In this lecture we will discuss the thermal phases of the gas in the ISM.
Classification according to temperature and ionization state. We can identify four main component: molecular clouds, cold neutral medium, warm neutral medium, warm ionized medium, and the hot ionized medium.
Simple model of the ISM: description of the ISM with a limited set of such states rather than as a smooth continuum of conditions.
The ISM is described in terms of thermodynamic properties: density, temperature, pressure, etc.
Most of the mass in the ISM is found in the form of clouds of cold, neutral gas with K and density and in giant molecular clouds with K and density .
Molecular clouds (molecular hydrogen).
30% of the mass, but only 0.05 % of the volume.
Gravitationally bound, unstable, sites of star-formation.
Main observational tracer: mm-wavelength emission lines from CO molecules.
Cold neutral medium
Distributed in sheets, filaments, occupy 1-5 % of the ISM volume.
Main observational tracer: UV and optical absorption lines towards bright background sources.
Most of the volume is occupied by hotter and less dens gas.
(1) warm neutral component
(2) warm ionized component
(3) hot diffuse component
Pressure equilibrium (all phases, but in molecular clouds):
Solide phase of the ISM.
Grain size: few microns to scale of macro-molecules.
Interstellar dust is mixed with all the gas phases (but the hottest ones).
Dust grains are rensponsbile for interstellar extinction, sites of chemistry in space.
In order to understand its important role: mass versus luminosity.
Magnetic fields & cosmic rays
The neutral atomic gas is mainly neutral hydrogen (symbol: HI).
Most of the gaseous hydrogen in the ISM (about 97%) is neutral; a small percentage (about 3%) is ionized.
Observations of HI clouds at 21 cm (via hyperﬁne ground state transitions of hydrogen) in emission and absorption provide information about temperatures and column densities.
The brightness of the emission provides a measurement of the HI column density
where is the atomic hydrogen density.
Decrease in the brightness of a background source at 21 cm is proportional to where is the temperature of the cloud.
An image of a large Galactic H I supershell (white region at center). Credit: McLure-Grifﬁths, University of Minnesota.
In the next lectures, we will describe in detail the following components in the ISM:
Chapter 1 of “Physical Processes in the Interstellar Medium”, by Spitzer
Chapter 3 of “The Physics and Chemistry of the Interstellar Medium”, by Tielens
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