Oxygen-rich dust in astrophysical environments
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Transcript of Oxygen-rich dust in astrophysical environments
Oxygen-rich dust in astrophysical environments
Ciska Kemper
UCLA
Oxygen-rich astromineralogy
Silicate astromineralogyCompositionDegree of crystallinity in astrophysical
environmentsProcessing of silicates
Carbonate astromineralogyDiscoveryFormation mechanism?Implications for solar system carbonates?
Infrared spectroscopyAstronomical 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
amorphous olivine (Fe,Mg)2SiO4
amorphous pyroxene (Fe,Mg)SiO3
metallic iron Fe
enstatite MgSiO3
forsterite Mg2SiO4
diopside (Ca,Mg)SiO3
water ice H2O
hydrous silicates silicate + H2O
carbonates (Ca,Mg)CO3
silica SiO2
spinel MgAl2O4
Mg(0.1)Fe(0.9)O
corundum Al2O3
Silicates
olivine: forsterite pyroxene: enstatite
crystalline versus amorphous
Si-O stretch
O-Si-O bend
lattice modes
Si-O stretch
O-Si-O bend
AGB star: OH 127.8+0.0 (Kemper et al. 2002)
Si-O O-Si-Olattice
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:10-7 M yr-1
Contrast: features are best seen when Tam = Tcryst
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)
11-18 %
diffuse ISM <0.4 %
starforming regions
small
Herbig Ae/Be, T Tau stars
5-8 %
Debris disks ?
Solar system Very 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 = I0e-
Sgr A* has intrinsic emission and absorptionUse Quintuplet as template
• WC Wolf Rayet stars: no silicates• Same dust composition along line of sight
Linear combination of absorption coefficients
ai
Fitting procedure Fit2 fit to 10 micron absorptionEvaluation of residuals
Laboratory spectraAmorphous 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
Results
Results: composition
Composition of amorphous silicates:olivine (MgFeSiO
4)
: 85%
pyroxene (MgFeSi2O
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 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 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!
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 (0.4-1.5 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 IDPsMessenger 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 silicatesCrystalline 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)
Kemper et al. (2002)
Kemper et al. (2002)
Abundance of dust componentsdust species M/M fraction
amorphous olivine 4.7 · 10-2 94 %
forsterite > 2.0 · 10-3 > 4.0 %
clino-enstatite > 5.5 · 10-4 > 1.1 %
water ice 3.6 · 10-4 0.72 %
diopside 2.8 · 10-4 0.56 %
calcite 1.3 · 10-4 0.26 %
dolomite 7.9 · 10-5 0.16 %
27 % 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
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
CO32-
atmosphere
silicates
CO2
Ca2+CaCO3
Carbonates are lake sediments
In Planetary Nebulae
Around 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 unlikely
Important alternative formation mechanism!
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
Hydrous silicates in young star HD 142527 (Malfait et al. 1999)
Carbonate inventory
Also found towards young star NGC 1333-IRAS 4
Inventory of environments: formation mechanismISO LWS (45-200 μm) database
SIRTF: 6.8, 11, 14 and 92(?) μm
SOFIA?
Carbonates towardsNGC 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 constraintsAqueous alteration as a formation
mechanism can be excludedCarbonate 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 constantsAstronomical instruments for mid- and far-
infrared spectroscopy, broad band
Back up sheets
Jäger et al. 1998
AFGL 4106
Molster (2000)
Annealing of silicates
Mg2SiO4 (Fabian et al. 2000)