Astromineralogy of Protoplanetary Disks

(and other astrophysical objects)

Steve Desch

Melissa Morris

Arizona State University

Outline

I. Mineral Opacities: all minerals emit infrared radiation with different dependendes on wavelength

II. Basics of Radiative Transfer: emission from comets / dusty disks can be calculated

III. Observations of Disks and Comets: different astrophysical systems have different spectra that tells us about their mineralogy

IV. Implications for Solar System Formation

Astrophysical Context

Protoplanetary Disk:

young (< 3 Myr) disk around new protostar, about 98% H2 and He gas, 1% H2O and 0.5 % “dust”. Total mass ~ 0.1 Msun

Astrophysical Context

Debris Disk:

somewhat older (~ 10 - 100 Myr) disk around main-sequence star, made entirely of dust shed from asteroids / planetesimals / comets. Total Mass ~ Mmoon

Astrophysical Context

Zodiacal Light / Exo-zody Disk:

Much older (~ 0.1 - 10 Gyr) disk around main-sequence star, made entirely of dust shed from asteroids / planetesimals / comets. Total mass << 1 Mmoon

Astrophysical Context

Comets: dirty snowballs

Mineral Opacities

Transitions between solid state vibrational and bending modes lead to emission / absorption of photons in the infrared, at wavelengths character-istic of the mineral.

Silicates: 10 m, 18 m

FeS: 23 m

Al2O3: 13 m

SiO2: 8.6 m, 20.5 m

Mineral OpacitiesShape and peak wavelength of the feature can further diagnose the chemical composition and crystallinity of the mineral.

From Laboratory Astrophysics Group, AIU Jena

Crystalline Olivine

Mineral OpacitiesShape and peak wavelength of the feature can further diagnose the chemical composition and crystallinity of the mineral.

From Laboratory Astrophysics Group, AIU Jena

Amorphous Olivine

Mineral Opacities

Amorphous Olivine vs. Amorphous Pyroxene

Mineral Opacities

Phyllosilicates

Basics of Radiative Transfer

Emission from a silicate particle only departs from a blackbody if the particles are small.

Cabs = a2 Qabs

Qabs = 4 x Im [ (m2 - 1) / (m2 + 2) ] (x << 1)

Qsca = (8 x4 / 3) | (m2 - 1) / (m2 + 2) |2 (x << 1)

x = 2a / , m = n + i kEmission can be predicted only if particle size, shape, and complex index of refraction known.

Basics of Radiative Transfer

Emission from optically thin systems (debris disks, comets) straightforward if temperature known.

I = B(Tp) [1 - e-], = np a2 Qabs() L

Temperature determined by balance between absorption of starlight and emission of infrared.

Qabs() (L / 4r2) d = Qabs() B(Tp) d

Essential to have complex index of refraction in optical as well as infrared!

Basics of Radiative Transfer

Emission from optically thick protoplanetary disks more complicated: sum of optically thick blackbody disk emission plus emission features from hotter, optically thin layer on disk surface.

H ~ T1/2

disk

= dH/dr - H/r

(L/ 4r2) / 2 = T4disk

Feedbacks between glancing angle and disk structure and temperature.

Protoplanetary Disks

crystalline silicates

optically thick disk emission

optically thin, hot surface layer

Spitzer data of HD 143006

Protoplanetary Disks Spitzer data of HD 143006

One possible model fit to data, including 3% phyllosilicates

Same model but replacing 3% phyllosilicates with amorphous olivine / pyroxene

Protoplanetary Disks

Apai et al. 2005

Protoplanetary Disks

Minerals already identified in T Tauri and Herbig Ae/Be disks:

•Amorphous silicates (olivine / pyroxene)

•Crystalline silicates (olivine / pyroxene)

•SiO2

•FeS

•(as well as nano-diamonds and PAHs)

We expect that phyllosilicates will be discovered in debris disks in the near future...

Debris Disks

Okamoto et al. 2004

beta Pictoris (12 Myr) debris disk: dust concentrated into belts. Crystalline silicates only at center (< 6 AU)

Debris Disksspectrum of Solar System zodiacal dust (Reach et al. 2003)

contains crystalline silicates

possibly phyllosilicates???

Cometary Spectra

Hale-Bopp

comet NEAT/Q4

Cometary Spectra

Definitely crystalline silicates (but almost enitrely in long-period comets)

Cometary Spectra

Probably crystalline silicates

Implications for Solar System Formation

Interstellar dust < 0.2% crystalline (Kemper et al. 2004)

Cometary / Protoplanetary Disk dust ~ 50% crystalline (see Wooden, Desch, Harker & Keller 2006, PP V)

Silicate dust was annealed.

Must have attained temperatures > 1000 K. (Hallenbeck et al. 2000).

These temperatures typical only < 1 AU from star.

Large-scale radial transport? (Bockelee-Morvan et al. 2002)

Or transient heating in situ (e.g., by shocks)? (Harker and Desch 2002)

Conclusions

Infrared spectra of protoplanetary disks, debris disks, zodiacal disks, comets, etc., can be used to characterize the mineralogy of dust in those objects.

Laboratory measurements of optical constants of minerals (complex index of refraction, from visual to infrared) + radiative transfer modeling =

size, shape, composition, crystallinity of dust grains.

Interstellar medium amorphous, but protoplanetary disks and comets have crystalline silicates, implying that dust in protoplanetary disks is thermally processed.

Astromineralogy is a growing field with lots of opportunity for progress!