Submillimeter Submillimeter Observations of Debris Observations of Debris

DisksDisks

Wayne HollandWayne HollandUK Astronomy Technology Centre, Royal Observatory EdinburghUK Astronomy Technology Centre, Royal Observatory Edinburgh

With Jane Greaves, Mark Wyatt, Bill Dent and a cast of many…With Jane Greaves, Mark Wyatt, Bill Dent and a cast of many…

Observing Debris DisksObserving Debris Disks

Large submillimetre telescope (JCMT) and sensitive camera (SCUBA)

Debris disks must be cold (e.g. Pluto temperature) so observe at long wavelengths

can attain sub-mJy noise levels but it takes a long time…

Thus can detect < 1 Lunar-mass of dust around stars within ~10pc

Submillimeter ObservationsSubmillimeter Observations

• Submm observations pick up the thermal emission from cold dust grains

– Sensitive to large scales (10’s to 100’s AU) so can probe Solar System-sized regions

– Signal is optically thin (so traces mass)

– Stellar photosphere signal is very weak so minimal calibration errors

Vega Debris DiskVega Debris Disk

• 25 hours of integration time; peak flux is 16 mJy at 850μm

– Two peaks seen in an otherwise face-on disk structure

– Clumps can be explained by migration of a Neptune mass planet

– Possible “spiral-arm” type structure?

Size of Pluto’s orbit

Diversity of DisksDiversity of Disks (to same physical scale) (to same physical scale)

τ Ceti ε Eridani Vega Fomalhaut β Pictoris

Key ResultsKey Results

• Submillimeter observations have revealed:

– Disk structures that are 1–3 times the size of the Solar System

– Disk masses that are tiny – up to several tens of lunar masses – trace the more massive comet population

– Dust grains that are a few microns up to centimetres (or more) in size

Evidence for Planets?Evidence for Planets?

• Two kinds of evidence

– Inner holes in most of the disks• dust is ejected by the planets

– Structure within the dust belts• if due to planets, ‘clumps’ should be associated

with particular resonances• could pinpoint the planet position, in advance of

imaging missions!

Limitations…Limitations…

• Observations are sensitive to mJy fluxes... but still need to observe for tens of hours to get deep images– especially at 450μm…

• Observing with 8–15’’ beams limits us to nearby stars... but it’s the only way to detect outer bounds of planetary systems– finding mostly large examples!– only one is as small as the Sun’s Kuiper Belt,

with r ~ 50AU

New Generation ArraysNew Generation ArraysArray module

SCUBA-2 is a new generation imaging array

for the JCMT

• Disk observations will take minutes instead of many hours…

• SCUBA-2 will be capable of detecting many more disks

• Due on the telescope in less than a year…

SCUBA-2 in the lab

SCUBA-2 Debris Disk SurveySCUBA-2 Debris Disk Survey

Survey aims:• The aim of the survey is to perform an unbiased

search of 500 nearby main sequence stars for disk emission at 850μm

Survey logistics :• The survey has been awarded 400 hours of

JCMT time from late-2006 for ~2 years

SCUBA-2 Debris Disk SurveySCUBA-2 Debris Disk Survey

Scientific goals:• To determine unbiased statistics on the incidence of

debris disks around nearby stars• To constrain disk masses and temperatures for far-IR

detections (e.g. ISO, Spitzer)

Size of Pluto’s orbit

Size of Pluto’s orbitSize of Pluto’s orbit

Grey body fit at 55K

SCUBA-2 Debris Disk SurveySCUBA-2 Debris Disk Survey

Scientific goals (cont):• To discover numerous disks too cold to be

detected in the far-IR

• To be the basis of source lists for future observing campaigns (e.g. using ALMA and JWST)

• To provide limits on the presence of dust that are vital to future planets detection missions (e.g. Darwin/TPF)

SCUBA-2 Debris Disk SurveySCUBA-2 Debris Disk Survey

Survey plan:

• 500 stars comprising 100 nearest observable stars from JCMT in spectral types A, F, G, K and M

• All stars will be imaged at 850μm to the confusion limit (~0.7 mJy)

• Unbiased surveys so far show that detection rates increase sharply with lower flux limits as we probe into the mass function

• Disks with significant structure will be targets for further deep imaging at 450μm

SCUBA-2 Debris Disk SurveySCUBA-2 Debris Disk SurveySelection criteria:• Unbiased sample for each spectral type so

can distinguish between detection rates of 5,10, 25 and 50% when dataset is subdivided

• Stellar type with 100 stars of A, F, G, K, and M

• Stellar age arises naturally, with 150 stars < 1 GYr and 350 stars 1–10 Gyr

• Stellar multiplicity arises naturally, with one-third of stars having a companion

• Presence of a planetary system, with ~20 samples having one or more planets from radial velocity estimates

SummarySummary

• Debris disks so far imaged have been very diverse in size, morphology and properties

• There is growing evidence that planets (or planetary systems) have played a major role in shaping the disk structure

• Large unbiased surveys are about to revolutionise this field of astronomy by substantially extending the number of known disks