Editing Interstellar medium (section)

=== Heating mechanisms ===

; Heating by low-energy [[cosmic ray]]s: 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 [[electron]]s 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 by grains: The [[ultraviolet]] radiation emitted by hot stars can remove [[electron]]s from dust grains. The [[photon]] is absorbed by the dust grain, and some of its energy is used to overcome the potential energy barrier and remove the electron from the grain. This potential barrier is due to the binding energy of the electron (the [[work function]]) and the charge of the grain. The remainder of the photon's energy gives the ejected electron [[kinetic energy]] which heats the gas through collisions with other particles. A typical size distribution of dust grains is ''n''(''r'')&nbsp;∝&nbsp;''r''{{sup|−3.5}}, where ''r'' is the radius of the dust particle.<ref>{{cite journal | author1=Mathis, J.S. | author2=Rumpl, W. | author3=Nordsieck, K.H. | title=The size distribution of interstellar grains|journal=Astrophysical Journal | volume=217 | pages=425 | bibcode=1977ApJ...217..425M | doi=10.1086/155591| year=1977 }}</ref> Assuming this, the projected grain surface area distribution is ''πr''{{sup|2}}''n''(''r'')&nbsp;∝&nbsp;''r''{{sup|−1.5}}. This indicates that the smallest dust grains dominate this method of heating.<ref>{{cite journal | author1=Weingartner, J.C. | author2=Draine, B.T. | title=Photoelectric Emission from Interstellar Dust: Grain Charging and Gas Heating | journal=Astrophysical Journal Supplement Series | volume=134 | issue=2 | pages=263–281 | bibcode=2001ApJS..134..263W | doi=10.1086/320852| arxiv=astro-ph/9907251 | year=2001 | s2cid=13080988 }}</ref>
; Photoionization: When an electron is freed from an atom (typically from absorption of a UV photon) it carries kinetic energy away of the order ''E''{{sub|photon}}&nbsp;−&nbsp;''E''{{sub|ionization}}. This heating mechanism dominates in H&nbsp;II 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 [[ion]]s, 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-ray]]s 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]] (H<sub>2</sub>) can be formed on the surface of dust grains when two [[Hydrogen|H]] atoms (which can travel over the grain) meet. This process yields 4.48&nbsp;eV of energy distributed over the rotational and vibrational modes, kinetic energy of the H<sub>2</sub> 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 less 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. A measure of efficiency in the heating is given by the accommodation coefficient:
<math display="block">\alpha = \frac{T_2 - T}{T_d - T}</math>
where ''T'' is the gas temperature, ''T<sub>d</sub>'' the dust temperature, and ''T''<sub>2</sub> the post-collision temperature of the gas atom or molecule. This coefficient was measured by {{harvard citation|Burke|Hollenbach|1983}} as ''α''&nbsp;=&nbsp;0.35.

; Other heating mechanisms: A variety of macroscopic heating mechanisms are present including:
:* [[Gravitational collapse]] of a cloud
:* [[Supernova]] explosions
:* [[Stellar wind]]s
:* Expansion of [[H II region|H&nbsp;II regions]]
:* [[Magnetohydrodynamic]] waves created by supernova remnants