Assessing atomic data accuracy along isoelectronic sequences

Assessing atomic data accuracy along isoelectronic sequences Peter Young* George Mason University, VA Uri Feldman ArtepInc, 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) Team members Ken Dere Giulio Del Zanna Enrico Landi Helen Mason Peter Young http://chiantidatabase.org http://pyoung.org/talks/aas2014

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 http://pyoung.org/talks/aas2014

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 Σnj = 1) • Method is to plot the nj along an isoelectronic sequence • Set temperature to be Tmax of ion • Use density 1010 cm-3 njcomputed with CHIANTI IDL routines pop_solver.pro & show_pops.pro http://pyoung.org/talks/aas2014

Li-like ions Example of a ‘good’ case http://pyoung.org/talks/aas2014

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 http://pyoung.org/talks/aas2014

C-like ions: the “iron bump” • For 2p3 levels an apparent anomaly for iron is seen http://pyoung.org/talks/aas2014

High-Z ions vs. low-Z ions • The excited ground configuration levels of high-Z ions gain significant population only at high densities Consider 2s2p33P0 (index=10) level of C-like ions => High-Z ion models are much more sensitive to the accuracy of weak transitions Density=1010 cm-3 Weak 3P0 – 3P0 transition is dominant excitation channel http://pyoung.org/talks/aas2014

Fe XXI 2s 2p33P0 • Why is the population for Fe XXI 2s 2p33P0 enhanced? • Fe XXI has 621 levels; Mn XX has 20 levels The ground level excitation rate is much higher for Fe XXI http://pyoung.org/talks/aas2014

Collision strength comparison • Big difference in collision strengths for weak ground level excitation Iron bump caused by enhanced direct excitation rates cascading from high levels Mn XX: Zhang & Sampson (1996, ADNDT, 63, 275) – distorted wave Fe XXI: Badnell & Griffin (2001, J.Phys.B, 34, 681) – R-matrix http://pyoung.org/talks/aas2014

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) already in CHIANTI IDL> pop_processes, ‘o_6’, lev=3, /verbose http://pyoung.org/talks/aas2014

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) http://pyoung.org/talks/aas2014

C-like 2p4 levels • Minor elements show large differences (e.g., P X vs. S XI) http://pyoung.org/talks/aas2014

P X & S XI: 2p41S0 level • Factor 10 difference in level population for 2p41S0level – why? • Error for P X oscillator strength The Z&S data have been replaced for the abundant elements, but not the minor elements http://pyoung.org/talks/aas2014

Assessing atomic data accuracy along isoelectronic sequences