High-resolution Imaging of Debris Disks

Jane Greaves

St Andrews University, Scotland

why debris? why long λ?

• debris is the ‘fallout’ of comet collisions

– dust must be continually regenerated or will blow away, spiral into star...

– shows that bodies at least km in size formed!• and are still there• sign that planets are likely?

– comet belts define the outer edges of planetary systems

• is the outer Solar System typical in size / content?

• far-IR/submm observations pick up the thermal emission from cool dust grains

– modelling the SED shows the grains are a few microns up to centimetres (or more) in size

– temperatures tens

of AU orbits

– signal is optically thin • (so traces mass)

– signal is >> the photosphere in submm

progress

• a lot of it, since excess found for Vega by IRAS!

• now Spitzer... getting near Solar System dust level

• imaging is key:

– size scale of system– structure of cometary belt

• planet perturbations!– holes cleared by planets

rogues gallery

τ Ceti ε Eridani Vega (α Lyr) Fomalhaut (α PsA) β Pic

big discoveries

• the Solar System is small

– for 5 debris disks imaged around Sun-like stars:

AU Mic (M1) rout < 70 AU (submm) t ~ 0.01 Gyr

ε Eri (K2) rout = 100 AU t = 0.85 Gyr

τ Ceti (G8) rout = 55 AU t = 10 Gyr

HD 107146 (G2) rout = 150 AU t ~ 0.1 Gyr

η Corvi (F2) rout = 150 AU t ~ 1 Gyr

• was our history of planet formation affected by having a compact disk around the Sun?

• the debris disk fraction may be high

– ~50% for A stars– ~10% for F/G/K stars

• some of which are older than the Sun!

– but perhaps as many more cold disks?

• submm detected

• planning future surveys...

• planets on very large orbits

– e.g. at ~100 AU in Fomalhaut system?• ~3x orbit of Neptune

story so far

• every system looks different!

• few are symmetrical!– high fraction of perturbing planets?

• potential for unique method to detect distant planets

• (unless you prefer decades of astrometry...)

– ‘icy Neptunes’, not ‘hot Jupiters’– high angular resolution very important

• how it works:– dust of certain size trapped in resonances– identify clump patterns, e.g. 2:1, 3:2 ...

• hence planet location– reality check: rotation of pattern

3:2

e = 0.3

e = 0.2

e = 0.1

really planet detection?

• central holes might be argued away– grain sublimation...? (doesn’t quite work)

• perturbed rings inexplicable without planet!– e.g. massive comet blow-ups too rare

• modelling of dust trapping can be quite exact• radius, eccentricity, position + direction of orbit• minimum mass of planet

• rotation of clump pattern is the clincher(this is not more indirect than radial

velocity!)

epsilon Eridani

• nearest Solar-ish analogue– K2V star 3.2 pc away– but only 0.85 Gyr old

• 5 years of SCUBA data • (by accident!)

• well resolved ring ~ face-on– dust peaks 65 AU out– centre offset from star!

• forced by inner gas giant?

proper motion

• star has moved 5’’ to right over 5 years– pick out real ring clumps...versus fixed high-z galaxies

ring rotation

• proper motion plus rotation leads to characteristic shifts

– tentative!!!!

• but systematic, ~2’’ counter-clockwise

– if ok, planet at ~40 AU

future plans

Interferometers can...

• pick out clumps precisely– and so the fraction of trapped dust, planet mass

• detect rotation after much less time

– 2’’ very hard with single dish!• boring waiting 5 years....

– quick with sub-arcsec resolution!

SMA and ALMA

• stars within 10 pc are great for SMA!– e.g. bright disks of Vega + Fomalhaut...

0.1’’ rotations per year

• ALMA from ~2008

– great for fainter and more distant debris disks

– also young disks... see a Jupiter in formation

summary

• debris disks give unique insight to planetary systems

• imaging with high resolution is the key for use as a planet detection method

• hence ground-based long-wavelength interferometers are the way of the future