oscillator strengths and predissociation widths for rydberg transitions in co between 930 and 935 Å...

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

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

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

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

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

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

Our Previous Measurements on CO

Far Ultraviolet Spectroscopic Explorer Observations

HD 203374A (Sheffer et al. 2003, ApJ, 597, L29)

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

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

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

Measurements at the SOLEIL Synchrotron

• Used the DESIRS beamline with a VUV FTS

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

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

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

Measurements at the SOLEIL Synchrotron

Preliminary results

Measurements at the SOLEIL Synchrotron

Preliminary results

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)  

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