Spitzer IRS Spectroscopy of IRAS-Discovered Debris Disks · • The moon and terrestrial planets...
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Spitzer IRS Spectroscopy of IRAS-Discovered Debris Disks
Christine H. Chen (NOAO)IRS Disks Teamastro-ph/0605277
A Possible Planet in the β Pic Disk
STIS/CCD coronagraphic images of the β Pic disk. The half-width of the occulted region is 15 AU. At the top is the disk at a logarithmic stretch. At bottom is the disk normalized to the maximum flux, with the vertical scale expanded by a factor of 4 (Heap et al. 2000)
Observed Dwarp = 70 AU48 MJup brown dwarf at
Why Study Debris Disks?
• How common is the architecture of our solar system (terrestrial planets, asteroid belt, Jovian planets, and Kuiper Belt)?
• What were the physical conditions in the early solar system?
• How do the physical conditions of the disk impact the formation of planets and subsequent orbital evolution of planets and smallbodies?
• How does material become processed in the early solar system, mix in the disk and become incorporated into larger bodies?
Giant Planet Formation and Migration in Our Solar System
• The moon and terrestrial planets were resurfaced during a short period (20-200 Myr) of intense impact cratering3.85 Ga called the Late Heavy Bombardment (LHB)
• Apollo collected lunar impact melts suggest that the planetary impactorshad a composition similar to asteroids
• Size distribution of main belt asteroids is virtually identical to that inferred for lunar highlands
• Formation and subsequent migration of giant planets may have caused orbital instabilities of asteroids as gravitational resonances swept through the asteroid belt, scattering asteroids into the terrestrial planets. Strom et al. (2005)
Mid- to Far-Infrared Spectra of Dust Debris Around Main Sequence Stars
• IRAS observations discovered more than 100 main sequence stars with unresolved excess and grain temperatures (Tgr = 50 – 125 K), similar to the Kuiper Belt in our Solar System, and fractional infrared luminosities (LIR/L* = 10-5 – 10-3)
• Dust grain lifetimes are shorter than the ages of the systems suggesting that the material is replenished from a reservoir such as collisions between parent bodies or sublimation of comets.
• These objects typically have Fν(10 μm) < 1 Jy, making the majority of systems too faint to be studied spectroscopically from the ground.
⇒ IRS 5.5 – 35 μm spectroscopy of 59 main sequence stars with IRAS 60 μm excess. (Observations of the first 19 objects observed are published in Jura et al. 2004)
Single Temperature Black Body Fits
SED modeling suggests that the dust is located in a thin ring which can be modeled assuming a single temperature distribution
What Could Create Central Clearings in Disks?
• Radiation pressure if the grains are small (disk is collisionallydominated)
• Sublimation if the grains are icy
• Gas-grain interactions in disks with gas:dust ratios 0.1 – 10 (Takeuchi & Artymowicz)
• Gravitational scattering of dust grains out of the system
• Trapping grains into mean motion resonances (Liou & Zook; Quillen & Thorndike 2002)
Are Circumstellar Dust Grains Icy?• Sublimation temperature of
water in a vacuum is Tsub = 150 K.
• The grain temperatures inferred from black body fits to the infrared excess peak at 110 – 130 K
• Sublimation lifetimes are a sensitive function of grain temperature. For example, dust grains with a = 3.5 micron and Tgr = 70 K, have a lifetime of 1.3×107Gyr (!) while grains with a = 16 μm and Tgr = 160 K have a lifetime of 7.4 minutes.
Dust in Pericenter Alignment with a Planet Around Fomalhaut?
• The Fomalhaut disk ansapossess a brightness asymmetry which may be caused by secular perturbations of dust grain orbits by a planet with a = 40 AU and e = 0.15 which forces grains into an elliptical orbit with the star at one focus (Stapelfeldt et al. 2005)
Kalas et al. (2005)
Dust in Mean Motion Resonances Around ε Eri?
Quillen & Thorndike (2002) model of dust captured into 5:3 and 3:2 exterior mean motion resonances of a 30 M planet with e = 0.3 and a = 40 AU.
Greaves et al. (2005)
Predicted Time Evolution of Debris
• If dust producing collisions remove two comets from the cloud, then the evolution of the number of comets
• With the solution
• For long times
• If internal collisional processes dominate grain loss, we can expect the number of visible grains to be proportional to the number of comets left in the system, corresponding to a 1/t dependence of the dust amount.
• If other processes dominate grain removal, such as Poynting-Robertson Drag, a Nc2 (corresponding to 1/t2) dependence should be expected
tttN sc 2
)( ≈)0(2 c
Observed Decay of Fractional Infrared Luminosity in Debris Disks Sample
• The upper envelope of the relation between fractional infrared luminosity and age can be fit with a 1/t power law. The 1/t2 power law does not produce a bad fit (only ηCrv is inconsistent).
• The scaling factor for our fit constrains (LIR/L*)oto =0.4 Myror (LIR/L*)oto2 =60 Myr2
Collisional Cascades in Planetesimal Disks
• In a minimum mass solar nebula, 1000 – 3000 km-sized bodies are expected to grow on timescales,tP = 15 – 20 Myr (D/30 AU)3
(Kenyon & Bromley 2004)
• If debris disks are self-stirred by forming planets and if dust is generated in collisionalcascades, then an outward propagating ring of dust emission should be observed.
Silicate Emission Features
• Predominantly associated with intermediate-age disks with ages
• The shape of the 10 μm Si-O and 20 μmO-Si-O bending mode features can be used to diagnose grain size
• The peak and the width of the features are dependent on the vacuum volume fraction
Kessler-Silacci et al. 2006
• Infrared spectroscopy of comets and analysis of comet dust grains from STARDUST suggest that comets possess crystalline silicates
• How does material processed at high temperatures near the sun mix in a proto-planetary disk become incorporated into cold bodies such as comets?
HR 7012 (β Pic Moving Group)• Silicates and
amorphous carbon: Tgr = 520 K
• Cool black body continuum,: Tgr = 200 K
• Large amorphous olivine and pyroxene grains: 1.1 – 5.0 μm
• Sub-micron sized enstatite and cristobalite grains
⇒ Recent Collision?
η Tel (β Pic Moving Group)
• Amorphous silicate Tgr = 370 K
• Two black body continua: Tgr = 115 K and 370 K
• Large olivine grains: 3.0 μm
HD 113766 (Lower Centaurus Crux)• Silicate and
amorphous carbon: Tgr = 600 K
• Cool black body continuum: Tgr = 200 K
• Large amorphous olivine and pyroxene grains: 1.5 μm
• Sub-micron forsterite grains
⇒ Recent Collision?
HR 3927 (field A0V star)
• Silicate temperature: Tgr = 290 K
• Cool black body component: Tgr = 80 K
• Large spherical olivine and forsteritegrains: 3.1 and 8 μm
η Crv (Field F2V Star)
• Silicate: Tgr = 360 K
• Cool black body component: Tgr = 120 K
• Large olivine, crystalline forsteriteand enstatite grains: 3.5, 8, and 1 μm
Conclusions1. The majority of observed debris disks do not possess spectral features,
suggesting that their grains are too cool and/or too large (a > 10 μm) to produce spectral features. Detailed modeling of objects with spectral features requires the presence of large, warm, amorphous silicates with Tgr= 290 – 600 K, in addition to cool black bodies with Tgr = 80 – 200 K, and the presence of crystalline silicate mass ratios 0-76%.
2. The IRS spectra of debris disks (without spectral features) are generally better fit using a single temperature black body than with a uniform disk. Stellar radiation pressure (in collisionally dominated systems), sublimation if the grains are icy, gas drag, and/or the presence of a perturbing body may contribute to the presence of inner holes in these disks.
3. The peak in the estimated black body grain temperatures Tgr = 110 – 120K suggests that sublimation of icy grains may produce the central clearings observed.
4. Since parent body masses typically are less than the mass of the Earth, it appears that planet formation efficiently consumes most of the mass of the primordial disk.
Spitzer IRS Spectroscopy of IRAS-Discovered Debris DisksA Possible Planet in the b Pic DiskWhy Study Debris Disks?Giant Planet Formation and Migration in Our Solar SystemMid- to Far-Infrared Spectra of Dust Debris Around Main Sequence StarsSingle Temperature Black Body FitsWhat Could Create Central Clearings in Disks?Are Circumstellar Dust Grains Icy?Dust in Pericenter Alignment with a Planet Around Fomalhaut?Dust in Mean Motion Resonances Around e Eri?Predicted Time Evolution of DebrisObserved Decay of Fractional Infrared Luminosity in Debris Disks SampleCollisional Cascades in Planetesimal DisksSilicate Emission FeaturesGrain GrowthCrystalline GrainsHR 7012 (β Pic Moving Group)η Tel (β Pic Moving Group)HD 113766 (Lower Centaurus Crux)HR 3927 (field A0V star)η Crv (Field F2V Star)Conclusions