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

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 70m 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 ~10-100m

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

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 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

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 70m 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 70m 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 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