Ionization and Recombination with Electrons: Laboratory Measurements and Observational Consequences

1. Ionization and Recombination with Electrons: Laboratory Measurements and Observational Consequences Daniel Wolf Savin Columbia Astrophysics Laboratory

2. Collaborators Warit Mitthumsiri, Michael Schnell – Columbia University Mark Bannister – Oak Ridge National Lab (ORNL) Martin Laming, Enrico Landi – Naval Research Laboratory Andreas Wolf – Max Planck Institute for Nuclear Physics Alfred Müller, Stefan Schippers – University of Giessen

3. Outline • Motivation • Types of Cosmic Plasmas • Electron Impact Ionization (EII) • Dielectronic Recombination (DR) • Future Needs

4. Spectra observations can be used to infer properties of the cosmos. The aim of laboratory astrophysics is to reduce atomic physics uncertainties so that discrepancies between spectral observations and models tells us something about the properties of the observed sources and cannot be attributed to errors in the atomic data used in the models.

5. Ionization balance calculations are used to infer the properties of cosmic objects. For example, to infer relative abundances, we note that Rewriting this gives Clearly, accurate ionization and recombination data are needed for reliable ionization balance calculations to get reliable relative abundances.

6. Outline • Motivation • Types of Cosmic Plasmas • Electron Impact Ionization (EII) • Dielectronic Recombination (DR) • Future Needs

7. Collisionally-ionized (stars, galaxies,...) Ionization due to electrons. In equilibrium an ion forms at Te ~ Ip/2. High Te DR dominant recombination process. Photoionized (PNe, IGM, XRBs, AGN,…) Ionization due to photons and resulting electrons. In equilibrium an ion forms at Te ~ Ip/20. Low Te DR dominant recombination process. Cosmic plasmas can be divided into two broad classes:

8. Outline • Motivation • Types of Cosmic Plasmas • Electron Impact Ionization (EII) • Dielectronic Recombination (DR) • Future Needs

9. Electron impact ionization (EII) e- + O7+→ e- + e- + O8+ EII requires Ek > Eb.

10. Published recommended EII rate coefficients have yet to converge.

11. Errors in EII data translate directly into errors in predicted line ratios. In collisional ionization equilibrium (CIE) we have Rewriting gives Errors in either the ionization or recombination data will affect predicted or interpreted line ratios involving ions q and q+1.

12. We are carrying out a series of new EII measurements at ORNL. Ionization data can be collected for collision energies 3-2000 eV. (Bannister 1996, Phys. Rev. A 54, 1435)

13. We have carried out preliminary measure-ments for EII of Be-like C2+→ C3+ Ground-state (2s21S0) IP = 47.89 eV Metastable (2s2p 3P) IP = 41.39 eV Lifetime = 9.7 ms (J=1) ≥ 200s (J=0,2)

14. Initial C2+ EII measurements are discrepant with theory. Arrows indicate threshold for metastable and ground-state C2+. Metastable fraction inferred by comparing electron impact excitation data (using same ion source) to theory. Curve shows configuration-average distorted-wave theory for our mixed state ion beam.

15. Extracted ground state cross section is a factor of 2 smaller than published theory. Lotz formula used for energy dependence of EII cross sections σG and σM. Fit to lab data gives σG and σM (solid curves). Also shown are distorted wave theory (dashed curve, Younger, 1981) and the recommended data (dash-dot curve, Bell et al., 1983)

16. Outline • Motivation • Types of Cosmic Plasmas • Electron Impact Ionization (EII) • Dielectronic Recombination (DR) • Future Needs

17. Dielectronic Recombination (DR) e- + Fe23+ ↔ (Fe22+)** → (Fe22+)* + hn Energy conservation requiresΔE = Ek + Eb. Both ΔEand Ebquantized Ekquantized. Low temperature DR occurs forEk<< ΔE. High temperature DR occurs forEk~ ΔE.

18. DR theory for L- and M-shell ions are theore-tically and computationally challenging. Until recently modelers have had few modern calculations to use. Comparisons show these data to have factor of 2 or more uncertainties. (Savin et al. 2002, ApJ, 576, 1098)

19. In photoionized gas DR uncertainties affect predicted temperature and gas stability. Using XSTAR and varying the low Te DR data for Fe17+ to Fe23+ by a factor of 2. Phase diagram Temperature Line emission seen from ions predicted to form in region of thermal instability. (Savin et al. 1999, ApJS, 123, 687)

20. In electron-ionized plasmas DR errors affect predicted relative abundances. Using older DR data inferred relative abundances in the solar corona can be a factor of 5 smaller or 1.6 times larger. (Savin & Laming 2002, ApJ, 566, 1166)

21. We are carrying out a series of DR measure-ments using the Test Storage Ring (TSR).

22. Schematic of the electron cooler

23. Measurements can be carried out for low and high temperature DR. DR of O-like Fe XIX forming F-like Fe XVIII (Savin et al. 1999, ApJS, 123, 687; 2001, ApJ, 576, 1098)

24. We can use these data to produce Maxwellian rate coefficients for plasma modeling. Pre-experiment Post-experiment Measurements are used to benchmark modern DR theory which is then used to calculate DR for other ions in the tested isoelectronic sequence.

25. Even with benchmarking modern DR theory has still not converged for all L-shell ions

26. Current AGN spectral models over-predict the ionization stages of M-shell iron ions. Models that match spectral features from abundant 2nd row elements, over-predict the average Fe charge stage. This is believed to be due to the absence of low Te DR data for M-shell Fe (Kraemer et al. 2004; Netzer 2004). (Netzer et al. 2003, ApJ, 599, 933)

27. Published laboratory work supports that poor Fe M-shell DR data is the cause. DR of Fe XVI forming Fe XV Published DR data were for tokamaks, stars, etc. and did not attempt to treat properly the low energy DR resonances. This is an example of how better communica-tion between atomic physics and astro-physics could have predicted this problem (Müller 1999, Int. J. Mass Spectrom. 192, 9)

28. We are carrying out further M-shell Fe DR measurements to address this issue. DR of Fe XV forming Fe XIV

29. Conclusions • Significant errors exist in EII data base. • Much experimental and theoretical EII work needs to be done. • L-shell DR data has improved recently but room remains for theoretical improvement. • More L-shell benchmark DR measurements needed. • Lots of experimental and theoretical work is needed to improve the M-shell DR data. • More accurate structure calculations are needed for low-lying autoionization levels.

30. We have added a beam attenuation cell to determine directly the metastable fraction. If the electron capture cross section for metastable and ground state ions differ significantly, then one state will be lost first as the target gas density increases. Plot of the log of ion current vs. target gas density is bi-linear (slopes proportional to capture cross sections) and can be used to infer relative populations of ground-state and metastable ions. (Zuo et al. 1995, ApJ, 440, 421)

31. These DR uncertaintes also affect predicted line emission. XSTAR spectra for gas at log(ζ)=2.1 erg cm s-1 (Savin et al. 2000, AIP CP547, 267)

32. In CIE, errors in DR data translate directly into errors in predicted line ratios. In CIE we have Rewriting gives Errors in either the ionization or recombination data will affect predicted or interpreted line ratios involving ions q and q+1.

33. Theory has also has a problem with high charge states for L-shell ions.

34. Recent AGN Observations have indicated the importance of Fe M-shell DR. A new AGN spectral feature at λ≈ 16-17 Å has been identified as being due to absorp-tion in M-shell iron ions. (Sako et al. 2001, A&A, 365, L168)