Ay 102 Physics of the Interstellar Medium supplemental material Hillenbrand – Winter Term 2019-2020
ISM Heating and Cooling (putting it all together now)
Calculations in the Galactic Ecosystem • Need to consider all permutations of the possible interactions (and reactions) among the atoms, ions, free electrons, and photons. • Collisional interactions: two-body / many-body. • Electrons: ionization / recombination excitation /de-excitation. • Photons: emission and absorption. • Not all processes are relevant for a given neutral gas or plasma, so trick is to identify the most important phenomena, i.e. what dominates heating + cooling .
Dust J. Williams
slide courtesy of A. Glassgold
Gas Example: Near a Photon Source Dopita & Sutherland
Gas: Example Shielded from any Source Klessen & Glover
Gas Example: Generic Place in the Galactic Wilderness Dickey et al. (1983) (emission and absorption from the same place on the sky!) VLA Greenbank
Gas Temperature is Determined by Balance between Heating ( Γ ) and Cooling ( Λ ) Near (but not too near) a source of UV radiation CNM/WNM + WIM phases Dopita & Sutherland
Summary of the Important Processes Primary Primary HIM WIM CMM CNM WNM slide courtesy of T. Greve
The next couple of slides are the most important of those contained in the large slide deck below.
Gas Heating Requirements
slide courtesy of T. Bisbas photoelectric dominates except deep inside clouds (this axis also scales to optical depth and density)
Gas Cooling Requirements • Collisions with high enough frequency to populate excited states: • from below, i.e. C lu • from above, i.e. C ul or recombination / cascade. • Energy exchange that is lower than the thermal energy of the gas (otherwise would be heating). • Transportation of the excitation energy by radiation, which must happen before next C ul . • Escape of photons from the medium, i.e. τ < 1.
Gas Cooling Mechanisms Different for the different gas phases: • CMM (molecular) • CNM/WNM (atomic) • WIM (ionized) • HIM (highly ionized) Recall that if CNM turns into CMM, stars can form, which then produce WIM and HIM from the inside-out!
T. Bisbas CII and OI dominates except deep inside clouds (this axis also scales to optical depth and density)
Dopita & Sutherland The ISM has Equilibrium Phases (not a continuum of n, T) Draine
View the ISM as being composed of numerous small (spherical!) clouds of molecular gas, each with an ionized halo (WIM) maintained by the interstellar UV background, surrounded by a neutral zone (WNM/CNM) that is heated by interstellar X-rays, and embedded in a diffuse hot ISM (HIM). Once star formation occurs, the centers of the molecular cloud cores become WIM, and soon after the massive stars explode as supernovae.
LOTS OF DETAIL ON THE VARIOUS PROCESSES FOLLOWS. WE HAVE ALREADY DISCUSSED THEM INDIVIDUALLY, BUT THE MATERIAL HERE PUTS IT ALL IN THE BROADER CONTEXT. I AM GIVING YOU THIS FOR COMPLETENESS AND YOUR GREATER APPRECIATION OF THE ISM
Ingredients for Heating è Cooling • Have dust and gas at different: – composition and relative abundances – densities and temperatures • Heating is determined by: – proximity to source of photons, and spectrum of source – local density and degree of shielding – regular photo and kinetic (collisional) processes – any dynamic (shock or turbulent) processes – details of the relevant heating (and cooling) mechanisms. • To first order, there is greater heating in the galactic plane and towards the galactic center – higher density n and “metallicity” z/z ⦿ gas – more photons, γ .
Review of Heating Processes
slide courtesy of A. Glassgold
- Details - If the electrons reach the surface of the grain with enough energy, can escape into the gas phase, heating that too! slide courtesy of A. Glassgold
Gas Heating Requirements
Heating via Photo-ionization J. Graham • However, this is effective only where the photons can penetrate.
Not all Photon Encounters Produce Heat Besides photo-ionization, which is a total absorption of the photon, remember that there are also other photon scattering processes. ‘ ‘
slide courtesy of T. Bisbas
slide courtesy of T. Bisbas
slide courtesy of T. Bisbas
slide courtesy of T. Bisbas
slide courtesy of T. Bisbas
slide courtesy of T. Bisbas photoelectric dominates except deep inside clouds
Summary of the Important Processes Primary Primary HIM WIM CMM CNM WNM slide courtesy of T. Greve
Review of Cooling Processes
slide courtesy of A. Glassgold
Gas Cooling Requirements • Collisions with high enough frequency to populate excited states: • from below, i.e. C lu • from above, i.e. C ul or recombination / cascade. • Energy exchange that is lower than the thermal energy of the gas (otherwise would be heating). • Transportation of the excitation energy by radiation, which must happen before next C ul . • Escape of photons from the medium, i.e. τ < 1.
Gas Cooling Mechanisms Different for the different gas phases: • CMM (molecular) • CNM/WNM (atomic) • WIM (ionized) • HIM (highly ionized) Recall that if CNM turns into CMM, stars can form, which then produce WIM and HIM from the inside-out!
CMM Klessen & Glover At cold temperatures, the most important coolants are CO rotational emission and a C I fine-structure line 23.4 K above the ground level. • As temp rises above a few tens of K, the CII line at 91.2 K above ground also contributes significantly.
CNM/WNM In neutral regions CII and OI dominate • Almost all carbon is in the form of C II, while almost all oxygen is in O I. • Si II, S II and Fe II are abundant but their fine structure transitions can be only excited by collisions with electrons and neutrals at high temperatures. • At low temperatures, only the upper fine-structure level of C II at 91.2 K is excited. • In warm neutral gas, the O I fine- structure level is at 228 K is populated. Draine
WIM In ionized regions O II, O III, N II, N III, Ne II, Ne III, and S III dominate • Draine
HIM In hot regions, which element dominates the cooling varies as a function of temperature. Breakdown by species Draine
HIM Composition also matters for CIE
T. Bisbas CII and OI dominates except deep inside clouds
Summary of the Important Processes Primary Primary HIM WIM CMM CNM WNM slide courtesy of T. Greve
Heating and Cooling Equilibrium CNM/WNM • In typical diffuse gas: – Dominant heating mechanism is photoelectric effect on dust. – Dominant cooling mechanism is forbidden line emission, especially low level “fine structure” lines of • [OI] at 146 μm ( 3 P 0 è 3 P 1 ) and 63μm ( 3 P 1 è 3 P 2 ) • [CII] at 158μm. – Detailed heating rate and cooling rate calculations for a typical density n ~100 cm -3 suggest typical gas kinetic temperatures of ~ 70-100 K. • In denser gas: – Cosmic rays dominate the heating, with H2 formation, turbulence, and gas-grain collisions growing in importance. – CI and then CO cooling mechanisms become important.
Heating and Cooling Equilibrium WIM Draine
but does depend on abundance WIM Draine, presumably
and also depends on density WIM Draine, presumably
Need to Consider Proximity to a UV Source vs Only the Standard ISRF P. Hartigan
Dynamical Effects May also be Important
Dopita & Sutherland The ISM has Equilibrium Phases (not a continuum of n, T Draine
Global Model for the ISM The “two-phase” model considers only neutral and ionized gas • (CNM plus WNM, and WIM): – heating dominated by photoelectric effect – cooling dominated by line radiation. “Three-phase” model includes the HIM, originating in dynamical • phenomena such as the injection of energy from supernovae but also need ``photon leakage” from HII regions. More recent recognition that full ``five-phase” treatment needed. • . • Heiles / Elmegreen
View the ISM as being composed of numerous small (spherical!) clouds of molecular gas, each with an ionized halo (WIM) maintained by the interstellar UV background, surrounded by a neutral zone (WNM/CNM) that is heated by interstellar X-rays, and embedded in a diffuse hot ISM (HIM). Once star formation occurs, the centers of the molecular cloud cores become WIM, and soon after the massive stars explode as supernovae.
Elmegreen
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