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

Ay 102: Physics of the Interstellar Medium

supplemental material Hillenbrand – Winter Term 2019-2020

Warm Ionized Medium

Greater Orion star-forming region (~30 x 50 pc at d=400 pc)

LMC (d=50 kpc)

[OI] Hα [SII]

NGC 346 in the SMC (d=61 kpc)

(REST)

Everything we think we know about star formation in galaxies!

[SII]

Random higher redshift

• bject

solar core this room

Studied via:

• HII (ionized H)
• metal lines

(moderately ionized and ``hot”)

High T Wide range of n Ionized Gas

WIM

Important neutral species

The low-level lines of OI (oxygen) and SI (sulfer) are good probes of the CNM/WNM. The higher-level lines are “hotter” and can probe WIM gas.

Important singly ionized species

[OII] and [SII] are commonly used to study photo-heated or shocked gas, along with [NII] (not shown).

Important doubly ionized species

[OIII]

Ionized Gas and HII Regions

• K. Luhman

• J. Williams

Consider Wien side of

based on J. Williams (re-ionized) (if re-ionized) (if cascades) (if cascades all the way down) (if cascades all the way down and is excited to higher n, rather than ionized)

• 0.85
• 0.85

NOTE: This is the inverse concept of what in Ay101 we would have called κbf (bound-free opacity)

• r σbf (bound-free

cross section). Instead it is quantifying the free-bound process with α = < σ v >.

Can recombine to any energy level, depending on KE of the collision, and thus on T

Recombination Caveat

• Free electrons are more likely to collide with

each other than with the ions.

• Electrons have the maxwell-boltzmann velocity

distribution at Te, and e- -- e- collisions have cross section 10-13 cm-2, so 1/nσv ~ few weeks.

• Recombination time ~ 103 years

n ~ 100 cm-3 and

Osterbrock & Ferland

Practical Application:

Note that the values across rows do not change much, so HI-line ratios are relatively insensitive to temperature. Thus, if you expect these ratios, you can use observed line ratios to measure the extinction!

Osterbrock & Ferland

Practical Application:

Note that the values across rows do not change much, so HI-line ratios are relatively insensitive to temperature. Thus, if you expect these ratios, you can use observed line ratios to measure the extinction!

Osterbrock & Ferland

Practical Application:

More change across rows for HeII lines compared to HI

Summarizing Recombination (of H)

• J. Williams

Photoionization is balanced by recombination, which can occur to any level, and is followed by quick downward radiative transition from the particular excited level all the way to the ground level.

es

Summarizing Photo-Ionization (of H)

Because of high A (spontaneous de-excitation) values, can consider all HI to be in the ground state 12S, so all ionized e- are considered to come from only n=1. Number of ionizing photons is NLyC =

• J. Graham

For hydrogen, Z = 1 and σν = 6.33 x 10-18 cm2 (ν/νο)−3.

Equilibrium

• ISM is not in LTE, so Saha Equation for ionization vs

recombination (e.g. from stars) is not valid.

• Must write down the rates.
• Can also consider charge neutrality, ne = Σ Zi ni
• Assume that the time scale for change in the ion,

electron, and neutral densities (ni, ne, nn) is longer than the time scales for ionization and recombination.

based on J. Williams

HII regions are (almost) completely ionized. Do define an ionization fraction f = ne/ntotal

based on J. Williams =

=

Define Stromgren radius such that:

(really #/sec not just #)

Stromgren Sphere Parameters

Osterbrock & Ferland

Sizes given in the rightmost column are for n = 1 and Rs scales as n-2/3.

Osterbrock & Ferland

HII regions have sharp boundaries!

photon.

based on K. Luhman

HII regions have sharp boundaries!

Not all of the Lyman Continuum Photons Escape Due to Re-Absorption

Also, ionizing spectrum becomes “harder” further away from the ionizing source as the “softer” photons have already been absorbed.

β = ”escape fraction”

Osterbrock & Ferland

HeII Regions

Osterbrock & Ferland

HeII Regions

(O6 star) (B0 star)

• C. Hirata

very!hard!sources.!!Further!ionizations!involve!the!inner!shell!and!are!only!acces rays!or!extremely!hot!gas.! ! Zone:! ! H!I! ! H+/He0! He+! ! He2+!

!

! ! Hydrogen! H!I! ! H!II! ! H!II! ! H!II! ! Helium! He!I! ! He!I! ! He!II! ! He!III! ! ! Carbon! C!II! ! C!II! ! C!III/C!IV! C!IV/C!V! Nitrogen! N!I! ! N!II! ! N!III! ! N!IV/N!V! Oxygen! O!I! ! O!II! ! O!II/O!III! O!IV/O!V! Neon! ! Ne!I! ! Ne!II! ! Ne!II/Ne!III! Ne!III/Ne!IV! Magnesium! Mg!II! ! Mg!III! ! Mg!III! ! Mg!III/Mg!IV! Silicon!! Si!II! ! Si!III! ! Si!III—V! Si!V! Sulfur! ! S!II! ! S!II! ! S!III—V!! S!V/S!VI! Iron! ! Fe!II! ! Fe!III! ! Fe!III/Fe!IV! Fe!V/Fe!VI! ! You!should!be!aware!that:! !

HeI/HeII + Other Ion Pairs are Analogs to HI/HII

The higher χi (ionization potential) species are not found in HII regions associated with massive stars. These require more energetic sources of ionizing photons such as AGN, producing lots of xrays.

Dopita & Sutherland

(this week in WIM) (some later week in HIM)

τν =∫ ανdν

Opacity at High Energies

Collisional Excitation of Metal Lines

In addition to H and possible He ionization/recombination….

(because the H is fully ionized there are lots of free electrons flying around)

Also Collisional De-excitation

(recall the importance of critical density!)

Structure of an HII Region

Dopita & Sutherland

Expansion of an HII Region

Osterbrock & Ferland

Besides Stromgren sphere-like HII Regions, also have ionized gas on larger scales, called PDRs for Photon-Dominated Regions a.k.a. Photo-Dissociation Regions.

• P. Hartigan

Draine