Nuclear spin of H3+ in diffuse molecular clouds

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Presentation transcript:

Nuclear spin of H3+ in diffuse molecular clouds Kyle N. Crabtree, Nick Indriolo, Holger Kreckel, Brian A. Tom, and Benjamin J. McCall University of Illinois

66th OSU International Symposium on Molecular Spectroscopy Interstellar clouds T. P. Snow and B. J. McCall, Annu. Rev. Astron. Astrophys. (2006), 44, 367-414. 21 June 2011 66th OSU International Symposium on Molecular Spectroscopy

Temperature in diffuse molecular clouds: H2 p-H2; J = 0 o-H2; J = 1 DE 170 K UV measurements of H2 absorption 99% of H2 in J=0 (para) and J=1 (ortho) levels Profile fitting gives accurate column densities for N(0) and N(1)  T01 B. L. Rachford et al., ApJ (2002), 577, 221-244. 21 June 2011 66th OSU International Symposium on Molecular Spectroscopy

Temperature in diffuse molecular clouds: H3+ 36685 Å R(1,1)u 36681 Å R(1,1)l 37155 Å High-resolution IR spectroscopy of H3+ Column densities N(1,0), N(1,1)  T(H3+) 21 June 2011 66th OSU International Symposium on Molecular Spectroscopy

H3+/H2 temperature discrepancy Average T01 in diffuse molecular clouds: 70 K (N = 66) Average T(H3+) in diffuse molecular clouds: 30 K (N = 18) Only 2 sight lines in common Recent observations expand this number to 6 Target Obs. p3 p2 T(H3+) T01 DT z Per UKIRT 0.65(4) 0.68(6) 25(3) 58(6) 33 X Per 0.66(5) 0.69(4) 24(4) 57(4) HD 154368 Gem. S. 0.69(6) 0.76(7) 22(4) 51(8) 29 HD 73882 VLT 0.67(4) 0.76(5) 23(3) 51(6) 28 HD 110432 0.60(2) 0.57(3) 30(2) 68(5) 38 l Cep UKIRT/Keck 0.57(7) 0.54(3) 34(10) 73(4) 39 21 June 2011 66th OSU International Symposium on Molecular Spectroscopy

H3+/H2 temperature discrepancy 21 June 2011 66th OSU International Symposium on Molecular Spectroscopy

Chemistry of H3+ Formation: H2 + cosmic ray  H2+ + e- (slow) H2+ + H2  H3+ + H (fast) Thermalization: H3+ + H2  H2 + H3+ Destruction via dissociative recombination (DR): H3+ + e-  H2 + H or 3H Nuclear spin dependence? 1st order approximation: #H3+-H2 collisions prior to H3+ DR: kt n(H2) n(H3+) (10-9)(102) # collisions = = = 100 kdr n(e-) n(H3+) (10-7)(10-2) 21 June 2011 66th OSU International Symposium on Molecular Spectroscopy

66th OSU International Symposium on Molecular Spectroscopy H3+ + H2  H2 + H3+ Sid Nuclear spin selection rules Conservation of angular momentum 1 “identity” H5+ 3 “hop” Recent laboratory measurements show that kH/kE = 0.5 at low temperatures (<130 K) 6 K. N. Crabtree, B. A. Tom, B. J. McCall, J. Chem. Phys. (2011) 134, 194310. K. N. Crabtree et al., J. Chem. Phys. (2011) 134, 194311. “exchange” 21 June 2011 66th OSU International Symposium on Molecular Spectroscopy

H3+ + H2 rate coefficients f (T,Sid) Parameter Value(s) Trot 10-160 K Tcoll Sid 0.1-0.9 Shop 0-1 Sexch koooo kooop koopo koopp kopoo kopop koppo koppp kpooo kpoop kpopo kpopp kppoo kppop kpppo kpppp Shop/Sexch = 0.5 21 June 2011 66th OSU International Symposium on Molecular Spectroscopy

Model: H3+ + H2 reaction only 10 K p2 linked to Tkin No agreement; H3+ + H2 reaction not solely responsible for T discrepancy 20 K Sid=0.1 30 K 40 K Sid=0.9 50 K etc 80 K 70 K 60 K 21 June 2011 66th OSU International Symposium on Molecular Spectroscopy

Chemistry of H3+ Formation: H2 + cosmic ray  H2+ + e- (slow) H2+ + H2  H3+ + H (fast) Thermalization: H3+ + H2  H2 + H3+ Destruction via dissociative recombination (DR): H3+ + e-  H2 + H or 3H Nuclear spin dependence? ke,o = o-H3+ DR xe = Electron fraction (1.5 x 10-4) f = Molecular fraction (0.9) ke,p = p-H3+ DR 21 June 2011 66th OSU International Symposium on Molecular Spectroscopy

66th OSU International Symposium on Molecular Spectroscopy DR rates equal Good agreement when Sid = 0.9 B. J. McCall et al., Nature (2004), 422, 500-502. 21 June 2011 66th OSU International Symposium on Molecular Spectroscopy

66th OSU International Symposium on Molecular Spectroscopy p-H3+ DR = 10× o-H3+ DR No agreement F. dos Santos et al., J. Chem. Phys (2007), 127, 124309. 21 June 2011 66th OSU International Symposium on Molecular Spectroscopy

66th OSU International Symposium on Molecular Spectroscopy Summary H3+ in diffuse molecular clouds is nonthermal The ortho:para ratio of H3+ lies between its nascent value and its thermalized value Competition between thermalization (H3+ + H2) and DR Low-temperature state-specific DR rate coefficients for H3+ needed Future: modeling environments in which H2 column densities are unavailable, but H3+ column densities are 21 June 2011 66th OSU International Symposium on Molecular Spectroscopy

66th OSU International Symposium on Molecular Spectroscopy Acknowledgements Kisam Park More information: K. N. Crabtree et al., ApJ (2011) 729, 15. http://bjm.scs.illinois.edu 21 June 2011 66th OSU International Symposium on Molecular Spectroscopy