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PDF hosted at the Radboud Repository of the Radboud University Nijmegen The following full text is a publisher's version. For additional information about this publication click this link. http://hdl.handle.net/2066/147352 Please be advised that this information was generated on 2017-12-05 and may be subject to change. MNRAS 454, 507–530 (2015) doi:10.1093/mnras/stv1989 The OmegaWhite survey for short-period variable stars – I. Overview and first results S. A. Macfarlane,1,2‹ R. Toma,3 G. Ramsay,3 P. J. Groot,1 P. A. Woudt,2 J. E. Drew,4 G. Barentsen4 and J. Eisloffel¨ 5 1Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, NL-6500 GL Nijmegen, the Netherlands 2Astrophysics,Cosmology and Gravity Centre, Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa 3Armagh Observatory, College Hill, Armagh BT61 9DG, UK 4School of Physics, Astronomy & Mathematics, University of Hertfordshire, College Lane, Hatfield, Hertfordshire AL10 9AB, UK 5Thuringer¨ Landessternwarte, Sternwarte 5, D-07778 Tautenburg, Germany Accepted 2015 August 24. Received 2015 August 20; in original form 2015 July 20 Downloaded from ABSTRACT We present the goals, strategy and first results of the OmegaWhite survey: a wide-field high- cadence g-band synoptic survey which aims to unveil the Galactic population of short-period http://mnras.oxfordjournals.org/ variable stars (with periods <80 min), including ultracompact binary star systems and stellar pulsators. The ultimate goal of OmegaWhite is to cover 400 deg2 along the Galactic plane reaching a depth of g = 21.5 mag (10σ), using OmegaCam on the VLT Survey Telescope (VST). The fields are selected to overlap with surveys such as the Galactic Bulge Survey and the VST Photometric Hα Survey of the Southern Galactic Plane for multiband colour information. Each field is observed using 38 exposures of 39 s each, with a median cadence of ∼2.7 min for a total duration of two hours. Within an initial 26 deg2, we have extracted the light curves of 1.6 million stars, and have identified 613 variable candidates which satisfy at Radboud University on December 14, 2015 our selection criteria. Furthermore, we present the light curves and statistical properties of 20 sources which have the highest likelihood of being variable stars. One of these candidates exhibits the colours and light-curve properties typically associated with ultracompact AM CVn binaries, although its spectrum exhibits weak Balmer absorption lines and is thus not likely to be such a binary system. We also present follow-up spectroscopy of five other variable candidates, which identifies them as likely low-amplitude δ Sct pulsating stars. Key words: methods: data analysis – methods: observational – techniques: photometric – surveys – binaries: close – Galaxy: bulge. have an orbital period (P )of≤70 min, implying that the sec- 1 INTRODUCTION orb ondary star cannot be a main-sequence star (Rappaport, Joss & Time domain astrophysics has been transformed over the last Webbink 1982). Furthermore, these hydrogen-deficient objects are decade. Whereas in the past, photometric data of individual objects predicted to be the strongest known sources of gravitational wave were painstakingly obtained using high-speed photometry or ded- radiation (GWR) in the passband of the satellite observatory eLISA icated long-term projects, now a whole series of synoptic projects (Roelofs, Nelemans & Groot 2007; Amaro-Seoane et al. 2013), and have been developed to observe large areas of sky over short time- as such are important calibrators that provide verification of the ex- scales giving photometric data on thousands or millions of objects. istence and detectability of GWR. Moreover, the evolution of these The diversity of goals of these projects is considerable, ranging from binary systems is influenced by the emission of GWR in addition detecting transiting exo-planets (e.g. the main aim of the ‘Super- to the mass transfer phase. Therefore, the study of UCBs will also WASP’ project; Pollacco et al. 2006) to discovering supernova out- help to answer key questions of late-stage binary evolution. Ear- bursts (e.g. supernova discoveries in the Palomar Transient Factory lier estimates of their intrinsic numbers suggested a relatively high (PTF); Law et al. 2009). foreground contribution from UCBs to the gravitational wave sig- In the field of Galactic binary research, one key goal has been to nal from merging supermassive black holes within the eLISA band. discover individual systems with astrophysically interesting proper- However, in a series of papers using SDSS and PTF data (Roelofs ties. Of particular interest are ultracompact binaries (UCBs) which et al. 2009; Rau et al. 2010;Carteretal.2013; Levitan et al. 2015), it is now clear that the predicted number density of AM CVn binaries (semidetached and mass transferring UCBs) in the solar neigh- E-mail: [email protected] bourhood is 5 ± 3 × 10−7 pc−3, a factor 50 lower than previous C 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society 508 S. A. Macfarlane et al. Table 1. Overview of recent high-cadence synoptic surveys. Survey Field location Total sky coverage Depth Variability sensitivity (deg) (deg2)(V mag) (min) OmegaWhitea |b|≤10 400 21.5 >6 RATSb |b|≤30 46 22.5 >5 RATS-Keplerc 6 ≤|b|≤21 49 22.5 >5 FSVSd |b|≥20 23 24.0 >24 DLSe |b| < 10 21 25.5 >15 Keplerf 15 ≤|b|≤25 116 20.0 >1or> 30 SuperWASPg All sky All sky 15.0 >10 Notes. aThis paper, bRapid Temporal Survey (Ramsay & Hakala 2005), cRamsay et al. (2014), dFaint Sky Variability Survey (Groot et al. 2003), eDeep Lens Survey (Becker et al. 2004), fBorucki et al. (2010), gWide Angle Search for Planets (Pollacco et al. 2006). estimates by Nelemans et al. (2001). Other surveys which have re- period δ Sct stars in the Galactic field have a dominant pulsation cently had success in discovering new AM CVn systems include the period of ∼26 min (Kim et al. 2010). Downloaded from Catalina Real-Time Transient Survey (CRTS; Drake et al. 2009), One survey that set out to discover short-period systems was and the All Sky Automated Survey for Supernovae (e.g. Wagner the Rapid Temporal Survey (RATS; Ramsay & Hakala 2005;Bar- et al. 2014). clay et al. 2011). RATS was carried out using the 2.5-m Isaac The emission line method for finding AM CVn systems, on Newton Telescope (INT) on La Palma and the 2.2-m Max Planck which the SDSS work is based, is most sensitive to systems with Gesellschaft telescope (MPG/ESO) between 2003–2010, and cov- http://mnras.oxfordjournals.org/ 2 Porb ≥ 30 min. More recently, the PTF survey has been identify- ered a total of 46 deg favouring fields near the Galactic plane. The ing outbursting AM CVns which have orbital periods shorter than fact that no AM CVn was discovered was (in retrospect) not a sur- this, but still greater than 22 min (Levitan et al. 2015). From the prise since the latest space densities imply that only a few systems (at total number of 43 known AM CVns, only 6 have periods shorter best) are expected in a survey covering this area. However, the sur- than 20 min. It is those systems with the shortest orbital period vey did discover many new and interesting short-period variables, (5 min < Porb < 20 min) which are predicted to be the strongest including a possible hybrid sdBV pulsator exhibiting long-period g emitters of gravitational waves. Determining their population size modes (Ramsay et al. 2006) and a dwarf nova discovered through is important for both the development of eLISA and for deducing its high-amplitude quasi-periodic oscillations (QPOs) in quiescence the relative importance of the three postulated formation channels (Ramsay et al. 2009). of these binaries (for a detailed review, see Solheim 2010). A way The OmegaWhite Survey, which is now being conducted using at Radboud University on December 14, 2015 to identify those AM CVn stars with the shortest orbital periods OmegaCam on the VLT Survey Telescope (VST), has a similar is through their photometric behaviour as they show a periodic observing strategy to RATS (see Table 1 for a comparison of similar modulation on, or close to, the orbital period (e.g. the first PTF stellar wide-field surveys). However, OmegaWhite aims to cover a AM CVn discovered; Levitan et al. 2011). Variations are expected much wider area (400 deg2) at lower average Galactic latitudes due to eclipses, ellipsoidal variations, irradiation, superhumps, or (|b| < 5 deg). In this paper we present: the OmegaWhite observing anisotropic disc hotspot emission in the system. The detection of strategy, the reduction pipeline, our method for identifying objects these especially short-period systems is the main motivation behind of interest, an outline of those variables identified in ESO semester the OmegaWhite survey. 88 (2011 Dec.–2012 Apr.), and a discussion of the science goals However, short time-scale photometric variations can also orig- which can be investigated using these data. inate from physical changes within the internal structure or atmo- sphere of the source. Examples of such sources include flare stars or fast pulsating stars such as δ Scuti stars (δ Sct, Breger 2000)or SX Phoenicis variables (e.g. Rodr´ıguez & Lopez-Gonz´ alez´ 2000). 2 OBSERVATIONS At even shorter periods, rapidly oscillating Ap or Am star systems (roAp, roAm), and pulsating white dwarfs (ZZ Ceti variables) ex- OmegaWhite observations are taken in g band using the wide-field hibit periodic variations on amplitudes ranging from a per cent or instrument OmegaCAM (Kuijken 2011) on the VST (Capaccioli & less up to several tens of per cent on time-scales of a few to tens of Schipani 2011).