ISM Lecture 13

H2 Regions II:

Diffuse molecular clouds;

C+ => CO transition

13.1 Diffuse molecular clouds

Diffuse clouds clouds with total visual extinction AV 1 mag

If AV 0.3 mag virtually all hydrogen in atomic form diffuse atomic clouds CNM

If 0.3 mag AV 1 mag significant fraction of hydrogen is molecular

Note:

with NH=N(H)+2N(H2)

AN

V

H

cm18 1021 2.

Van Dishoeck 1998 in Mol. Astr.Snow & McCall 2006, ARAA

Observations of diffuse clouds

Observed primarily by absorption lines at visible (since 1900’s) and UV wavelengths (since 1970’s)

Classical example: line-of-sight towards Oph Spectra show sharp interstellar lines super-imposed on

broad stellar lines

Observed species

Many atomic lines information on depletion (see Chap. 7)

Molecules detected: H2, HD, CH, CH+, C2, CO, OH, CN, NH, HCl, C3

Not detected: N2 (?), H2O, H2O+, MgH, NaH, SH+, …

Interstellar H2 lines towards Ophiuchi

Copernicus data 1970’s FUSE data >1999

Physical conditionsa. Rotational excitation of H2

H2 lines out of J = 0-7 detected with Copernicus + FUSE

Population distribution non-thermal Low J: excitation dominated by collisions

sensitive to T and nH

Abundance H+ large enough that ortho/para exchange rapid and J=1/J=0 gives Tkin

High J: energy levels lie very high (> 1000 K) not populated by collisions at T = 40 - 80 K populated by optical pumping process through B X and C X systems proportional to interstellar radiation field IUV

Formation process may play a small role as well

Observed H2 rotational excitation

Spitzer & Cochran 1973

ln NJ/gJ

H2 excitation

b. C2 rotational excitation

Like H2, C2 has no permanent dipole moment so excited rotational levels long-lived

Excitation similar to H2, but by radiation around 1 m rather than UV

Advantage of C2: observable from ground

Van Dishoeck & de Zeeuw 1984

b. C2 rotational excitation

C2 excitation Low-J population: sensitive to T High-J population: determined by optical pumping + collisional de-excitation => sensitive to nH and Ired

c. Other diagnostics

CO rotational excitation Small dipole moment => lowest levels can be

populated by collisions even at low densities => sensitive to T and nH

C, C+, O fine-structure excitation Fine-structure populations determined by collisions =>

sensitive to T and nH

See Chap. 4 for critical densities

Overall results: T~25-50 K, nH~100-500 cm-3

O I , C I and C II fine structure lines

Thermal balance

Similar to H I clouds Heating: photoelectric emission from dust +

photoionization of large molecules/PAHs Cooling: fine structure excitation and emission

from [C II]

13.2 Chemistry in diffuse clouds

Detailed models needed to understand observed abundances of molecules Started with Kramers & ter Haar 1946, Bates & Spitzer

1951 Gas-phase ion-molecule reactions are very rapid at

low temperatures Herbst & Klemperer 1973

Modern view Neutral-neutral reactions also significant at low T Grain surface formation minor role in diffuse clouds (except

for H2); major role in dense clouds

Tielens Chap. 8.7-8.8

Ion-molecule collisions

Interaction potential (induced dipole +

centrifugal barrier):

Veff has maximum value:

Critical impact parameter:

Rate coefficient is independent of T:

V Re

R

b

Reff ( )

2

4

2 2

22 2

v

( ) / (8 ) b e2 2 2 2v

1

2

42 2 2 2 22

2

1 4

v vv

FHG

IKJ( ) / (8 )

/

b e be

crit

k v be

= v cm scrit2 3 12 10

29

Langevin rate

=polarizability

Networks of chemical reactions

Formation of bonds Radiative association: X+ + Y XY+ + h Grain surface: X + Y:g XY + g

Destruction of bonds Photo-dissociation: XY + h X + Y Dissociative recombination: XY+ + e X + Y

Rearrangement of bonds Ion-molecule reactions (fast): X+ + YZ XY+ + Z Neutral-neutral reactions (slow): X + YZ XY + Z

Carbon chemistry

Need to have ions and molecules to start ion-molecule chemistry

I.P. of C < 13.6 eV carbon mostly C+

C+ + H2 CH2+ + h possible at low T (initiating

reaction) Once CH2

+ formed, rapid ion-molecule reactions lead to CH, C2, …

C+ + H2 CH+ + H: endothermic by 0.4 eV

Carbon chemistry and its coupling with oxygen

Oxygen chemistry

I.P. of O > 13.6 eV oxygen mostly O Ionization provided by cosmic rays

H2 or H + C.R. H2+ or H+ + C.R. + e

H2+ + H2 H3

+ + H (very fast)

H+ or H3+ can react with oxygen

H+ + O H + O+ , O+ + H2 OH+ + H H3

+ + O OH+ + H2 Once OH+ formed, rapid ion-molecule reactions lead to

OH, H2O and CO Note that OH abundance proportional to cosmic ray

ionization rate CR => can use observed OH abundance to determine CR

Oxygen chemistry and its coupling with carbon

Depth dependence of major species Per cloud

Van Dishoeck & Black 1986edge center

13.2 Translucent clouds

Clouds with 1 mag AV 5 mag “translucent clouds”

Intermediate between diffuse clouds and dense molecular clouds

Not self-gravitating Thin enough for optical absorption lines,

but thick enough for mm emission lines of CO

High-latitude clouds

Discovered by CO emission (Magnani et al. 1985) Seen as IRAS 100 m cirrus AV=1-2 mag => similar to translucent clouds

Example: high latitude cloud toward HD 210121; mapped in CO and optical absorption lines toward star T~15-30 K; nH=1000 cm-3

High latitude cloud toward HD 210121

Gredel et al. 1992

CO J=1-0 map

CO formation and destruction

CO is most abundant molecule after H2 and is easily observed through (sub-) mm lines

Good tracer of H2

At edge of cloud, most of carbon is C+

Transition C+ C CO with increasing depth CO is very stable (De = 11.09 eV 1118 Å)

can only be dissociated at 912 Å < < 1118 Å

CO photodissociation

Like H2, CO has no direct dissociation channels dissociation through line absorption self-shielding, but at greater depth than H2 because of smaller abundance

At AV 1-2 mag, CO / H2 increases from 10–7 to 10–4

Self-shielding of CO and H2

Photodissociation rates

- Note that H2 lines can shield CO UV lines: mutual shielding

Densities of major species in translucent cloud

T=15 KnH=1500 cm-3

IUV=1

Edge Center

Column densities with AV

Increase in CO/H2 at AV=1-2 mag from 10-7 to 10-4

Exact location and sharpness transition depend on Strength UV radiation field Density Gas-phase carbon abundance

13.4 Photon-dominated regions (PDRs)

Diffuse and translucent clouds are examples of PDRs, I.e., clouds in which UV photons control the physical and chemical state of the cloud

Traditionally, PDRs are dense molecular clouds located close to an OB star, in which the UV radiation field is enhanced by a factor of 105 w.r.t. average interstellar radiation field Example: Orion Bar

PDRs show very strong atomic fine-structure lines E.g. [C II] 158m, [C I] 610 m, [O I] 63 m

And submillimeter lines of molecules E.g. CO 7-6, HCO+ 4-3

Tielens Chap 9

PDR structure

Orion Bar PDR

Yellow: H2 v=1-0Blue PAHRed: CO

Note layered structure!

(0,0)=2A Ori

M17 Edge-on ionization front

M17: CO vs [C I]

- [C I] peaks deeper into cloud than CO, contrary to PDR models => evidence for clumpy cloud?

Keene et al. 1985

NGC 1977: uniform vs. clumpy model

[ C II] emission