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

Low-T, High-n (relative to rest of ISM)

solar core

Studied via:

• CO, most readily
• Other molecules

at high density

• H2 lines at warm

temperatures

SFE = SFR/M_gas Mgas = atomic + molecular SFR from UV merging galaxies è compression

• f ism

è gas turns from atomic to molecular è induced star formation

Molecular Gas

M 51 (face-on) Note pattern of gas density relative to the stellar light Arm : Interarm contrast ~30:1 (compare to 2.5:1 for HI)

Molecular Gas

Milky Way (edge-on, from the inside) Similar to HI maps but now measuring cold, dense gas

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

Molecules

Transitions between energy levels:

• 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

transitions can be radiative (either consuming or producing photons),

• r non-radiative (involving collisions).

rotational levels within a vibrational level

Rotational Modes: Linear Molecules

+ J. Williams

where B = h / 8 π2 I

Rotational Modes: Linear Molecules

• J. Williams

Rotational Modes: Nonlinear Molecules “Symmetric Top”

• J. Williams

• J. Williams

Rotational Modes: Nonlinear Molecules “Symmetric Top”

Rotational Modes: Nonlinear Molecules “Spherical Top” (Special Case of Symmetric)

IA = IB = IC

No dipole changes under rotation, so no rotational mode radiation. Vibrations though!

• J. Williams

Rotational Modes: Nonlinear Molecules “Asymmetric Top”

Others

e.g. “seeds of life”

Molecular Astrophysics is Complicated!

Now Vibrational Modes

Vibrational Modes

Tielens + J. Williams

CHON molecular bands in infrared

(X is generic for some random heavier atom)

Dopita & Sutherland

rotational levels within a vibrational level

Now Ro-Vibrational Modes

(accounting for both vibration and rotation)

Now Ro-Vibrational Modes

(accounting for both vibration and rotation)

(Boogert et al. 2002)

Ro-Vibrational Modes

(accounting for both vibration and rotation)

Sieghard “Fundamentals of vibration-rotation spectroscopy” Kwok ΔJ = +/- 1 between Δv=any vibrational levels

Evidence that this formalism actually describes reality data

Kwok

model

Also CO ro-vibrational bands in the ultraviolet

designated as 2S+1Λ+/-

Ω,g/u

Also, Electronic States

figure by W.-F. Thi

designated as 2S+1Λ+/-

Ω,g/u

è ground state It is actually even a little more complicated than this……

Also, Electronic States

Dopita & Sutherland

vibrational levels within an electronic level

Ro-Vibrational Including Electronic States

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

• H2 = molecular hydrogen – the most important
• CO = carbon monoxide – the most observed

Why H2 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 H2 Molecule & Lines

Dopita & Sutherland

• no dipole
• quadrupolewith ΔJ = +2
• para = even transitions
• ortho = odd transitions
• light and small molecule

S(0) S(1) S(2) S(3)

Rotational- Vibrational Ladders for H2

Lyman-alpha “Photon Pumping” of Hot H2

These are electronic transitions Dopita & Sutherland

FUSE spectrum of the hot LMC star Sk-67-166 (Tumlinson et al 2002) Derive N(H2)= 5.5x1015 cm-2 (compare to 2x1021 cm-2 for Av = 1 mag) (Ly 4-0) Here seen in absorption, they indicate ubiquitous presence of translucent, diffuse clouds. Sensitive to very low column densities, N(H2) > 1014 cm-2

Lots of Hot H2 Lines in UV part of the spectrum

• 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
• utflows and supernovae with n > 104 cm-3 , T > 2000 K

(but < 4000-5000 K since by then H2 would dissociate).

“Collisional Pumping”

• f Warm H2 lines

(seen in near- and mid-infrared)

Cold H2 ? Needs a Tracer = CO

• In the cold ISM, H2 is shieldedby HI. Since HI is photoionizedat 13.6 eV

this takes all the would-bephoto-dissociatingphotons that would

• therwise be availableto destroy the H2 (>4.48 eV).
• H2 can be also `self-shielded’ from standard ISRF when at N > 1020 cm-2
• However, because of the lack of a dipolemoment -- either electric or

magnetic -- cold H2 is not detectabledirectlysince don’t populateJ=2.

• CO is used as a tracer, even though CO/H2 is only ~10-5. Best available.

However, 12CO lines are opticallythick, so they can not be used to derive the density. Less abundant 13CO is thin, so can be used to estimate N.

• CO is excited via collisions with H2 , requiringn > 3x102 cm-3, and it

radiates by spontaneousde-excitation through its rotation levels.

• The low de-excitation rate or Aul however, means that CO traces H2 to
• nly a maximum densityof ncrit ~ 4x104 cm-3. At higher densities, the

excited levels stay populatedsince Clu/ Aul> 1.

• After CO saturates, need different tracers such as: CS, HCO+, HCN,

H2CO+, and NH3.

The Important CO Lines

Δν = 1 ~4.67 μm Δν = 2 ~2.3 μm J=1-0 around 115 GHZ (millimeter)

M.D. Smith

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.

33.2 K 16.6 K 5.53 K

Molecular Gas Probes Star Formation

Star Formation Occurs in Molecular Clouds

• J. Graham

Channel Maps = Movie in Velocity

12CO then 13CO saturates, leaving C18O and other tracers.

• J. Graham

At the Higher Densities

CS 3-2: Has critical density of 1.5 x106 cm-3 and Tex = 90% of Tkin at n=3x106 cm-3. These are around the same value. NH3 1-1: Has critical density of 2x103 cm-3 and Tex => Tkin above n = 106 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 << Tkin, the density must be much larger than the critical density in order for the line to be visible.

• R. Pogge

Tkin

At the Colder Temperatures

figure by W.-F. Thi

CO “freezes out” and is no longer in the gas phase where we can

• bserve it.

Goal is to find appropriate lines è N

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)

https://iopscience.iop.org/article/10.1086/680323/pdf (invaluable article)

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