harvey lisztlondon, january 2006 harvey liszt nrao, charlottesville

Harvey Liszt London, January 2006

What do we learn from studyingWhat do we learn from studyingH3

+ in the diffuse ISM

Harvey Liszt

NRAO, CHARLOTTESVILLE


Hydrogen-bearing molecules in diffuse ISM

• H2, HD & H3+ (hydrogen/H ratios?)(hydrogen/H ratios?)

• All explained by embarrassingly-simple notions of static equilibrium at low n(H)

• But why?– Such equilibrium is VERY slow to occur– The chemistry of trace species cannot be

understood in these terms

• Not to be ungrateful, but we mustn’t become complacent.


H2 turns us us on …


H2 turns us us on …

EBV =0.07 mag, N(H)~4x1020 cm-2


Protonate, diffuselydiffusely protonate



•C

opernicus



•C

opernicus

• N(CH) ~ 2-4x1013 cm-2

• => N(H2) < 1021 cm-2

• But N(H)~7-10x1021 cm-2



•C

opernicus

• N(CO)/N(H) < 1-5x10-6

• N(CH) ~ 2-4x1013 cm-2

• => N(H2) < 1021 cm-2

• But N(H)~7-10x1021 cm-2


But what about CO emission?


“Getting it”• Previously (1974, actually) we learned that

H I clouds are full of H2

• In the mid-1990’s we found out that they are full of trace polyatomics (more later)

• Ca. 1999 - now we learn that diffuse gas has a relatively high abundance of H3

+

• DEAL WITH IT!

• Let’s move on and talk in these terms


Nature makes H2 … can we?



• <TK> = 70-80K• <nTK> = 1-3,000=> n ~ 12-40/cc



• <TK> = 70-80K• <nTK> = 2-3,000=> n ~ 25-40/cc


What did that take?• Wolfire et al. (1995) heating/cooling

– Small grains/PAH heat gas, neutralize atomic ions

• Spitzer’s (1978) H2 - formation rate

– H2 forms as H atoms land on large grains

– 3x10-18 n(H) n(H I) TK1/2 cm-3 s-1

– Slights details of grain conditions!

• Lee et al. (1996) H2 - shielding factors

• EQUILIBRIUM with ambient radiation

• In other words, an exercise in model-building


Chemistry of H3+ in partly-atomic gas


Chemistry of H3+ in partly-atomic gas

• Every primary ionization creates an electron (duh)(duh)

• Every e gets two chances at knocking off an H3+

<= Chain stops if < 10% of H is H2, n(H3+) ~ n(e)-2


H2, H3+, HD & H


Taking a breather …• H2, H3

+ & HD can be understood in the very simplest terms– Late-time equilibrium of optically transparent, low

density, quiescent gas clots– Atomic-ion neutralization by PAH very important– One twist, enhanced low-level ionization of H, H2

• HD needs the kick every bit as much as H3+

• Dense dark molecular gas ain’t needed – or wanted for that matter

• So what’s the rub?


Taking a breather …• H2, H3

+ & HD can be understood in the very simplest terms– Late-time equilibrium of optically transparent, low

density, quiescent gas clots– Atomic-ion eutralization by PAH very important– One twist, enhanced low-level ionization of H, H2

• HD needs the kick every bit as much as H3+

• Dense dark molecular gas ain’t needed – or wanted for that matter

• So what’s the rub?


H3+ is no silver bullet

• Abundances of trace molecules which were unexplained before H3

+ remain unexplained





N(HCO+)=2.4x1012

N(H2) ~1x1021





• CO + H3+ will not explain HCO+

– Instead, HCO+ + e CO

• O + H3+ competes with O+ + H2

only for n(H) > few hundred/cc N(HCO+)=2.4x1012

N(H2) ~1x1021


How quickly does H2 form?


Time evolution of H3+



• High abundances of H3+ and

effects of varying H do not appear until the last minute


Summing up: H2, H3+ & HD

• They are trying as hard as they can to convince us they’re in a profoundly simple state of equilibrium with the ambient radiation field at low density and extinction

• But we’re wise to them!!!!– HOW DID THEY GET TO EQUILIBRIUMHOW DID THEY GET TO EQUILIBRIUM

• Diffuse clouds have tricks up their sleeves– ALL THOSE TRACE MOLECULESALL THOSE TRACE MOLECULES


Summing up: H2, H, H33++ & HD & HD

• Locally H2 seems easy, but lacks context

• What is a diffuse (or “H I”)(or “H I”) ‘cloud’?• Size &/or column density spectra, volume filling factor,

total amount of gas in diffuse ISM?

• Duration?

• External dynamics?– Mechanisms which carry material between ISM phases?

• Internal dynamics?– ‘Turbulence’?

» Disappointingly few clues in absorption profiles

» Trace molecule chemistry speaking to us in loud grunts


Summing up: HH22,, H3+ & HD

• Both HD and H3+ want the same change

– Gratifying to see that ionization balance works• In diffuse gas ionization balance is EVERYTHING

– Confirming contribution of PAH

• Isn’t this paradoxical, wanting more ionizing radiation?

– Is the enhanced low-level H-ionization really H?• If not, what then?

• Whatever it is, does it extend beyond diffuse ISM?– 35 year history of dense cloud chemistry with low H

» Is it even possible to invert the chemistry to check H?


Summing up: epistemology• We are left with TWO puzzles

– How to understand the trace molecules?• H3

+ per se doesn’t help

– Why doesn’t the study of hydrogen (which is after all the dominant gas constituent) in its so many forms – H I, D I, H+, H2, HD, H3

+ – provide more insight?

• Observation of neutral diffuse gas alone is a closed system, lacking needed information– Answers must come from outside


The end, thanks for having me


H2, H3+, HD & H

harvey lisztlondon, january 2006 harvey liszt nrao, charlottesville

Documents

harvey lisztlondon

hd h slide

chemistry of h

breather h

copernicus slide

cc slide

charlottesville slide

atomic gas slide