Oxygen-rich dust in astrophysical environments
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Oxygen-rich dust in astrophysical environmentsCiska KemperUCLA
Oxygen-rich astromineralogySilicate astromineralogyCompositionDegree of crystallinity in astrophysical environmentsProcessing of silicatesCarbonate astromineralogyDiscoveryFormation mechanism?Implications for solar system carbonates?
Infrared spectroscopyAstronomical spectraISO 2-200 m spectroscopy (1995-1998)Ground-based N- and Q-bandSIRTF: 5-40 m spectroscopySOFIA Laboratory spectroscopyGrain properties: size, shapeRadiative transfer: absorption, emission and (multiple) scattering by grains
amorphous olivine(Fe,Mg)2SiO4amorphous pyroxene(Fe,Mg)SiO3metallic ironFeenstatiteMgSiO3forsteriteMg2SiO4diopside(Ca,Mg)SiO3water iceH2Ohydrous silicatessilicate + H2Ocarbonates(Ca,Mg)CO3silicaSiO2spinelMgAl2O4Mg(0.1)Fe(0.9)Ocorundum Al2O3
Silicatesolivine: forsteritepyroxene: enstatitecrystalline versus amorphousSi-O stretchO-Si-O bendlattice modesSi-O stretchO-Si-O bend
AGB star: OH 127.8+0.0 (Kemper et al. 2002)Si-OO-Si-Olattice
Post-AGB star: MWC 922 (Molster 2000)
Crystallinity as a function of density?Sylvester et al. (1999)t
Models for 20% crystallinity
Kemper et al. (2001)20% crystallinityMass loss:10-7 M yr-1
Contrast: features are best seen when Tam = TcrystAbsorptivity determines T: in NIR kam > kcryst, in mid-IR almost equalStar radiates in NIR: the amorphous dust is warmer for t1: inner grains heat outer grains in MIR, T difference disappears and contrast improves
Crystallinity determined by:Condensation temperatureTemperature and energy release during processing history:UV radiationIon bombardmentGrain-grain collisions (grain growth)ShocksOnce formed, crystalline materials can exist at low T
Crystallinity correlates with grain growth, in old starsMolster et al. (1999)
and in young starsBouwman et al. (2001)
The life cycle of silicatescrystallinity
Evolved (AGB, PN, RSG)11-18 %diffuse ISM
Silicates in the diffuse ISMGalactic Center line-of-sight: Large beam and crowded field many sourcesThermal emission and absorption local to GC sourcesAbsorption by dust in diffuse ISM
ObservationsFrom Vriend (1999), see also Lutz et al. (1996) and Chiar et al. (2001)
Optical depth in 10 micron featureOptical depth t from continuum subtractionI(l) = I0(l) e-t
Sgr A* has intrinsic emission and absorptionUse Quintuplet as templateWC Wolf Rayet stars: no silicatesSame dust composition along line of sight
Linear combination of absorption coefficientst = aiki
Fitting procedure Fit2 fit to 10 micron absorptionEvaluation of residualsLaboratory spectraAmorphous silicates: Dorschner et al. (1995)Good fit to OH 127.8+0.0Composition and structure knownCrystalline silicates: Koike et al. (1999, 2000)Complete set of all detected crystalline silicates: forsterite, enstatite, diopside
- Results: compositionComposition of amorphous silicates:olivine (MgFeSiO4) : 85%pyroxene (MgFeSi2O6) : 15%Crystallinity
- Silicate producing starsExplanations:Dilution by other sources of amorphous silicate dust: Supernovae or dust formation in ISMFast amorphisation in ISM conditionsAGB stars and red supergiantsCrystalline fraction: 11-18% of dust ejected into diffuse ISM is crystallineBut we observe in diffuse ISM:
Dilution by supernova silicatesSupernovae seem to be a significant source of dust (Dunne et al. 2003, Morgan et al. 2003): 60-75% of interstellar dust is coming from SNeLittle 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 2.5-4The 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 dustAmorphisation rateTo 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 rateFor a destruction rate of 2x10-8 yr-1 we find that amorphisation occurs on a time scale of 2 Myr
Amorphisation processesIon bombardments can cause amorphisationExperimental studies at low energy (4-60 keV) show amorphisation, but low fluxesHigher energies (0.4-1.5 MeV): no amorphisation for light weight ions. Iron?
Recent processingThe 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 IDPsMessenger et al. (2003) studied 1031 subgrains taken from a handful IDPs6 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
- The life cycle of silicatesCrystalline silicates are ubiquitous. They are found around young stars and old stars.The presence of a disk seems to enhance annealing and grain growthThe silicates in the diffuse interstellar medium are highly amorphous: degree of crystallinity
Planetary nebulae are formed by post-main-sequence starsCarbonates in Planetary Nebulae
NGC 6302 (Molster et al. 2001)
Koike et al. (2001)
Kemper et al. (2002)
Kemper et al. (2002)
Kemper et al. (2002)
Abundance of dust components27 % of calcium is depleted into calcite, dolomite and diopside10 % of water is contained in the solid phase
But what does it mean to find carbonates?
Carbonates on Mars
CarbonatesOn earth, carbonates are formed through aqueous alterationEarth, (Mars-)meteorites and interplanetary dust particles (IDPs) Used as a tool to disentangle the formation history of the Solar System
CO32-atmospheresilicatesCO2Ca2+CaCO3Carbonates are lake sediments
In Planetary NebulaeAround NGC 6302: 70 M of carbonatesOn planets, carbonate/silicate mass ratio 1/100Around PNe: the formation and subsequent shattering of a sufficiently large planetary system is unlikelyImportant alternative formation mechanism!
Formation of PN carbonatesGas phase condensation: CaO (gas) + CO2 (gas) CaCO3 (solid)Interaction between silicate grains and CO2 and H2O in the gas phase: hydrous silicatesInteraction between silicate grains and a mobile ice layer of CO2 and H2O
Hydrous silicates in young star HD 142527 (Malfait et al. 1999)
Carbonate inventoryAlso found towards young star NGC 1333-IRAS 4 Inventory of environments: formation mechanismISO LWS (45-200 m) databaseSIRTF: 6.8, 11, 14 and 92(?) mSOFIA?
Carbonates towardsNGC 1333-IRAS 4
Ceccarelli et al. (2002)
Conclusions: carbonatesThe carbonates calcite and dolomite are identified in two planetary nebulae and towards a young stellar objectDo not violate abundance constraintsAqueous alteration as a formation mechanism can be excludedCarbonate formation in the solar system?
Conclusions: astromineralogyMIR and FIR spectroscopy have opened the field of astromineralogyProbes astrophysical conditionsProvides clues to understand the formation of planetary systems
What do we need?Laboratory study of dust condensation, chemical alteration and processing under astrophysical conditionsComparison with Solar System mineralogyDatabase of optical constantsAstronomical instruments for mid- and far-infrared spectroscopy, broad band
Back up sheets
Jger et al. 1998AFGL 4106
Annealing of silicatesMg2SiO4 (Fabian et al. 2000)