A&A 508, 1057–1065 (2009) Astronomy DOI: 10.1051/0004-6361/200912010 & c ESO 2009 Astrophysics A submillimetre search for cold extended debris disks in the β Pictoris moving group, R. Nilsson1, R. Liseau2,A.Brandeker1, G. Olofsson1, C. Risacher3, M. Fridlund4, and G. Pilbratt4 1 Department of Astronomy, Stockholm University, AlbaNova University Center, Roslagstullsbacken 21, 10691 Stockholm, Sweden e-mail: [ricky;alexis;olofsson]@astro.su.se 2 Onsala Space Observatory, Chalmers University of Technology, 43992 Onsala, Sweden e-mail: [email protected] 3 SRON, Postbus 800, 9700 AV Groningen, The Netherlands e-mail: [email protected] 4 ESA Astrophysics Missions Division, ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands e-mail: [email protected]; [email protected] Received 9 March 2009 / Accepted 13 October 2009 ABSTRACT Context. Previous observations with the Infrared Astronomical Satellite and the Infrared Space Observatory, and ongoing observations with Spitzer and AKARI, have led to the discovery of over 200 debris disks, based on detected mid- and far infrared excess emission, indicating warm circumstellar dust. To constrain the properties of these systems, e.g., to more accurately determine the dust mass, temperature and radial extent, follow-up observations in the submillimetre wavelength region are needed. Aims. The β Pictoris moving group is a nearby stellar association of young (∼12 Myr) co-moving stars including the classical debris disk star β Pictoris. Due to their proximity and youth, they are excellent targets when searching for submillimetre emission from cold, extended, dust components produced by collisions in Kuiper-Belt-like disks. They also allow an age independent study of debris disk properties as a function of other stellar parameters. Methods. We observed 7 infrared-excess stars in the β Pictoris moving group with the LABOCA bolometer array, operating at a central wavelength of 870 μm at the 12-m submillimetre telescope APEX. The main emission at these wavelengths comes from large, cold dust grains, which constitute the main part of the total dust mass, and hence, for an optically thin case, make better estimates on the total dust mass than earlier infrared observations. Fitting the spectral energy distribution with combined optical and infrared photometry gives information on the temperature and radial extent of the disk. Results. From our sample, β Pic, HD 181327, and HD 172555 were detected with at least 3σ certainty, while all others are below 2σ and considered non-detections. The image of β Pic shows an offset flux density peak located near the south-west extension of the disk, similar to the one previously found by SCUBA at the JCMT. We present SED fits for detected sources and give an upper limit on the dust mass for undetected ones. −3 Conclusions. We find a mean fractional dust luminosity f¯dust = 1.1 × 10 at t ≈ 12 Myr, which together with recent data at 100 Myr −α suggests an fdust ∝ t decline of the emitting dust, with α>0.8. Key words. circumstellar matter – planetary systems – planetary systems: formation 1. Introduction continuously replenished by collisions of larger bodies (see e.g. Backman et al. 1995). Deeper and more targeted observations by The first large and unbiased survey of the infrared (IR) sky, the Infrared Space Observatory (ISO), Spitzer,andAKARI have conducted by the Infrared Astronomical Satellite (IRAS), re- revised and extended this list of debris disk candidates to en- vealed that approximately 15% of nearby main-sequence stars − compass over 200 stars (Matthews et al. 2007; Rhee et al. 2007; have an excess of 12 100 μm emission corresponding to a lu- UK ATC Debris Disk Database 2009). minosity at least 2 × 10−5 times higher than that expected from a pure stellar photosphere (Backman & Paresce 1993; Backman Although some of the most nearby debris disks have been & Gillett 1987; Aumann et al. 1984). This indicates that these imaged in the optical and IR, most are spatially unresolved and stars are surrounded by warm circumstellar dust, interpreted as characterised solely on fits of the spectral energy distribution originating in a disk of debris left over from planet formation, (SED) to stellar synthetic spectra and a handful of IR photom- which is being heated by stellar optical and ultraviolet radiation, etry data points. To better constrain the SED, and with this the and re-radiating at mid- and far-IR wavelengths. The short or- temperature and radial extent of the disk, complementary sub- bital lifetime of small dust particles suggests that they are being millimetre (submm) photometry is needed. Submm observations probe the Rayleigh-Jeans tail of the thermal radiation, where Based on observations with APEX, Llano Chajnantor, Chile (ESO the measured integrated flux is roughly (sometimes with non- programmes 079.F-9307(A) and 079.F-9308(A)). negligible correction) proportional to the temperature and the Table 3 is only available in electronic form at mass of the dust. Since the most efficient submm emitters are http://www.aanda.org large and cool grains, which also dominate the mass of the disk Article published by EDP Sciences 1058 R. Nilsson et al.: A submm search for cold extended debris disks in the BPMG Table 1. Observed objects and integration time. Target RA Dec Spectral type Distance Integration time (h m s) (◦) (pc) (h:min) HD 15115 02 26 16.24 +06 17 33.2 F2 45 1:45 HD 39060 (β Pic) 05 47 17.09 –51 03 59.5 A6V 19.3 10:45 HD 164249 18 03 03.41 –51 38 56.4 F5V 47 4:00 HD 172555 18 45 26.90 –64 52 16.5 A7V 29 2:00 HD 181296 19 22 51.21 –54 25 26.2 A0Vn 48 1:45 HD 181327 19 22 58.94 –54 32 17.0 F5/F6V 51 2:08 HD 191089 20 09 05.21 –26 13 26.5 F5V 54 1:23 (Zuckerman 2001), observations with instruments like the Large Max-Planck-Institut für Radioastronomie, and the European APEX BOlometer CAmera (LABOCA, Siringo et al. 2007)on Southern Observatory at an altitude of 5100 m in the Chilean the Atacama Pathfinder EXperiment (APEX, Güsten et al. 2006) Andes, has been equipped with the LABOCA bolometer array. telescope (operating at 870 μm) or SCUBA (Holland et al. 1999) It operates at a central wavelength of 870 μm (345 GHz) and a at the James Clerk Maxwell Telescope (JCMT, Prestage 1996) bandwidth of 150 μm (60 GHz), covering a 11.4 field-of-view (operating at 850 μm) provide good estimators for the total dust with its array of 295 bolometers; each with a 18. 6 full-width at mass (assuming an optically thin disk at these wavelengths). half-maximum (FWHM) beam. Another advantage of observations in the submm regime is Observations were performed between July 27 and August 5, the ability to investigate cool dust created in very extended de- 2007, employing a spiral pattern scan in order to recover fully bris disks or belts located some hundreds of AU from the star sampled maps from the under-sampled bolometer array (Siringo (akin to the Solar system’s Kuiper Belt), compared to the inner et al. 2007). Each 7.5 min long scan gives a raw map with − warm dust at 1 100 AU radius studied in IR surveys. The larger uniform noise distribution for an area of about 4 around the disk region probed by submm, compared to IR, makes it feasible central position of the source. Targets and observation data are to potentially resolve nearby disks, and morphologically deter- presented in Table 1. Additional calibration measurements to mine outer disk radii, dynamical interaction with unseen plan- determine the pointing accuracy were made on Jupiter. Uranus ets, etc. (e.g. Wyatt 2008), even though the angular resolution in and Neptune were observed for absolute flux calibration together the submm in general is lower than for shorter wavelengths. with secondary calibrators B13134, G5.89 and IRAS 16293, Young, nearby stars, with a confirmed mid- and far-IR ex- while several skydips in-between target observations yielded the cess are prime targets for a submm search for cold extended correction for atmospheric opacity (with a zenith opacity rang- disks. The β Pictoris Moving Group (BPMG, Zuckerman et al. ing between 1.0 and 3.0). 2001) is a nearby stellar association of 30 identified young For data reduction, we used mainly the Java-based software (∼12 Myr) co-moving member stars (Zuckerman & Song 2004; MiniCRUSH, a special version of CRUSH (Comprehensive Barrado y Navascués et al. 1999), harbouring the perhaps most Reduction Utility for SHARC-2, Kovács 2008) adapted for studied of all debris disk systems, namely β Pictoris (Smith & the APEX bolometers, while IDL and Matlab were used for Terrile 1984), and also e.g., AU Mic. In this paper, we present a further analysis and calculations. The reduction procedure in study of 7 main-sequence IR excess stars in the BPMG, observed MiniCRUSH started with a correction for the atmospheric opac- at 870 μm with LABOCA at APEX in Chile. The stars were se- ity calculated from observed sky temperature during the skydips, lected from the sample of currently known members of BPMG ◦ with cryostat temperature drifts taken into account. Each tar- fulfilling the following criteria: (1) located at δ<−50 , thus, get scan was corrected with linearly interpolated opacity values southern stars that are not accessible to JCMT, but that will be from its nearest preceding and nearest subsequent skydip scan. observable with ALMA; and (2) previously detected IR excess. Thereafter, the target scans were similarly flux calibrated from However, several of the stars previously observed at 850 μm, neighbouring calibrator measurements.