Transcript An Evolutionary Model of KB

Modeling Planetary Systems
Around 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 Hawley
February 23, 2006
AST 591: Journal Club
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
Outline
Context and Introduction
Observations
Spectral Energy Distributions
Debris Disk Modeling
Evolutionary Model
Summary
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
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
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)
Observations
6 targets, 5 of which have excess (3)
emission at 70m but 3 excess at 33
m
Taken using MIPS (Multiband Imaging
Photometer for Spitzer) at 24 and 70 m
bands
Spectral Energy Distributions
Expected photospheric emission found using
Kurucz model on published photometry
Predicted magnitudes found using method
outlined in Cohen et al. (2003)
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 ~10100m
Radiation Pressure and
Poynting-Robertson Drag
Particles <~1m have blow out time of
<100yr
Particles >~1m subject to slow P-R
drag, destroyed after 106-107 years
– Short compared to age of systems,
implying object are being replenished
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)
HD 8907 - closer look
Used disk model from Wolf &
Hillenbrand (2003) and LevenbergMarquardt algorithm for best-fit
Assumptions
– n(r)r-1, n(a)a-3.5, amax=1mm, Rout=100AU
Vary parameters: Rin, amin, Mdust
This model gives Rin of 42.5 AU compared to 48
AU of simple blackbody model
Warm Dust Mass
Masses on order of 10-6 M
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
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
•Results are within factor of 2-3 of predicted 70m
excesses for the targets, except HD 13974
•Present solar system dust mass 30% of HD
145229
HD 13974 - closer look
Binary system (period=10days)
Model would suggest much higher 70m
excess than observed
– No KB bodies?
– Neptune-like planet to perturb and cause
collisions?
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
Summary
FEPS is allowing a more complete database of debris systems
5 sources have excess emission at 70m, 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