Interstellar Medium - AstroWiki

## Phases ### The Different Phases of Medium ![[ISM_phasesTable.png|align:center]] The ISM has 6 Major Phases: - [[#HII Regions]] - [[#Hot Ionized Medium (HIM)]] - [[#Warm Ionized Medium (WIM)]] - [[#Warm Neutral Medium (WNM)]] - [[#Cold Neutral Medium (CNM)]] - [[#Molecular Clouds]] > [!note] Three-Phase Model > There is also a three phase model that describes the ISM gas as... > > - **Diffuse Cloud** - colder dense phase of neutral and molecular hydrogen > - **Intercloud Gas** - warmer phase of rarefied neutral and ionized gas > - **Coronal Gas** - very hot phase heated by supernovae > > The downside of this model is that the relative proportions of the phases and their subdivisions are still not well understood. Given the properties outlined in the table above, we can create a useful temperature-density plot that shows how the gas evolves over time. The gas (roughly plotted) mostly follows a straight line on the log-log plot with a slope of $-1$. ![[ISM_phasesPlot_combined.png|align:center]] This means that the different phases of the ISM are in pressure equilibrium, if we assume no magnetic fields exist and if $n\propto T^{-1}$ for all phases. If the phases weren't in equilibrium, the pressure balance would cause the phases to blend together. $T \propto n^{-1} \implies P \propto n T \propto \text{constant} \implies \delta P = 0$ Roughly speaking, we can also organize the phases of the ISM in space by their densities and temperatures. In the cold regions, there is the [[#Molecular Clouds|molecular clouds]]. These regions are surrounded by more diffuse and warmer mediums, moving up until everything is surrounded by the [[#Hot Ionized Medium (HIM)]]. \[ Basically, (cold + dense) $\longrightarrow$ (hot + diffuse) \] ![[ISM_phasesStructure.png|align:center|450]] > [!image] [Phase Structure](https://www.researchgate.net/publication/237450083_A_modified_theoretical_S_-_D_relation_for_supernova_remnants_I_The_case_of_constant_temperature_within_the_supernova_remnant) > > ![[ISM_phasesStructure2.png|align:center|550]] > > (Left) A segment of the average small interstellar cloud. The central part represents the cold core of the cloud (CNM). The middle layer is the warm neutral medium (WNM), and the outer layer represents the warm ionized medium (WIM). Concentrations and temperatures for each of these cases are given. > > (Right) A region of the interstellar matter of a size of 30 × 40 pc. The shock wave of the supernova is expanding from the upper right corner of the picture. The cold core radii (black circles) are in the interval of 0.4 -1 pc. All of the clouds with cores have warm wrapping (dotted areas) with radii around 2.1 pc. Some clouds are too small to have cold cores. Cloud envelopes inside the SNR are compressed and destroyed (from M&O). ### Molecular Clouds | ISM %Mass | ISM % Volume | Scale Height ${\rm [pc]}$ | Temperature ${\rm [K]}$ | Density ${\rm \left[ \frac{particles}{cm^{3}} \right]}$ | State of Hydrogen | Observational Traces | | ---------------------- | :----------: | :-----------------------------------------: | :--------------------------: | :---------------------------------------------------------: | :---------------: | ---------------------------------------------------- | | $\sim 30 \%$ | lt; 1 \%$ | $80$ | $10-20$ | $10^{2}-10^{6}$ | molecular | Radio and IR molecular emission and absorption lines | A (mostly) [[Binding Energy#Gravitational Binding Energy|gravitationally bound]] cloud of gas composed primarily of molecular hydrogen and [[#Dust|dust]]. Since the region has "cores" that are unstable to collapse, they tend to become sites for star formation. If enough dust is present and the region is dense enough to block visible light from background sources like stars, [[Nebulae#Emission nebula|emission nebulae]] or [[Nebulae#Reflection nebula|reflection nebulae]], it can also be called a "[[Nebulae#Dark Nebula]]". Very filamentary and chaotic-looking structure, which arises from a combination of gravitational collapse and turbulence (large scale). ### Cold Neutral Medium (CNM) *(Also known as **HI Clouds**)* | ISM %Mass | ISM % Volume | Scale Height ${\rm [pc]}$ | Temperature ${\rm [K]}$ | Density ${\rm \left[ \frac{particles}{cm^{3}} \right]}$ | State of Hydrogen | Observational Traces | | :----------------------: | :----------: | :-----------------------------------------: | :--------------------------: | :---------------------------------------------------------: | :----------------------------------: | ---------------------------------------------- | | $\sim 50 \%lt;br>(w/ WNM) | $1-5 \%$ | $100-300$ | $50-100$ | $20-50$ | [[#Atomic Hydrogen\|neutral atomic]] | HI [[Spectral Features#21 cm line]] absorption | **Notes:** - Traced by the *absorption* of the [[Spectral Features#21cm line]] (1420 MHz) of [[#Atomic Hydrogen|HI]] as the result of a low-probability transition (electron spin flip) in the hyperfine structure of the ground energy level of H - Serve as excellent tracers of spiral structure ### Warm Neutral Medium (WNM) *(Also known as **HI Clouds**)* | ISM %Mass | ISM % Volume | Scale Height ${\rm [pc]}$ | Temperature ${\rm [K]}$ | Density ${\rm \left[ \frac{particles}{cm^{3}} \right]}$ | State of Hydrogen | Observational Traces | | :----------------------: | :----------: | :-----------------------------------------: | :--------------------------: | :---------------------------------------------------------: | :----------------------------------: | -------------------------------------------- | | $\sim 50 \%lt;br>(w/ CNM) | $10-20 \%$ | $300-400$ | $6000-10000$ | $0.2-0.5$ | [[#Atomic Hydrogen\|neutral atomic]] | HI [[Spectral Features#21 cm line]] emission | **Notes:** - Traced by the *emission* of the [[Spectral Features#21cm line]] (1420 MHz) of [[#Atomic Hydrogen|HI]] as the result of a low-probability transition (electron spin flip) in the hyperfine structure of the ground energy level of H - Form at the boundaries of [[#Molecular Clouds|molecular clouds]] and [[#HII Regions]]. - Serve as excellent tracers of spiral structure ### Warm Ionized Medium (WIM) *(Also known as the **Diffuse Gas**)* *(Also known as part of the **Intercloud Gas**)* | ISM %Mass | ISM % Volume | Scale Height ${\rm [pc]}$ | Temperature ${\rm [K]}$ | Density ${\rm \left[ \frac{particles}{cm^{3}} \right]}$ | State of Hydrogen | Observational Traces | | ---------------------- | :----------: | :-----------------------------------------: | :--------------------------: | :---------------------------------------------------------: | :---------------: | ------------------------------------------------------------------------------------- | | $\sim 30 \%$ | $20-50 \%$ | $1000$ | $8000$ | $0.2-0.5$ | ionized | [[Spectral Features#H-alpha]] emission and [[Neutron Star#Pulsar\|pulsar]] dispersion | Primarily surrounding [[Spectral Classes#O]] & [[Spectral Classes#B]] stars and hot [[Stellar Classes#White Dwarf|WDs]]. ### Hot Ionized Medium (HIM) *(Also known as **Coronal Gas**)* | ISM %Mass | ISM % Volume | Scale Height ${\rm [pc]}$ | Temperature ${\rm [K]}$ | Density ${\rm \left[ \frac{particles}{cm^{3}} \right]}$ | State of Hydrogen | Observational Traces | | ---------------------- | :----------: | :-----------------------------------------: | :--------------------------: | :---------------------------------------------------------: | :---------------------------------------: | --------------------------------------------------------------------------------------- | | Very little | $30-70 \%$ | $1000-3000$ | $10^{6}-10^{7}$ | $10^{−4}-10^{−2}$ | ionized (metals also highly ionized) | X-ray emission; absorption lines of highly ionized metals, primarily in the ultraviolet | **Notes:** - Comprised of highly ionized atoms, primarily heated by [[Stellar Explosions#Supernova|SNe]] explosions and stellar winds. - Occupies the majority of the volume in the ISM, as seen in the broad emission features in [[#Ionized Hydrogen|HI]] spectra of extragalactic sources. - Observed through UV absorption lines and in the soft x-ray background ### HII Regions | ISM %Mass | ISM % Volume | Scale Height ${\rm [pc]}$ | Temperature ${\rm [K]}$ | Density ${\rm \left[ \frac{particles}{cm^{3}} \right]}$ | State of Hydrogen | Observational Traces | | ---------------------- | :----------: | :-----------------------------------------: | :--------------------------: | :---------------------------------------------------------: | :---------------: | ---------------------------------------------------------------------------------------- | | | lt; 1 \%$ | $70$ | $8000$ | $10^{2}-10^{4}$ | ionized | [[Spectral Features#H-alpha]] emission, pulsar dispersion, and radio recombination lines | **HII Regions** are... - Dense, hot regions of [[#Ionized Hydrogen|singly ionized hydrogen]] found around hot, young stars ([[Spectral Classes#O]] & [[Spectral Classes#B]] stars) - The gas is ionized by the star's strong stellar winds (observed by the [[Spectral Features#H-alpha]] line) In galaxies, HII regions are found in: - the arms of [[Galaxy Classification#Spiral Galaxy|spiral galaxies]] - chaotically in [[Galaxy Classification#Irregular Galaxy|irregular galaxies]] - nowhere in [[Galaxy Classification#Elliptical Galaxy|elliptical galaxies]] *(Also see [[Question 73|Strömgren Radius]].)* --- ## Components (https://www.astro.princeton.edu/~burrows/classes/204/ISM.pdf) ### Atomic Hydrogen A single atom of hydrogen ($\ce{HI = H_{1} = _{1}^{1}H}$), composed of a proton ($p$) and electron ($e^{-}$) with a neutron charge. **Notes:** - Comprises $\sim 60 \%$ (by mass) of matter in the ISM - Primarily heated by photoionization and cooled by line emission - Detectable through UV absorption (see [[Spectral Features#Lyman Series]]) or the [[Spectral Features#21cm line]] - Other isotopes of hydrogen like deuterium ($\ce{D = _{1}^{2}H}$) or tritium ($\ce{D = _{1}^{3}H}$) can also be present, but in significantly smaller quantities. - By its symmetry, atomic hydrogen does not have any rotational modes, only translational and vibrational. ### Ionized Hydrogen Charged [[#Atomic Hydrogen|atomic hydrogen]] ($\ce{HII = H^{+} = H_{1}^{+} = _{1}^{1}H^{+}}$), or in other words, just a proton ($p$). **Notes:** - For volume hydrogen gas to become completely ionized, it must exceed temperatures of $T \gtrsim 10^{4} \; {\rm K}$. Though, a fraction of the gas might start becoming ionized near $T \sim 7000 \; {\rm K}$) ### Molecular Hydrogen Molecular hydrogen ($\ce{H_{2}}$) is a diatomic gas comprised of two hydrogen atoms, united with a covalent bond. **Notes:** - Most abundant molecule in ISM - Degrees of Freedom: 3 translational + 2 rotational + 1 vibrational = 6 total - There is no [[Spectral Features#Balmer Series]] equivalent because the binding energy of molecular hydrogen is $\sim 4.75 \; {\rm eV}$. Any absorption of energies that high would just unbind the molecule. - Finding $\ce{H_{2}}$ was usually done by measuring $\ce{CO}$ with millimeter telescopes and then constraining the $\ce{CO-H_{2}}$ correlation. Now with [[Instruments#JWST]], this can be done better, since we have a [[Electromagnetic Spectrum#Mid-IR]] integrated field unit (IFU) in space. ### Hot Gas Hot Gas ($T > 10^{6} \; {\rm K}$) is produced by fast stellar winds, [[Stellar Explosions#Nova|novae events]], and [[Stellar Explosions#Supernova|supernovae events]]. Over time, it uses [[Bremsstrahlung Radiation|bremsstrahlung radiation]] as a cooling mechanism, emitting x-ray radiation in the process. Some of the gas can be ejected into the galactic halo, or leave the galaxy entirely. ### Dust Composition depends strongly on environment; micron-scale grains of silicates, hydrocarbons, ice and/or other molecules like $\ce{CO}$. Absorbs in [[Electromagnetic Spectrum|UV]] and emits [[Blackbody Radiation|thermally]] in far-IR (near IR remains fairly unaffected). Typically produced by fairly evolved stars shedding their outer layers. However: We've seen very high redshift objects obscured by dust, suggesting another mechanism (maybe SNe?) can produce dust at earlier times. ### Ice Volatiles are a group of chemical elements and compounds that can be readily vaporized. **Examples:** - nitrogen (${\rm N}_{2}$) - carbon dioxide ($\rm{CO_2}$) - ammonia ($\rm{NH_3}$) - hydrogen (${\rm H}_{2}$) - methane ($\rm{CH_4}$) - sulfur dioxide (${\rm SO}_{2}$) - water ($\rm{H_2O}$) ### Cosmic Rays **Cosmic rays** are a collection of energetic protons, electrons, and heavier nuclei traveling near the speed of light through the ISM. They are mostly thought to be produced through [[Stellar Explosions#Supernova|supernovae]] events as the particles are accelerated to relativistic speeds by shock acceleration; however, there could be a variety of other high energy astrophysical sources as well. **Notes:** - Serve as am important source for atmospheric ionization (detected via air showers) - Lower energy CRs originate and are confined to the galaxy - Higher energy CRs originate from extragalactic sources - The charged CRs can be modulated by galactic, solar, and terrestrial electromagnetic fields. ### Radiation Field (including [[Cosmic Microwave Background|CMB]], starlight, and dust)