1. 2007 June 19, 10:49 AM62 nd ISMS, OSU, Columbus, OHY Sheffer 1 Astronomical Detections of VUV Transitions of CH Y. Ronny Sheffer & Steve R. Federman Dept. Physics & Astronomy U. Toledo (OH) 62 nd ISMS, OSU, Columbus, OH June 19, 2007

2. 2007 June 19, 10:49 AM62 nd ISMS, OSU, Columbus, OHY Sheffer 2 Introduction Important in diffuse ISM chemistry Among very first astronomical detections ~1940 together with CH + and CN. Herzberg & Johns (1969) mapped a few Rydberg bands in the VUV, but can they be seen in astronomical spectra?

3. 2007 June 19, 10:49 AM62 nd ISMS, OSU, Columbus, OHY Sheffer 3 Internal Structure Strongest optical line @4300.313 Å is a doublet A 2 Δ–X 2 Π transition, which is normally used for N(CH) measurements. Two equal-population Λ components are 1.4 km/s apart (blended). Higher states B 2 Σ – and C 2 Σ + produce 0–0 bands at 3889 and 3140 Å, respectively. All non-A bands have unblended transitions from the Λ components of the ground state. Even higher states “E” 2 Π, F 2 Σ +, G( 2 Σ +, 2 Π, 2 Δ)=3d, and 4d found by Herzberg & Johns produce bands near 1557, 1549, 1370, and 1270 Å.

5. 2007 June 19, 10:49 AM62 nd ISMS, OSU, Columbus, OHY Sheffer 5 The Mystery Cardelli et al. (1991) listed 3 unidentified VUV features toward ζ Oph: UID1, UID2, and UID3 @ 1368.74, 1369.13, and 1370.87 Å. UID2 was noted to have a wider appearance, and was suggested to be a VUV analog of the optical DIBs. The strength (W λ ) of UID2 was shown to have better correlation with CO than with atomic species. Federman et al. (1995) presented another UID @ 1271 Å.

6. 2007 June 19, 10:49 AM62 nd ISMS, OSU, Columbus, OHY Sheffer 6 Mystery Solved (1) Watson (2001) re-analyzed original photographs from Herzberg & Johns to infer the characteristics of the 3d state ( 2 Σ +, 2 Π, 2 Δ components), which is responsible for the VUV G–X band near 1370 Å, and suggested that UID1, UID2, and UID3 are G–X lines, and that λ1271 is a 4d–X transition. Explained that the width of UID2 can be the result of strong predissocoation of the 3d Rydberg state. Estimated oscillator strength (f-value) for this band from the observed total N(CH) toward ζ Oph.

7. 2007 June 19, 10:49 AM62 nd ISMS, OSU, Columbus, OHY Sheffer 7 Mystery Solved (2) We assembled a sample of 12 sight lines from HST spectra, all with known N 4300 (CH) values and produced spectrum syntheses based on known cloud component structures from λ4300. Derived UID f-values and predissociation (Γ) widths for all sight lines, and found them to be in good agreement under the assumption that CH was the parent molecule. Confirmed Watson’s solution, albeit f-values had to be increased by a factor of 2 relative to his estimates.

10. 2007 June 19, 10:49 AM62 nd ISMS, OSU, Columbus, OHY Sheffer 10 From Mystery to Mastery… Now that CH is known to show VUV band(s), what other CH bands might be lurking in outer space? We searched for more bands from the lab detections of Herzberg & Johns… Voila #1! We found new absorption lines near 1549 Ǻ that belong to the F 2 Σ + –X band of CH. Aside: F–X lines are seen between C IV λλ 1548, 1550 …

12. 2007 June 19, 10:49 AM62 nd ISMS, OSU, Columbus, OHY Sheffer 12 From Mystery to Mastery… Voila #2! We also found new absorption lines near 1694 Å that belong to the D 2 Π–X band of CH. None were found from a so-called E–X band (1557 Å).

15. 2007 June 19, 10:49 AM62 nd ISMS, OSU, Columbus, OHY Sheffer 15 Results: f-values Based on known N/2(CH) from A–X, we derived new f-values for D–X, F–X, 3d–X, and 4d–X. All values agree with previous ranges of theoretical predictions for CH, showing that this molecule has been correctly identified as the parent. 3d–X is found to have the largest f-value among all bands of CH.

17. 2007 June 19, 10:49 AM62 nd ISMS, OSU, Columbus, OHY Sheffer 17 Results: widths For cases with strong predissociation we also derived lifetimes (from Γ). All wider features are nicely fit by single transitions with Lorentzian wings, showing no need for an association with optical DIBs. A lot more theoretical and lab work is needed for additional comparison with measured values.

18. 2007 June 19, 10:49 AM62 nd ISMS, OSU, Columbus, OHY Sheffer 18 P.S. Oh no, another UID… So far we identified VUV absorption from intersystem bands ( 12 CO, 13 CO), A–X bands for rare CO isotopologues, Rydberg bands below 1000 Å ( 12 CO), and now CH Rydberg features... HST spectra show at least one more UID feature likely to be from a molecular species. It is at λ rest = 1400.87 Å. There is but a weak correlation with N(CH), and no ID from theoretical studies of CH electronic states. If not CH, perhaps belongs to the next hydrocarbon, CH 2.

19. 2007 June 19, 10:49 AM62 nd ISMS, OSU, Columbus, OHY Sheffer 19 The End. Thank you!