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Modeling Planetary SystemsAround Sun-like Stars
Paper: Formation and Evolution of Planetary Systems: Cold Outer Disks Associated with Sun-like Stars, Kim, J.S., et al. 2005, ApJ 632, 659.
Wendy HawleyFebruary 23, 2006
AST 591: Journal Club
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Scope of Study
Presents five Sun-like stars with characteristics of exo-KBs
Models debris disks and discusses implications for our Solar System
Models one star with emission consistent with photosphere
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Outline
Context and Introduction
Observations
Spectral Energy Distributions
Debris Disk Modeling
Evolutionary Model
Summary
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Context
Previous Work:– Meyer et al. (2004) : debris disk around
Sun-like stars– Cohen et al. (2003): data analysis with
Kurucz model– Wolf & Hillenbrand (2003): dust disk
models
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Introduction
Why study other planetary systems?– Puts our Solar System in context
Debris systems in our Solar System– Asteroid belt (2-4 AU) - zodiacal dust cloud– Kuiper Belt (30-50 AU) - beyond Neptune
Other systems can be used to help model ours
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Spitzer Space Telescope
Data taken from FEPS (Formation and Evolution of Planetary Systems)
Previous studies done using Infrared Astronomical Satellite (IRAS) and Infrared Space Observatory (ISO)
Detection of new systems with Spitzer
More info: Meyer et al. (2004)
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Observations
6 targets, 5 of which have excess (3) emission at 70m but 3 excess at 33 m
Taken using MIPS (Multiband Imaging Photometer for Spitzer) at 24 and 70 m bands
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Spectral Energy Distributions
Expected photospheric emission found using Kurucz model on published photometry
Predicted magnitudes found using method outlined in Cohen et al. (2003)
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Debris Disk Models
Assumptions:– Optically thin disk in thermal equilibrium– Temperature depends on distance from
star– Max. Temp. ~100 K, Min. Equilibrium
Distance 10 AU for grains of radius ~10-100m
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Radiation Pressure and Poynting-Robertson Drag
Particles <~1m have blow out time of <100yr
Particles >~1m subject to slow P-R drag, destroyed after 106-107 years– Short compared to age of systems,
implying object are being replenished
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Simple Blackbody Grain Models
Based on Tc (excess color temperature) calculated from Planck formula– Ax : emitted grain cross-sectional area– Grain luminosity– Grain mass
Rin found from formula used by Backman and Paresce (1993)
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HD 8907 - closer look
Used disk model from Wolf & Hillenbrand (2003) and Levenberg-Marquardt algorithm for best-fit
Assumptions– n(r)r-1, n(a)a-3.5, amax=1mm, Rout=100AU
Vary parameters: Rin, amin, Mdust
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This model gives Rin of 42.5 AU compared to 48 AU of simple blackbody model
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Warm Dust Mass
Masses on order of 10-6 M
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Age Determination
Age bins rather than specific ages used
Inferred from chromospheric and coronal activity– Indicated respectively by CaIIH and K
emission and X-ray luminosity
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Solar System Evolutionary Model
Model from Backman et al. (2005)
Assumptions:– Rin=40 AU, Rout=50 AU
– Starting mass of KB 10 M
– P-R induced “zodiacal” dust cloud extending inward
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•Results are within factor of 2-3 of predicted 70m excesses for the targets, except HD 13974•Present solar system dust mass 30% of HD 145229
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HD 13974 - closer look
Binary system (period=10days)
Model would suggest much higher 70m excess than observed– No KB bodies?– Neptune-like planet to perturb and cause
collisions?
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Possible Planets?
Dust depletion occurring inside Rin
– Sublimation and grain “blowout” ruled out– Planet preventing P-R drift
– Planet would be >Mjupiter and have a semimajor axis of 10-20 AU, plus exterior belt of planetesimals
– More work to be done through direct imaging and constraints on low-mass companions
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Summary
FEPS is allowing a more complete database of debris systems
5 sources have excess emission at 70m, indicating exo-KBs
SED modeling indicated log(LIR/L*)-5.2, color temperatures 55 to 58 K, Rin 18 to 46 AU
Solar system model within a few factors of observed fluxes
HD 13974 either doesn’t have KB-like objects or they have been ejected from the system
Dust depletion <Rin due to Jupiter-like planet at 10-20 AU