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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.
II.
III.
IV.
V.
Motivation
Types of Cosmic Plasmas
Electron Impact Ionization (EII)
Dielectronic Recombination (DR)
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.
Ionization balance calculations are used to
infer the properties of cosmic objects.
For example, to infer relative abundances, we
note that
n
q nA
Iline  nA nH line.
Rewriting this gives
nA  Iline .
nH line nq
nA
Clearly, accurate ionization and recombination
data are needed for reliable ionization balance
calculations to get reliable relative abundances.
Outline
I.
II.
III.
IV.
V.
Motivation
Types of Cosmic Plasmas
Electron Impact Ionization (EII)
Dielectronic Recombination (DR)
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.
II.
III.
IV.
V.
Motivation
Types of Cosmic Plasmas
Electron Impact Ionization (EII)
Dielectronic Recombination (DR)
Future Needs
Electron impact ionization (EII)
e- + O7+ → e- + e- + O8+
EII requires Ek > Eb.
Published recommended EII rate coefficients
have yet to converge.
Errors in EII data translate directly into errors
in predicted line ratios.
In collisional ionization equilibrium (CIE) we have
ne nqCq  ne nq 1 q 1.
Rewriting gives
nq1
nq

Cq
 q1
.
Errors in either the ionization or recombination
data will affect predicted or interpreted line ratios
involving ions q and q+1.
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 measurements for EII of Be-like C2+ → C3+
Ground-state (2s2 1S0)
IP = 47.89 eV
Metastable (2s2p 3P)
IP = 41.39 eV
Lifetime = 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 configurationaverage 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.
II.
III.
IV.
V.
Motivation
Types of Cosmic Plasmas
Electron Impact Ionization (EII)
Dielectronic Recombination (DR)
Future Needs
Dielectronic Recombination (DR)
e- + Fe23+ ↔ (Fe22+)** → (Fe22+)* + hn
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.
DR theory for L- and M-shell ions are theoretically 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.
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)
In electron-ionized plasmas DR errors affect
predicted relative abundances.
Variation
Line Ratio
Using older DR data
inferred relative
abundances in the
solar corona can be a
factor of 5 smaller or
1.6 times larger.
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
(Savin & Laming 2002, ApJ, 566, 1166)
We are carrying out a series of DR measurements using the Test Storage Ring (TSR).
Schematic of the electron cooler
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)
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.
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 Mshell Fe (Kraemer
et al. 2004; Netzer 2004).
(Netzer et al. 2003, ApJ, 599, 933)
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 communication between atomic
physics and astrophysics could have
predicted this problem
DR of Fe XVI forming Fe XV
(Müller 1999, Int. J. Mass Spectrom. 192, 9)
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 bilinear (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.
XSTAR spectra for gas at log(ζ)=2.1 erg cm s-1
(Savin et al. 2000, AIP CP547, 267)
In CIE, errors in DR data translate directly into
errors in predicted line ratios.
In CIE we have
ne nqCq  ne nq 1 q 1.
Rewriting gives
nq1
nq

Cq
 q1
.
Errors in either the ionization or recombination
data will affect predicted or interpreted line ratios
involving ions q and q+1.
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.
A new AGN spectral
feature at λ ≈ 16-17 Å
has been identified as
being due to absorption in M-shell iron
ions.
(Sako et al. 2001, A&A, 365, L168)