Oscillator Strengths for Rydberg Transitions in CO between 925 and 956 Å S.R. Federman, Y. Sheffer (Univ. of Toledo) M. Eidelsberg, J.L. Lemaire, J.H. Fillion, F. Rostas (Obs. de Paris, Meudon) J. Ruiz (Univ. de Málaga) 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 isotopomers Isotope Charge Exchange – favors 13 C 16 O –It has lower zero-point energy – 13 C C 16 O → 12 C C 16 O – 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: 12 C/ 13 C = 70±7; 16 O/ 18 O = 560±25; 16 O/ 17 O = 1900±200 RatioX Per  Oph A χ Oph  Oph N( 12 C 16 O)/N( 13 C 16 O)73±12125±23117±35167±15 N( 12 C 16 O)/N( 12 C 18 O)3000± ±500…1550±440 N( 12 C 16 O)/N( 12 C 17 O)8700±3600……≥ 5900

Introduction Problems Detailed models [e.g., van Dishoeck & Black (1988)] can reproduce either the isotopomeric ratios or the total column density, but not both with the same model Models for diffuse molecular clouds produce too little CO (increasing the rate constant for C + + OH – Dubernet et al – lessens problem)

Introduction Problems Detailed models [e.g., van Dishoeck & Black (1988)] can reproduce either the isotopomeric ratios or the total column density, but not both with the same model Models for diffuse molecular clouds produce too little CO (increasing the rate constant for C + + OH – Dubernet et al – lessens problem) Solution? Suggest that part of the problem lies 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 recent determinations based on electron energy loss and laser absorption –But these f-values tend to be larger than those used in chemical models, with the differences increasing with increasing band strength

Our Previous Measurements on CO Far Ultraviolet Spectroscopic Explorer Observations HD A (Sheffer et al. 2003, ApJ, 597, L29)

Our Previous Measurements on CO Far Ultraviolet Spectroscopic Explorer Observations HD A (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 12 C 16 O, 13 C 16 O, and 13 C 18 O (967 – 972 Å) –There is significant mixing among bands, but sum of f-values independent of isotopomer –First measurements on 13 C 18 O –Our 12 C 16 O results consistent with larger values from the most recent laboratory and astronomical studies

Newest Results on CO 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) formally called F – X (0-0) band] –Studied 12 C 16 O, 13 C 16 O, and 13 C 18 O at pressures usually ranging from 2 to 14 mTorr; pressures up to 60 mTorr used for weak E – X (1-0) band of 13 C 18 O –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 –The E – X (0-0) band, whose f-value is well determined, used to determine CO column density

Oscillator Strengths for CO Comparison of Results for W – X Bands (f-value × 10 3 ) Italics: measurements at 20 K Reference(0-0)(1-0)(2-0)(3-0) Present Results ( 12 C 16 O)16.6± ± ± ±1.4 Sheffer et al ( 12 C 16 O)…15.8±2.023±519.8±2.4 Eidelsberg et al ( 12 C 16 O)12.1± ± ± ±1.6 Stark et al. 1992, 1993, 1994 ( 12 C 16 O)12.9± ± ± ±1.5 Yoshino et al ( 12 C 16 O)13.6± ± ± ±2.6 Present Results ( 13 C 16 O)15.1± ± ± ±1.4 Eidelsberg et al ( 13 C 16 O)13.2± ± ± ±1.9 Present Results ( 13 C 18 O)13.8±2.0perturbed29.7± ±2.4 Eidelsberg et al ( 13 C 18 O)13.2± ± ± ±1.9

Conclusions Observe fractionation among isotopomers in diffuse clouds –Selective Isotopic Photodissociation dominant process –Available models not adequate –Need to include new reaction rates and f-values Our f-values indicate larger values – consistent with other recent measurements – than what is used in models for many important Rydberg transitions –Enhances self shielding from photodissociation Future work –Finalize J-dependent predissociation widths –New measurements with SOLEIL, the next generation synchrotron source in France –Analyze interstellar data on CO and H 2 to constrain models further