Oxygen-rich dust in astrophysical environments

1. Oxygen-rich dust in astrophysical environments Ciska Kemper UCLA

2. Oxygen-rich astromineralogy • Silicate astromineralogy • Composition • Degree of crystallinity in astrophysical environments • Processing of silicates • Carbonate astromineralogy • Discovery • Formation mechanism? • Implications for solar system carbonates?

4. Infrared spectroscopy • Astronomical spectra • ISO 2-200 μm spectroscopy (1995-1998) • Ground-based N- and Q-band • SIRTF: 5-40 μm spectroscopy • SOFIA … • Laboratory spectroscopy • Grain properties: size, shape… • Radiative transfer: absorption, emission and (multiple) scattering by grains

6. Silicates olivine: forsterite pyroxene: enstatite Si-O stretch O-Si-O bend Si-O stretch O-Si-O bend lattice modes crystalline versus amorphous

7. Si-O O-Si-O lattice • AGB star: OH 127.8+0.0 (Kemper et al. 2002)

8. Post-AGB star: MWC 922 (Molster 2000)

9. Crystallinity as a function of density? t Sylvester et al. (1999)

10. Models for 20% crystallinity

11. 20% crystallinity Mass loss: 10-7M yr-1 Kemper et al. (2001)

12. Contrast: features are best seen when Tam = Tcryst • Absorptivity determines T: in NIR kam> kcryst, in mid-IR almost equal • Star radiates in NIR: the amorphous dust is warmer for t<<1 • t>>1: inner grains heat outer grains in MIR, T difference disappears and contrast improves

13. Crystallinity determined by: • Condensation temperature • Temperature and energy release during processing history: • UV radiation • Ion bombardment • Grain-grain collisions (grain growth) • Shocks • Once formed, crystalline materials can exist at low T

14. Crystallinity correlates with grain growth, in old stars… • Molster et al. (1999)

15. …and in young stars • Bouwman et al. (2001)

16. The life cycle of silicates crystallinity

17. Silicates in the diffuse ISM • Galactic Center line-of-sight: Large beam and crowded field  many sources Thermal emission and absorption local to GC sources Absorption by dust in diffuse ISM

18. Observations • From Vriend (1999), see also Lutz et al. (1996) and Chiar et al. (2001)

19. Optical depth in 10 micron feature • Optical depth t from continuum subtraction I(l) = I0(l)e-t Sgr A* has intrinsic emission and absorption Use Quintuplet as template • WC Wolf Rayet stars: no silicates • Same dust composition along line of sight • Linear combination of absorption coefficients t =  aiki

20. Fitting procedure • Fit 2 fit to 10 micron absorption Evaluation of residuals • Laboratory spectra Amorphous silicates: Dorschner et al. (1995) • Good fit to OH 127.8+0.0 • Composition and structure known Crystalline silicates: Koike et al. (1999, 2000) • Complete set of all detected crystalline silicates: forsterite, enstatite, diopside

21. Results

22. Results: composition • Composition of amorphous silicates: • olivine (MgFeSiO4) : 85% • pyroxene (MgFeSi2O6) : 15% • Crystallinity • <0.4 % of silicates in diffuse ISM are crystalline • Crystallinity of 0.2% gives best fit to the 10 micron absorption feature

23. AGB stars and red supergiants Crystalline fraction: 11-18% of dust ejected into diffuse ISM is crystalline But we observe in diffuse ISM: <0.4 % Silicate producing stars • Explanations: Dilution by other sources of amorphous silicate dust: Supernovae or dust formation in ISM Fast amorphisation in ISM conditions

24. Dilution by supernova silicates • Supernovae seem to be a significant source of dust (Dunne et al. 2003, Morgan et al. 2003): 60-75% of interstellar dust is coming from SNe • Little is known about the dust composition in SN remnants => 22 micron feature: protosilicates ?! • For 0% crystallinity of the SN silicates, the dust from other stellar ejecta is diluted by a factor of 2.5-4 • The combined crystallinity of the stellar ejecta contributing to the ISM should then be 3-7%: dilution may contribute but is not sufficient!

25. Dunne et al. (2003) • Arendt et al. (1999)

26. Amorphisation of crystalline dust • Amorphisation rate To go from 11-18% crystallinity in stellar outflows to 0.2% in the diffuse ISM, the amorphisation rate should be 75 times faster than the destruction rate For a destruction rate of 2x10-8 yr-1 we find that amorphisation occurs on a time scale of 2 Myr

27. Ion bombardments can cause amorphisation Experimental studies at low energy (4-60 keV) show amorphisation, but low fluxes Higher energies (0.4-1.5 MeV): no amorphisation for light weight ions…. Iron? Amorphisation processes

28. Recent processing • The low crystallinity of silicates in the diffuse ISM suggest that very few AGB grains survive the diffuse ISM. • Crystallinity seen in our own solar system occurred locally, and are not AGB grains which survived the diffuse ISM unaltered. • Exchange of crystalline silicates between dense environments (dense ISM, star forming regions, young stars and the solar system) is not ruled out, but excursions to the diffuse ISM are very unlikely.

29. Silicates in IDPs Messenger et al. (2003) studied 1031 subgrains taken from a handful IDPs 6 of these 1031 have non-solar oxygen isotopic ratios, and originate from AGBs or RSGs. Mineralogy is known for 3 of these 6 extrasolar grains: 1 forsterite and 2 GEMS grains. • 1 out of 6 ≠ <0.4% • Is this single forsterite grain the lucky one that survived the amorphisation processes in the ISM?? • Maybe the grain is amorphitized in the ISM and annealed in the local environment, without altering the chemical (isotopic) composition?

30. The life cycle of silicates • Crystalline silicates are ubiquitous. They are found around young stars and old stars. • The presence of a disk seems to enhance annealing and grain growth • The silicates in the diffuse interstellar medium are highly amorphous: degree of crystallinity <0.4% • Very few AGB and RSG grains survive the diffuse ISM unaltered. Is the high amorphisation rate explained by ion bombardments? • Crystallinity of silicates in the solar system is caused by local processes: Condensation or annealing.

31. Carbonates in Planetary Nebulae • Planetary nebulae are formed by post-main-sequence stars

32. NGC 6302 (Molster et al. 2001)

33. Koike et al. (2001)

34. Kemper et al. (2002)

35. Kemper et al. (2002)

36. Kemper et al. (2002)

37. Abundance of dust components 27 % of calcium is depleted into calcite, dolomite and diopside 10 % of water is contained in the solid phase But what does it mean to find carbonates?

38. Carbonates on Mars

39. Carbonates • On earth, carbonates are formed through aqueous alteration • Earth, (Mars-)meteorites and interplanetary dust particles (IDPs) • Used as a tool to disentangle the formation history of the Solar System

40. atmosphere CO2 CO32- silicates Ca2+ CaCO3 Carbonates are lake sediments

41. In Planetary Nebulae • Around NGC 6302: 70 M of carbonates • On planets, carbonate/silicatemass ratio  1/100 • Around PNe: the formation and subsequent shattering of a sufficiently large planetary system is unlikely • Important alternative formation mechanism!

42. Formation of PN carbonates • Gas phase condensation: • CaO (gas) + CO2 (gas)  CaCO3 (solid) • Interaction between silicate grains and CO2 and H2O in the gas phase: hydrous silicates • Interaction between silicate grains and a mobile ice layer of CO2 and H2O

43. Hydrous silicates in young star HD 142527 (Malfait et al. 1999)

44. Carbonate inventory • Also found towards young star NGC 1333-IRAS 4 • Inventory of environments:formation mechanism • ISO LWS (45-200 μm) database • SIRTF: 6.8, 11, 14 and 92(?) μm • SOFIA?

45. Carbonates towardsNGC 1333-IRAS 4

46. Ceccarelli et al. (2002)

47. Conclusions: carbonates • The carbonates calcite and dolomite are identified in two planetary nebulae and towards a young stellar object • Do not violate abundance constraints • Aqueous alteration as a formation mechanism can be excluded • Carbonate formation in the solar system?

49. Conclusions: astromineralogy • MIR and FIR spectroscopy have opened the field of astromineralogy • Probes astrophysical conditions • Provides clues to understand the formation of planetary systems

50. What do we need? • Laboratory study of dust condensation, chemical alteration and processing under astrophysical conditions • Comparison with Solar System mineralogy • Database of optical constants • Astronomical instruments for mid- and far-infrared spectroscopy, broad band