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Oscillator Strengths and Predissociation Widths for Rydberg Transitions in CO
between 930 and 935 Å
S.R. Federman, Y. Sheffer (Univ. of Toledo)M. Eidelsberg, J.L. Lemaire, F. Rostas (Obs. de Paris,
Meudon and Univ. de Cergy-Pontoisse)J.H. Fillion (Univ. UMPC, Paris VI)
This research was supported by NASA and the CNRS-PCMI program
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Introduction
Background
• CO observed in many astronomical environments
– Diffuse and dark, molecular interstellar clouds
– Circumstellar shells of asymptotic giant branch stars and planetary nebulae
– Circumstellar disks around newly formed stars
– Comets and planetary atmospheres
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Introduction
Processes affecting mix of isotopologues
• Isotope Charge Exchange – favors 13C16O
– It has lower zero-point energy
– 13C+ + 12C16O → 12C+ + 13C16O – ΔE (ΔE/k ≈ 35 K)
• Selective Isotopic Photodissociation – favors more abundant isotopic variant
– Dissociation occurs through line absorption at far UV wavelengths
– More abundant variant has lines that are more optically thick, shielding itself from further dissociation
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Introduction
Hubble Space Telescope results on diffuse interstellar clouds
IS Ratios: 12C/13C = 70±7; 16O/18O = 560±25; 16O/17O = 1900±200
Ratio X Per Oph A χ Oph Oph
N(12C16O)/N(13C16O) 73±12 125±23 117±35 167±15
N(12C16O)/N(12C18O) 3000±600 1000±500 … 1550±440
N(12C16O)/N(12C17O) 8700±3600 … … ≥ 5900
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Introduction
Problems
• Detailed models can reproduce either the isotopologic ratios or the total column density, but not both with the same model
• Models for diffuse molecular clouds produce too little CO
Solution?
• Part of the problem may lie in adopted oscillator strengths (f-values), which now seem too small for many important transitions
– Small f-values lessen amount of self shielding, but need more self shielding
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Our Previous Measurements on CO
Federman et al. (2001, ApJS, 134, 133)
• Used the Synchrotron Radiation Center of the Univ. of Wisconsin-Madison
• Derived f-values for the B – X (0-0), B – X (1-0), C – X (0-0), C – X (1-0), and E – X (0-0) bands (above 1075 Å)
– Our results agree with other determinations based on electron energy loss and laser absorption
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Our Previous Measurements on CO
Far Ultraviolet Spectroscopic Explorer Observations
HD 203374A (Sheffer et al. 2003, ApJ, 597, L29)
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Our Previous Measurements on CO
Eidelsberg et al. (2004, A&A, 424, 355)• Used the SU5 beam line
at the SuperACO Synchrotron in Orsay
• Derived f-values for the K – X (0-0), L′ – X (1-0), L – X (0-0) bands for 12C16O, 13C16O, and 13C18O (967 – 972 Å)– There is significant mixing among bands, but sum of
f-values independent of isotopologue– First measurements on 13C18O
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Our Previous Measurements on CO
Eidelsberg et al. (2006, ApJ, 647, 1543)• Published additional data acquired on SU5 beam line
– Focus on the W – X (v′-0; v′=0-3) bands as well as E – X (1-0) and B – X (6-0) bands [B – X (6-0) formerly called F – X (0-0) band]
– Studied f-values and predissociation widths for 12C16O, 13C16O, and 13C18O
– Analysis based on profile syntheses that adjusted the band oscillator strength and line width (instrumental, thermal, and predissociation) in a non-linear least-squares fashion
– Allowed for J-dependent predissociation widths
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Oscillator Strengths for CO
Comparison of Results for W – X Bands (f-value × 103)
Italics: measurements at 20 K
Reference (0-0) (1-0) (2-0) (3-0)
Eidelsberg et al. 2006 (12C16O) 16.6±1.6 16.0±1.3 30.0±2.3 19.7±1.4
Sheffer et al. 2003 (12C16O) … 15.8±2.0 23±5 19.8±2.4
Eidelsberg et al. 1991 (12C16O) 12.1±1.2 13.5±1.4 25.8±2.6 16.3±1.6
Stark et al. 1992, 1993, 1994 (12C16O) 12.9±1.3 14.8±1.5 30.0±3.0 14.9±1.5
Yoshino et al. 1995 (12C16O) 13.6±2.0 14.8±1.5 20.4±3.1 17.0±2.6
Eidelsberg et al. 2006 (13C16O) 15.1±0.7 16.1±2.8 30.4±1.3 18.7±1.4
Eidelsberg et al. 1991 (13C16O) 13.2±1.3 16.1±1.6 27.9±2.8 18.7±1.9
Eidelsberg et al. 2006 (13C18O) 13.8±2.0 perturbed 29.7±4.2 15.4±2.4
Eidelsberg et al. 1991 (13C18O) 13.2±1.3 16.0±1.6 27.9±2.8 18.6±1.9
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Measurements at the SOLEIL Synchrotron
• Used the DESIRS beamline with a VUV FTS
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Measurements at the SOLEIL Synchrotron
The FTS (de Oliveira et al. 2009, Rev. Sci. Instru., 80, 043101)
• Resolving power as high as 750,000
• Based on wave front division instead of amplitude division
• Relies on modified bimirror configuration requiring only flat mirrors
• Path difference scanning through translation of one reflector
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Measurements at the SOLEIL Synchrotron
Preliminary results
• Calibration band and line profile
– Used the B – X (0,0) and (1,0) bands for calibration [perturbations affect the E – X (0,0) band and W – X bands too strong]
– Used an Airy function for the line shape
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Measurements at the SOLEIL Synchrotron
Preliminary results
• Initial studies of series of bands between 930 and 935 Å in 12C16O, 13C16O, 13C18O, and 12C17O (first measurements on 12C17O)
• Significant mixing is present
– In 12C16O have interactions among 4pπ(2), II 1Π, 4pσ(2), 5pπ(0), 5pσ(0), and I 1Π
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Measurements at the SOLEIL Synchrotron
Preliminary results
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Measurements at the SOLEIL Synchrotron
Preliminary results
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Measurements at the SOLEIL Synchrotron
Preliminary results
§ integrated limitsa this value represents only the f-value of the R branch of 5pb this is sum of II1Π, 4pσ(2), and 5pπ(0)c this is sum of 5pσ(0) and I1Π
Band λ (Ǻ) f-values
(upper level)limits ( ×10-3 )
Present E91 S91 Y95
4p(2) 929.7-930.8 7.30 6.3 7.3(0.7) 6.1(0.9)
II1 930.922-932.230 4.59 930.75§- 930.75§-
4pσ(2) 931.160-932.140 3.13 21.6(2.2)
5p(0) 931.640-933.400 10.85
Σ = 18.57b 43.9 932.58§
5p(0) 932.617-933.900 17.86 8.4(0.8)a
933.05§-
I1 933.150-934.500 7.89 16.5(1.6)
Σ = 25.75c 934.5§ 934.5§
929.7-934.5 51.62 50.2 53.8(5.4)
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Future Work
• Complete analysis on these bands, including a set of predissociation widths
• With the improved spectral resolution, refine results for the W – X bands
• Perform measurements with a cooled free jet (≈ 30K)• A special focus on transitions in 12C17O• Study many of the bands between 885 and 972 Å,
especially those thought to be important for photodissociation in interstellar space