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)