Ionization and ion chemistry in molecular clouds

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1 Ionization and ion chemistry in molecular clouds John H. Black Department of Earth & Space Sciences and Onsala Space Observatory Chalmers University of Technology Sweden Cosmic Ray Interactions: Bridging High and Low Energy Astrophysics Lorentz Center, Leiden 2011 March 14

2 What is a molecular cloud? concentration of gas & dust isolated in projection on the sky and in Doppler velocity hierarchical structure, turbulence weakly ionized: e/h 2 =10-9 to 10-4 magnetic field coupled to gas via charged particles only physical & chemical state far from equilibrium

3 What is a molecular cloud? composition H 2 He CO, O, H dust (1% by mass) a great variety of other molecules, atoms

4 Regimes of the neutral interstellar medium (Snow & McCall) - shaded area now accessible in UV with HST/Cosmic Origins Spectrograph

5 Types of clouds Diffuse/translucent observable by absorption spectroscopy AV<6 mag, NH<10 22 cm -2 ionization by UV starlight not gravitationally bound partly atomic: H and H2, C + and CO Dense/dark observable by mm emission lines NH>10 22 cm -2 cosmic-ray ionization gravitationally bound fully molecular: H2, CO photon-dominated region (PDR)

6 The dark cloud Barnard 68 (color image from J. Alves, European Southern Observatory).

7 lambda Ori nebula (fig. 15 from Dolan & Mathieu 2001 Astron. J., 121, 2124). Dots mark the pre-main-sequence stars; the grayscale image is far-infrared brightness at 60 micron wavelength

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9 B35 - contours of CO line emission

10 CO line emission from a translucent cloud HD169454

11 What do cosmic rays do to interstellar matter? ionize hydrogen and helium, produce e- drive a rapid ion-neutral chemistry even at T<10 K heating (excess energy of secondary electrons) pressure destroy or modify dust particles

12 Cosmic-ray-induced chemistry CR HeH + H2 H2 + H2 H3 + O OH + H2 H2O + H2 H3O + H He e - H2 e - e - e - e - e - e - 2H 3H O OH H2O H2 CR O H H + O + D D + H H2 HD

13 Transient versus terminal ions H2 +, OH +, H2O +, HeH + are transient: they react on nearly every collision with the most abundant neutrals, H2 or H transient ions cannot be thermalized population distributions over their quantum states governed by the energetic formation processes low steady-state abundances H3 + and H3O + wait around for less abundant reactants O, CO, e - and thus build up higher steady-state abundances

14 J.K. Becker, J.H. Black, M. Safarzadeh, F. Schuppan 2011 in prep.

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17 First results from Herschel/HIFI PRISMAS programme: hydrides in diffuse molecular gas NEW! - widespread HF, H 2 O +, OH +, H2Cl + Context: interstellar ion chemistry and the superthermal ionization rate Perspective: new steps toward a pan-chromatic view of interstellar matter: Herschel, HST (new Cosmic Origins Spectrograph), ALMA...γ-rays, cosmic rays

18 PRobing InterStellar Molecules with Absorption line Studies is a guaranteed-time Key Programme with HIFI (Heterodyne Instrument for the Far-Infrared) on board Herschel Space Observatory Principal Investigator: Maryvonne Gerin (ENS, Paris)

19 Herschel Space Observatory launched from Kourou, French Guiana on 2009 May 14 aboard an ESA Ariane 5 rocket; after initial problems, HIFI remains alive and well!

20 Submm-wave absorption spectroscopy GHz with the Herschel/HIFI instrument Toward compact submm-wave sources in star-forming regions, we observe a mixture of emission and absorption lines of molecules distances D 2 to 8 kpc common hydrides and reactive ions for the first time Observed intensity I(ν) = I source (ν) exp ( τ los (ν) ) + B ν (T ex ) ( 1 exp( τ source (ν) ) so far we have analyzed only the clean absorption in line-of-sight, diffuse clouds. simple absorption measurements yield accurate column densities column densities fractional abundances if we can use a surrogate tracer of H 2 (for example, CH) Galactic rotation allows us to sample a large range of Doppler velocities (and thus many, distinct clouds) over these lines of sight some spectra are further complicated by intrinsic hyperfine structure

21 SURPRISE! OH + and H2O + were easy to detect in absorption toward distant continuum sources We expected smaller abundances because both ions react on every collision with H2

22 Gerin et al toward G Neufeld et al toward W49

23 Interstellar oxygen chemistry (cf. van Dishoeck & Black 1986) Cosmic-ray ionizations of H and H 2 produce H +, H + 2, and H+ 3, which drive a rapid ion-neutral chemistry: or followed by H + + O O + + H O + + H 2 OH + + H H O OH+ + H 2 OH + + H 2 H 2 O + + H H 2 O + + H 2 H 3 O + + H. Both ions are also removed by free electrons. When the abundance of free electrons is limited by starlight photoionization of carbon to make C +, we expect n(oh + ) n(h 2 O + ) 0.12 ( ) 1/2 = /T f(h 2 ) The observations suggest N(OH + )/N(H 2 O + ) > 4; therefore, the molecular fraction in the ion-containing gas f(h 2 ) < 0.06.

24 Preliminary Conclusions There is an important component of neutral gas of low molecular fraction, f(h2)<0.1, where OH + and H2O + are abundant Observed OH+ /H suggests a cosmic-ray ionization rate of (0.6 to 2.4) s -1 Time-dependent effects may be important in controlling the low molecular fraction

25 Nearby interstellar matter (D<800 pc) OH 1.00 Normalized Flux OH Velocity (km/s) Visible/near-UV absorption spectra of HD (Destree, Snow, Black 2010, in preparation)

26 H3 + toward the Galactic Center Goto et al. (2002), Oka et al. (2005) abundant metastable (J,K) =(3,3) negligible (J,K)=(2,2) implies hot (250 K), dilute (<200 cm -3 ) gas?

27 H3 + energy levels (J,K) - (1,0), (3,3), (5,5), and (6,6) levels are highly metastable (no spontaneous emission) - (4,4) is quasi-metastable with a slow (A=3X10-9 s -1 ) transition to (3,1) at 218 GHz, a possible maser (Black 1998) - all levels can be re-shuffled by reactive collisions with H2 (cf. Oka & Epp 2004, ApJ, 613, 349) - the formation process may be important in populating the highly excited states - in some environments, radiative pumping through the vibrationrotation transitions may be important

28 H3O + toward Sgr B2: highly excited metastable states in absorption Herschel/HEXOS: Lis, Bergin, Schilke, et al., in prep.

29 H3O + population diagram

30 H3O + theoretical model spectra excitation by formation pumping implies an enhanced ionization rate (in this case maybe X-rays rather than cosmic rays)

31 H3O + population distribution with formation-pumping Tex=700 K Tkinetic=150 K

32 Example: UV-pumped CO in sunlight J.H. Black & J. L. Fox (in preparation)

33 GRB080607: H2 and CO at z= Prochaska et al. (2009, ApJ, 691, L27), Sheffer et al. (2009, ApJ, 701, L63) show that H2 is pumped by the UV afterglow Black (2009, in prep.) predicts the UV pumping in CO, which produces submm-wave emission, too CO J= THz (rest) red-shifted to GHz at z=3.0363

34 Afterglows of γ-ray Bursts at High Redshift GRB is the most distant known source in the Universe at z=8.26 UKIRT discovery images from Tanvir et al. astro-ph: mm/submm-wave afterglow? Castro-Tirado et al. report a λ=3 mm source at the burst position with flux density 0.2 mjy (GCN Circular 9273, ) consistent with Bock et al. upper limit of 0.7 mjy at CARMA Prospects for ALMA to probe high-z galaxies

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36 LAMDA - the Leiden Atomic and Molecular Database [ ] will be expanded RADEX - a non-lte radiative transfer code has been released in a public version (van der Tak et al. 2007, A&A, 468, 627) RADEX includes: radiative pumping in arbitrary radiation fields; source/sink terms in the rate equations; helpful scripts for visualizing results; enhanced source code due soon

37 Panchromatic perspective: coupling of mm/ submm transitions to infrared/visible/uv Formation and destruction processes: constrain abundances of symmetry species (e.g. ortho/para) and affect excitation in some cases Collisional cross section data often lacking

38 The usual rate equations for steady-state populations are dn i dt = j>i n j A ji k<i n i A ik + l i (n l C li n i C il ) = 0 But this ignores source and sink terms as well as coupling to external radiation and it yields relative populations only. We recommend dn i dt = j>i n j ( A ji + I ν B ji ) k<i [n i (A ik + I ν B ik ) n k I ν B ki ] + l i (n l C li n i C il ) = F i (T form ) n i D i The state-specific formation rate F i is a model of the formation process (e.g. a Boltzmann distribution at some formation temperature T form ). It permits different spin-modifications to be treated together even without reactive interchange processes. For properly chosen F i and D i, the solutions are number densities. No apologies for crude radiative transfer!

39 Radex can do really big non-lte calculations: e.g. CH3OH with 2742 levels, transitions Tkinetic=150 K 30 CH3OH (bigtest1) - non-lte Rotational Diagram 25 ln(n u / g u ) [cm-2] Upper state energy E u /k [K]

40 Interest in NH3 absorption measurements at high redshift - cosmology (constancy of me/mp, etc.) - properties of interstellar matter in distant galaxies NH3 has been measured in a galaxy at z= toward B by Henkel et al. (2005, A&A, 440, 893) a fully non-lte calculation of the NH3 excitation with Radex revises some results: - a small column density, N=1.75X10 13 cm -2 in the narrow component, (7X10 13 total) - a smaller density n(h2)=1000 cm -3, not questions about the ammonia thermometer

41 redshifted inversion lines (Henkel et al. 2005) computed spectra are sensitive to: density and formation temperature

42 some line intensities are sensitive to excitation by the background continuum, e.g. (3,0)-(2,0); (3,1)-(2,1); (3,2)-(2,2) near 1.76 THz

43 NH3 in a hot core n(h2)=10 5 cm -3, Tk=150 K, N(NH3)=5X10 16 cm -2

44 cf. Wilson, Henkel, Hüttemeister 2006, A&A, 460, 533 absorption in the (18,18) inversion line in Sgr B2(M) Note: there is no high kinetic temperature in the model, yet a high excitation temperature persists for the highly excited metastable states.

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47 Submm-wave absorption spectroscopy GHz with the Herschel/HIFI instrument

48 HF is widespread in the diffuse molecular gas

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50 HF velocity structure follows HCO + HF abundance: consistent with HF/H based on well-known CH - H2 correlation: CH/H2 =

51 Detection of widespread HF confirms: theoretical chemical model of Neufeld et al. F is unique in its ability to react directly with H 2 at low temperature in the interstellar gas F + H 2 HF + H HF comprises most of the gaseous fluorine HF can be used as a surrogate tracer of H 2 we can compare directly with CH and H 2 O distributions

52 Reactive molecular ions OH+ and H2O + should be among the most direct tracers of cosmic-ray ionization in the interstellar gas Absorption observations at 972 and 1115 GHz toward distant continuum sources reveal that both ions are widespread and abundant Herschel first results: Gerin et al. (2010, A&A, 518, L110; Neufeld et al. (2010, A&A, 521, L10)

53 Discovery of interstellar chloronium H2Cl + (Lis, Pearson, Neufeld, et al. 2010, A&A, 521, L9) 1974 Now

54 Conclusions: HCl/H2Cl + = 1 to 10 is consistent with models; however, observed column densities of H 2 Cl + > cm -2 are significantly larger than predictions?

55 What next?

56 HST/COS observations of Cyg OB2 Schulte 8a (Snow et al. 2010, ApJ, 720, L190)

57 Keck/HIRES and HST/COS observations of Cyg OB2 8a (Snow et al. 2010)

58 now 2013 ALMA completing the panchromatic survey In the future, there are a few stars with dense, ionized winds bright enough to be observed in high-resolution absorption spectra with the Atacama Large Millimetersubmillimeter Array (ALMA) - some of the same stars used as background sources in the UV, visible, and near-infrared.

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