The Debris Disk Legacy Survey

FIGURE 1: Debris disks seen with SCUBA, including (l-to-r) τ Ceti, ε Eridani, Vega (α Lyr), Fomalhaut (α PSa) and η Corvi. The disks are shown to the same physical scale i.e. as if all at one distance; actual distances are 3 to 18 pc. Sketches at the bottom demonstrate the disk orientations, and the star symbols are at the stellar positions. The spectral types and stellar ages are (l-to-r) G8 V / 10 Gyr, K2 V / 0.85 Gyr, A0 V / ~ 0.4 Gyr, A3 V / 0.3 Gyr and F2 V / ~ 1 Gyr. The images are at 850 μm except for η Corvi at 450 μm.

INTRODUCTION

How diverse are planetary systems, why, and where does the Solar System fit in the picture? These questions are the drivers for the Debris Disk Survey (DDS). Many main-sequence stars are surrounded by dusty "debris" which are fed by colliding asteroids and comets. The dust emission traces the regions where planet growth at least reached kilometre-sized bodies, and is thus a signpost to stars possessing planetary systems. Furthermore, imaging cleared and/or perturbed regions within the disks is a unique method to uncover distant planets, from Saturn-like orbits out to beyond Neptune. This tells us much about planet formation scenarios different from the Solar System, for example leading to huge comet populations -- we can even calculate whether these pose a bombardment threat to life on any inner terrestrial planets

SCUBA has left a great legacy of debris disk discoveries -- a 'rogues gallery' of nearby systems is shown in Figure 1. The SCUBA-2 observations will build up the big picture, by making the first ever unbiased search for debris disks around nearby stars. The submillimetre flux is negligibly affected by the stellar photosphere or background cirrus, unlike in the far-infrared, and thus we can survey any stars visible to the JCMT.

SCIENCE GOALS

There are five main goals for the survey:

(i) to determine unbiased statistics on the incidence of disks around nearby stars;
(ii) to constrain disk masses and temperatures for far-IR detections (e.g., from the IRAS, ISO and Spitzer satellites);
(iii) to discover numerous disks too cold to detect in the far-IR;
(iv) to be the basis of source lists for future high-resolution observing campaigns using e.g., ALMA and JWST;
(v) to provide limits on the presence of dust that are vital to future exo-Earth detection missions like Darwin/TPF.

SURVEY PLAN

	FIGURE 2

The survey will use SCUBA-2 at 850 micron in band 2/3 weather to look at 500 nearby stellar systems in which the primaries are 100 each of the spectral types A, F, G, K, and M observable from the JCMT. Subgiants are included for spectral types A, F and G, while only dwarfs are included in the K and M targets. Because the survey is unbiased, young and old stars, singles and multiples, and stars with and without giant planets will be included in their natural proportions. Thus the survey can uncover the factors that produce substantial belts of planetesimals, whether this outcome is most affected by stellar type or evolutionary time or environment. The distance limits range from 10 pc for M stars out to ~ 40 pc for A stars, but in effect the survey is mass-limited, because the less luminous stars are expected to have cooler, fainter dust and so need to be closer to compensate. Every star will be observed down to an rms noise level of 0.7 mJy where background confusion becomes significant. The total time award is 390 hours, mainly within two years but with a small reserve for more detailed follow-up imaging at 450 micron of new disk discoveries.

Figure 2 shows current debris detection rates for stars of different spectral types, and thus masses. One of our aims is to test models of how planetesimal formation and destruction depend on the dynamical times for stars of different mass. Note that the far-infrared (Spitzer, ISO, IRAS) detection rates do not always agree with the rates found so far in the submm (with SCUBA), implying that the full picture of hot to cold disks is not understood. The SCUBA-2 survey will robustly distinguish detection rates such as 5, 10, 25 or 50 % between any of the categories of stars when divided by age, type, binarity or planet-host status.

The legacy products of the survey will include archived data of all 500 fields; images of all the disks; a catalogue of fluxes and limits; and plots of all the spectral energy distributions. The survey papers will cover disk parameters including dust temperatures, masses, spectral indices, and characteristic disk radii, plus the results from systematic modeling of the masses and sizes of colliding planetesimals, and masses and locations for perturbing planets (where the disk structure is well-resolved). Some ancillary data products will also emerge from the survey, including a catalogue of background sources not associated with the star, and a flux list for nearby stars where the photosphere is detected but there is no debris disk. These products will be used for extragalactic science, providing a sample of high-z objects with a nearby bright guide star for follow-up, and for stellar science, testing atmospheric models at very long wavelengths.

Area Coverage and Numbers of Targets Planned for the Survey

	FIGURE 3

Team Members: Jane Greaves (St Andrews, archive contact, coordinator), Wayne Holland (UKATC, UK coordinator) and Brenda Matthews (HIA, Canada coordinator), and Pierre Bastien (Montreal), Chas Beichman (Caltech), Andy Biggs (Edinburgh), Mike Barlow (UCL), Harold Butner(JAC), Bill Dent (UKATC), Frossie Economou (JAC), James Di Francesco (HIA), Carsten Dominik (Amsterdam), Laura Fissel (Toronto), Per Friberg (JAC), Mark Halpern (UBC), Rob Ivison (UKATC), Ray Jayawardhana (Toronto), Tim Jenness (JAC), Doug Johnstone (HIA), J J Kavelaars (HIA), Jonathon Marshall (Open University), Neil Phillips (Edinburgh), Gerald Schieven (JAC), Ignas Snellen (Leiden), Derek Ward-Thompson (Cardiff), Bernd Weferling (JAC), Glenn White (Open University), Mark Wyatt (Cambridge), Jeremy Yates (UCL) and Ming Zhu (JAC).

For the most up-to-date list of survey members see the Survey membership Web Page .