ISM Lecture 13 H 2 Regions II: Diffuse molecular clouds; C + => CO transition

13.1 Diffuse molecular clouds  Diffuse clouds  clouds with total visual extinction A V  1 mag  If A V  0.3 mag  virtually all hydrogen in atomic form  diffuse atomic clouds  CNM  If 0.3 mag  A V  1 mag  significant fraction of hydrogen is molecular  Note: with N H =N(H)+2N(H 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:  H 2, HD, CH, CH +, C 2, CO, OH, CN, NH, HCl, C 3  Not detected:  N 2 (?), H 2 O, H 2 O +, MgH, NaH, SH +, …

Interstellar H 2 lines towards  Ophiuchi  Copernicus data 1970’s  FUSE data >1999

Physical conditions a. Rotational excitation of H 2  H 2 lines out of J = 0-7 detected with Copernicus + FUSE  Population distribution non-thermal  Low J: excitation dominated by collisions  sensitive to T and n H  Abundance H + large enough that ortho/para exchange rapid and J=1/J=0 gives T kin  High J: energy levels lie very high (> 1000 K)  not populated by collisions at T = K  populated by optical pumping process through B  X and C  X systems  proportional to interstellar radiation field I UV  Formation process may play a small role as well

Observed H 2 rotational excitation Spitzer & Cochran 1973 ln N J /g J

H 2 excitation

b. C 2 rotational excitation  Like H 2, C 2 has no permanent dipole moment so excited rotational levels long-lived  Excitation similar to H 2, but by radiation around 1  m rather than UV  Advantage of C 2 : observable from ground Van Dishoeck & de Zeeuw 1984

b. C 2 rotational excitation  C 2 excitation  Low-J population: sensitive to T  High-J population: determined by optical pumping + collisional de-excitation => sensitive to n H and I red

c. Other diagnostics  CO rotational excitation  Small dipole moment => lowest levels can be populated by collisions even at low densities => sensitive to T and n H  C, C +, O fine-structure excitation  Fine-structure populations determined by collisions => sensitive to T and n H  See Chap. 4 for critical densities  Overall results: T~25-50 K, n H ~ 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 H 2 ); major role in dense clouds Tielens Chap

Ion-molecule collisions  Interaction potential (induced dipole + centrifugal barrier):  V eff has maximum value:  Critical impact parameter:  Rate coefficient is independent of T: 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 + + H 2  CH h possible at low T (initiating reaction)  Once CH 2 + formed, rapid ion-molecule reactions lead to CH, C 2, …  C + + H 2  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  H 2 or H + C.R.  H 2 + or H + + C.R. + e  H H 2  H H (very fast)  H + or H 3 + can react with oxygen  H + + O  H + O +, O + + H 2  OH + + H  H O  OH + + H 2  Once OH + formed, rapid ion-molecule reactions lead to OH, H 2 O 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 1986 edge center

13.2 Translucent clouds  Clouds with 1 mag  A V  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  A V =1-2 mag => similar to translucent clouds  Example: high latitude cloud toward HD ; mapped in CO and optical absorption lines toward star  T~15-30 K; n H =1000 cm -3

High latitude cloud toward HD Gredel et al CO J=1-0 map

CO formation and destruction  CO is most abundant molecule after H 2 and is easily observed through (sub-) mm lines   Good tracer of H 2  At edge of cloud, most of carbon is C +  Transition C +  C  CO with increasing depth  CO is very stable (D e = eV  1118 Å)  can only be dissociated at 912 Å < < 1118 Å

CO photodissociation  Like H 2, CO has no direct dissociation channels  dissociation through line absorption  self-shielding, but at greater depth than H 2 because of smaller abundance  At A V  1-2 mag, CO / H 2 increases from 10 –7 to 10 –4

Self-shielding of CO and H 2 Photodissociation rates - Note that H 2 lines can shield CO UV lines: mutual shielding

Densities of major species in translucent cloud T=15 K n H =1500 cm -3 I UV =1 Edge Center

Column densities with A V  Increase in CO/H 2 at A V =1-2 mag from to  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 10 5 w.r.t. average interstellar radiation field  Example: Orion Bar  PDRs show very strong atomic fine-structure lines  E.g. [C II] 158  m, [C I] 610  m, [O I] 63  m  And submillimeter lines of molecules  E.g. CO 7-6, HCO Tielens Chap 9

PDR structure

Orion Bar PDR Yellow: H 2 v=1-0 Blue PAH Red: CO Note layered structure! (0,0)=  2 A 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