Interstellar medium

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This article is about interstellar medium. For Steven Spielberg's upcoming sci-fi film, see Interstellar (film).

The interstellar medium (or ISM) is the name astronomers give to the tenuous gas and dust that pervade interstellar space. Whilst the ISM refers to the matter (interstellar matter, also abbreviated by ISM) that exists between the stars within a galaxy, the energy, in the form of electromagnetic radiation, that occupies the same volume is called the interstellar radiation field (or ISRF).

The ISM consists of an extremely dilute (by terrestrial standards) plasma, gas and dust, consisting of a mixture of ions, atoms, molecules, larger dust grains, electromagnetic radiation, cosmic rays, and magnetic fields. The matter consists of about 99% gas and 1% dust by mass. It fills interstellar space. This mixture is usually extremely tenuous, with typical gas densities ranging from a few hundred to a few hundred million particles per cubic meter. As a result of primordial nucleosynthesis, the gas is roughly 90% hydrogen and 10% helium by number, with additional elements ("metals" in astronomical parlance) present in trace amounts.

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, molecular clouds, and replenish the ISM with matter and energy through planetary nebulae, stellar winds, and supernovae. In turn, 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.

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[edit] The history of interstellar space

The nature of the interstellar medium has received the attention of astronomers and scientists over the centuries. However, they first had to acknowledge the basic concept of "interstellar" space. The term appears to have been first used in print by Francis Bacon in 1626 where he wrote: "The Interstellar Skie.. hath .. so much Affinity with the Starre, that there is a Rotation of that, as well as of the Starre." (Sylva §354–5). Later, natural philosopher Robert Boyle surmised: "The inter-stellar part of heaven, which several of the modern Epicureans would have to be empty." (1674 Excell. Theol. ii. iv. 178)

Before modern electromagnetic theory early physicists postulated that an invisible luminiferous aether existed as a medium to carry lightwaves. It was assumed that this aether extended into interstellar space, as R. H. Patterson wrote in 1862, "This efflux occasions a thrill, or vibratory motion, in the ether which fills the interstellar spaces" (Ess. Hist. & Art 10).

The advent of deep photographic imaging allowed Barnard to produce the first images of dark nebula silhouetted against the background star field of the Galaxy. In 1904 Hartmann detected spectroscopic absorption lines towards a pair of binary stars that could not have come from the stars themselves. The growing evidence for interstellar material led William Henry Pickering to comment in 1912 that "While the interstellar absorbing medium may be simply the ether, yet the character of its selective absorption, as indicated by Kapteyn, is characteristic of a gas, and free gaseous molecules are certainly there, since they are probably constantly being expelled by the Sun and stars..."

The same year Victor Hess's discovery of cosmic rays, highly energetic charged particles that rain down on the Earth from space, led others to speculate whether they also pervaded interstellar space. The following year the Norwegian explorer and physicist Kristian Birkeland wrote: 'It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar [sic] systems or nebulae, but in "empty" space.' (See "Polar Magnetic Phenomena and Terrella Experiments", in The Norwegian Aurora Polaris Expedition 1902-1903 (publ. 1913, p.720).

In 1930, Samuel L. Thorndike notes that ".. it could scarcely have been believed that the enormous gaps between the stars are completely void. Terrestrial aurorae are not improbably excited by charged particles from the Sun emitted by the Sun. If the millions of other stars are also ejecting ions, as is undoubtedly true, no absolute vacuum can exist within the galaxy".

[edit] Interstellar Matter

[edit] The Three Phase Model

In 1969 Field, Goldsmith, & Habing put forward the a static two phase equilibrum model to explain the observed properties of the ISM. Their ISM consisted of a cold dense phase (T<300K), comprised of clouds of neutral and molecular hydrogen, and a warm intercloud phase (T~1,000K), comprised of rarefied neutral and ionized gas. McKee and Ostriker later added a dynamic third phase that represented the very hot (T = 1,000,000K) gas which had been expelled from supernovae and HII regions and constituted most of the volume of the ISM. Their 1977 paper formed the basis for further study over the past quarter-century. However, the relative proportions of the phases and their subdivisions are still a matter of considerable contention in scientific circles.

The following table shows a breakdown of the properties and origin of the components of the three phases.

Interstellar medium (ISM) phases
Component Fractional
Volume		 Temperature
(K)		 Density
(atoms/cm³)		 State
Molecular clouds < 1 % 20 - 50 103 - 106 hydrogen molecules
Cold Neutral Medium (CNM) 1-5% 50 - 100 1 - 103 neutral hydrogen atoms
Warm Neutral Medium (WNM) 10-20% 1000 - 5000 10-1 - 10 neutral hydrogen atoms
Warm Ionized Medium (WIM) 20-50% 103 - 104 0.01 ionized hydrogen
H II regions ~10% 104 102 - 104 ionized hydrogen
Coronal gas
Hot Ionized Medium (HIM)		 30-70% 106 - 107 10-4 - 10-2 highly ionized
(both hydrogen and trace metals)

[edit] Structures

Features prominent in the study of the interstellar medium include molecular clouds, interstellar clouds, supernova remnants, planetary nebulae, and similar diffuse structures.

[edit] Interstellar Extinction

The medium is also responsible for extinction and reddening, the decreasing light intensity and dominant observable wavelengths of a star as the light travels through the medium. These effects are caused by scattering and absorption of photons and allows the ISM to be observed with the naked eye in a dark sky. The rifts that can be seen in the band of the Milky Way are caused by absorption of background starlight from the uniform disk of stars by molecular clouds within a few thousand light years.

Far ultraviolet light is absorbed effectively by the neutral components of the ISM. For example, a typical absorption wavelength of atomic hydrogen lies at about 121.5 nanometers, the Lyman-alpha transition. Therefore, it is nearly impossible to see light emitted at that wavelength from a star farther than a few hundred light years from Earth, because most of it is absorbed during the trip to Earth by intervening neutral hydrogen.

[edit] Interstellar Radiation Field

The interstellar radiation field (ISRF) is the sum total of all electromagnetic radiation travelling through interstellar space.

[edit] Heating of the Interstellar Medium

[edit] Heating by Low Energy Cosmic Rays

The first mechanism proposed for heating the ISM was heating by low energy cosmic rays. 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. Cosmic rays are an efficient heating source able to penetrate in the depths of molecular clouds.

[edit] 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. Since the size distribution of dust grains is:

n(r) \propto r^{-3.5}

where r is the size of the dust particle, the grain area distribution is:

r^2 n \propto r^{-1.5}

This indicates that the smallest dust grains dominate this method of heating.

[edit] Photoionization

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.

[edit] 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.

[edit] Chemical Heating

Molecular hydrogen (H2) can be formed on the surface of dust grains when 2 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 transfered from de-excitation of the hydrogen molecule through collisions heats the gas.

[edit] 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 remnant 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. A measure of efficiency in the heating is given by the accommodation coefficient:

\alpha = \frac{T_2 - T}{T_d - T}

where T is the gas temperature, Td the dust temperature, and T2 the post-collision temperature of the gas atom/molecule. This coefficient was measure by Burke & Hollenbach (1983) as α = 0.35.

[edit] Other Heating Mechanisms

A variety of macroscopic heating mechanisms are present including:

  • Gravitational collapse of a cloud
  • Supernova explosions
  • Stellar Winds
  • Expansion of HII regions
  • Magnetohydrodynamic waves created by supernova remnants


[edit] Cooling of the Interstellar Medium

[edit] Fine Structure Cooling

This process is dominant in most regions of the ISM, 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 upper levels, which will eventually de-excite through photon emission, which will carry the energy out of the region.

[edit] 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.


[edit] References

  • Burke J.R. and Hollenbach D.J. 1983. ApJ, 265, 223.
  • Dyson, J. (1997) Physics of the Interstellar Medium, London: Taylor & Francis
  • Field, G. B., Goldsmith, D. W., & Habing, H. J. (2001). "Cosmic-Ray Heating of the Interstellar Gas". Astrophysical Journal 155: L149. (NASA Astronomy Abstract Service Entry)
  • Ferriere, D. (2001). "The Interstellar Environment of our Galaxy.". Reviews of Modern Physics 73 (4): 1031-1066. (astro-ph preprint)
  • Lequeux J. The Interstellar Medium. Springer 2005.
  • Pickering, W. H. (1912). "Solar system, the motion of the, relatively to the intersteller absorbing medium". Monthly Notices of the Royal Astronomical Society 72: 740. (NASA Astronomy Abstract Service Entry)
  • Thorndike, S. L. (1930). "Interstellar Matter". Publications of the Astronomical Society of the Pacific 42 (246): 99. (PASP online copy)
  • Spitzer, L. (1978) Physical Processes in the Interstellar Medium New York: Wiley.
  • Wisconsin H-Alpha Mapper Survey

[edit] See also

[edit] External links

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