Ay 102 Physics of the Interstellar Medium supplemental material Hillenbrand – Winter Term 2019-2020
solar core Low-T, High-n (relative to rest of ISM) Studied via: - CO, most readily - Other molecules at high density - H2 lines at warm temperatures
merging galaxies SFE = SFR/M_gas è compression Mgas = atomic of ism + è molecular gas turns from atomic SFR from UV to molecular è induced star formation
Molecular Gas Arm : Interarm contrast ~30:1 (compare to 2.5:1 for HI) Note pattern of gas density relative to the stellar light M 51 (face-on)
Molecular Gas Similar to HI maps but now measuring cold, dense gas Milky Way (edge-on, from the inside)
Molecular Gas Milky Way velocity vs longitude è galactic structure e.g. n (R, θ , z)
Molecular Gas è Star Formation Young star clusters trace the Giant Molecular Clouds The Orion GMC/s (in green) and recently formed stars
Molecular Gas -- a hierarchy of physical structures
Channel Maps = Movie in Velocity
Molecular Gas -- main diagnostics
Transitions between energy levels: Molecules - rotational - vibrational - electronic
Molecules
Molecules
Molecules
Molecules
Molecular Energy Levels • Recall the “Grotrian diagrams” for the energy levels of atoms/ions showing the n, l layout, and the transitions labelled by their spectroscopic terms. • For molecules there are “Jablonksi diagrams” illustrating the rotational, vibrational, and electronic energy levels, and the transitions between them. As for atoms/ions, the rotational levels vibrational level transitions can be radiative (either within a consuming or producing photons), or non-radiative (involving collisions).
Rotational Modes: Linear Molecules where B = h / 8 π 2 I + J. Williams
Rotational Modes: Linear Molecules J. Williams
Rotational Modes: Nonlinear Molecules “Symmetric Top” J. Williams
Rotational Modes: Nonlinear Molecules “Symmetric Top” J. Williams
Rotational Modes: Nonlinear Molecules “Spherical Top” (Special Case of Symmetric) I A = I B = I C No dipole changes under rotation, so no rotational mode radiation. Vibrations though!
Rotational Modes: Nonlinear Molecules “Asymmetric Top” J. Williams
Molecular Astrophysics Others is Complicated! e.g. “seeds of life”
Now Vibrational Modes
Vibrational Modes CHON molecular bands in infrared (X is generic for some random heavier atom) Tielens + J. Williams
Now Ro-Vibrational Modes (accounting for both vibration and rotation) rotational levels vibrational level within a Dopita & Sutherland
Now Ro-Vibrational Modes (accounting for both vibration and rotation) (Boogert et al. 2002)
Ro-Vibrational Modes (accounting for both vibration and rotation) Δ J = +/- 1 between Δ v=any vibrational levels Sieghard “Fundamentals of Kwok vibration-rotation spectroscopy”
Evidence model that this formalism data actually describes reality Kwok
Also CO ro-vibrational bands in the ultraviolet
Also, Electronic States designated as 2 S+1 Λ +/- Ω , g / u
Also, Electronic States designated as 2 S+1 Λ +/- Ω , g / u ground state è It is actually even a little more complicated than this…… figure by W.-F. Thi
Ro-Vibrational vibrational levels electronic level Including within an Electronic States Dopita & Sutherland
Example: OH Levels in FIR and radio Recall that the quantum number “F” includes nuclear spin.
Molecules: More Vocabulary • Fluorescence : following absorption of a photon, there can be non-radiative “vibrational relaxation”, followed by almost immediate re-emission of a photon. • Phosphorescence : following absorption and some “vibrational relaxation”, have a non-radiative “intersystem crossing” to a different spin state, followed by more vibrational relaxation, and eventual photon re-emission. The ISC involves a forbidden transition with slow time scales, so the radiation is delayed. CO level population fluorescence pumping
Two Key Molecules • H 2 = molecular hydrogen – the most important • CO = carbon monoxide – the most observed
Why H 2 is So Important? • Most abundant molecule. • Relatively robust molecule, compared to others which are much more readily photo-dissociated. • At low temperature, it’s the most stable form of H. • Can form in primordial hot gas via H + H - but typical production is on ~10K dust grains. • Destroyed by UV photons • 4.5 eV for photo-dissociation • 15.6 eV for photo-ionization
The H 2 Molecule & Lines S(3) S(2) S(1) S(0) • no dipole • quadrupolewith ΔJ = +2 • para = even transitions • ortho = odd transitions • light and small molecule Dopita & Sutherland
Rotational- Vibrational Ladders for H 2
Lyman-alpha “Photon Pumping” of Hot H 2 These are electronic transitions Dopita & Sutherland
Lots of Hot H 2 Lines in UV part of the spectrum Here seen in absorption, they indicate ubiquitous presence of translucent, diffuse clouds. Sensitive to very low column densities, N(H 2 ) > 10 14 cm -2 (Ly 4-0) FUSE spectrum of the hot LMC star Sk-67-166 (Tumlinson et al 2002) Derive N(H 2 )= 5.5x10 15 cm -2 (compare to 2x10 21 cm -2 for Av = 1 mag)
“Collisional Pumping” of Warm H 2 lines (seen in near- and mid-infrared ) • Following J=2-0 de-excitation that radiates in the mid-infrared, collisions can populate low-J excited levels. Subsequently, have ro-vibrational transitions e.g. v=1-0 S(1) in near-infrared at 2.2um. • This happens in shocked regions such as molecular outflows and supernovae with n > 10 4 cm -3 , T > 2000 K (but < 4000-5000 K since by then H 2 would dissociate).
Cold H 2 ? Needs a Tracer = CO • In the cold ISM, H 2 is shieldedby HI. Since HI is photoionizedat 13.6 eV this takes all the would-bephoto-dissociatingphotons that would otherwise be availableto destroy the H 2 (>4.48 eV). • H 2 can be also `self-shielded’ from standard ISRF when at N > 10 20 cm -2 • However, because of the lack of a dipolemoment -- either electric or magnetic -- cold H 2 is not detectabledirectlysince don’t populateJ=2. • CO is used as a tracer, even though CO/H 2 is only ~10 -5 . Best available. However, 12 CO lines are opticallythick, so they can not be used to derive the density. Less abundant 13 CO is thin, so can be used to estimate N. • CO is excited via collisions with H 2 , requiringn > 3x10 2 cm -3 , and it radiates by spontaneousde-excitation through its rotation levels. • The low de-excitation rate or A ul however, means that CO traces H 2 to only a maximum densityof n crit ~ 4x10 4 cm -3 . At higher densities, the excited levels stay populatedsince C lu / A ul > 1. • After CO saturates, need different tracers such as: CS, HCO + , HCN, H 2 CO+, and NH 3 .
The Important CO Lines Δν = 1 ~4.67 μm Δν = 2 ~2.3 μm J=1-0 around 115 GHZ (millimeter) M.D. Smith
Molecular Gas Probes Star Formation 33.2 K 16.6 K 5.53 K Simulated CO (J=1-0) through (J=9-8) emission contours for progenitor disk galaxy. While lower CO transitions trace the bulk of the molecular gas, higher lying transitions with relatively high critical densities probe only the nuclear star forming regions. Panels are 12 kpc on a side, and scale on bottom is in units of K- km s − 1.
Star Formation Occurs in Molecular Clouds J. Graham
Channel Maps = Movie in Velocity
12 CO then 13 CO saturates, leaving C 18 O and other tracers.
J. Graham
At the Higher Densities T kin CS 3-2: Has critical density of 1.5 x10 6 cm -3 and T ex = 90% of T kin at n=3x10 6 cm -3 . These are around the same value. NH 3 1-1: Has critical density of 2x10 3 cm -3 and T ex => T kin above n = 10 6 cm -3 . These differ by 3 orders of magnitude! Why the difference? Stimulated emission is more important at low frequencies compared to at high frequencies. If T = hv/k << T kin , the density must be much larger than the critical density in order for the line to be visible. R. Pogge
At the Colder Temperatures CO “freezes out” and is no longer in the gas phase where we can observe it. figure by W.-F. Thi
Goal is to find appropriate lines è N https://iopscience.iop.org/article/10.1086/680323/pdf (invaluable article) The basic form of this should look famililiar with N ~ ∫ T * dv Some lines are optically thick though, so need correction factor (sometimes called 1/ β )
The Interstellar Molecular Soup (well over 200 molecules) http://www.astro.uni-koeln.de/cdms/molecules For the historical record, including 9 found in 2019, see http://www.astrochymist.org/astrochymist_ism.html
Studying Chemistry in Other Galaxies is More Challenging http://www.astro.uni-koeln.de/cdms/molecules
Molecule Formation
Molecule Formation movie for water https://www.youtube.com/watch?v=X_jSenHTqFw
One of the More Important Molecules -- at ~280 K
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