Ay 102 Physics of the Interstellar Medium supplemental material - - PowerPoint PPT Presentation

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Ay 102 Physics of the Interstellar Medium supplemental material - - PowerPoint PPT Presentation

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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

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Ay 102 Physics of the Interstellar Medium

supplemental material Hillenbrand – Winter Term 2019-2020

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SLIDE 2

ISM Heating and Cooling

(putting it all together now)

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SLIDE 3

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
  • r plasma, so trick is to identify the most important

phenomena, i.e. what dominates heating + cooling.

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SLIDE 4
  • J. Williams

Dust

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SLIDE 5

slide courtesy of A. Glassgold

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SLIDE 6

Dopita & Sutherland

Example: Near a Photon Source

Gas

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SLIDE 7

Gas: Example Shielded from any Source

Klessen & Glover

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SLIDE 8

Example: Generic Place in the Galactic Wilderness

Gas

Dickey et al. (1983)

VLA Greenbank

(emission and absorption from the same place on the sky!)

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SLIDE 9

Dopita & Sutherland

Near (but not too near) a source of UV radiation CNM/WNM + WIM phases

Gas Temperature is Determined by Balance between Heating (Γ) and Cooling (Λ)

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SLIDE 10

slide courtesy of T. Greve

CMM CNM WNM HIM WIM

Primary Primary

Summary of the Important Processes

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SLIDE 11

The next couple of slides are the most important of those contained in the large slide deck below.

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SLIDE 12

Gas Heating Requirements

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SLIDE 13

slide courtesy of T. Bisbas

photoelectric dominates except deep inside clouds (this axis also scales to optical depth and density)

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SLIDE 14

Gas Cooling Requirements

  • Collisions with high enough frequency to populate excited

states:

  • from below, i.e. Clu
  • from above, i.e. Cul 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 Cul.

  • Escape of photons from the medium, i.e. τ < 1.
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SLIDE 15

Gas Cooling Mechanisms

  • 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!

Different for the different gas phases:

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SLIDE 16
  • T. Bisbas

CII and OI dominates except deep inside clouds (this axis also scales to optical depth and density)

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SLIDE 17

Dopita & Sutherland

The ISM has Equilibrium Phases

Draine

(not a continuum of n, T)

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SLIDE 18

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.

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SLIDE 19
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SLIDE 20

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

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SLIDE 21

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.

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SLIDE 22

Review of Heating Processes

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SLIDE 23

slide courtesy of A. Glassgold

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SLIDE 24

slide courtesy of A. Glassgold

  • If the electrons reach the surface of the

grain with enough energy, can escape into the gas phase, heating that too!

  • Details
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Gas Heating Requirements

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Heating via Photo-ionization

  • J. Graham
  • However, this is effective only where

the photons can penetrate.

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SLIDE 27

‘ ‘

Besides photo-ionization, which is a total absorption

  • f the photon, remember

that there are also other photon scattering processes.

Not all Photon Encounters Produce Heat

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SLIDE 28

slide courtesy of T. Bisbas

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SLIDE 29

slide courtesy of T. Bisbas

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SLIDE 30

slide courtesy of T. Bisbas

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SLIDE 31

slide courtesy of T. Bisbas

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SLIDE 32

slide courtesy of T. Bisbas

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SLIDE 33

slide courtesy of T. Bisbas

photoelectric dominates except deep inside clouds

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SLIDE 34

slide courtesy of T. Greve

CMM CNM WNM HIM WIM

Primary Primary

Summary of the Important Processes

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SLIDE 35

Review of Cooling Processes

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SLIDE 36

slide courtesy of A. Glassgold

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SLIDE 37

Gas Cooling Requirements

  • Collisions with high enough frequency to populate excited

states:

  • from below, i.e. Clu
  • from above, i.e. Cul 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 Cul.

  • Escape of photons from the medium, i.e. τ < 1.
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SLIDE 38

Gas Cooling Mechanisms

  • 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!

Different for the different gas phases:

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SLIDE 39

Klessen & Glover

CMM

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.

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SLIDE 40

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

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SLIDE 41

WIM

Draine

In ionized regions O II, O III, N II, N III, Ne II, Ne III, and S III dominate

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SLIDE 42

Draine

HIM

Breakdown by species

In hot regions, which element dominates the cooling varies as a function of temperature.

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SLIDE 43

Composition also matters for CIE

HIM

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SLIDE 44
  • T. Bisbas

CII and OI dominates except deep inside clouds

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SLIDE 45

slide courtesy of T. Greve

CMM CNM WNM HIM WIM

Primary Primary

Summary of the Important Processes

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SLIDE 46
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SLIDE 47

Heating and Cooling Equilibrium

  • 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 (3P0 è 3P1) and 63μm (3P1 è 3P2)
  • [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.

CNM/WNM

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SLIDE 48

Draine

WIM Heating and Cooling Equilibrium

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SLIDE 49

WIM

but does depend

  • n abundance

Draine, presumably

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SLIDE 50

WIM

and also depends

  • n density

Draine, presumably

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Need to Consider Proximity to a UV Source vs Only the Standard ISRF

  • P. Hartigan
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SLIDE 52

Dynamical Effects May also be Important

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SLIDE 53
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SLIDE 54

Dopita & Sutherland

The ISM has Equilibrium Phases

Draine

(not a continuum of n, T

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SLIDE 55

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

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SLIDE 56

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.

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SLIDE 57

Elmegreen