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Assessing atomic data accuracy along isoelectronic sequences
Peter Young*George Mason University, VA
Uri FeldmanArtep Inc, MD
*Work funded by NSF and NASA
The CHIANTI atomic database
• Atomic data and software package for modeling emission lines– ions and neutrals for elements up to zinc– software written in IDL and Python
• First released in 1996 (Dere et al. 1997, A&AS)• Latest version: 7.1 (Landi et al. 2013, ApJ)• Citations: 1943
• Large effort gone into benchmarking atomic data against observations
• CHIANTI atomic data are used in other codes (CLOUDY, APED, MOCASSIN, XSTAR)
Dr Peter Young (GMU) http://pyoung.org/talks/aas2014 2
http://chiantidatabase.org
Team membersKen DereGiulio Del ZannaEnrico LandiHelen MasonPeter Young
The CHIANTI atomic database
• CHIANTI contains atomic data for solving level balance equations for atoms and ions
• Principal data-sets are:– electron excitation rates– radiative decay rates– experimental energy levels
Dr Peter Young (GMU) http://pyoung.org/talks/aas2014 3
Level populations
• Level balance equations are
• αji are atomic rate coefficients (radiative decay rates, electron excitation rates, etc.)
• nj are the level populations (normalized so that Σ nj = 1)
Dr Peter Young (GMU) http://pyoung.org/talks/aas2014 4
nj computed with CHIANTI IDL routines pop_solver.pro & show_pops.pro
Method is to plot the nj along an isoelectronic sequence- Set temperature to be Tmax of ion- Use density 1010 cm-3
He-like ions
An atomic data error clearly seen for N VI
Dr Peter Young (GMU) http://pyoung.org/talks/aas2014 6
• 2-photon transition had been incorrectly assigned• Fixed with recent
v7.1.4 release
C-like ions: the “iron bump”
• For 2p3 levels an apparent anomaly for iron is seen
Dr Peter Young (GMU) http://pyoung.org/talks/aas2014 7
High-Z ions vs. low-Z ions
• The excited ground configuration levels of high-Z ions gain significant population only at high densities
Consider 2s2p3 3P0 (index=10) level of C-like ions
Dr Peter Young (GMU) http://pyoung.org/talks/aas2014 8
=> High-Z ion models are much more sensitive to the accuracy of weak transitions
Si IX (log T = 6.0) Fe XXI (log T = 7.1)
Level ni qi,10 niqi,10 ni qi,10 niqi,10
3P0 0.10 6.9(-12) 6.9(-13) 1.0 6.9(-13) 6.9(-13)3P1 0.31 5.9(-10) 1.8(-10) 9.2(-4) 8.0(-11) 7.4(-14)
Weak 3P0 – 3P0 transition is dominant excitation channel Density=1010 cm-3
Fe XXI 2s 2p3 3P0
• Why is the population for Fe XXI 2s 2p3 3P0 enhanced?
• Fe XXI has 621 levels; Mn XX has 20 levels
Dr Peter Young (GMU) http://pyoung.org/talks/aas2014 9
Mn XX Fe XXIContrib. Level Process Contrib. Level Process56.8% 1 e exc. 49.2% 1 e exc.42.8% 2 e exc. 26.0% 25 cascade
4.9% 2 e exc.3.9% 26 cascade
The ground level excitation rate is much higher for Fe XXI
Collision strength comparison
• Big difference in collision strengths for weak ground level excitation
Dr Peter Young (GMU) http://pyoung.org/talks/aas2014 10
Mn XX: Zhang & Sampson (1996, ADNDT, 63, 275) – distorted waveFe XXI: Badnell & Griffin (2001, J.Phys.B, 34, 681) – R-matrix
Iron bump caused by1. enhanced direct excitation
rates2. cascading from high levels
Implementation
With CHIANTI 8 (coming soon!) we will distribute an IDL routine to plot isoelectronic level populations
For investigating population processes of a specific level, use
Dr Peter Young (GMU) http://pyoung.org/talks/aas2014 11
IDL> ch_plot_iso_pops, ‘o_6’, 3
(plots isoelectronic populations for the level identified as no. 3 for O VI -> 1s2 2p 2P3/2)
IDL> pop_processes, ‘o_6’, lev=3, /verbose
Into level Out of level
Radiative decay 0.5 % 100.0 %
Electron excitation 99.5 % 0.0 %
already in CHIANTI
Summary
• Studying level populations along isoelectronic sequences is a good way of assessing atomic data accuracy.
• The method focuses attention on those atomic data that most influence the populations (and hence emissivities) of the ions.
• For Z ≥ 20 large atomic models are critical for modeling weak ground-level excitations.
• Software will be distributed with CHIANTI 8.
Dr Peter Young (GMU) http://pyoung.org/talks/aas2014 12
See online talk for He- to Na-like sequences(http://pyoung.org/talks/aas2014)
C-like 2p4 levels
• Minor elements show large differences (e.g., P X vs. S XI)
Dr Peter Young (GMU) http://pyoung.org/talks/aas2014 13
P X & S XI: 2p4 1S0 level
• Factor 10 difference in level population for 2p4 1S0 level – why?
• Error for P X oscillator strength
Dr Peter Young (GMU) http://pyoung.org/talks/aas2014 14
Transition Zhang & Sampson (1996) CHIANTI
2s 2p3 1P1 - 2p4 1S0 0.388 3.49
The Z&S data have been replaced for the abundant elements, but not the minor elements