Stellar and laboratory XUV/EUV line ratios in Fe XVIII and Fe XIX
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Lawrence Livermore National Laboratory, Livermore CA, USA # also at AIRUB, Bochum, Germany
Chandra spacecraft observes CapellaComparison of observed XUV (13-18 Å), EUV (90-120 Å), VUV (1100 Å) line intensities with predictions by the APEC model XUV appears brighter than expected - APEC model incorrect or - Capella peculiar? Interstellar absorption involved
Laboratory study under way: Electron beam ion trap, two flat-field spectrometers Detection efficiency calibration (experimental data) Modeling of the excitation process (using the FAC Flexible Atomic Code by M. F. Gu) ! ! ! ! (monoenergetic electron beam vs. Maxwellian electron energy distribution)Density and temperature effects
Possible insights
APiP 21 July 2011
Elmar Träbert #, Peter Beiersdorfer, Joel H. T. Clementson
Stellar and Laboratory XUV/EUV Line Ratios in Fe XVIII and Fe XIX
NASA project funding
see Poster
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L62 DESAI ET AL. Vol. 625
Fig. 1. The observed-to-predicted ux ratios of strong lines in the X-ray,EUV, and FUV spectral regions. Shown for comparison are the ratios obtainedusing the APEC, CHIANTI, and SPEX spectral codes and the FAC rates. Thedensity is Ne 1010 cm 3, except for SPEX. Top: Comparison for Fe xviiilines, normalized to
93.92. The X-ray lines plotted here are
14.208,
15.625,
and
16.071. Bottom: Comparison for Fe xix lines, normalized to
108.37.The X-ray lines plotted are 13.518, 14.664, and 15.079.
Fig. 2. The observed-to-predicted ux ratios of X-ray lines using FACand APEC. Lines from Table 1 excluding heavily blended Fe xviii
16.004
are shown. Note the 3d 2p lines are between 14 and 15 for Fe xviii and Ashortward of 14 for Fe xix. Ratios are calculated at Ne 1010 cm 3. Dash- Adotted lines represent agreement within a factor of 2. Top: Comparisonfor Fe xviii, normalized to
14.208. There are no published FAC models for
Fe xviii 4d 2p lines around 11.4 . Bottom: Comparison for Fe xix, nor- Amalized to
13.518.
Observed flux (Capella) / predicted flux as presented by Desai et al., ApJ 625, L59 (2005)Watch out for log scales!
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Transmission for Capella
50 100 150 200 250 300 350 400Wavelength
0.2
0.4
0.6
0.8
Interstellar extinction: 8% loss of EUV signal vs. XUV signal
(A)o
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Fe XIX O-like Fe XVIII F-like
VUVEUV
XUV
VUV
EUV
XUV
M1M1 M1
E2
2s 2p
2s 2p
2s 2p nl
2
2
5
6
4
2s 2p
2s 2p
2s 2p nl
2
2
4
5
3
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The Livermore electron beam ion trap is the archetypical EBIT
E TIAL B
20 Years of
LIVERMOREsince 1986
Spectroscopy
Electron collector
Several layers of cooling and cryogenic shields at the temperatures of liquid He and liquid N2
Superconducting magnets (pair of Helmholtz coils, B = 3 T)
Drift tubes at electrostatic potentials trap ions axially; with openings for optical access
Electron gun
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0
500
1000
1500
2000
2500
0 5 10 15 20 25
Ionization potential of Fe ions
Charge state q+
Electron beam
Electron energy
Electron beam energy relative to IP determines the highest charge state present.
IP (e
V)
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G. V. Brown et al., Astrophys. J. Suppl. Ser. 140, 588 (2002)(Fe XVIII - Fe XXIV in an EBIT)
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What to expect in a spectrum?
Atomic structure Element abundance Collisional excitation f(T, n)Radiative de-excitation Photoionization and -excitation --> Ionization balance f(T), spectral intensity distribution, "emissivity"
Data basesKelly & Palumbo, CHIANTI, NIST ASD, Mewe/Kaastra/Liedahl, ...
tend to be grossly incomplete, not up to date, sometimes faulty - but eventually improving
ModelingHULLAC ! Hebrew University Lawrence Livermore Atomic CodeAPEC ! Astrophysical Plasma Emission Code FAC ! ! Flexible Atomic Code (M. F. Gu)produce thousands of levels and tens of thousands of transitions
... need benchmarking (testing some testable parameters such as key level energies and some transition rates)
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86 KOTOCHIGO
13.4 13.5 13.6 13.7 13.8Wavelength (Angstroms)
0
100
200
300
400
500
600
700
Cou
nts
Fe XIX (HULLAC98)Capella
Fe XIX (Lab)
Fe XIX (Kotochigova)
Figure 1. Chandra spectrum of Capella (black line) in the spectral regionbetween 13.4 and 13.8 ¯ (Desai et al. 2005) shown in comparison with threespectral models. The three models for Fe xix (in magenta) use data from theAPEC code v1.3 (Smith et al. 2001) with only the wavelengths changed. Ne ix(dark blue) and other Fe L-shell (light blue) lines in the region are shadedfor the observed spectrum. Upper panel: model using the Fe xix wavelengthsfromHULLAC (D. Liedahl 1997, private communication).Middle panel: Fe xixwavelengths include the experimentallymeasured values reported inBrown et al.(2002). Lower panel: Fe xix wavelengths are from this work and Kotochigovaet al. (2007) using the MDFS method. Adapted from Brickhouse (2007).
S. Kotochigova et al., The Astrophysical Journal Supplement Series, 186:85—93, 2010
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13 14 15 16 17 18
Capella vs. APECT = 6 MK
Wavelength (A)
Flux
sig
nal
o
XUV flux seen is higher than the model prediction (tied to EUV)
Interpretation A: XUV/EUV excess
Interpretation B: XUV underprediction by APEC (and FAC etc.)
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H 7 components
Ly
Principal quantum number n
1
2
3
α
s p d
Calculate line ratioall n=2-3 vs n=1-3
O VIII 102 Å vs 16.0 Å
EUV
XUV
ββ
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12 14 16 18 20 22
Coun
ts
O VII
O VIII
0
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80 90 100 110 120 130 140
Coun
ts
Wavelength (Å)
O VIII
Spectra of CO2 (mostly oxygen lines in the regions shown) dispersed with a 1200 l/mm grating in a R=5.6 m flat-field grating spectrometer and recorded with a CCD camera at an EBIT
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0
50
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80 90 100 110 120 130 140
SFFS CO2
Coun
ts
Wavelength (Å)
O VIII
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0
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12 14 16 18 20 22 24
Coun
ts
O VII
O VII
O VIIO VIII
O VIII
0
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500
13 14 15 16 17 18
Coun
ts
Wavelength (A)
XVIII XVII
O
OO
O
XVII + XIX
XIX
XIXXIXXIX
XIX
XVII
XVIII
XVIII
o
a
b
EBIT reference spectrum of oxygen
EBIT spectrum with Fe (CO)5 injection
Spectra dispersed with an R=44.3 m 2400 l/mm flat-field grating and recorded with an MCP-based detector.
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SFFS Fe I2
Cou
nts
Wavelength (Å)
Fe XVIII
Fe XIXFe XIX
Fe XIX
Fe XIX
Fe XIX
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500
400
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01716151413
Cou
nts
Wavelength (A)o
Fe Electron beam energy 2 keV
Plenty of lines in EBIT - mostly Fe, some O - what about stellar spectra?
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0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
90 95 100 105 110 115 120 125
Capella vs APEC & EBIT
Cap
ella
exc
ess
ratio
Density dependence
APEC
Wavelength (A)o
Experiment with error barsFe XIX Fe XVIII
APEC open circles
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0.1
1
10
12 13 14 15 16 17 18 19 20
APEC
& E
BIT
vs. C
apel
laR
atio
Wavelength (Å)
APEC vs. Capella
EBIT Fe XIXEBIT Fe XVIIIEBIT experiment vastly exceeds
Capella flux - something is wrong in this analysis! --> EBIT has an electron beam, Capella has a thermal plasma; need to simulate this difference.
APEC falls short of Capella flux - may be a modeling problem
XUV EBIT and XUV APEC compared to Capella
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0
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0 5 10 15 20 25
Ionization potential of Fe ions
Electron beam
Electron energy
Maxwellian
Number of electrons
XUV
EUVVUV
IP (
eV)
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1
1,5
2
2,5
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3,5
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13 14 15 16 17 18
Capella XUV excess at 6MK
Cap
ella
exc
ess
ratio
Wavelength (A)
Agreement much improved by using Maxwellian model, but the data slope points to a systematic problem.
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13 14 15 16 17 18
Fe XVIIIFe XIX
T = 7 MK
Flux
sig
nal
Wavelength (A)o
Assuming a higher temperature (7 MK instead of 6 MK) improves the agreement with models.
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Conclusions (preliminary):
Relative calibration XUV / EUV achieved relatively simply Calibration within each range good to ± 10% (maybe) (Chandra LETGS / HETGS are better known)
Transfer electron beam / Maxwellian via FAC code (M. F. Gu) seems reasonable; details are still being worked on
Interpretation of Chandra spectra not fully achieved; the spectrum is possibly richer than previously assumed; modeling approach of varying the experimental wavelengths seems dubious; alternative: additional blending lines from whatever elements
XUV / EUV excess seems different for Fe XIX and Fe XVIII
Possible interpretation: underlying temperature 6 MK may be too low
Moreover, the APEC (HULLAC, FAC) model may well be incomplete and insufficient
! ! ! ! ! ... much more work needs to be done If you have questions or suggestions see the poster!