydrocarbons. Most promising route is insertion reactions (c.f. early work by Suzuki), e.g. C 2, C 3, C 2 H 2 & C 3 H 2 can be made with gas-phase reactions: C CH C 2 H C CH 4 C 2 H 3 H C 2 H 2 C 2 H H C 2 H 3 e C 2 H 2 H C 2 H e C 2 H C C 2 H 2 C 3 H H C 3 H e C 3 H C 3 H H 2 C 3 H 3 hv C 3 H 3 e C 3 H 2 H N.B. Not all routes and branches have been shown. Compared to the mono-carbon species (CH, CH, CH 4 ), the gas phase chemistry of multi-carbon species is quite promising.

8 C. More Complex Chemistry: Nitrogen IP(N) = 14.5 ev: The most abundant nitrogen species in diffuse regions is atomic N. Some reactions may have barriers at low T, e.g., H 3 N NH H 2, NH 2 H N H 2 NH H NH 3 and NH 4 interact weakly or not at all: standard ion-molecule sequence ends at NH 2. Ion-molecule chemistry doesn't work as well for N as for C & O. NH 3 is probably made on grains.

9 The CN radical can be made by neutral reactions from carbon radicals, e.g., CH N CN H C 2 N CN C Both CN & HCN can also be made by ion-molecule reactions, e.g., C NH CN H CN H 2 HCN H H 3 CN HCN H 2 HCN H 2 HCNH H HCNH e HCN, HNC H HCN e CN H 2 Polyacetylene chains can also be built up along these lines, e.g., C 2 H 2 HCN --> HC 3 N 2 H 2. This type of chemistry is particularly appropriate to the circumstellar envelopes of AGB stars such as IRC

10 2. Ionization of Interstellar Clouds Promising as it is, ion-molecule chemistry is difficult to test because chemical models are extremely complex. For example, the UMIST program ( has ~500 species and ~3500 reactions, few of which have been measured in the laboratory. Furthermore, the chemistry depends on the temperature and density, so the results are sensitive to the dynamical and thermal model. Testing ion-molecule chemistry naturally hinges on the problem of the ionization of interstellar clouds, i.e., signature molecular ions. In earlier lectures we developed simple closed-form theories for the ionization fraction, e.g., in Lecture 7, we gave the theory for photo and collisionally ionized atomic hydrogen: (α 2 k c )x e2 (G/n H k c )n e G/n H = 0. In the next we generalize this to include heavy atomic and molecular ions.

11 2. Ionization of Interstellar Clouds

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15 Note on the Role of Proton Affinity NH 3 Si A molecular ion AH will usually easily transfer Its proton to another molecule B if it gets better bound. The determining quantity is the proton affinity which, aside from the fact that its defined in terms of the enthalpy per particle, is essentially the binding energy. Thus the energy yield (difference between initial & final energies) of a proton transfer reaction is, E(AH B BH A) = [pa(b) - pa(a)] In the first entry in the table for H, one finds the binding energy of the H 2 molecule, 2.65 ev. From the entry for H 2, one sees that H 3 has a binding similar to H 2 itself. Exothermic proton transfers go from bottom to top. Proton Affinity (ev) NH 2 H 2 CO CH H 2 O C 2 OH C CO CH 4 CO 2 N 2 O H 2 O 2 N 3 H

16 3. The Discovery of Interstellar and H 3 H 2 is the first ion produced by cosmic rays in molecular gas, but its abundance is very low because it is destroyed rapidly by both atomic and molecular hydrogen. In the latter case, it produces H 3, which is much more abundant since it is not destroyed by either atomic or molecular hydrogen. However it is destroyed by many neutral species, especially by proton transfer, thereby generating a rich ion-molecule chemistry. HCO, the first molecular ion discovered in dense regions (Snyder et al. 1970) is produced this way, H 3 CO HCO H, and was the surrogate for H 3 and the signature for interstellar ion-molecule chemistry until H 3 was discovered in the ISM in Why did this take 25 years?

17 Discovery of H 3 J. J. Thomson Phil. Mag H 2 H 3 H

18 H 3 in Laboratory Hydrogen Plasmas A. J. Dempster (discoverer of 235 U) established that H 3 is abundant in lab plasmas (Phil Mag ) H 3 is produced by the reaction H 2 H 2 H 3 H Hogness & Lunn (Phys Rev ) The exothermic nature and rate of this reaction had been known since the 1930 s, but appreciation of its possible interstellar role had to wait 30 years for a short note in ApJ by Martin et al. (next page);10 more years for serious consideration; and 25 more years for it to be detected in the ISM.

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20 Early Work on Interstellar H 3 Martin et al. (ApJ ) suggest significant interstellar abundance of H 3 Solomon & Werner (ApJ ): CRs ionize cloud interiors; estimate n(h 3 )/n H ~ 10-6 Glassgold & Langer ( ApJ 179 L ) balance CO cooling with CR heating in dense clouds, including chemical heating from H 3 (~ 8 ev per ionization) Watson (ApJ 182 L ): isotope fractionation via H 3 HD H 2 D H Watson (ApJ 183 L ) and Herbst & Klemperer (ApJ ) develop ion-molecule chemistry, with H 3 generating many other progenitor ions by proton transfer, e.g., H 3 CO HCO H: H 3 is the universal protonator

21 Detecting Interstellar H 3 Symmetric rotor: forbidden pure rotational spectrum No electronic states or spectrum Vibrational spectrum is the only tool: NIR absorption spectroscopy for both lab and astronomical detection ν 2 ν 1 Theoretical spectrum, with lines detected by Oka s lab experiment (PRL ) ν 2

22 Rotational Structure Energy levels are those of a symmetric top: E = B J(J1) (C-B) K 2 B = cm -1 C = cm -1 Observed ν 2 = 1-0 transitions J J=K=0 is forbidden by the exclusion principle, so the J = 1, K = 1 state of para H 3 is the lowest rotational state; the J = 1, K = 0 of ortho H 3 is only E/k = 32.9 K higher. These are the only levels populated for T 5-50 K. They have ~ equal populations since the higher statistical weight of ortho (I = 3/2) compared to para (I =1/2) compensates approximately for the Boltzmann factor. ortho I=3/2 para I=1/2 K K=0 K=1

23 Discovery of Extraterrestrial H 3 First detection of H 3 in space was in the aurora of the Jovian planets (Trafton et al. ApJ 343 L ). First interstellar detections towards massive, IR-bright, deeply embedded YSOs, W33A and AFGL2136, in the ortho-para doublet. N(H 3 ) 4 x cm -2,T = K Geballe & Oka Nature

24 Subsequent Detections of H 3 Geballe, Hinkle, McCall, & Oka have detected H 3 along many lines of sight, originally towards distant IR-luminous embedded YSOs and more recently towards some close bright stars for which extensive UV observations are available. McCall et al. Nature Cyg OB2 #12 diffuse Geballe et al. ApJ Galactic center diffuse and dense (also Cyg OB2 #12) McCall et al. ApJ luminous embedded YS0s dense McCall et al. ApJ diffuse clouds McCall et al. Nature ζ Per diffuse cloud Quotation marks have been used for some of the diffuse & dense designations where the lines of sight are long and very likely involve inhomogeneous gas.

25 H 3 in Dense Clouds 1.02 R(1,1) u R(1,0) R(1,1) l AFGL AFGL Wavelength (Å) N(H 3 ) = McCall, cm -2 Geballe, Hinkle, & Oka ApJ 522, 338 (1999)

26 Diffuse Clouds 1.01 HD McCall, et al. ApJ 567, 391 (2002) R(1,1) u R(1,0) R(1,1) l Wavelength (µm) GC IRS H 3 Column Density (10 14 cm -2 ) HD Cyg OB2 5 HD WR 104 WR 121 HD χ 2 Ori P Cygni HD ζ Oph Cyg OB2 12 WR E(B-V) (mag)

27 Cygnus OB2 12 and ζ Per N(H 3 ) cm -2 similar to columns for dense clouds N(H 3 ) 8 x cm -2 ζ Per is a well studied cloud and nicely exemplifies the conundrum posed by the observations of H 3

28 Confronting Theory & Observation for ζ Per Earlier we derived the following expression for the local abundance of H 3 x(h 3 ) = ς 2 n H x(h2) βx kx and showed that, for a dense cloud with n H =2500 cm -3, the H 3 abundance is ~10-8. Thus if the cloud has a linear dimension of 1-2 pc, the H 3 column is ~ cm -2 the same order as observed for dense and diffuse clouds. The problem arises when the case of the well studied diffuse cloud towards ζ Per is considered. From the UV observations (Copernicus, Savage et al. ApJ ; HST Cardelli et al. ApJ ), we have: N H = 1.6x10 21 cm -2 n H ~ 250 cm -3 L ~ 6.4x10 18 cm N(H 2 )/N H =0.3 N(C )/N H =10-4 e O

29 N(H or 3 Theory & Observation for ζ Per All of these results are average quantities and need to be converted into local quantities with some realistic model. In the absence of a model, and to illustrate principles, we integrate the above expression for x(h 3 ) to obtain ) = ς 2 x(h2) ds βx kx N(H e 3 ) O ς L 2 ς 2 x(h ds βx 1 x(h β x e 2 ) 2 e ) for diffuse clouds If we further assume that β is roughly constant, we can solve for the CR ionization rate of H 2 in terms of the measured H 3 column 1 β N(H3 ) x(h2) ς 2 L x e

30 Discussion of H 3 in ζ Per But there is a problem in that the cloud average of the abundance ratio of H 2 to electrons is variable, one increasing and the other decreasing, so that the ratio rapidly increases going into a cloud (or into a dense inhomogeneity of the cloud). What McCall et al. (and everyone else) do is replace these abundance by column density ratios: ς 2 N(H β L 3 ) N(C N(H 2 ) / ) / N N H H Substituting the measured column densities leads to: ς s

31 H 3 in ζ Per (concluded) This value is 10 times larger than that obtained from: (1) demodulated measurements of the local CR intensity (2) chemical modeling of OH using ion-molecule reactions. This is a challenge to interstellar chemistry. Some of the explanations that come to mind: a. ζ CR is larger for the ζ Per l.o.s. due to cosmic ray sources in the Per OB association b. very low-energy CRs (< 2 MeV) are screened from thicker clouds since the range of a 2-MeV proton is 4x10 21 cm -2. c. some other ionizing radiation operates in diffuse clouds d. spatial in homogeneities may increase <n(h2)/ne> above 3,000. If the large ζ CR is sustained, it has important implications for the heating and ion-molecule chemistry of diffuse clouds, the ones most amenable to detailed study.

32 20 Years of Uncertainty About the Dissociative Recombination Rate Measured values of β (H 3 ) have fluctuated, and the theory has been uncertain. Lab experiments on the afterglow of decaying H plasma ranged from < to 2x10-7 cm 3 s -1 (at 300 K). The small rate coeffcients originally led McCall & Oka to suspect that the large ζ was really an incorrect large β. There always were concerns that the measurements were wrong because the ions were were in excited rovib states. Ion storage ring experiments consistently give large values.

33 Cryogenic Ion Storage Ring Results β for rotationally cold H 3 vs. electron temperature β(30 K) = 2.6 x 10-7 cm 3 s -1 β(300 K) = 6.8 x 10-8 cm 3 s -1 T -1/2 For details of McCall s experiments, see Appendix A of these notes, or McCall et al. Nature and Phys Rev A

34 New Chemical Model for ζ Per Le Petit, Roueff, & Herbst (A&A ) use a 3-component model to calculate the abundances of 18 measured species, plus the rotational population of H 2. There are 2 uniform phases, with these parameters, plus an arbitrary number of MHD shocks. The dense phase is tiny (100 AU) and the diffuse phase has L = 5 pc. The shocks and the dense phase are introduced to solve other problems (the old one of CH and C 2, C 3 ) & not the H 3 abundance.

35 Chemical Model for ζ Per By varying the temperature (which affects the H O charge exchange rate), they get factor of 3-4 agreement with all measurements using an intermediate value of the CR ionization rate, 2.5x10-16 s -1. This is at the expense of under-predicting the H 3 abundance by 3 and over-predicting many others. Thus their CR ionization rate would be only 30% smaller than McCall s if they fit the H 3 measurement exactly (and of course get other things wrong). Therefore, this model does not really solve the problem raised by the H 3 observations. But it does suggest that somewhat larger CR ionization rates may be tolerated in ion-molecule chemistry of diffuse clouds.

36 Future Work on H 3 Search for H 3 in more UV-accessible sightlines comparisons with ζ Per meaning of non-detections in ο Per & ζ Oph Better observations and modeling of ζ Per cloud and nearby lines of sight Observations of H 3 in more reddened sources decrease in ionization rate

37 Closing Thought

38 Appendix A: Cryogenic Ion Storage Ring Experiment to Measure Dissociative Recombination of H 3 CRYRING ion storage ring, Manne Siegbahn Laboratory in Stockholm Cold H 3 ions injected from the Berkeley supersonic expansion ion source In a straight section of the storage ring, the ions passed collinearly through an approximately homogeneous electron beam Electron energy is chosen so that ion electron velocity matching is achieved H 3 ions are lost to dissociative recombination with electrons and collisions with residual gas molecules Total number of neutral reaction products (H H H and H 2 H) are measured Electron energy varied by changing the cathode voltage in the electron cooler, and the neutral fragment signal recorded as a function of the longitudinal energy difference between electrons and ions ( detuning energy )

39 Berkeley Supersonic Ion Source Gas inlet 2 atm H 2 Pulsed nozzle design Supersonic expansion leads to rapid cooling Discharge from ring electrode downstream Similar to sources for laboratory spectroscopy in Saykally group Skimmer employed to minimize arcing to ring Pinhole flange/ground electrode Insulating spacer -900 V ring electrode Solenoid valve Skimmer H 3

40 Spectroscopy of H 3 Source Infrared Cavity Ringdown Laser Absorption Spectroscopy Confirmed that H 3 produced is rotationally cold, as in interstellar medium McCall, et al. Nature 422, 500 (2003)

41 CRYRING Ion Storage Ring 50 m Very simple experiment Complete vibrational relaxation Control H 3 e - impact energy -Rotationally hot ions produced -No rotational cooling in ring

42 Cryogenic Ion Storage Ring Result McCall et al. Nature Note structure (resonances) in the cross-section.

43 Recent Theoretical Work Significantly better than earlier attempts, but not without problems

44 Appendix B: D Isotope Fractionation The ion-molecule reactions are all fast in this problem (except when endothermic). Reactions not discussed here: very fast dissociative recombination, photodissociation of HD (little self-shielding), and neutral destruction of molecular ions. Transforming D into HD: H D --> D H D H 2 --> HD H endothermic by ~41 K Fractionating H 3 : H 3 HD --> H 2 D H 2 exothermic H 2 D H 2 --> H 3 HD endothermic by ~ 150 K Passing on the fractionation H 2 D CO --> DCO H 2 HCO HD --> DCO H 2 Observations: a. FUV: absorption lines of D and HD in diffuse clouds, b. mm: DCO /HCO ratios up to ~0.1 in dense clouds plus many other deuterated molecules, e.g., D 3 H Importance of Low Temperatures D HD, DCO etc., is favored by low T because reversing fractionation is endothermic.