[Paper Review] The Distribution of Thermal Pressures in the Interstellar Medium

E. B. Jenkins, Todd M. Tripp|arXiv (Cornell University)|Sep 20, 2001

Astrophysics and Star Formation Studies1 references3 citations

TL;DR

This study uses high-resolution UV absorption spectra from the Hubble Space Telescope's STIS instrument to measure thermal pressures in the neutral interstellar medium via fine-structure levels of neutral carbon (C I). It finds that while most gas resides at moderate pressures (10³–10⁴ cm⁻³ K), a small but ubiquitous fraction exists at very high pressures (>10⁵ cm⁻³ K), likely due to transient, small-scale compressions from supersonic turbulence, with implications for gas dynamics and molecular formation.

ABSTRACT

It is generally recognized that the interstellar medium has a vast range of densities and temperatures. While these two properties are usually anticorrelated with each other, there are nevertheless variations in their product, i.e., the thermal gas pressure divided by the Boltzmann constant k. In neutral gas, the relative populations of neutral carbon atoms in the excited fine-structure states can give a direct measure of a local thermal pressure. A picture of the distribution function for thermal pressures in H I regions is now arising from a survey of interstellar C I absorption features in the UV spectra of 21 early-type stars, observed with a wavelength resolving power of 200,000 by the STIS instrument on the Hubble Space Telescope. Most of the gas is within the range 1000 < p/k < 10,000 cm^{-3}K, but there is also evidence for some of the material being at much higher pressures, i.e., p/k > 10^5 cm^{-3}K. While the fraction of gas at these elevated pressures is quite small, it seems nearly ubiquitous. This phenomenon may arise from small-scale, short-lived density enhancements that are produced by converging flows of material in supersonic turbulence.

Motivation & Objective

• To map the distribution of thermal pressures in the neutral interstellar medium using fine-structure transitions of neutral carbon (C I).
• To determine the kinematical and physical conditions of interstellar gas by analyzing C I excitation states in UV absorption spectra.
• To investigate the origin of high-pressure gas components that deviate from equilibrium thermal distributions.
• To assess the role of turbulence in generating transient, high-pressure structures in the ISM.

Proposed method

• High-resolution UV spectroscopy (λ/Δλ = 200,000) of 21 early-type stars using the STIS instrument on the Hubble Space Telescope.
• Simultaneous fitting of 300 linear equations across 9 C I multiplets to derive column density profiles for C I, C I*, and C I** as functions of radial velocity.
• Use of f1 = N(C I*)/N(C I_total) and f2 = N(C I**)/N(C I_total) as diagnostic variables to represent thermal pressure in a phase-space diagram.
• Kinematical segregation of absorption components into quiescent (within allowed velocity range), high-velocity, and low-velocity categories to isolate pressure contributions.
• Comparison of observed f1–f2 points with theoretical constant-temperature tracks to infer thermal pressures (p/k).
• Application of the 'center of mass' principle to interpret blended components as superpositions of distinct pressure regions.

Experimental results

Research questions

• RQ1 What is the distribution of thermal pressures in the neutral interstellar medium as traced by C I fine-structure levels?
• RQ2 Why do most C I absorption components lie above the theoretical pressure tracks in f1–f2 diagrams?
• RQ3 What physical mechanisms can produce the observed high-pressure gas components (p/k > 10⁵ cm⁻³ K) that are not in thermal equilibrium?
• RQ4 How do kinematical components (e.g., peculiar motions) correlate with pressure distributions in the ISM?
• RQ5 Can supersonic turbulence explain the existence of transient, high-pressure structures in the ISM?

Key findings

• The median thermal pressure for the entire sample is p/k = 2240 cm⁻³ K at a temperature of ~40 K.
• 65% of the C I column density resides in the pressure range 10³.⁰–10³.⁵ cm⁻³ K, and 28.5% in 10³.⁵–10⁴.⁰ cm⁻³ K.
• 3.7% of the C I column density is found at pressures exceeding 10⁴.⁰ cm⁻³ K, with a significant fraction at p/k > 10⁵ cm⁻³ K.
• All high-pressure components (f2 > 0.3) with p/k ~ 10⁵–10⁶ cm⁻³ K are associated with a single, isolated, negative-velocity component in front of λ Cep, likely compressed by stellar wind.
• The observed offset of data points above theoretical pressure tracks indicates a nearly ubiquitous superposition of low-pressure gas (10³–10⁴ cm⁻³ K) with a small fraction of high-pressure gas (p/k > 10⁵ cm⁻³ K).
• The high-pressure gas is likely produced by short-lived, small-scale compressions from converging flows in supersonic turbulence, with compression timescales much shorter than thermal relaxation times.