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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 3 P 0 (E = 0 cm -1, g = 1) 3 P 1 (E = 16.4 cm -1, g = 3) 3 P 2 (E = 43.4 cm -1, g = 5) C I C I* C I** Upper Electronic Levels Collisionally Induced Transitions Optical Pumping (by Starlight) Spontaneous Radiative Decays E/k = 23.6 K E/k = 62.4 K

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C I Absorption Features in the UV Spectrum of λ 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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Column density per unit velocity [10 13 cm -2 (km s -1 ) -1 ] Velocity (km s -1 ) C I C I* C I** λ Cep

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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 f1 f2 Then consider the plot: Collision partners at a given density and temperature are expected to yield specific values of f1 and f2

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n(H) = 10 cm -3 n(H) = 100 cm -3 n(H) = 1000 cm -3 n(H) = 10 4 cm -3 n(H) = 10 5 cm -3 Collisional Excitation by Neutral H T = 100 K

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Collisional Excitation by Neutral H Plus Optical Pumping by the Average Galactic Starlight Field n(H) = 10 cm -3 n(H) = 100 cm -3 n(H) = 1000 cm -3 n(H) = 10 4 cm -3

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Collisional Excitation by Neutral H Plus Optical Pumping by 10X the Average Galactic Starlight Field n(H) = 10 cm -3 n(H) = 100 cm -3 n(H) = 1000 cm -3 n(H) = 10 4 cm -3

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Tracks for Different Temperatures n(H) = 100 cm -3 T = 30 K T = 60 K T = 120 K T = 240 K

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Tracks for Different Temperatures T = 30 K T = 60 K T = 120 K T = 240 K p/k = 10 4 cm -3 K

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(Back to simple f1f2 diag.)

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Cloud 1 Cloud 2 A Theorem on how to deal with superpositions

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C I-weighted “Center of Mass” gives Composite f1,f2

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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 = T = 20K T = 40K T = 80K T = 160K H II reg.

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T = 20K T = 40K T = 80K T = 160K H II reg. Note: HISA-land is down here

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V LSR V Differential Galactic Rotation Positive Velocities Negative Velocities Allowed Velocities Sun TargetKinematicsKinematics Column density per unit velocity [10 13 cm -2 (km s -1 ) -1 ] Velocity (km s -1 ) C I C I* C I** λ Cep (heliocentric)

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Allowed Velocities Composite f1 = 0.203, f2 = T = 20K T = 40K T = 80K T = 160K H II reg.

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Positive Velocities Negative Velocities Composite f1 = 0.231, f2 = for both velocity intervals T = 20K T = 40K T = 80K T = 160K H II reg.

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eff = 0.72 Barytropic index (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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Observed composite f1, f2 Log-normal distribution of H I mass fraction vs. n(H), with γ eff = 5/3 H I C I

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Observed composite f1, f2 H I C I Log-normal distribution of H I mass fraction vs. n(H), with γ eff = 5/3

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Observed composite f1, f2 H I C I Log-normal distribution of H I mass fraction vs. n(H), with γ eff = 5/3

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Observed composite f1, f2 H I C I Log-normal distribution of H I mass fraction vs. n(H), with γ eff = 5/3

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Observed composite f1, f2 H I C I Log-normal distribution of H I mass fraction vs. n(H), with γ eff = 5/3

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Observed composite f1, f2 H I C I Log-normal distribution of H I mass fraction vs. n(H), with γ eff = 5/3

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Obs. Model for a random mixture of high and low pressure gas Obs.

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Pressure Distribution Function p/k (cm -3 K) Relative Mass Fraction H I mass fraction 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. The width and central pressure of this peak are not well known, but the height of the peak is well determined.

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Pressure Distribution Function p/k (cm -3 K) Relative Mass Fraction C I mass fraction H I mass fraction

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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 Blue = neg. vel. Red = pos. vel.

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HS 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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p/k (cm -3 K) Relative Mass Fraction Implications on the Existence of Small Neutral Stuctures T cool = 15,000 yr (for T = 60 K) T cool = 2,500 yr Rapid Compression

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High pressure component mass fraction is low (~10 -3 ), relative to most of the gas. It has n(H I) ~ 10 3 −10 4 cm -3 and T ≥ 100 K. T cool ≤ 2500 yr, which implies a typical dimension of only pc (i.e., 50 AU), or less, if crossing-time velocities are of order 10 km s -1 and the compression is nearly adiabatic. Implications on the Existence of Small Neutral Stuctures

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