Oxygen-rich dust in astrophysical environments Ciska Kemper UCLA

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?

Infrared spectroscopy Astronomical spectra ISO μm spectroscopy ( ) 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

amorphous olivine(Fe,Mg) 2 SiO 4 amorphous pyroxene(Fe,Mg)SiO 3 metallic ironFe enstatiteMgSiO 3 forsteriteMg 2 SiO 4 diopside(Ca,Mg)SiO 3 water iceH2OH2O hydrous silicatessilicate + H 2 O carbonates(Ca,Mg)CO 3 silicaSiO 2 spinelMgAl 2 O 4 Mg (0.1) Fe (0.9) O corundumAl 2 O 3

Silicates olivine: forsteritepyroxene: enstatite crystalline versus amorphous Si-O stretch O-Si-O bend lattice modes Si-O stretch O-Si-O bend

AGB star: OH (Kemper et al. 2002) Si-O O-Si-O lattice

Post-AGB star: MWC 922 (Molster 2000)

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

Models for 20% crystallinity

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

Contrast: features are best seen when T am = T cryst Absorptivity determines T: in NIR  am  >  cryst, in mid-IR almost equal Star radiates in NIR: the amorphous dust is warmer for  <<1  >>1: inner grains heat outer grains in MIR, T difference disappears and contrast improves

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

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

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

The life cycle of silicates Evolved (AGB, PN, RSG) % diffuse ISM<0.4 % starforming regions small Herbig Ae/Be, T Tau stars 5-8 % Debris disks? Solar systemVery high crystallinity

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

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

Optical depth in 10 micron feature Optical depth  from continuum subtraction I  = I 0  e -  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  a i  

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

Results

Results: composition Composition of amorphous silicates: olivine (MgFeSiO 4 ) : 85% pyroxene (MgFeSi 2 O 6 ) : 15% Crystallinity <0.4 % of silicates in diffuse ISM are crystalline Crystallinity of 0.2% gives best fit to the 10 micron absorption feature

Silicate producing stars Explanations: Dilution by other sources of amorphous silicate dust: Supernovae or dust formation in ISM Fast amorphisation in ISM conditions 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 %

Dilution by supernova silicates Supernovae seem to be a significant source of dust (Dunne et al. 2003, Morgan et al. 2003): % 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 The combined crystallinity of the stellar ejecta contributing to the ISM should then be 3-7%: dilution may contribute but is not sufficient!

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

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

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

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.

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?

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.

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

NGC 6302 (Molster et al. 2001)

Koike et al. (2001)

Kemper et al. (2002)

Abundance of dust components dust speciesM/M  fraction amorphous olivine4.7 · % forsterite> 2.0 · > 4.0 % clino-enstatite> 5.5 · > 1.1 % water ice3.6 · % diopside2.8 · % calcite1.3 · % dolomite7.9 · % 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?

Carbonates on Mars

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

CO 3 2- atmosphere silicates CO 2 Ca 2+ CaCO 3 Carbonates are lake sediments

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

Formation of PN carbonates Gas phase condensation: CaO (gas) + CO 2 (gas)  CaCO 3 (solid) Interaction between silicate grains and CO 2 and H 2 O in the gas phase: hydrous silicates Interaction between silicate grains and a mobile ice layer of CO 2 and H 2 O

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

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

Carbonates towards NGC 1333-IRAS 4

Ceccarelli et al. (2002)

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?

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

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

Back up sheets

Jäger et al AFGL 4106

Molster (2000)

Annealing of silicates Mg 2 SiO 4 (Fabian et al. 2000)