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Target Selection for the SUNS and DEBRIS Surveys for Debris Discs in the Solar Neighbourhood

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Target Selection for the SUNS and DEBRIS Surveys for Debris Discs in the Solar Neighbourhood Mon. Not. R. Astron. Soc. 000, 1–?? (2009) Printed 18 November 2009 (MN LATEX style file v2.2) Target selection for the SUNS and DEBRIS surveys for debris discs in the solar neighbourhood N. M. Phillips1, J. S. Greaves2, W. R. F. Dent3, B. C. Matthews4 W. S. Holland3, M. C. Wyatt5, B. Sibthorpe3 1Institute for Astronomy (IfA), Royal Observatory Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ 2School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife, KY16 9SS 3UK Astronomy Technology Centre (UKATC), Royal Observatory Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ 4Herzberg Institute of Astrophysics (HIA), National Research Council of Canada, Victoria, BC, Canada 5Institute of Astronomy (IoA), University of Cambridge, Madingley Road, Cambridge, CB3 0HA Accepted 2009 September 2. Received 2009 July 27; in original form 2009 March 31 ABSTRACT Debris discs – analogous to the Asteroid and Kuiper-Edgeworth belts in the Solar system – have so far mostly been identified and studied in thermal emission shortward of 100 µm. The Herschel space observatory and the SCUBA-2 camera on the James Clerk Maxwell Telescope will allow efficient photometric surveying at 70 to 850 µm, which allow for the detection of cooler discs not yet discovered, and the measurement of disc masses and temperatures when combined with shorter wavelength photometry. The SCUBA-2 Unbiased Nearby Stars (SUNS) survey and the DEBRIS Herschel Open Time Key Project are complimentary legacy surveys observing samples of ∼500 nearby stellar systems. To maximise the legacy value of these surveys, great care has gone into the target selection process. This paper describes the target selection process and presents the target lists of these two surveys. Key words: solar neighbourhood – stars: statistics – circumstellar matter – surveys – stars: distances 1 INTRODUCTION tained simultaneously, and deep images at 450 µm will be proposed to follow-up 850 µm detections. The solar neighbourhood is an ideal testing ground for the study of debris discs and planetary systems. Proximity max- The Disc Emission via a Bias-free Reconnaissance in imises dust mass sensitivity and can allow systems to be spa- the Infrared/Sub-millimetre (DEBRIS) Herschel Open Time tially resolved. Systems near the Sun span a wide range of Key Program will image 446 systems (356 in common with arXiv:0911.3426v1 [astro-ph.SR] 18 Nov 2009 stellar parameters e.g., mass, age, metallicity, multiplicity. SUNS) at 110 and 170 µm using the PACS (Photodetec- Whilst determining these parameters may not be easy, the tor Array Camera and Spectrometer; Poglitsch et al. 2008) diversity included in volume limited samples makes them instrument, with follow-up of around 100 systems at 250, ideal for legacy surveys where one may wish to investigate 350 and 500 µm using the SPIRE (Spectral and Photo- trends as a function of many system parameters. metric Imaging Receiver; Griffin et al. 2008) instrument. This paper presents five all-sky volume limited samples This survey is primarily driven by the 110 µm band, which of nearby stellar systems with main-sequence primaries of has the highest dust mass sensitivity for cold discs such spectral type A,F,G,K,M. These form the basis of the target as the Kuiper-Edgeworth belt of our Solar System. The lists of two complimentary surveys for debris discs using the intended flux RMS at 110 µm is 1.2mJy/beam, which is SCUBA-2 (Submillimetre Common User Bolometer Array twice the predicted extragalactic confusion limit. 170 µm 2; Holland et al. 2003; Audley et al. 2004) camera on the images are taken simultaneously with a predicted RMS of James Clerk Maxwell Telescope (JCMT), and the Herschel 1.7mJy/beam, equal to the predicted extragalactic confu- space observatory (Pilbratt 2008). sion limit in this band. The SCUBA-2 Unbiased Nearby Stars survey (SUNS; The primary goals of these surveys are statistical: In Matthews et al. 2007) is a large flux-limited survey of 500 general how do debris disc properties vary with stellar mass, systems at 850 µm. The target flux RMS is 0.7mJy/beam, age, metallicity, system morphology (multiplicity, compo- equal to the extragalactic confusion limit of the JCMT at nent masses, separations), presence of planets etc. To be able 850 µm. Shallow 450 µm images of varying depth will be ob- to answer so many questions, and to minimise the risk of un- c 2009 RAS 2 N. M. Phillips et al. foreseen selection effects, large samples and simple, clearly -2 A defined target selection criteria are required. Volume limited 0 F G samples satisfy these requirements, and as well as maximis- K 2 M ing the proximity of the targets, the stars nearest the Sun isochrones are very widely studied. For example, nearby stars are the 4 main targets of radial velocity, astrometry and direct imag- 6 ing planet searches. The majority of SUNS and DEBRIS tar- V 8 gets also have photometry at 24 and 70 µm from the MIPS M (Multiband Imaging Photometer and Spectrometer) instru- 10 ment on the Spitzer space telescope, which ceased operation 12 at the end of March 2009. This large spectral coverage, from 24 to 850 µm for over 300 systems will be an incredible re- 14 source for detailed spectral energy distribution modelling of 16 systems with debris discs. 18 Given that we are considering the closest systems to 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 the Sun, substantial effort was required to compile the sam- B - V ples presented here. Late M-type stars within 10 pc are still Figure 1. Johnson B,V absolute colour-magnitude diagram for being discovered (e.g. Henry et al. 2006), and complete ho- system primaries. Overlaid are [Fe/H] = 0.0, [α/Fe] = 0.0 mogeneous datasets covering the spectral type and distance isochrones from the Dartmouth Stellar Evolution Database (Dot- ranges we consider do not exist. We have tried to make our ter et al. 2008) with ages of 0.25, 0.5, 1, 2, 4, 8 Gyr (with turn-offs sample selection using the most complete and accurate data going from left to right). The photometry is mostly converted available at the time of the DEBRIS proposal submission in from Tycho photometry (Tycho-2 or TDSC) using transforma- October 2007. tions for unreddened main-sequence stars. For most M-type tar- gets Johnson B,V photometry from various sources was used (see text). Note that primaries in some close binaries are not individ- ually resolved in this photometry. 2 SELECTION CRITERIA Our systems all have primaries (defined here as the compo- in age and composition. The fact that several interesting ob- nent with the brightest visible magnitude) which we believe jects (e.g. with known IR excess, or planets) are considered are main-sequence (i.e. hydrogen burning) stars. The sample here as secondaries does not affect the statistical usefulness is split into 5 volume limited subsamples based on spectral of the sample, although it has the disadvantage that such type: A, F, G, K, M. In the rest of this paper we use the objects may not be observed by these surveys (see below). term “X-type system” to mean “system with X-type pri- The number of systems in each subsample was deter- mary”. Using separate subsamples is necessary due to the mined by the selection criteria for SUNS, which required steep nature of the stellar mass function, which for example 100 systems in each subsample in the declination range means that a single volume limited sample would contain −40◦ <δ < +80◦. Hence the all-sky samples presented over 100 times as many M-type systems as A-type systems. here contain roughly 123 (100×2/(sin 80◦ +sin 40◦)) systems The choice of using spectral types to split the sample, rather each. The SUNS sample sizes were chosen to allow detection than stellar mass, is purely practical as, with the exception rates for various subsets e.g. planet hosts to be distinguished of certain binary systems, stellar masses cannot be directly (see Matthews et al. 2007). determined observationally. Using spectral types does how- The DEBRIS target list comprises the nearest systems ever have the effect that the subsamples cover quite different presented here (all-sky), subject to a cut in the predicted ranges in logarithmic mass space. 110 µm cirrus confusion level towards each system. The con- The early type, upper mass, limit of A0 is chosen as fusion prediction was taken from the Herschel Confusion stars of earlier type are too rare in the solar neighbourhood Noise Estimator (HCNE), which is part of the Herschel to build a suitably large sample. A conservative late type Observation Planning Tool (HSPOT). Systems with total limit of M7.0 was chosen to avoid the inclusion of any brown predicted confusion for point-source detections greater than dwarfs, and also to improve the completeness of the M-type 1.2mJy/beam, corresponding to twice the predicted extra- sample. M-type stars span the largest log M range of any of galactic confusion limit, were rejected. To maximise the our spectral classes, so making a cut at M7.0 will not restrict number of systems observed, DEBRIS will not image sec- the statistical usefulness of the sample. ondary components in systems where they will not fit in the We do not discriminate against multiple star systems, PACS point-source field of view (FoV) (150′′ × 50′′ with un- and they are included naturally within the volume limits. We constrained orientation) with the primary. This will affect consider common proper motion stars (with compatible par- between 20 and 49 systems depending on the actual field allax where available) as members of the same system, with orientations.
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