Transcript High Resolution Observations of Debris Disks

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)
ε Eri (K2)
τ Ceti (G8)
HD 107146 (G2)
η Corvi (F2)
•
rout < 70 AU (submm)
rout = 100 AU
rout = 55 AU
rout = 150 AU
rout = 150 AU
t ~ 0.01 Gyr
t = 0.85 Gyr
t = 10 Gyr
t ~ 0.1 Gyr
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