Lecture 10: "Chemistry in Dense Molecular Clouds"

Transcription

1 Lecture 10: "Chemistry in Dense Molecular Clouds"

2 Outline 1. Observations of molecular clouds 2. Physics of dense clouds 3. Chemistry of dense clouds: C-, O-, N-chemistry

3 Types of molecular clouds Diffuse clouds: Tkin~100 K, n ~100 cm -3 Translucent: Tkin~ K, n ~ cm -3 Dark dense clouds: Tkin~ K, n ~ cm -3

4 CO (1-0) survey of Milky Way Taurus-Auriga Perseus Auriga B5 L1449 IC348 NGC1333 TMC-1 T Tau Taurus Ungerechts & Thaddeus

5 Prestellar cores Barnard 68 Visible Infrared Dust (1%), gas (99%) Typical mass ~ M sun, size < 1 pc, n >10 4 cm -3, T ~10 K Dynamically "quiet", t ~ 1 10 Myr Clumpy

6 Ward-Thompson et al. 1994, 1999 General scheme: physics Bonnor-Ebert spheres: hydrostatic equilibrium of isothermal clouds bound by external pressure Density profile: flat in the center, steep decrease toward the edge Many cores are highly asymmetric Some may have already preprotostars/show signs of infall

7 General scheme: physics Heating through cosmic rays/fuv Cooling through dust and molecular line radiation (C, CO, OH) Increasing temperature toward the edge Observed T is too low in the center: non-thermal pressure support (B-field) or ongoing collapse Alves et al. 2001

8 Velocity structures Barnard 68 L(NH 3 ) ~ 5x10 21 cm 2 s -1 L(NH 2 D) ~ 3x10 20 cm 2 s -1 Complex velocity pattern: rotation, infall, turbulence (outflows?) In some cores, angular momentum seems to be smaller on smaller scale forming protostars?

9 Observations of dense clouds Dust continuum mapping accurate Tdust Tgas ~ 10 K strongest molecular lines are at mm (rotation) Barnard nh ~ 10 4 cm -3 higher J-lines can be subthermally excited 68 Masses ~ M sun optically thick lines Line widths are thermal Intensevily observed since 70's, "classical" source: TMC-1S or TMC-1CP Sofar detected ~ 60 species, mostly with single-dish antennas Structures of individual cores are studied with radiointerferometers, e.g. B68, L1544, etc.

10 Molecules in dense clouds Depletion of C-bearing species: CO, CCS, CS,... Non-depletion of N2H + and NH3: - depletion Barnard of CO: CO + N2H + HCO + + N2 does not work - slow formation 68 of N2 Abundant carbon chains Abundant negative ions Abundant deuterated species Simple organics: HCOOH, CH3OH,...

11 Ices in dense clouds Spitzer IRS spectrum Barnard 68 H 2 O HCOOH NH 4 + +? Silicate CO 2 5 Ices are observed in absorption against background stars Dominated by H2O, CO, CO2, Complex ices: HCOOH, CH3OH, In starless clouds ~10 50% of heavy elements are in ices Up to 99% of the heavy elements may be frozen out in the center

12 Molecules as probes Tracer Properties Quantity 12 CO Optically thick lines H2, NH3 Symmetric species Temperature 13 CO, C 18 O, CS, CCS, H2CO Large dipole moment Density HCO +, H 13 CO +, N2H +, C, C + Charged species Ionization H2CO, organics Complex species Surface processes HDCS, D2CS, DCO +, DCN, NH2D, H2D + Deuterated species Deuterium fractionation Semenov et al. (2010)

13 General scheme: chemistry Dense core: C + is converted to C and CO Early times: CCS and HCnN are abundant Late times: N2H +, H2D +, NH3, CO is absent in the center CCS traces outer shell, NH3 traces central region Suzuki et al. 1992

14 Dust and molecules in B68 No traces of a protostar Two components CO is frozen in the center N2H + is sitting there CS is concentrated toward a smaller clump different 'chemical' age accross B68? Alves et al. 2002

15 Long carbon chains in TMC-1 Barnard 68 Langer et al. 1997

16 Negative ions in TMC-1 C 6 H + e - C 6 H - + hν k <10-7 cm 3 s -1 Barnard 68 Effective for molecules with large e- affinities <10% of anion/neutral (predicted by Herbst 1981) Discovered in clouds with predicted abundances (McCarthy et al. 2006, Bruencken et al. 2007) McCarthy et al. 2006

17 DC 3 N/HC 3 N ~ Deuterium chemistry HDCS/H2CS ~ 0.3 D2CS/H2CS ~ 0.3 Barnard 68 Langer et al CCD/CCH ~ 0.01 DCN/HCN ~ N2D + /N2H + ~ D2CO/H2CO ~ DC 5 N/HC 5 N ~ 0.02 Efficient deuterium enrichment in cold cores: D/H in molecules > [D/H] ~ 10-5 Schoerb et al. 1981

18 'Early' vs 'Late Time' molecules C 2 S vs HC 3 N C 2 S vs NH 3 When C is available, CCS and HCnN are quickly produced At later times surface chemistry catches up: NH3 is formed, CO is frozen in the center CCS traces outer shell, NH3 traces central region where CO freezes out onto dust grains Suzuki et al. 1992

19 Effect of external environment Barnard 68 CCS is more readily present in isolated, less evolved cores Effect of environment: Radiation or Temperature? Needs C to be synthesized FUV/CRP destruction of CO? Foster et al. 2009

20 Typical timescales Chemical time: > years Collision with dust grains: once in ~1 day at 10 K and 10 4 cm -3 Life-time of a cloud: ~1 10 x 10 6 years Low-mass star formation: ~10 6 years Chemistry is slow and can be affected by evolution

21 Early chemical models First gas-phase ion-molecule chemistry models: Herbst & Klemperer 1973, Watson 1976, Dalgarno & Black 1977, Prasad & Huntress 1980, Millar et al No photoprocesses, only H2 formation on grains Grain-surface chemistry gradually included by Allen & Robinson 1978, Tielens & Hagen 1982, d Hendecourt et al. 1985, Herbst & Hasegawa 1993

22 Modern gas-phase networks Most recent models contain ~5000 gas- phase reactions between ~450 (up to 13 atoms): - UMIST: - Ohio State Univ.: research.html - Kinetic Database for Astrochemistry:

23 More detailed chemical models Models with evolving physics: nh and T vary with time (e.g., Tarafdar et al. 1985, Aikawa et al. 2009) Depth-dependent models: FUV intensity is accurately calculated (e.g. Lee et al. 1996) Chemo-dynamical models: cycling of gas parcels from inner to outer to inner regions (e.g., Boland & de Jong 1982, Xie et al. 1995, Willacy et al. 2002)

24 Chemical models with surface processes Enough density in the center to build up icy mantles: t c = [V(πr 2 n d )] -1 ~ 10 9 /nh(cm -3 ) years ~10 5 years if nh is 10 4 cm -3 A grain accomodates ~ 1 species per day Migrating species: H, H2, D, but also O, C, N Low density active surface hydrogenation: NH3, H2O, CH3OH, etc. High density O-, C-, N-chemistry on grains: hydrocarbons, cyanopolyynes, etc. D Hendecourt et al. 1985, Herbst & Hasegawa 1993, Shalabiea & Greenberg 1994, Aikawa et al , Garrod & Herbst 2008,...

25 Problems of chemical models HC 3 N C 2 S Modeled abundances are uncertain by factors of >3 Vasyunin et al. (2004, 2007), Wakelam et al. (2005, 2006)

26 Pause

27 Oxygen chemistry I.P. of O > 13.6 ev oxygen mostly neutral Ionization provided by cosmic rays: H2 H +,H2 +, H3 + (rapid) Then: H + + O H + O + (+227 K) O + + H2 OH + + H H3 + +O OH + +H2 Once OH + formed, rapid ion-molecule reactions lead to OH, H2O and CO

28 Formation of water H 2 + CRP H e - H H 2 H H H O OH + + H 2 OH n + + H 2 OH n H H 3 O + + e - H 2 O + H; OH + 2H, etc

29 Oxygen chemistry

30 A problem of observed low O2 Goldsmith et al Factor >100 discrepancy between observed and modeled O 2 Solution: Allow freeze-out of O on dust grains (Bergin et al. 2000) converted to water ice rather than O2 ice

31 Carbon chemistry I.P. of C < 13.6 ev carbon mostly C + Reactions with H3 + are also important C + + H2 CH2 + + hν possible at low T (initiating reaction) CH2 + rapid ion-molecule reactions lead to CH, C2, C2H, C2H2,... C + + H2 CH + + H: endothermic by 0.4 ev

32 Gas-phase formation of hydrocarbons C + + H 2 CH 2 + CH H 2 CH H CH H 2 /O CH 5+ /HCO + + H 2 CH e- CH 3 + H 2 CH 3 + O H 2 CO CH H 2 O CH 3 OH + X 2 (too low rate, Luca et al. 2002) CH 3 OH e- CH 3 OH + H (3 ± 2%, Geppert et al. 2006)

33 Carbon chemistry

34 Nitrogen chemistry I.P. N > 13.6 ev nitrogen mostly neutral Nitrogen chemistry: N + H3 + NH2 + + H does not occur (barrier?) N + + H2 NH + + H (~+100 K) So, it starts with neutral-neutral chemistry linked to carbon: CH, C2 + N CN + H, C CH3 + + N H2CN + + H H2CN + + e HCN or HNC + H

35 Nitrogen chemistry

36 Sulfur chemistry Marka et al Modeling supports the observed evidence that CCS/NH3 decreases with the age of the cloud CCS is produced when C is still in the gas, NH3 forms on dust grain surfaces later

37 Deuterium fractionation e- H 2, H H 3 + H 2 HD H 2 D + HD, H 2, D, H e- e- H 2 HD H 2 e- D 2, D D 2 H + D + 3 HD Barriers No barriers CO, N 2, O CO, N 2, O CO, N 2, O CO, N 2, O HCO +, N 2 H +, OH + DCO +, HCO +, N 2 D +, N 2 H +, OD +, OH + DCO +, N 2 D +, OD + Courtesy of H. Roberts Mass difference between HD and H2 fractionation via H2D +, D2H +, D3 +, ~ K At steady-state: H 2 D + /H 3 + ~ 0.1 D/H of e.g. N2D +, DCO+ Freeze-out DR of H3 + isotopologues many D atoms surface addition reactions

38 Surface chemistry: water O O O O 2 O 3 H 2H H OH H 2 O 2 OH H H 2 O 2H H 2 O H 2 O H Tielens & Hagen 1982

39 Surface chemistry: HCOOH and CH3OH OH

40 Observations vs Theory: TMC1-S About 75% agreement for 50 observed molecules in TMC1

41 The End