Glossary

7.3 Heating and cooling processes in the ISM

2 min readโ€ขjuly 25, 2024

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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  • 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)

  • 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)

  • 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)

  • 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

  • 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)

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

  • 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

  • achieved when balance between heating and cooling rates expressed as ฮ“heating=ฮ›cooling\Gamma_{heating} = \Lambda_{cooling}

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

  • 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)

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

  • 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)

Key Terms to Review (14)

Bremsstrahlung cooling: Bremsstrahlung cooling refers to the process through which hot, ionized gas loses energy via the emission of radiation when charged particles, such as electrons, are deflected by ions. This radiation loss is significant in the interstellar medium, as it helps regulate temperatures and influences the thermal balance of astrophysical environments. Understanding this cooling process is crucial for comprehending how energy dynamics operate in space.
Chemical heating: Chemical heating refers to the process by which energy is released during chemical reactions, particularly in the context of interstellar gas and dust. This energy release plays a significant role in heating the interstellar medium (ISM), impacting star formation and the thermal balance of various cosmic structures.
Cosmic ray heating: Cosmic ray heating refers to the process by which high-energy particles, primarily protons and atomic nuclei from outer space, collide with the interstellar medium (ISM) and transfer their energy to gas and dust in the ISM. This process plays a significant role in influencing the thermal balance of the ISM, affecting its temperature and the dynamics of star formation.
Dust cooling: Dust cooling refers to the process by which interstellar dust grains absorb energy from their surroundings and then re-radiate that energy as thermal radiation, primarily in the infrared spectrum. This cooling mechanism plays a crucial role in regulating the temperature of the interstellar medium (ISM) and influences star formation by removing heat from dense regions, allowing gas to collapse and form stars.
H II regions: H II regions are large clouds of ionized hydrogen gas that occur in the interstellar medium, primarily found around young, hot stars. These regions are important because they are sites of active star formation and play a critical role in the evolution of galaxies, contributing to the chemical enrichment of the universe as they interact with surrounding material.
Line emission cooling: Line emission cooling refers to the process where gas in space, particularly in the interstellar medium (ISM), loses energy through the emission of specific wavelengths of light (or photons) as electrons transition between different energy levels in atoms or ions. This mechanism is crucial in regulating the temperature of the ISM, especially in regions where gases become ionized or heated, as it allows the gas to cool and return to thermal equilibrium.
Photoelectric heating: Photoelectric heating refers to the process where energy from photons, particularly ultraviolet and visible light, is absorbed by particles in the interstellar medium (ISM), leading to an increase in thermal energy. This process is significant in understanding how regions of space heat up, especially in areas with high levels of radiation, impacting the physical state and dynamics of the ISM.
Radiation Pressure: Radiation pressure is the pressure exerted by electromagnetic radiation on a surface, resulting from the momentum transfer of photons when they collide with that surface. This pressure plays a significant role in various astrophysical processes, including the dynamics of stellar atmospheres and the behavior of interstellar gas and dust.
Recombination Cooling: Recombination cooling refers to the process by which the temperature of a gas decreases as free electrons recombine with ions to form neutral atoms, releasing energy in the form of photons. This cooling mechanism is particularly significant in astrophysical contexts, especially within the interstellar medium (ISM), where it plays a crucial role in regulating the temperature of ionized gases and influencing star formation processes.
Stellar winds: Stellar winds are streams of charged particles ejected from the outer layers of a star into space. These winds play a crucial role in shaping the environment around stars and influence the interstellar medium by contributing to its composition and dynamics. They can also affect the evolution of stars, particularly massive ones, by stripping away their outer layers and altering their luminosity.
Supernova feedback: Supernova feedback refers to the processes through which the explosive death of a massive star impacts its surrounding environment, particularly in the interstellar medium (ISM). This phenomenon can inject energy and momentum into the ISM, influencing heating, gas dynamics, and the formation of new stars. The interplay between supernova feedback and other heating and cooling processes is crucial for understanding the lifecycle of galaxies and the evolution of the ISM.
Thermal equilibrium: Thermal equilibrium is a state in which two or more bodies or systems exchange no net heat energy because they are at the same temperature. In this condition, energy flow ceases, leading to a stable configuration where all interacting components maintain consistent thermal properties. Understanding thermal equilibrium is crucial for exploring how stars maintain their internal structure and how different phases of matter interact in various environments.
Thermal instability: Thermal instability refers to a condition where a system becomes unstable due to variations in temperature, leading to rapid changes in pressure and density. In astrophysics, this phenomenon often plays a crucial role in the heating and cooling processes within interstellar matter, as well as influencing the behavior of pulsating and cataclysmic variable stars. It is fundamentally linked to how energy is distributed and released in these systems, affecting their evolution and dynamics.
X-ray heating: X-ray heating refers to the process by which high-energy X-rays from various astrophysical sources, like supernova remnants or accreting black holes, transfer energy to the interstellar medium (ISM), causing an increase in temperature. This heating mechanism plays a crucial role in regulating the thermal balance and dynamics of the ISM, influencing star formation and the overall evolution of galaxies.
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