This is part of the HERMES Public Outreach pages, created by Juliette Voyez (Paris 7 University)

The Interstellar Medium

Contents

  1. The Interstellar Medium
    1. Introduction
    2. The dust
    3. The gas
      1. Different phases
      2. HII regions
      3. Molecular clouds
    4. Heating and cooling processes
      1. Heating mechanisms
      2. Cooling mechanisms
        1. Dust cooling
        2. Gas cooling
    5. References

Introduction

The interstellar medium (ISM) is the gas and dust that pervade interstellar space: the matter that exists between the stars within a galaxy. It fills interstellar space and blends smoothly into the surrounding intergalactic space.

The interstellar medium consists of an extremely dilute mixture of ions, atoms, molecules, larger dust grains, cosmic rays, and (galactic) magnetic fields. The matter consists of about 99% gas (mostly hydrogen) and 1% dust by mass. Densities range from 10-3 atom/cm3 to 107 molecules/cm3 with an average value in the Milky Way Galaxy of one particle per cubic centimeter.

The ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales. Stars form within the densest regions of the ISM, the cores of molecular clouds, and replenish the ISM with matter and energy through e.g. planetary nebulae, stellar winds, and supernovae. This interplay between stars and the ISM helps determine the rate at which a galaxy depletes its gaseous content, and therefore its lifespan of active star formation.

The dust

The interstellar dust is composed of extremely small grains, a typical size is 0.1 micrometer. Temperatures drop to about 10 Kelvins in the inner cores of molecular clouds. The grains' chemical composition is varied: graphite, silicates, carbonates. The absorbed radiation is reemitted in the far infrared. Grains absorb and redden the stellar light which is called interstellar extinction, therefore dust can be observed by reddening (bright regions) in near-infrared wavelengths and by extinction in visible light (FIG 1) (dark regions).

The dust seen by extinction (left) and reddening (right)

FIG 1: The dust seen by extinction (left) and reddening (right) in Barnard 68. VLT ANTU + FORS 1 - NTT + SOFI, ESO PR Photo 02c/01, 2001.

The gas

Different phases

The gas component, mostly hydrogen, is present in two forms, atomic or molecular. There are also some traces of heavier elements.

The atomic gas can be ionized by the UV energetic photons emitted by young hot stars, forming HII regions.

In 1977, McKee and Ostriker proposed a model of the interstellar gas, quickly presented below (FIG 2).

The different phases of the interstellar gas

H2 Molecular Clouds

CNM: Cold Neutral Medium

WNM: Warm Neutral Medium

WIM: Warm Ionized Medium

HIM: Hot Ionized Medium

SNR: Supernova Remnants

FIG 2: The different phases of the interstellar gas.

HII regions

NGC 604, a supergiant HII region in M33

FIG 3: NGC 604, a supergiant HII region in M33. H. Yang (UIUC), HST, NASA, 2002.

The strong UV radiation from the young hot stars ionized the surrounding gas. The emission is dominated by the hydrogen Hα line, at 656.3 nm, giving these regions their characteristic reddish color. HII regions (FIG 3) are often associated with giant molecular clouds from which they form together with stars.

Molecular clouds

M16, the Eagle Nebulae, a giant molecular cloud

FIG 4: M16, the Eagle Nebulae, a giant molecular cloud.

A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2).

Giant Molecular Clouds

Vast assemblages of molecular gas with masses of 104–106 times the mass of the sun are called Giant Molecular Clouds (GMC, FIG 4). The clouds can reach tens of parsecs in diameter and have an average density of 102–103 particles per cubic centimetre (the average density in the solar vicinity is one particle per cubic centimetre). They are cold regions, where the temperature is about ten Kelvins. Substructure within these clouds is a complex pattern of filaments, sheets, bubbles, and irregular clumps.

The densest parts of the filaments and clumps are called "molecular cores", whilst the densest molecular cores are called "dense molecular cores" and have densities of 104–106 particles per cubic centimeter.

Small Molecular Clouds

Isolated gravitationally bound small molecular clouds with masses less than a few hundred times the mass of the sun are called Bok globules. The densest parts of small molecular clouds are equivalent to the molecular cores found in GMCs and are often included in the same studies.

Heating and cooling processes

Depending on the temperature, density, and ionization state of a portion of the ISM, different heating and cooling mechanisms determine the temperature of the gas.

Heating mechanisms

Heating by low-energy cosmic rays: The first mechanism proposed for heating the ISM was heating by low-energy cosmic rays. Cosmic rays are an efficient heating source able to penetrate in the depths of molecular clouds. Cosmic rays transfer energy to gas through both ionization and excitation and to free electrons through Coulomb interactions. Low-energy cosmic rays (a few MeV) are more important because they are far more numerous than high-energy cosmic rays.

Photoelectric heating in grains: The ultraviolet radiation emitted by hot stars can remove electrons from dust grains. The photon hits the dust grain, and some of its energy is used in overcoming the potential energy barrier (due to the possible positive charge of the grain) to remove the electron from the grain. The remainder of the photon's energy heats the grain and gives the ejected electron kinetic energy.

Photoionization of the gas: When an electron is freed from an atom (typically from absorption of a UV photon) it carries kinetic energy away of the order: Ephoton − Eionization. This heating mechanism dominates in HII regions, but is negligible in the diffuse ISM due to the relative lack of neutral carbon atoms.

X-ray heating: X-rays remove electrons from atoms and ions, and those photoelectrons can provoke secondary ionizations. As the intensity is often low, this heating is only efficient in warm, less dense atomic medium (as the column density is small). For example in molecular clouds only hard X-rays can penetrate and X-ray heating can be ignored. This is assuming the region is not near an X-ray source such as a supernova remnant.

Chemical heating: Molecular hydrogen (H2) can be formed on the surface of dust grains when two H atoms (which can travel over the grain) meet. This process yields 4.48 eV of energy distributed over the rotational and vibrational modes, kinetic energy of the H2 molecule, as well as heating the dust grain. This kinetic energy, as well as the energy transferred from de-excitation of the hydrogen molecule through collisions, heats the gas.

Grain-gas heating: Collisions at high densities between gas atoms and molecules with dust grains can transfer thermal energy. This is not important in HII regions because UV radiation is more important. It is also not important in diffuse ionized medium due to the low density. In the neutral diffuse medium grains are always colder, but do not effectively cool the gas due to the low densities. Grain heating by thermal exchange is very important in supernova remnants where densities and temperatures are very high. Gas heating via grain-gas collisions is dominant deep in giant molecular clouds (especially at high densities). Far infrared radiation penetrates deeply due to the low optical depth. Dust grains are heated via this radiation and can transfer thermal energy during collisions with the gas.

Other heating mechanisms: A variety of macroscopic heating mechanisms are present including:

Cooling mechanisms

Dust cooling

Interstellar dust contributes the bulk of the total cooling radiation via far-infrared continuum radiation. The brightest line is often the [CII] 158 micron line. It contributes only upto about 2% to the total cooling.

Gas cooling

Fine structure cooling: The process of fine structure cooling is dominant in most regions of the Interstellar Medium, except regions of hot gas and regions deep in molecular clouds. This occurs most efficiently with abundant atoms having fine structure levels close to the fundamental level such as: CII and OI in the neutral medium and OII, OIII, NII, NIII, NeII and NeIII in HII regions. Collisions will excite these atoms to higher levels, which will eventually de-excite through photon emission, which will carry the energy out of the region.

Cooling by permitted lines: At higher temperature more levels than fine structure levels can be populated via collisions. For example, collisional excitation of the n=2 level of hydrogen will release a Lyα photon upon de-excitation. In molecular clouds, excitation of rotational lines of CO is important. Once a molecule is excited, it eventually returns to a lower energy state, emitting a photon which can leave the region, cooling the cloud.

References

hermesWiki: InterstellarMedium (last edited 2009-07-23 15:19:13 by JulietteVoyez)