Molecules in the interstellar medium

The ISM: composition, mass, energy flow,and synthesis of the interstellar molecules

Composition of the ISM

• The interstellar medium (ISM)consists of gas and dustexisting over a wide range ofphysical conditions

• About 1/2 of the ISM mass inour Galaxy is molecular in form

• The ISM is powered by energyemitted by stars (SN, giantstars, novae, etc.)

• The 5 ISM components:“coronal” gas, warm intercloudmedium (WIM), HII regions,neutral hydrogen (HI) clouds,and complexes of giantmolecular clouds (GMCs)

Coronal gas and the WIM

• Coronal gas is observed as far-UV absorption lines of highlyionized atoms and in the formof a soft X-ray background

• It is hot (log T> 6), rarified (logn < -1.5), and has a filling factoras large as 0.5

• It is a product of hot gasesejected in stellar explosionsand winds

• The WIM occupies most of therest of the Galaxy. It is seen asbroad emission features in HIspectra of extragalactic sources

• The WIM has: log T<4 and-1.0< log n < 0

From D. Burrows, PSU

Neutral hydrogen (HI) clouds

• Roughly another half of the mass ofthe ISM is in HI clouds

• HI is observed at 21 cm (1420 MHz)as the result of a low-probabilitytransition (electron spin flip) in thehyperfine structure of the groundenergy level of H

• HI clouds have log T<2. Because oftheir abundance, they have beenused as excellent tracers of spiralstructure

• HII regions are small, roughlyspherical clouds of hot (log T~4)ionized H, centered on hot stars(sources of ionizing UV radiation)

Molecular clouds• GMCs are complex (sizes 20-200 pc), massive (103-107Msun),

cool (T<10K) regions, which are sites of chemical anddynamical activity leading to star formation

• Their mean density estimates vary from 102 to 103 cm-3

• GMC structure is best traced by mapping the emission ofcollision excited 12CO molecule

Morphology of the ISM

• Temperatures and densitiesof the main components ofthe ISM keep them in anapproximate pressureequilibrium

• GMCs and HI clouds arecomparable in total massand are more massive thanthe intercloud gas (WIM),which contains much moremass than the coronal gas

• Most of the galactic volumeis taken by the coronal gasand the WIM, followed by HIclouds and the GMCs, whichare the most compact ISMstructures

-4 -2 0 +2 +4 +6 log n (cm-3)

6

4

2

0

Log

T (K

)

HII regions

Warm/hotmolecular cores

Cold darkclouds Globules

HIclouds

Intercloudgas

Coronalgas

2 3 4log (P/k)

Dynamics of the ISM

• Energy and mass relationships inthe ISM can be schematicallydepicted as a pressure-densitydiagram of its main components

• They are located on the line ofdynamical equilibrium (T,P vs. n)

• Coronal and HII phases (not shown)are not in equilibrium (both fed bystellar activity)

• Global pressure balance existsbetween the WIM (fed by coronalgas) and HI phases of the ISM

• In the region of instability, neutral,atomic gas reradiates energy undercompression --> spontaneouscollapse

• Stability of GMCs is maintained bygravity (no consequence for theglobal ISM equilibrium)

-2 0 +2 log n (cm-3)

log

P

12,000K

Coronalgas

10,000K

WIM

Ioni

zed

gas

9,000K

7,500K

3,000K

1,000K

5,000K

unstable

Molecu

les an

d dus

t

Giant m

olecu

lar

cloud

s

260K

160K

50K

30K

HI

Mass flow,cooling

The basic interstellar chemistry

Production of molecules in the interstellarspace

Fertilization of the ISM

• Interstellar chemistry beginswith the formation of dustgrains in the outflows from giantstars

• In the process, all Si and Fe,50% of C and 20% of O getlocked up in dense dust graincores and become chemicallyinert

• Over 120 molecules have beenidentified in the ISM. It is clearthat the interstellar chemistry iscarbon-dominated (e.g.cyanopolyynes, PAH’s)

• Molecules form in a variety ofenvironments (diffuse ISM,circumstellar shells, GMCs)

What do we get out of spectral lines?

• Most of the interstellar molecules aredetectable at mm wavelengthsthrough emission/absorptiongenerated by their rotational states

• Solid state molecules (ices) aredetectable in IR (vibrational states)

• From radiative transfer:

• Abundance is derived by integratingTB over velocities

• Kinetic temperature, Tk can becomputed, if environment is collision- dominated: Tk~Tul. Usually obtainedfrom brightness of 12CO line

• Densities are obtained fromunsaturated lines with τν>>1

!

I" = B"Tul (1# e#$" ); $" = nl%"

&TB = (Tul #Tbg )(1# e#$" ); h" << kT

&TB = Tul #Tbg; $" >>1

&TB = nl%"Tul ; $" <<1

Molecule formation - I

• Chemical bonding involves sharingelectrons, therefore it useselectromagnetic, not nuclear forces

• Stable molecule has to have lowerEM potential energy than the sumof separate atoms that make it up

• Electrostatic repulsion of electronsform a barrier called the activationenergy

• Excitation and energy release by amolecule occurs through one of thethree mechanisms: electronic,vibration, and rotation with rotationbeing important in the gas phaseand vibration dominating in thesolid state environment (ices ontop of dust grains)

Distance between atoms

EM p

oten

tial e

nerg

y

Separateatoms

Molecule

Activationenergy

Electronic Vibration Rotation

Molecule formation - II

• Gas-phase, ion-molecule: cosmic rays ionize H2, make H2+, H+ (also

He+), which react with H2 and CO (most abundant) and createsimple neutral molecules and metal ions through successivereactions [A++B-->C++D]. Important in both diffuse and dense clouds

• Shock-induced: gas heating caused by shock wave overcomesactivation barriers, enables neutral-neutral reactions [A+B-->C+D]that are impossible in cold clouds, may cause grain disruption.Important in dense, hot star-forming regions

• Circumstellar: “freeze-out” chemistry in circumstellar envelopes,gas-phase molecules stick to grains, which develop icy mantles.Reactions proceed on grain surfaces (hydrogenation of O, C, N,formation of H2 most common, because of H mobility)

• Dust grain surfaces: shield molecules from UV radiation field,produce H2 through catalysis: H+H+grain-->H2+grain (reaction isexothermic, energy is used to detach H2 from surface). H2 drivesmuch of gas-phase chemistry

Processes on dust grains - I• Adsorption or sticking efficiency is

thought to be high for dust grains.Sticking occurs, when anapproaching particle loses energyto the surface’s lattice

• Scanning or mobilty of particles onsurfaces is necessary to producechemical reactions (quantumtunneling, τq =4h/ΔE, or thermalhopping, τh=ν-1exp(TB/T))

• Desorption: micro-continuous-->exothermic reaction liberatesmolecule (possibly alsoneighboring molecules) fromsurface; macro-continuous-->explosive liberation of moleculesby mantle destruction by energeticphotons or cosmic rays; violentdesorption --> collectivedestruction of grains by shockwaves

D. Witt, 2000

adsorptiondesorptionH

O

CH3OH

H2OCH4

CO2

diffusion NH3 CO

CH3OH

reactionSilicate core

Adapted from:Ehrenfreund et al, 2005

Processes on dust grains - II• Atoms of H scan the grain’s surface fast compared to other atoms, because

of their much higher mobility• Most common reactions: hydrogenation of C, N, O, and H2 molecule

formation in clouds containing enough H:

• In high H-abundance clouds, these molecules and CO accreted from gasphase form grain mantles. Low H-abundance limits hydrogenation andallows formation of more complex molecules:

• In absence of atomic H, mantles are formed out of atoms of C, N, O whichwill react to form O2, CO, and possibly NO

• Generally, surface reactions produce simple molecules, while UV irradiationleads to formation of complex molecules

!

C" CH4

: methane

N" NH3

: ammonia

O" H2O : water

!

O+ H" OH+C

# $ # COH+3H

# $ # # CH3OH : methanol

CH +H" CH2

+O# $ # H2CO : formaldehyde

C + H" CH+COH +4H

# $ # # # CH3CH2OH : ethanol

Molecule formation pathways

Adapted from: Turner & Ziurys 1988

Interstellar cloud

Shell of embeddedstar/protostar

Shell of isolatedevolved star

2 body gas phasereactions

Surface reactionson grains

Many-body gasphase reactions

Surface reactionson grains

Elemental condensation

Many-body gasphase reactions

Surface reactionson grains

Elemental condensation

Diatomicmolecules

Diatomic/polyatomicmolecules

Interstellar grains

Diatomic/polyatomicmolecules

Interstellargrains

Further gas phase/surface reactions

Ejection intothe ISM

Ejection in

to

surro

unding cloud

Mass-loss to

the ISM

Mass-loss tothe ISM

Polyatomicmolecules

Molecules/grains in starforming clouds

MoleculesIn clouds

Grainsin clouds

Accretion

?

Interstellargrains

Molecules innon-star formingclouds

Grains in nonstar formingclouds

Moleculesin clouds

Surfacereactions

Break-upof grains

Examples of molecules in the ISM

J. Wisniewski

Whittet et al. 1996

The Grand Scheme

Homework #3,4Deadline: Apr 6

• Write a paper on the following subject: “Organic material fromspace on Earth: delivery, composition, and relevance to theorigins of life”. Technical specifications are the same as forpaper #1

• Download a set of radial velocity data from the course webpageand analyze it to estimate the five Keplerian orbital parametersof a planet around a star for which these measurements havebeen made. Given the model orbit, estimate a minimum mass ofthe planet and its distance from the star. Use information onorbit determination included in the course lecture on the radialvelocity method. As before, the more accurate your answers,the better grade you get!