Ionization and Recombination with Electrons: Laboratory Measurements and Observational

Consequences

Daniel Wolf Savin

Columbia Astrophysics Laboratory

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

Outline

I. Motivation

II. Types of Cosmic Plasmas

III. Electron Impact Ionization (EII)

IV. Dielectronic Recombination (DR)

V. Future Needs

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.

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.

Ionization balance calculations are used to infer the properties of cosmic objects.

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Outline

I. Motivation

II. Types of Cosmic Plasmas

III. Electron Impact Ionization (EII)

IV. Dielectronic Recombination (DR)

V. Future Needs

Cosmic plasmas can be divided into two broad classes:

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.

Outline

I. Motivation

II. Types of Cosmic Plasmas

III. Electron Impact Ionization (EII)

IV. Dielectronic Recombination (DR)

V. Future Needs

Electron impact ionization (EII)

e- + O7+ → e- + e- + O8+

EII requires Ek > Eb.

Published recommended EII rate coefficients have yet to converge.

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.

Errors in EII data translate directly into errors in predicted line ratios.

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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)

We have carried out preliminary measure-ments for EII of Be-like C2+ → C3+

Ground-state (2s2 1S0)IP = 47.89 eV

Metastable (2s2p 3P)IP = 41.39 eVLifetime = 9.7 ms (J=1)

≥ 200s (J=0,2)

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.

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)

Outline

I. Motivation

II. Types of Cosmic Plasmas

III. Electron Impact Ionization (EII)

IV. Dielectronic Recombination (DR)

V. Future Needs

Energy conservation requires ΔE = Ek + Eb.

Both ΔE and Eb quantized Ek quantized.

Low temperature DR occurs for Ek << ΔE.

High temperature DR occurs for Ek ~ ΔE.

Dielectronic Recombination (DR)

e- + Fe23+ ↔ (Fe22+)** → (Fe22+)* + h

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)

In photoionized gas DR uncertainties affect predicted temperature and gas stability.

(Savin et al. 1999, ApJS, 123, 687)

Using XSTAR andvarying the low Te DR data for Fe17+ to Fe23+ by a factor of 2.

Line emission seen from ions predicted to form in region of thermal instability.

Temperature

Phasediagram

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)

Line RatioVariation

Minimum Maximum

Mg VI/Ne VI 0.60 1.11

Mg VII/Ne VII 0.67 1.22

Mg IX/S IX 0.33 1.29

Mg IX/S X 0.51 1.64

Si IX/S IX 0.20 1.01

Si IX/S X 0.36 1.14

Si X/S X 0.43 1.60

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

Schematic of the electron cooler

Measurements can be carried out for low and high temperature DR.

(Savin et al. 1999, ApJS, 123, 687; 2001, ApJ, 576, 1098)

DR of O-like Fe XIX forming F-like Fe XVIII

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.

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

Current AGN spectral models over-predict the ionization stages of M-shell iron ions.

(Netzer et al. 2003, ApJ, 599, 933)

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 (Kraemeret al. 2004; Netzer 2004).

Published laboratory work supports that poor Fe M-shell DR data is the cause.

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)

DR of Fe XVI forming Fe XV

We are carrying out further M-shell Fe DR measurements to address this issue.

DR of Fe XV forming Fe XIV

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.

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)

These DR uncertaintes also affect predicted line emission.

(Savin et al. 2000, AIP CP547, 267)

XSTAR spectra for gas at log(ζ)=2.1 erg cm s-1

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.

In CIE, errors in DR data translate directly into errors in predicted line ratios.

.11 qqeqqe nnCnn

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1

qq

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

Recent AGN Observations have indicated the importance of Fe M-shell DR.

(Sako et al. 2001, A&A, 365, L168)

A new AGN spectral feature at λ ≈ 16-17 Å has been identified as being due to absorp-tion in M-shell iron ions.