Fluctuations in ISM Thermal Pressures Measured from C I Observations

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Fluctuations in ISM Thermal Pressures Measured
from C I Observations

• Edward B. Jenkins
• Princeton University Observatory

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Fundamentals

• Most of the free carbon atoms in the ISM are
  singly ionized, but a small fraction of the ions
  have recombined into the neutral form.
• The ground electronic state of C I is split into
  three fine-structure levels with small energy
  separations.
• Our objective is to study the relative
  populations of these three levels, which are
  influenced by local conditions (density
  temperature.

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Fine-structure Levels in the Ground State of C I
Upper Electronic Levels
Optical Pumping (by Starlight)
Spontaneous Radiative Decays
E/k 62.4 K
C I
3P2 (E 43.4 cm-1, g 5)
Collisionally Induced Transitions
E/k 23.6 K
3P1 (E 16.4 cm-1, g 3)
C I
C I
3P0 (E 0 cm-1, g 1)
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C I Absorption Features in the UV Spectrum of ?
Cep Recorded at a Resolution of 1.5 km s-1 by
STIS on HST
From Jenkins Tripp (2001 ApJS, 137, 297)
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? Cep
C I
Column density per unit velocity 1013 cm-2 (km
s-1)-1
C I
C I
Velocity (km s-1)
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Most Useful Way to Express Fine-structure
Population Ratios

• n(C I)total n(C I) n(C I) n(C I)
• f1 ? n(C I)/n(C I)total
• f2 ? n(C I)/n(C I)total

f2
Then consider the plot
Collision partners at a given density and
temperature are expected to yield specific values
of f1 and f2
f1
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Collisional Excitation by Neutral H
T 100 K
n(H) 105 cm-3
n(H) 104 cm-3
n(H) 1000 cm-3
n(H) 100 cm-3
n(H) 10 cm-3
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Collisional Excitation by Neutral H Plus Optical
Pumping by the Average Galactic Starlight Field
n(H) 104 cm-3
n(H) 1000 cm-3
n(H) 100 cm-3
n(H) 10 cm-3
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Collisional Excitation by Neutral H Plus Optical
Pumping by 10X the Average Galactic Starlight
Field
n(H) 104 cm-3
n(H) 1000 cm-3
n(H) 100 cm-3
n(H) 10 cm-3
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Tracks for Different Temperatures
T 240 K
T 120 K
n(H) 100 cm-3
T 60 K
T 30 K
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Tracks for Different Temperatures
T 240 K
T 120 K
p/k 104 cm-3 K
T 60 K
T 30 K
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(Back to simple f1f2 diag.)
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A Theorem on how to deal with superpositions
Cloud 2
Cloud 1
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C I-weighted Center of Mass gives Composite
f1,f2
A Theorem on how to deal with superpositions
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Allowed Region for Composite Results
P/k ? ?
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Results

• Original observations reported by Jenkins Tripp
  (2001) included 21 stars.
• We have now expanded this survey to about 100
  stars by downloading from the MAST archive all
  suitable STIS observations that used the highest
  resolution echelle spectrograph (E140H).
• The archival results have somewhat lower velocity
  resolution because the standard entrance aperture
  was usually used (instead of the extremely narrow
  slit chosen for the Jenkins Tripp survey).

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Composite over all velocities and stars f1
0.217, f2 0.073
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H II reg.
T 160K
T 80K
T 40K
T 20K
Note HISA-land is down here
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Kinematics
? Cep
VDifferential Galactic Rotation
VLSR
C I
Target
Column density per unit velocity 1013 cm-2 (km
s-1)-1
Sun
C I
C I
Velocity (km s-1)
(heliocentric)
Positive Velocities
Allowed Velocities
Negative Velocities
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Allowed Velocities
Composite f1 0.203, f2 0.063
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Positive Velocities
Negative Velocities
Composite f1 0.231, f2 0.082 for both
velocity intervals
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Barytropic index
?eff 0.72
(Wolfire, Hollenbach, McKee, Tielens Bakes
1995, ApJ 443, 152)
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Gamma_eff on f1f2 (0.72)
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Gamma_eff on f1f2 (0.72, 0.90)
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Log-normal Distribution of Mass vs. Density
Relative Mass Fraction
n(H I) (cm-3)
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Log-normal distribution of H I mass fraction vs.
n(H), with ?eff 5/3
Observed composite f1, f2
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Log-normal distribution of H I mass fraction vs.
n(H), with ?eff 5/3
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Log-normal distribution of H I mass fraction vs.
n(H), with ?eff 5/3
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Log-normal distribution of H I mass fraction vs.
n(H), with ?eff 5/3
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Log-normal distribution of H I mass fraction vs.
n(H), with ?eff 5/3
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Log-normal distribution of H I mass fraction vs.
n(H), with ?eff 5/3
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Model for a random mixture of high and low
pressure gas
Obs.
Obs.
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Pressure Distribution Function
Note The width of this peak is a lower limit,
since the observations at each velocity probably
exhibit some averaging of pressure extremes along
the straight portion of the f1-f2 curve.
H I mass fraction
Relative Mass Fraction
The width and central pressure of this peak are
not well known, but the height of the peak is
well determined.
p/k (cm-3 K)
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Pressure Distribution Function
H I mass fraction
C I mass fraction
Relative Mass Fraction
p/k (cm-3 K)
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A Question to Consider About the High Pressure
Component

• Could this component arise simply from the action
  of radiation or mass loss from the target stars
  (or their associations) either of which could
  compress the gas?
• Probably not recall that negative velocity
  material behaved in much the same way as positive
  velocity material

Except for some gas parcels that have only high
pressures
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HS06246907
Galactic Coordinates l 145.7, b 23.4
Nearest O- or B-type star to the line of sight
43 Cam (V 5.14, spectral type B7IV), about 2
away
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Implications on the Existence of Small Neutral
Stuctures
Tcool 2,500 yr
Rapid Compression
Tcool 15,000 yr (for T 60 K)
Relative Mass Fraction
p/k (cm-3 K)
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Implications on the Existence of Small Neutral
Stuctures

• High pressure component mass fraction is low
  (10-3), relative to most of the gas.
• It has n(H I) 103 -104 cm-3 and T 100 K.
• Tcool 2500 yr, which implies a typical
  dimension of only 0.00025 pc (i.e., 50 AU), or
  less, if crossing-time velocities are of order 10
  km s-1 and the compression is nearly adiabatic.