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] H α [SII]

NGC 346 in the SMC (d=61 kpc)

Random higher redshift object [SII] (REST) Everything we think we know about star formation in galaxies!

solar core WIM this room High T Wide range of n Ionized Gas Studied via: - HII (ionized H) - metal lines (moderately ionized and ``hot”)

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

Consider Wien side of J. Williams

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

NOTE: This is the inverse concept of what in Ay101 we would have called κ bf (bound-free opacity) or σ bf (bound-free cross section). Instead it is quantifying the free-bound process with α = < σ v >. -0.85 Can recombine to any energy level, depending on KE of the collision, and thus on T -0.85

Recombination Caveat • Free electrons are more likely to collide with each other than with the ions. • Electrons have the maxwell-boltzmann velocity distribution at T e , and e - -- e - collisions have cross section 10 -13 cm -2 , so 1/nσv ~ few weeks. • Recombination time ~ 10 3 years n ~ 100 cm -3 and

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 Osterbrock & Ferland

Summarizing Recombination (of H) 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 J. Williams

Summarizing Photo-Ionization (of H) Because of high A (spontaneous de-excitation) values, can consider all HI to be in the ground state 1 2 S, so all ionized e - are considered to come from only n=1. Number of ionizing photons is N LyC =

For hydrogen, Z = 1 and σ ν = 6.33 x 10 -18 cm 2 ( ν / ν ο ) − 3 . J. Graham

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, n e = Σ Z i n i • Assume that the time scale for change in the ion, electron, and neutral densities (n i , n e , n n ) is longer than the time scales for ionization and recombination.

HII regions are (almost) completely ionized. Do define an ionization fraction f = n e /n total based on J. Williams

= = (really #/sec not just #) Define Stromgren radius such that: based on J. Williams

Stromgren Sphere Parameters Sizes given in the rightmost column are for n = 1 and Rs scales as n -2/3 . Osterbrock & Ferland

Osterbrock & Ferland HII regions have sharp boundaries!

HII regions have sharp boundaries! photon. based on K. Luhman

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”

HeII Regions Osterbrock & Ferland

HeII Regions (O6 star) (B0 star) Osterbrock & Ferland

HeI/HeII + Other Ion Pairs are Analogs to HI/HII very!hard!sources.!!Further!ionizations!involve!the!inner!shell!and!are!only!acces rays!or!extremely!hot!gas.! ! Zone:! ! H! I ! ! H + /He 0 ! He + ! ! He 2+! ! ! ! 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 ! ! The higher χ i (ionization potential) species are not You!should!be!aware!that:! ! found in HII regions associated with massive stars. These require more energetic sources of ionizing photons such as AGN, producing lots of xrays. C. Hirata

(this week in WIM) Opacity at High Energies τ ν = ∫ α ν d ν (some later week in HIM) Dopita & Sutherland

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