Some physical processes and their chemical tracers in near-stellar environments

1. Some physical processes and their chemical tracers in near-stellar environments David Williams University College London Department of Physics and Astronomy

2. Content of this talk • Hot Cores – comprise an integrated (chemical) history of stellar birth • Stellar jets from young stars – chemical probes of nearby clouds • Stellar Outflows/parental cloud interfaces – chemical tracers of dynamical mixing

3. Structure of this talk For each of these topics, I shall describe: • Physical situation • Resulting chemistry • What we can learn from the chemistry? • Some recent or current work

4. Cartoon by M Hogerheide, (after F Shu 1987) In this talk we are concerned with stages b), c), and d) of the cartoon.

5. Star Formation Sequence

6. Hot Cores (N.B. preceding talk by Malcolm Walmsley)

7. Hot Cores: Physical Situation • Small, dense, warm knots of gas associated with high mass star formation • Material shares the collapse in which the star is formed, but is not incorporated into the star • Increasing density promotes gas/dust interaction and ice formation • Radiation from young star warms and evaporates the ices, giving observed species

8. Physical Characteristics of Hot Cores • Small: 10-1 - 10-2 pc • Dense: > ~ 107 H2 cm-3 • Warm: > ~ 102 K • Opaque: AV >102 mag • Transient: < ~ 105 y • Location: < 0.1 pc from star

9. Hot cores: resulting chemistry The long low-temperature collapse promotes: • Hydrogenation, leading to high abundances of small saturated molecules • Fractionation leading to high D/H ratios, including multiple deuteration • Solid state chemistry (activation unclear) leading to complex hydrocarbons These give rise to a hot core chemistry that is distinct from that of quiescent molecular clouds

10. Hot cores:what can we learn from the chemistry? • Ices are a cumulative record of the chemistry occurring during the collapse • Hot core molecules reflect conditions in the cold high-density pre-stellar phase • Evaporation can tell us about the warm-upphase from ~10 K to ~300 K • from which we may be able to infer the “switch-on” time of the massive star, and possibly the onset of stellar winds

12. Hot Cores: Recent/current work • The “warm-up” phase • Impact of stellar winds on hot cores • Hot cores as probes of high red-shift objects

13. 1. The “warm-up” phase Warm-up phase of proto-star is not instantaneous but typically ~ 3 x 104 y for a very massive star (e.g. Bernasconi and Maeder 1996) Warm-up phase of hot cores from ~ 10 K to ~ 300 K must be of similar duration Situation is similar to laboratory TPD experiments

14. The Warm-up Phase Laboratory TPD data for mixed ices shows that icemorphology is important (Collins et al. 2004) These data can be incorporated into a time-dependent (single-point) hot core model (Viti et al. 2004)

16. TPD categories (Collings et al. 2004) A. “CO-like” shows four peaks: (i) monolayer desorption of from H2O amorphous ice (ii) multilayer desorption (iii) “volcano” desorption on H2O crystallization (iv) co-desorption with H2O B. “Intermediate”: shows peaks (iii) and (iv), and - if molecule is sufficiently mobile - possibly peak (i) C. “H2O like”: only peak (iv)

17. TPD experiments (Collings et al. 2004) Molecules are in one of several categories with respect to TDP from mixed ices • CO-like (examples: CO, CH4, N2 , O2) • Intermediate (examples: OCS, H2S, SO2) • H2O-like (examples: H2O, CH3OH, HCOOH, NH3)

18. Model of hot core chemistry: time-dependence and laboratory data • Use new lab data for molecules, + data inferredfor other species (total 40 species), to specify fraction of each species desorbed as function of temperature • Assume temperature rises according to a power law from 10 K to 300 K in a “contraction time” • Chemistry computed as a function of time; results depend on stellar mass

20. Results of new hot core model • Differential desorption effects are clearly evident in the results • Chemistry sensitive to stellar mass and to elapsed time • While power law may not be accurate, any monotonic function will give the same qualitative behaviour

21. The “Established Hot Core” phase This phase is affected by the earlier “warm-up” phase (whose existence is now confirmed by observations). Calculations of established hot core chemistry need to take account of the earlier “warm-up” phase and follow its evolution into the later phase.

22. Comparison of results for instantaneous and “warm-up” evaporation models

24. 2. Onset of winds of massive stars NS/CS, SO/CS, HCO/H2CO appear to be sensitive to presence of a shock in an evolving hot core (Viti et al. 2001) Hatchell & Viti 2002 measured NS/CS in sample of six hot cores, and found a low and near-uniform ratio for all six cores

25. Onset of winds of massive stars Implications of Hatchell & Viti data • Results are inconsistent with model with instantaneous desorption and no shock • Results are consistent with model in which an early shock removes ices, which later re-form and are subsequently thermally desorbed • Near-uniform ratio implies that sample hot cores all at similar evolutionary stage

27. 3. Hot Cores at high red-shift • High redshift galaxies may be very active in star formation – should have many hot cores • Are hot core signatures detectable in low metallicity massive stars at high z? • If so, what are the main chemical tracers? Detections should constrain metallicity(depends on previous generation of stars) and star formation rate at high red-shift

28. Hot cores at high redshift: assumptions • Metallicity: 1 >Z/ZMW≥ 1/10000 • Dust/Gas ratio ~ Z/ZMW • ~107 - 108 hot cores for high z starburst galaxy

29. Extragalactic Hot Cores: model predictions • Large range of metallicity and of number of hot cores per galaxy for which a variety of molecules should be detectable. • CO, CH3OH, H2S, SO, SO2, H2CO etc should be detectable with e.g. VLA at red-shift ~ 6.

30. Extragalactic Hot Cores: model predictions • Our models show that different molecules trace different conditions: e.g. SO is sensitive to the number of hot cores at very low metallicity, while SO2 is most sensitive to metallicity. • Observed HCN deficit at high z arises since star formation occurs in near-primordial gas in which N is very under-abundant (Lintott & Viti 2006 – poster).

32. Stellar Jets • Molecule formation inside the jet • Jet/cloud interactions

33. Stellar jets: Physical situation • Stellar jets initially atomic/ionic • Both molecular and atomic/ionic emission is seen in stellar jets in regularly-spaced knots • Can be interpreted as emission in a jet with periodic velocity variations • In HH1, emission knots in H2 and [FeII] coincide

36. Stellar jets: Resulting Chemistry What is origin of this H2? • Envelope? No. • Injection at source into a time-variable jet (Voelker et al. 1999), or • Formation directly in a dust-free atomic/ionic variable jet?

37. Stellar jets:What can we learn from their chemistry? From a chemical/dynamical model of a sinusoidal jet we can learn: • Gas density in jet • Gas density in environment • Velocity and amplitude • Mass loss rate in the jet

38. Stellar jets: Recent/current workH2 formation in the jet Chemistry in dust-free atomic gas (Raga, Williams & Lim 2005) Reactions forming H2: H + e → H- ; H- + H →H2 + e H + p → H2+; H2+ + H →H2 + p Species included: H, H+, He, He+, He++, C+, C++,C3+, O, O+, O++, H2, H-, H2+ in a network of about 20 reactions

39. Computations for the jet • Gas dynamic equations + cooling + chemistry are integrated self-consistently • Jet velocity has sinusoidal variation about mean velocity vJ to create regularly spaced shocks along jet • We vary velocity half-amplitude, initial density, and mass loss rate to simulate observed linear distribution of knots

40. Adopted parameters • Jet radius = 5x1014 cm • Jet mean velocity = 200 km s-1 • Period = 10 y • Environment density = 103 cm-3 • Environment temperature = 103 K

41. Emission maps • Emission maps in Hα and H2 1-0 calculated assuming that the outflow axis is on the plane of the sky • Maps can be compared with observations of HH 1 and HH 111 which have a chain of knots in both emission lines, of comparable intensity • Plausible choices for these objects are δV = 10 km s-1, nH = 2 x 105 cm-3, [dM/dt]jet =10-7 Mo y-1

43. Molecules arising from H2 in jets? • Given that significant quantities of H2 can be formed in a variable jet, what about other species? • Conventional interstellar chemistry depends on H2 as initiator of chemistry. • Calculations of abundances of molecules including H2CO, CH3OH, SiO within the jet are in progress

45. Jet/cloud interactions:Physical situation • HH objects are regions of shocked gas where a stellar jet impinges on a cloud • many molecular condensations associated with HH objects • often detected in NH3 and HCO+ emission; • lines often narrow, so condensation is not always shocked by stellar jet • condensations show anomalous chemistry cf. dark clouds

47. Jet/cloud interactions: resulting chemistry Radiation from the HH object desorbs ices from dust in the condensation, and triggers photochemistry in region previously dark (Girart et al. 1994) Models have supported this view (Taylor & Williams1996; Viti & Williams 1999; Viti et al. 2003; Girart et al. 2005)

49. HH-induced photochemistry: theoretical predictions Characteristic abundance enhancements in dense clump near HH object is predicted for HCO+, NH3, CH3OH, H2S, C3H4, H2CO, SO, SO2, H2CS, NS compared to cold dark clouds

50. Jet/cloud interactions:what can we learn from the chemistry? • Density structures within molecular clouds • Distribution of ices within the clouds • HH shock velocity • Shock-induced radiation field