Astrochemistry Lecture 7 Chemistry in star-forming regions

Transcription

1 Astrochemistry Lecture 7 Chemistry in star-forming regions Ewine F. van Dishoeck Leiden Observatory Spring 2008

2 Outline Introduction Observational techniques Chemical scenario Cold outer envelopes around YSOs Warm inner envelopes around YSOs Hot cores Protoplanetary disks Comets

3 7.1 Introduction Van Dishoeck & Hogerheijde 1999, Crete II Ceccarelli et al. 2007, PPV Star formation accompanied by enormous changes in physical parameters n, T response of chemistry? Which molecules are good diagnostics of various stages: collapse, outflow, hot cores? Molecular lines are used to probe physical conditions and dynamics need to choose right molecule! In which form are molecules incorporated into circumstellar disks and new planetary systems?

4 Star formation

5 7.2 Observational techniques Submillimeter emission (0.4-1 mm; GHz) Single-dish telescopes IRAM 30m, JCMT 15m, CSO 10m, APEX 12m, HHT 10m Typical beam sizes AU at 150 pc Interferometers IRAM 6 15m, CARMA 6 10m+9 6m, SMA 8 6m, Nobeyama 6 10m 100 GHz: AU at 150 pc 230 GHz: ~100 AU

6 Radiation at submm wavelengths Continuum: cold dust at K; steep spectrum with ν 3 Lines: pure rotational transitions of molecules Submillimeter observations probe warmer and/or denser gas associated with Young Stellar Objects (YSOs)

7 (Sub)millimeter: emission Pure rotational transitions Collisions radiation J=1 J=0 Higher transitions probe higher temperatures and densities n cr μ A ul /C ul μ 2 ν 3

8 M. Hogerheijde

9 Typical sizes Linear Size Angular size Taurus 140 pc Orion 450 pc 5 AU Inner disk 100 AU Outer disk 1000 AU YSO envelope AU=0.05 pc Cloud core

10 Submm lines Line strength abundances Line ratios temperatures, densities Line profiles kinematics

11 Infrared observations Far-IR emission/absorption: μm ISO-LWS 90, Herschel ~15 Mid-infrared emission/absorption: 3-45 μm Ground: VLT, Keck, UKIRT, IRTF, Space: ISO-SWS, Spitzer,

12 Infrared vs submillimeter Submillimeter: Very high spectral resolution (R>10 6, <0.1 km/s) Many gas-phase molecules with abundances down to w.r.t. H 2 Emission => map of region Infrared: Moderate spectral resolution (R~ ) Gases and solids with abundances down to w.r.t. H 2 Probe major reservoirs of C, N and O Molecules without permanent dipole moments (H 2, C 2 H 2, CH 4, CO 2, CH 3, ) Absorption => pencil beam line-of-sight; also emission

13 7.3 Chemical scenario Starless cores: Tª10-20 K, nª10 4 cm -3 Quiescent ion-molecule chemistry Radicals and carbon chains Pre-stellar cores and collapse: Tª10 K, nª cm -3 Heavy freeze-out of molecules onto grains Grain surface reactions produce new species: H O, C, N, CO H 2 O, CH 4, NH 3 H 2 CO, CH 3 OH O CO CO 2

14 Chemical scenario (cont d) Embedded YSO phase: Tª K, nª cm -3 Gas and dust heated by accretion luminosity, UV + X-rays young star Evaporation of molecules from grains: H 2 O, CH 3 OH, CH 4, complex organics?. Sequence according to evaporation temperatures Hot core chemistry: complex organics, HCN, Bipolar outflows: Tª K, n= cm -3 Interaction outflow with envelope shocks High-T chemistry: H 2 O,. Return icy mantles to gas: H 2 O, CH 3 OH, CH 4, Destroy grain cores: SiO,

15 Physical structure protostellar envelope Single dish beam High-T chemistry Evaporation ices Freeze-out Schöier et al Jørgensen et al. 2002

16 -Sublimation temperatures for pure ices as measured in lab; values in space are lower because of lower pressure Evaporation temperatures Species T evap (lab) (K) H 2 O 150 CH 3 OH 99 HCN 95 SO 2 83 NH 3 78 CO 2 72 H 2 CO 64 H 2 S 57 CH 4 31 CO 25 N 2 22 Mumma et al. 1993, PPIII

17 7.4 Cold outer envelopes: freeze-out At densities > 10 4 cm -3, freeze-out times scales become shorter than lifetime cores most species (except H 2, He, H 3+ ) freeze-out onto grains unless there is an efficient mechanism to return them Outer envelope cold significant freeze-out expected Since YSOs spatially unresolved in single-dish data, need multi-line data of each molecule to infer chemical gradients on scale smaller than beam Surveys by van Dishoeck, Blake et al. (1994, 1995), Jørgensen et al. (2002, 2004, 2005), Ceccarelli et al. (2000), Maret et al. (2004, 2006) Constrain T, n from dust continuum data Use line data to fit abundance Simplest model: constant abundance with radius More sophisticated models: drop, jump abundance profiles

18 Timescales

19 Example: modeling of CO lines toward L723 Jørgensen et al Adopt n(r) and T(r) from continuum; then constrain abundances - Overall philosophy: use simplest possible abundance models (constant, jump, drop) and add complexities only if needed

20 Physical structure from dust emission D=200 pc Ice evaporation JCMT beam Freeze-out SD beam Jørgensen et al. 2002

21 CO freeze-out: constant abundance Canonical CO abundance (Lacy et al. 1994) CO freezes out at low temp. ( 35 K) Objects with high envelope masses (younger?) show significantly higher degree of CO depletion => CO not a good tracer of H 2 mass

22 Drop abundance model n de T ev Constant Drop

23 L723: Constant abundance model Drop abundance model - Drop abundance much better fit to J=1-0 to 3-2 Jørgensen et al. 2004

24 Chemical structure confirmed by interferometry L483: 450 μm cont. N 2 H + C 18 O Jørgensen 2004

25 Anticorrelation CO and N 2 H + : models Jørgensen 2004

26 Oxygen chemistry: freeze-out Pure gas-phase Freeze-out + gas-grain chemistry OÆH 2 O gr Æsticks CÆCH 4 gr Æevaporates Bergin et al Gas-grain chemistry and differential evaporation can explain low H 2 O and O 2 in dark clouds, as found by SWAS and ODIN

27 Infrared: direct observation of ices Background star Embedded young star Flux Continuum due to hot dust Absorption by cold dust Wavelength

28 Infrared observations of ices HH 46: solar-mass YSO From 10 5 to <0.1 L sun objects! ISO ISO L1014: substellar YSO Spitzer Ground-based 8-m - H 2 O, CH 3 OH, CO from ground - Other species from space Boogert et al. 2004, 2008 Pontoppidan et al Öberg, Bottinelli et al. 2008

29 Direct observation ices Species High-mass YSO NGC 7538 IRS9 Low-mass YSO HH46 H 2 O ~10-4 w.r.t. H 2 CO CO CH 3 OH 9 6 H 2 CO <3 HCOOH 2 3 CH NH 3 10 OCN CO highly variable because of low evaporation temperature -CH 3 OH, NH 3, OCN - vary by order of magnitude from source to source

30 Different ice phases Comparison of observed CO, CO 2 line profiles with lab spectra multiple phases Polar : H 2 O-rich, hydrogen bonding Apolar or Non-polar : CO-rich, van der Waals bonding Segregated: CO 2 -rich, CH 3 OH-rich Possible explanations Diverse condensation: H-rich vs. O-rich gas Diverse evaporation: temperature gradient results in differential outgassing and restructuring ices

31 Different ice phases Non-polar Polar N 2 O 2 CO CO CO - Different ice phases can be seen in IR spectra!

32 CO 2 ice lab spectra Stretching mode 4.3 μm Bending mode 15.2 μm Ehrenfreund et al Note variations in band shape if molecule surrounded by different partners

33 CO 2 15 μm changes with heating H 2 O:CO 2 :CH 3 OH mixture Ehrenfreund et al Note that spectrum becomes similar to that of pure CO 2 when mixed ice is heated

34 Observations CO 2 15 μm - Same trend seen in observations Ice segregates, with nearly pure CO 2 clusters forming upon heating

35 Different ice phases in YSO envelopes

36 7.5 Warm inner envelope: evaporation Evaporation of ices directly observed in IR spectra Indirectly inferred from submm data as jump in abundance in inner envelope

37 Ice evaporation low-mass YSO s CO ice CO gas Pontoppidan et al 2003

38 Hot and cold methanol: Massive YSOs CH 3 OH 7 K -6 K band N6334 Hot: 200 K N7538 I1 Warm: K N7538 I9 Cold: 30 K Van der Tak et al 2000

39 Hot methanol around solar-mass protostar Hot CH 3 OH gas T~80 K IRAS See Exercises, problem 3 - Many different species but abundances low - Segregation O- and N-rich organics Van Dishoeck et al. 1995, Ceccarelli et al. 2000

40 Rotation diagrams IRAS : one temperature vd et al. 1995

41 Example: CH 3 OH jump abundance structure X J Jump Freeze-out

42 Abundance Pre-stellar core: Low temperature Depletion toward center...but not edge Protostellar core: Central heating ~ temperature gradient Thermal desorption toward center...outside (low T): depletion/no depletion regions as in pre-stellar stages Q: Is this an evolutionary sequence? Can chemistry constrain timescales? Current data suggest phase of heavy depletions (pre-+protostellar) lasts only ~10 5 yr Jørgensen et al. 2005

43 7.6 Hot cores Observations of most high-mass YSOs such as Orion-KL, SgrB2, G , W3(H 2 O), show high abundances of complex saturated organic molecules These are called hot cores, with characteristics Tª K, nª10 7 cm -3, size<0.1 pc Several low-mass YSOs show a similar chemical complexity Favorite model: evaporated molecules from grains into warm gas drive a rapid gas-phase chemistry for 10 5 yr Initial conditions: ice abundances, as constrained by observations

44 Typical hot core spectrum: Signpost of massive star formation G327.3 Massive YSO Gibb et al. 2000

45 Complex organics around solar-mass protostars IRAS Cazaux et al. 2003

46 Orion line surveys Tercero & Cernicharo 2007 Blake, Sutton et al Schilke. Comito et al. 1993, s

47 Orion line surveys 1980 s 1990 s Tercero & Cernicharo

48 Orion line surveys Tercero & Cernicharo

49 Q: how far does chemical complexity go? Prebiotic molecules?

50 Chemical differentiation in Orion Groesbeck Two high-mass YSOs, comparable L,M -Only 1.5 apart -Very different chemistry!

51 Line rich vs line poor sources Three nearby massive YSOs in W3 JCMT 345 GHz IRS5: Rich in SO, SO 2, H 2 O: Rich in complex mol. IRS4: Simple, PDR species Helmich & vd 1997

52 Chemical differentiation on small scales W3(H 2 O) W3(OH) - Chemical differentiation between O- and N-rich complex organics on few arcsec scale (few thousand AU) Wyrowski et al. 1999

53 Hot core chemistry Charnley et al. 1992, 1997 Wakelam et al. 2004, 2006 Hot gas-phase chemistry starting at t=0 from evaporated ices Which complex organics are first generation made in the ices, and which are second generation produced in the gas? Can complex organics, e.g. CH 3 OCH 3 /CH 3 OH, be used as chemical clocks? Data suggest tª yr since evaporation Differences between sources may be explained by different initial ice composition or different age (evolutionary stage)

54 Chemical structure envelope

55 Chemical Scenario Heavy freeze-out of molecules onto grains in cold pre-stellar phase Grain surface reactions produce new species Protostar heats surroundings Ice evaporation Hot core chemistry Fraction of ices and gas ends up in disks; remainder is dispersed

56 7.7 Inner disk chemistry (<10 AU) Large literature on chemical models of the solar nebula (=disk from which our solar system formed) since 1970 s E.g., Lewis, Prinn, Fegley, Lunine,. Models applied to large range of solar system observations E.g., CO/CH 4 planetary atmospheres, comet abundances, meteorites Chemistry thought to be dominated by thermal equilibrium (3-body) rather than kinetic (2-body) processes 1 AU: n~10 14 cm -3, T=800 K 15 AU: n~10 11 cm -3, T=80 K

57 Thermochemistry Principle chemistry CO + 3H 2 CH 4 + H 2 O N 2 + 3H 2 2NH 3 CO + H 2 O CO 2 + H 2 Reactions proceed more to the right at lower T Fischer-Tropsch reactions on metallic iron can also convert CO to CH 4 Lewis & Prinn 1980

58 Quench surface Equilibrium region: t chem < t mix Disequilibrium region: t chem >t mix Chemical reactions too slow to be equilibrated; composition similar to that at quench surface E.g., CO and N 2 dominant in solar nebula, since conversion to CH 4 and NH 3 in outer region kinetically inhibited

59 Other processes Ice condensation and evaporation: 4-20 AU (species and time dependent) Dust evaporation (<1 AU) Weak (accretion) shocks Lightning UV and X-rays from Sun/star Radial and vertical mixing Ionization by extinct radionuclides ( 26 Al, 60 Fe, )..

60 7.8 Outer disk chemistry 1D Radial transport models Consider chemical evolution of parcel of gas as it moves radially from >100 AU to few AU Include large gas-phase chemistry network (few hundred species, few thousand reactions) and gasgrain adsorption/desorption processes Chemistry dominated by temperature profile: virtually no gas-phase molecules >10 AU, active gas-phase chemistry <10 AU E.g., Bauer et al. 1997, Finocchi & Gail 1997, Gail , Willacy et al. 1998, Aikawa et al. 1997, 1999

61 Example Abundances after 3x10 6 yr Everything frozen out at >10 AU But this is NOT what is observed! Aikawa et al. 1999

62 Observations of molecules in disks JCMT CN 3-2 HCN 4-3 LkCa CO 3-2 HCO MWC 480 MWC 480 OVRO Dutrey et al Kastner et al Qi 2001 Thi et al. 2004

63 Some observational findings Simple gas-phase molecules observed Ion-molecule reactions (HCO + ) Photon processes (high CN/HCN) High deuterium fractionation (DCO + ) Low abundances complex species (H 2 CO, CH 3 OH) Data only sensitive to >50 AU Lines comes from warm K layer with n= cm -3 Disk-averaged abundances are depleted by factor of (using mass from dust continuum and assuming gas/dust=100) Solid CO and H 2 O detected in edge-on disks

64 Detection of DCO + in a circumstellar disk TW Hya face-on disk 1.1 μm 1.6 μm JCMT HCO + /2 H 13 CO + Scattered light => radius 200 AU Weinberger et al DCO + DCO + /HCO + =0.035 => gas in disks is cold with heavy depletions Dutrey et al Kastner et al van Dishoeck et al. 2003

65 Chemical structure of 2D flaring disks - Surface layer: molecules dissociated by UV photons - Warm intermediate layer: molecules not much depleted, rich chemistry - Cold midplane: molecules heavily frozen out Aikawa et al. 2002

66 midplane surface Vertical structure (R=200 AU) T gas is larger Freeze-out Ionization fraction: - Surface: 10-4 (C + ) - Intermediate: 10-9 (HCO + ) - Midplane: ~10-11 (H 3+,H 2 D + ) Photodissociation

67 7.9 Cometary chemistry Comets are thought to have formed in the outer part of the disk which formed our solar system Comets contain the least-modified original interstellar material Early optical observations probed mostly daughter molecules (CH, CN, C 2, CO +, OH, OH +, ). More recent IR and (sub)mm observations provide direct information on parent molecules, i.e., original composition of ices Lots of data from comets Halley, Hyakutake and Hale-Bopp

68 Parent-daughter model As comet approaches Sun, ices heat up and evaporate. Radiation from Sun photodissociates molecules: H 2 O Æ OH Æ O NH 3 Æ NH 2 Æ NH CH 4 Æ CH 2 Æ CH HCN Æ CN

69 -Similar abundances substantial fraction of cometary ices are unaltered ISM ices? Ice abundances Species Protostar ices Comets H 2 O CO CO CH 3 OH H 2 CO HCOOH NH CH

70 7.10 Summary Systematic trends in ices and gas-phase data allow chemical scenario during low- and high-mass star formation to be developed Astrochemistry has come of age Chemistry driven by heating of envelope, resulting in evaporation of ices which drive a hot core chemistry Increasingly sophisticated gas-grain chemistry models coupled with dynamics Different IR and submm features can be used as physical diagnostics of the earliest stages of star formation Chemistry in disks starting to be studied Inner disk: hot core chemistry at high densities Outer disk: layered chemical structure Comets provide clue to composition of material from which our solar system is made Large similarity between cometary and interstellar ices Laboratory data and theoretical studies of basic molecular processes remain essential for further progress.

71 Chemistry during star formation Van Dishoeck & Blake 1998

72 Lifecycle of gas and dust Based on Ehrenfreund & Charnley 2000 What are building blocks for life elsewhere in the Universe?