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Molecular Astrophysics
Interstellar chemistry
Brief history, astrophysical context

Translucent molecular clouds: unresolved problems
CH+: Non-Maxwellian velocity distributions, XDRs, PDRs H3+: X-ray driven chemistry, ionisation rate DIBs: change of ionisation balance

VLT/UVES Observations: Molecules in Magellanic Clouds
first optical detection of CH, CH+, CN beyond Galaxy

HST & FUSE observations of CO and H2 in the Galaxy
N(CO) vs. N(H2) Complemented by High R CH, CH+, CN (ESO CES & McDonald)


· Y. Sheffer, S. Federman, D. Lambert, D. Welty · Roland Gredel, MPIA Heidelberg


Interstellar molecules
I. Brief History
1922 1926 1934 1937-39 1951 1963 1970 1973 1975 2005 5780, 5797 stationary features (Heger 1922) Eddington, molecules cannot survive ISRF Merrill, several strong DIBs detected CH, CH+, CN: stationary optical absorption lines Bates & Spitzer, first models (Kramers & ter Haar 1946) Radio astronomy, OH, NH3 H2 Copernicus satellite, UV absorption lines Herbst & Klemperer, ion-molecule reactions X-ogen (HCO+) some 125 gas-phase molecules confirmed


Interstellar Medium

·

Phase transitions: H
· · · ·

+

H

Hot ionised HII: 5 105 K, 5 10-3 cm-3 Warm HI/HII: 8000 K, 0.3 cm-3 Cool atomic: 80 K, 30 cm-3 Cold molecular: 10-100 K, 100 - 103 cm
· · Diffuse giant molecular clouds Pressure equilibrium: nT = const

H2

-3


Interstellar Dust

·

Absorption & polarisation of starlight
· · · · Variation with wavelength: reddening E Visual extinction AV = -2.5 log FV/F0
»
B-V

AV/EB-V = 3.1 depends on dust properties Gas/Dust ratio: 0.01

Absorption law: FV/F0 = exp (- k x)


Molecular clouds

· · ·

Diffuse clouds and translucent clouds
· · Optical detections of molecules, DIBs Rich chemistry, large and complex organic molecules

Giant molecular clouds and isolated globules Hot cores, UCHII regions, PDRs


Translucent molecular clouds
· ·
­ ­ ­

Diffuse interstellar clouds
·

Translucent clouds, edges of giant molecular clouds
Av = 1 ­ 5 mag
­ Background stars still visible

Av < 1 mag, ionisations from ISRF dominate

T = 20 ­ 100 K n = 100 ­ 1000 cm-3


Interstellar chemistry

· · · ·

ion-molecule reactions
Ionising source: photons (diffuse clouds), X-rays, cosmic rays (dense clouds)

Dissociative & radiative recombination
free electrons required

Neutral-neutral reactions Initiation of gas phase chemistry: H2 required


Interstellar chemistry
I. H
· ·
­

H2
by radiative association, H(1s) + H(1s) on grain surface, H + H:gr H2 + gr
Very slow, forbidden in first order for homonuclear molecules

H2 + hv

II.

H2

H3
H2 + CR

+

H2+ H2+ + H2

H3

+

·

H3+ + e
·

dissociative recombination rate

H2 + H

Fast (Amano 1988, Larsson 2000) Slow (Plasil et al. 2003) minority view



Interstellar chemistry


Interstellar chemistry
· New Standard Model
Bettens 1995, 3785 reactions, 409 species = 10-17 s-1

·

Famous problems remain: DIBs, H3+, CH+
I. Carriers of Diffuse Interstellar Bands II. DIBs and H3+: impact on ionisation rates III. CH+ formation scenarios not understood


H

3

+

Geballe & Oka 1996; McCall et al. 2002
Very important detection Abundances too high if dissociative recombination is fast General correlation N(H3+) ~ E B-V


H
·
· ·

3

+

in Cyg OB2 No. 12

McCall et al. 1998
Formation in diffuse material, very long pathways L = 400 ­ 1200 pc, n = 10 cm-3

·

Cecchi-Pestellini & Dalgarno 2000
Dense clumps of gas embedded in diffuse material C2 formation at n = 7000 cm-3

·

Gredel, Black & Yan 2001
· · · Tkin = 35K, n = 600 cm-3 increased radiation field from OB stars X-ray induced chemistry

= 0.6 ­ 3 10-15 s-1


Cyg OB2

X-rays from Cyg OB2
­ ­ Chandra observations Waldron et al. 2004 Rapidly exanding stellar winds, shocks X-ray emission


X-ray induced chemistry
· Cool molecular clouds subjected to X-rays
Gredel, Lepp, Dalgarno, Black, Yan; various papers

· ·

M + Xray
· C++ + H2

M++
CH+ + H+

+ 2e

Energy deposition by fast secondary electrons
Coulomb losses to thermal electrons Ionisation and excitation of H and H2 He, n 2, 3 singlet and triplet S and P states and to 41P


H2 X-ray excitation

Electron im pact

continuous fluorescence (UV)
H+H

Collisions: H, H2, e E2 radiative cascade (IR)

· Different selection rules · UV: Lyman & Werner bands, continuous · NIR H2 emission · Optical emission: high v

· Tgas, n, IUV, vs, xe


X-ray induced chemistry

·
· · · ·

Radiation field in dense molecular clouds
Energetic, secondary electrons from X-ray ionisation H2 + e H2 * H2 H2(vJ) + UV-photons (Lyman and Werner bands) H2 (vJ) H2 + NIR-photons (E2 cascade)

·
·

Increased photoionisation and photodissociation rates
Explains C/CO ratio in dense clouds


Cyg OB2 No. 12
Detailed chemical model including X-ray ionisations

T, nH, ne constrained by observations Cool gas with T = 35 K, n = 600 cm-3

= 0.6 ­ 3 10

­15 s-1


Cyg OB2 No. 12

·

Model prediction:
· Observable amounts of H2O+ in absorption S/N > 1000 spectrum of Cyg OB2 no. 12


H

3

+

in the diffuse ISM

McCall et al. 2003
Large abundance towards Per High cosmic ray ionisation rate = 1.2 10-15 s-1 General solution !?


The ironic twists in H
I. Lepp et al. 1988, large molecules

3

+

Chemical models including photoelectric heating of LM: large, observable abundance of H3+
Wrong. slow recombination rate used

Model predictions did not stimulate observations to detect H3+

II. New laboratory measurements: H3+ + e very fast
Models: H3+ abundance too low to be detected Stimulated huge observational efforts to detect H3+

III. 2003: large abundance of H3+ detected in diffuse ISM Ionisation rate to be increased?


The CH+ problem
· · Nobs/Nmodel = 1000
· C+ + H2 CH+ + H E=0.4 eV

Thermal formation scenarios
­ ­ ­ Elitzur & Watson 1978, 1980: J-type shocks Pineau des Forets et al. 1986: C-type shocks Falgarone, dissipation of interstellar turbulence, boundary layers

CH+ formation in hot gas, T = 1000 ­ 4000 K


The CH+ problem
I. CH ­ CH+ velocity difference
· · · Detailed shock model towards z Oph (Draine 1986) Model result: v(CH) ­ v(CH+) = 3.4 km s-1 Very high R observations: Dv < 0.5 km s-1


The CH+ problem
· Spatially related stars: N(CH+) ~ EB-V
Tight correlation in single translucent clouds (Gredel et al. 2003; 2004)

·

Radial velocities agree within errors
Earlier results with v(CH) ­ v(CH+) > 4 km s-1 cannot be reproduced: upper limit to shock velocities

·

C2 observations
·

n, T

CH+ formation sites in cool gas


Interstellar CH
· Thermal models

+

Multiple shocks, model for dissipation of IS turbulence
Gredel, Pineau des Forets & Federman 2002 Model for dissipation of IS turbulence Criss-crossing, low-velocity shocks

·

Non-thermal models Non-Maxwellian velocity distributions
Gredel van Dishoeck & Black 1993, 1997 `broad` lines: CH+ highly reactive, no thermal profiles Super-thermal C+ b(CH+) = b(CH) = 1 ­ 2 km s-1


The CH+ problem
Special case: Pleiades
- - - - White 1984: very large CH+ abundances at low optical depths ISM very close to stars Very high UV ionisation rates CH+ produced in PDR


LMC & SMC observations
· Kueyen/UVES & archival data
7 SMC & 13 LMC sightlines, V = 11 ­ 14 mag Selected from Tumlinson (2002) FUSE H2 > 1019 cm-2 Reanalysis of FUSE · T01= 45 ­ 90 K · IUV = 3 ­ 10 (30 ­ 900 near 30 Dor and SW SMC) · nH = 100 ­ 600 cm-3 LMC: EB-V = 0.08 ­ 0.51 mag SMC: EB-V = 0.07 ­ 0.34 mag Galactic foreground absorption 0.02 ­ 0.06 mag


Interstellar Molecules in the Magellanic Clouds
· Why Magellanic Clouds?

Increased radiation fields, factors of ~ 5 Lower metallicities, factors of 2 (LMC) to 4-5 (SMC) Lower gas-to-dust ratios, factors of 3 (LMC) to 8 (SMC) Test models & expectations in different physical & chemical environment


Observing technique
Interstellar absorption lines bright, featureless continua


CH, CH+, CN


N(CH) - N(H2) relation

· ·

Danks, Federman & Lambert (1984), Mattila (1986), Rachford et al. (2002) van Dishoeck & Black (1989), nH = 500 ­ 1000 cm-3

Galaxy: N(CH) ~ N(H2)

LMC, LMC: same regression


Interstellar CH
· Very abundant in LMC and SMC · Equilibrium gas-phase chemistry in quiescent gas
­ C+ + H2 CH2+ CH CH/H2 correlation expected ­ removed by photodissociation N(CH) ~ 0.67 k1 x(C+) N(H2) n(H)/{IUV G0(CH) ...} Galaxy: CH/H2 reproduced by (IUV = 1) nH = 500 cm-3 ­ LMC/SMC: x(C+), IUV nH = 1200 ­ 2900 I
UV 2

­ Inconsistent with densities inferred from H


CH ~ KI, NaI CH+ ~ CH


CH+ in the Galaxy

CH+ in LMC formed in PDRs
CH3+ +e CH N(CH) ~ 0.67 k1 N(CH+) f(H2) n(H) / IUV G0(CH) CH+ + H
2

CH2+ + H2

·

N(CH)/N(CH+) nH = 100 ­ 1000 cm-3, consistent with densities from H2 analysis


The diffuse interstellar bands
· 226 DIBs confirmed, maybe up to 400
BD+63o1964 (Tuairisg et al. 2000) Confusion limit, >1 carrier responsible for a given feature

·

Carriers
Large C-bearing molecules in gas phase PAH and fullerene cations Ly induced 2-photon absorption by H2

·

Needed:
Use CH, CH+, CN, C2, CaI, CaII, NaI to determine variations in physical parameters


The diffuse interstellar bands
· Ehrenfreund et al. 2002
DIBs in the MCs Not too different from Galactic clouds despite low metallicity

·

Sollerman et al. 2005
DIBs towards SNIa in NGC 1448 Correlations with CaII and NaI
Local conditions affect DIB strength, in particular IUV SN absorptions: DIBs are readily formed and survive different physical & chemical environments universal carbon chemistry


DIB carriers

·
­

Large Molecules fundamental role in ionisation balance
Liszt 2003 PAH grain neutralisations
· · Heating balance: radiative and dielectronic recombination of charged ions PAH- + H+ PAH + H rapid destruction of protons

Ionisation rate must be significantly increased



Diffuse Interstellar Bands in MCs

·

Several detections
· · · · · No obvious correlation with N(HI) as in Galaxy Good correlation with EB-V On average weaker 5780, 5797, 6284 DIBs by factors of 10 in LMC, 20 in SMC, relative to HI by factors of 2 relative to EB-V W(6284)/N(HI) factors 30-70 below Galaxy C2-DIBs as strong as in Galaxy

·

No uniform scaling relations


FUV H2 & CO absorption lines

· HST: CO A-X band, 1229 ­ 1544 a · FUSE: H2 (2,0), (3,0), (4,0) Lyman bands · Saturation
­ 58 sightlines, 33 new results ­ cloud structure confined by CH ­ accurate N(H2), N(CO) ­ CH, CH+, CN ­ R = 170,000 ­ 220,000 ­ 66 sightlines, 43 new results

profile fitting

· ESO & McDonald data


FUV absorption lines of H

2

UV absorpt ion

H+H

· UV: Lyman & Werner bands · Trot, nH, IUV


FUV H2 & CO absorption lines


H2 thermal excitation in shocks

H+H

Collisions E2 radiative cascade (IR)

· X 1g+ v'J' collisional excitation by H, H2, He · Boltzmann distribution up to v'J' · NIR H2 emission · v'J' v''J'' E2 · C-type vs. J-type evolutionqry state · L(H2) ~ L(YSO)

· T, n, (vs)


H2 in interstellar shocks


N(CO) vs. N(H2)


N(CO) vs. N(H2)
· Break in quadratic relation
­ log N(H2) ~ 1.5 log N(CO) < 1014 cm-2 ­ log N(H2) ~ 3.1 log N(CO) < 1014 cm-2 ­ good connection to dark cloud values (CO from WCO; N(H2) inferred from AV ­ CO shielding parameter
· Break caused by change in CO photochemistry · Initiation of CO UV shielding · confirmation of vD&B1988 shielding function

­ Break: Low density vs. high density chemistry


N(CO) vs. N(H2)



CH vs. H2, CH+ vs. CO, H2


CH vs. H2, CH+ vs. CO, H2
· CH+ ~ CO for N(CO) < 1014 cm-2 · Similar break than for N(CO) vs. N(H2) · CH+ chemistry
­ non-Maxwellian velocity distributions
· Teff = Tkin + µ/3k (v v = 3 km + CO, CH
turb

)

2

· CH

­ Increase of CO by factors of 50 ­ 100 at low AV

· CH+: not in vDB and LePetit models · Sonnentrucker 2007 CO vs. H2


Summary
I. Diatomic molecules
First detection of CH, CH+ (SN1987A) & CN CH production in extensive PDRs together with CH Dense gas (CN-like CH) towards Sk 143 & Sk ­6702
+

II. Diffuse Interstellar Bands
5780, 5797, 6284 weaker by 10 (LMC) ­ 20 (SMC) relative to HI, weaker by 2 ­ 6 relative to EB-V IUV, metallicity C2-DIBs towards Sk143 and Sk-6702, similar strengths as in Galaxy no uniform scaling relations


Summary
· Direct determination of X = N(H2)/N(CO) · N(CO) ~ N(H2)
· · · · Importance of CO UV shielding Importance of CO production via CH+ LePetit 2006, Sonnentrucker 2007 Reproduction of observed CH+ levels in nonMaxwellian velocity fields


Outlook
DIBs: Jena, D. Huisken potential carriers UV spectra of