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Diffuse gas and the CO sky

Diffuse gas and the CO sky. Harvey Liszt (NRAO) Robert Lucas & Jerome Pety (IRAM). After the hunt …. OK. CO emission was found! Over much of the sky even! It’s the universally-accepted marker for “molecular gas”! whatever that is

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Diffuse gas and the CO sky

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  1. Diffusegas and the CO sky Harvey Liszt (NRAO) Robert Lucas & Jerome Pety (IRAM) IAP, September 2005

  2. After the hunt … • OK. • CO emission was found! • Over much of the sky even! • It’s the universally-accepted marker for “molecular gas”! whatever that is • But what does it, uh, like, you know, um, really mean, to observe CO? IAP, September 2005

  3. Its easy to be skeptical about CO Edith Falgarone Jacques Crovisier Bernard Lazereff IAP, September 2005

  4. CO emission doesn’t even represent CO perfectly IAP, September 2005

  5. But as we’ve been told • You go to the telescope to map the emission you have, not the emission you want IAP, September 2005

  6. Monoceros in EB-V , CO IAP, September 2005

  7. Monoceros IAP, September 2005

  8. Cygnus & Beyond in E(B-V) & CO IAP, September 2005

  9. N(HCO+)=2.4x1012 N(H2) ~1x1021 Cygnus and Beyond IAP, September 2005

  10. Diffuse lines of sight + TMC-1 … optical , radio EB-V CO IAP, September 2005

  11. EB-V =0.88 HCO+ ~ 2.2x1012 N(H2) ~ 1.1x1021 EB-V =1.65 HCO+ ~ 12x1012 N(H2) ~ 6x1021 What do optical diffuse lines of sight look like … radio? IAP, September 2005

  12. HCO+ IAP, September 2005

  13. Exhale • Much or most of the CO sky is diffuse gas • Likely signs of diffuse gas: • Weak or absent CO emission (but other absorption) • Large-ish 12CO/13CO ratios • Not seeing anything beside CO in absorption • Misleading signs of darkness in CO emission: • 12CO emission 3-7 K • 12CO/13CO ratios << 60, even as small as 4 • There’s lots more CO emission waiting to be found (but, like, why bother?) IAP, September 2005

  14. Exhale • Seeing an advanced chemistry (CS, HCN) more strongly in emission would be a sign of dense gas, but: • Truly dense dark gas is much less common than one might think • The chemistry is there whether you see it in emission or not. • We fail to find dense dark gas, we find only its chemistry • What does ‘early-time’ chemistry mean? IAP, September 2005

  15. Faith-based 12CO Chemistry ... if X(HCO+) = 2x10-9 IAP, September 2005

  16. Happy Birthday, Pierre • Thanks for having me. • When next you see a large-scale map of CO, think of me. • If you visit the US, DON’T EAT THE BEEF, we don’t do much testing. • And please be kind to radio astronomy IAP, September 2005

  17. Just what have Robert, Jerome & I done with all that telescope time? (to say nothing of 10-12 years of our lives) IAP, September 2005

  18. IAP, September 2005

  19. Reloaded … IAP, September 2005

  20. How much HCO+ is there? • N(HCO+)/EB-V ~ 3x1012 cm-2 mag-1 • N(HCO+)/N(H2) ~ 5x10-9if fH2=0.2 • Fully ½ of Ohishi+Irvine (1992) value for TMC-1 • Models: HCO+/OH ~ 10-3, HCO+/H2 ~ 10-10 IAP, September 2005

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