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HI Self - Absorption

HI Self - Absorption. Peculiar Puzzle and Potential Probe Predating the Arecibo Telescope Paul F. Goldsmith. The 21-cm Line of Atomic Hydrogen. Discovered in 1951 by Ewen and Purcell (2002 Gordon Lecture at Arecibo)

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HI Self - Absorption

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  1. HI Self - Absorption Peculiar Puzzle and Potential Probe Predating the Arecibo Telescope Paul F. Goldsmith

  2. The 21-cm Line of Atomic Hydrogen • Discovered in 1951 by Ewen and Purcell (2002 Gordon Lecture at Arecibo) • Seen in EMISSION, which was not surprising since theories indicated that atomic gas in Milky Way would be warm (T ~ 100 K)

  3. 1955: Clear Statement re. Absorption Features

  4. 1960: “Self-Absorption” Terminology Appears for the First Time

  5. Molecular Emission (OH) is Sometimes Associated With HI Self-Absorption Features

  6. HINSA Spectra in Arecibo Survey of Taurus (Li & Goldsmith 2003) Width of the HINSA features is so narrow that they must come from COLD (< 20K) gas! Line minimum is so deep that in some cases the gas must again be below 20 K!

  7. The Situation as we Now See it:

  8. OH (shaded) 13CO(red) & C18O(green)Show General, But Not Perfect Agreement in L1544

  9. Extremely Good Correlation of HI Narrow Self-Absorption and Molecular Emission Lines

  10. Globule CB45 mapped in HI (not bounded) then 13CO (also not bounded) ………

  11. Complete HI Self-Absorption Map Confirms Close Correlation with 13CO and C18O !

  12. Significance of Atomic Hydrogen Within Well-Shielded Molecular Clouds • HI destroyedby conversion to H2 on grain surfaces • HI formed by cosmic ray destruction of H2 • STEADY-STATE ABUNDANCE of HI is ~ 1 cm-3 corresponding to 10-4 fractional abundance w.r.t. total proton density (due to preponderance of H2 )

  13. HI Fractional Abundances are ~ 10 x Greater than Steady State Values

  14. HI Fractional Abundance Measures TIME since start of Atomic to Molecular Conversion

  15. How to Proceed From Here? • We have defined a large-scale survey of the Taurus region for HINSA to be carried out using ALFA – the L-band 7 element focal plane array • Map several hundred square degrees with good sensitivity • Compare with 12CO and 13CO maps from FCRAO (300 hours of telescope time already awarded) • ~ 100 hours of AO observing time required to get high sensitivity maps of COLD and WARM HI

  16. Central Portion of the Taurus Molecular Cloud

  17. The ALFA – TAUProject • When we compare the kinematics of cold and warm atomic gas -- are they the same? • What is the relationship of the atomic and the molecular gas? • What is the characteristic timescale for dark cloud cores as traced by residual HI? • Can we develop a scenario for the evolution from atomic to molecular material and then from molecular clouds to cores to young stars?

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