7.3 Heating and cooling processes in the ISM

The interstellar medium (ISM) is a complex system of gas and dust between stars. Its temperature is regulated by various heating and cooling processes, creating a delicate balance that shapes the ISM's structure and evolution.

Heating mechanisms like photoelectric effect and cosmic rays warm the gas, while cooling processes such as line emission and dust radiation remove heat. This balance determines the ISM's temperature, density, and ionization state, influencing star formation and galaxy evolution.

Heating Mechanisms in the ISM

Heating mechanisms in interstellar medium

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• Photoelectric heating occurs when UV photons interact with dust grains causing electrons to be ejected from grain surfaces transferring kinetic energy to gas particles (increases gas temperature)

• Cosmic ray heating involves high-energy particles (mostly protons) from outside solar system ionizing gas atoms and molecules producing secondary electrons that heat the gas (penetrates dense clouds)

• X-ray heating happens when X-rays from hot stars and compact objects are absorbed by gas leading to ionization and heating (important in vicinity of active galactic nuclei)

• Chemical heating releases energy through exothermic chemical reactions such as formation of H2 on dust grains (significant in dense molecular clouds)

Cooling Processes and Thermal Equilibrium

Cooling processes of interstellar gas

• Line emission cooling involves collisionally excited atoms and molecules emitting photons during electron transitions common coolants include CII, OI, CO, H2 (dominant in neutral atomic and molecular gas)

• Dust cooling occurs through collisions between gas particles and dust grains followed by thermal radiation from dust grains (efficient in dense regions)

• Recombination cooling happens when free electrons recombine with ions releasing excess energy as radiation (important in ionized regions)

• Bremsstrahlung (free-free) cooling results from electrons decelerating in ion fields emitting photons during deceleration (significant in hot ionized gas)

Thermal equilibrium in interstellar medium

• Thermal equilibrium achieved when balance between heating and cooling rates expressed as $\Gamma_{heating} = \Lambda_{cooling}$

• Determines temperature structure of ISM phases density distribution in different regions and ionization state of the gas

• Thermal instability arises from perturbations in equilibrium conditions leading to formation of distinct ISM phases (cold, warm, hot)

• Time scales for heating and cooling processes compared with dynamical time scales influence ISM evolution

Feedback processes for interstellar heating

• Supernovae feedback generates shock waves heating surrounding gas enriches medium with metals affecting cooling rates creates hot low-density bubbles (shapes large-scale ISM structure)

• Stellar winds input mechanical energy to ISM form wind-blown bubbles and shells compress surrounding medium (influence local ISM dynamics)

• Radiation pressure transfers momentum directly from photons to gas and dust important in high-luminosity environments (affects dust distribution)

• HII regions experience photoionization heating from massive stars expand as ionized bubbles (modify ISM composition and structure)

• Impact on ISM structure includes generation of turbulence triggering of star formation formation of galactic outflows and fountains

• Feedback regulation involves self-regulation of star formation maintenance of multi-phase ISM structure (crucial for galaxy evolution)