How Young Can a Cloud Be

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How Young Can a Cloud Be

• HINSA as a New Molecular Cloud Probe

Di Li
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Galactic HI Absorption
3
1954 Clear Statement re. Absorption Features
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Past Observations-Technical Limitations

• Haystack 120ft 25 0.5km/s (Myers et al.
  1978)
• Green Bank 140ft 21 0.4 km/s (Knapp
  1974)
• 76m Lovell Telescope 12 0.5 km/s (McCutcheon
  et al.1978 Montgomery et al. 1995 )
• Effelsberg 100m 9, 0.5 km/s (Wilson Minn
  1977 Batrla et al. 1983 Poppel et al. 1983)
• Arecibo 300m 4, 1 km/s (Baker Burton 1979,
  Bania Lockman 1984)
• VLA DRAO 1 - 1.5, 1.3 km/s (Van der
  Werf et al. 1988, 1989 Gibson et al. 2000)

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Arecibo Upgraded
Point-focused with secondary and tertiary Wider
bandwidth LO and better backends Central panel
patched Surface readjustment underway
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Detecting Cold HI - HI Narrow Self Absorption

• Called self-absorption in order to
  differentiate from absorption against continuum
  sources
• HI Self-Absorption is different from the usual
  meaning of self, as in the case of thick tracer
  like CO.
• In case of HI absorption caused by
  dark clouds, HINSA could be
  a less confusing alternative.

-10 -5 0 5 10 15 Vlsr
(km/s)

• (Li Goldsmith 2003 ApJ, 585, 823)

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HINSA Definition

• HI Absorption with corresponding molecular
  emission and
• ?V(HI Absorption) lt ?V(CO)

• (Li Goldsmith 2003 ApJ, 585, 823)

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Survey Statistics HINSA Detection Rate

• 23 sources with clear HI NSA
• Detection rate
• 77
• Similar Turbulent content HINSA vs. OH
• Average non-thermal line width (km/s)
• OH 0.83 HI 0.84
• The raw line width of HI is on average 1km/s,
  less than two channels in DRAO survey. The
  average depth is less than 2 RMS noise in DRAO
  survey.
• (Li Goldsmith 2003 ApJ, 585, 823)

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Extremely Good Correlation of HI Narrow
Self-Absorption and Molecular Emission Lines
(Goldsmith Li 2004 submitted to ApJ)
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The Real Globule?
The North condensation was previously unknown
and was found by mapping the HINSA and following
up with 13CO and C18O This morphology confirms
close connection between cold HI absorption and
molecular emission
position in CB catalog (Clemens Barvanis 1988)
(Goldsmith Li 2004 submitted to ApJ)
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Three-Component Radiative Transfer
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Two Useful Limiting Cases

• Without foreground absorber is equivalent to an
  emitting cloud at the temperature differential
• With optically thin foreground and TbTf

(Li Goldsmith 2003 ApJ, 585, 823 cf. Van der
Werf et al. 1988 )
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Column Density of HI

• Total HI column density derived from optical
  depth of the 21cm line
• Average HI column density
• If using C18O, the abundance HI/H2 is 0.15
• The third most abundant species inside molecular
  clouds after only H2 and He, but before CO!

8x1018 cm-2
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How Fast Does Star Form?

• Conventional View
• Core Ambipolar diffusion
• Collapse Inside-out
• Jets Deuterium Burning, Stellar energetic starts
  to take over
• Accretion Disk the termination of infall will
  determine the final mass of the new star.
• Fast Star Formation?
• Clouds form by HI stream interaction (Hartmann et
  al. 2001)
• Stars are formed on a dynamical time scale (Myr
  Elmegreen 2001).
• Clouds are dispersed by stellar energy output
  (Elmegreen 2000)

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Gas-Grain Reaction Network of H2 Formation

• The variable set
• The parameters set

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HI Fractional Abundance Measures TIME since the
start of Atomic to Molecular Conversion
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From Atoms to Stars

• HgtH2 gt Cloud Complex gt
  CoregtProtostars
• ALFA-Tau Spitzer c2d COMPLETE

• Goodman
• N. Ridge
• S. Schnee
• J. Foster
• D. Li
• (the COMPLETE team)

P. Goldsmith C. Brunt M. Heyer R. Snell G.
Narayanan D. Li H. Arce and GALFA consortium
N. Evans L. Allen P. Myers T. Burke (the
Spitzer legacy team)
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HINSA Conclusions

• HINSA with ?v lt 2 km s-1 is a widespread
  phenomenon associated with dark clouds .
• The narrow line atomic hydrogen has significant
  column density N(HINSA)8x1018 cm-2, making it
  the third most abundant species of molecular
  clouds after H2 and He.
• HINSA places a lower limit on molecular cloud
  core age at gt 5 million years
• HINSA could be a good tracer for Zeeman
  measurement.
• HINSA could provide a probe to galactic cosmic
  ray distribution.
• Large scale HI, CO, and infrared surveys would
  build a picture of cloud formation and their
  evolution coupled with star formation.

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Colder Gas in CNM

• Phases in neutral medium (Wolfire et al. 1995
  McKee Ostriker 1977 Field et al. 1962)
• Cold Neutral Medium T80 K
• Warm Neutral Medium T8000 K
• Existence for colder CNM (Tlt40 K)
• Continuum absorption Heiles 2001, Kalbella et
  al. 1985
• HI self-absorption (HISA) Knee Brunt 2001

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HI Self Absorption

• Cold feature revealed in deficiency of emission
• GSH139-03-69
• (Knee and Brunt, 2001)