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The ortho-H 2 abundance and the age of molecular clouds

The ortho-H 2 abundance and the age of molecular clouds. Laurent Pagani LERMA, UMR8112 du CNRS, Observatoire de Paris. This talk :. Low mass star formation Ortho-H 2 : its role in deuteration control formation and destruction Deuteration amplification needs CO depletion – a myth ?

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The ortho-H 2 abundance and the age of molecular clouds

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  1. The ortho-H2 abundance and the age of molecular clouds Laurent Pagani LERMA, UMR8112 du CNRS, Observatoire de Paris

  2. This talk : • Low mass star formation • Ortho-H2 : • its role in deuteration control • formation and destruction • Deuteration amplification needs CO depletion – a myth ? • How old is a cold cloud ?

  3. Low mass star formation

  4. Low mass star formation • Prestellar cores form either slowly: • Turbulence dissipation (Nakano 1998) • Ambipolar diffusion (Mouschovias 1991, Ciolek & Basu 2000, 2006,...) • or fast : • Supersonic turbulent flows -> local density enhancements (Klessen et al. 2000, Larson 2007, Hennebelle et al. 2007, 2008…)

  5. Ortho H2 : deuteration control • pH3+ + HD ➔ pH2D+ + pH2 + 232 K • pH3+ + HD ➔ oH2D+ + pH2 + 145 K • pH3+ + HD ➔ pH2D+ + oH2 + 62 K • pH3+ + HD ➔ oH2D+ + oH2- 25 K • etc. • H3+ + HD ➔ H2D+ + H2 + 232 K ??? • pH2D+ + oH2➔ oH2D+ + pH2 + 83 K ortho - H2 > 1 % ⇔ no H2D+ Pagani et al. 2009

  6. Ortho H2 : deuteration control • CH3+ + HD ➔ CH2D+ + pH2 + 375 K • CH3+ + HD ➔ CH2D+ + oH2 + 205 K • C2H2+ + HD ➔ C2HD+ + pH2 + 550 K • C2H2+ + HD ➔ C2HD+ + oH2 + 380 K • etc. ↪ no ortho-H2 control ! ↪ allows deuteration in warm regions (Parise et al. 2009) … and in cold regions too!

  7. Ortho H2 : fabrication/destruction • H2 mostly fabricated on grains with o/p ratio = 3:1 • ortho H2 destroyed via : • oH2 + H+ ➔ pH2 + H+(Honvault et al. 2011a, 2011b.) • oH2 + H3+ ➔ pH2 + H3+(Hugo et al. 2009)

  8. Ortho H2 : fabrication/destruction ≤1.5 My ! Full chemical model OSU/Nahoon style + H2 and H3+ spin state chemistry

  9. Do we need CO depletion to deuterate ? • [CO] = 1-2 × 10-4 & [HD] = 3 × 10-5 • H3+ + CO ➔ HCO+ + H2(main route) • H3+ + HD ➔ H2D+ + H2 (minor route) • H2D+ + CO ➔ HCO+, DCO+ + H2 • ↪ [DCO+] ≈ cst !!

  10. Do we need CO depletion to deuterate ? Steady-state model

  11. Do we need CO depletion to deuterate ? • Yes, we need CO depletion in general • …except for DCO+ • So, why the clouds are not full of DCO+ ? • But are they not ?

  12. Do we need CO depletion to deuterate ? NH3(1,1) CS 3-2 (Butner et al. 1995)

  13. How old is a cold cloud ? • So, no DCO+ outside cold depleted cores • Because ortho-H2 prevents deuteration ? • But ortho-H2 eventually disappears • So… clouds must be young (goslings ?)

  14. How old is a cold cloud ? • Deuteration model (Roueff et al. 2005) • + ortho/para spin state H chemistry • (Hugo et al. 2009) • + DR rates (Pagani et al. 2009) • + corrections (CD + oH2,…) • Undepleted • 𝛇 = 10-17s-1 • metallicity = 3.4 × 10-8 • Tkin = 10 K • nH = 2 × 104 cm-3 • Detection limit : • 0.1 K in 0.5 km/s • (DCO+ J:1-0) • Column density : • 1022 H2 cm-2

  15. How old is a cold cloud ? Dependence on Cosmic Ray Ionization rate

  16. Conclusions • Deuterium chemistry seems to say that prestellar cores and clouds are young : • clouds < 6-7 My (for normal CR rates and Metallicity) • clouds + prestellar core < 1.5 My (subm. A&A) • Role of C-bearing Deuterium chemistry needs clarification • Watson scheme is too simplistic : DCO+ • high Cosmic rays ionization rates impossible in dark clouds ? [e]

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