peter young* george mason university, va uri feldman artep inc, md *work funded by nsf and nasa

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


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)


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

Li-like ions

Example of a ‘good’ case


He-like ions

An atomic data error clearly seen for N VI


• 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


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


=> 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


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


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


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.


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)


P X & S XI: 2p4 1S0 level

• Factor 10 difference in level population for 2p4 1S0 level – why?

• Error for P X oscillator strength


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

peter young* george mason university, va uri feldman artep inc, md *work funded by nsf and nasa

Documents

seendr peter young gmuhttp

xstardr peter young

ionsdr peter young gmuhttp

n vidr peter young gmuhttp

atomic data accuracy

benchmarking atomic

ionsan atomic data error

level of c