Earth, Moon, and Planets (2005) 97: 261–268 DOI 10.1007/s11038-006-9072-z
Springer 2006
Serendipitous Asteroid lightcurve survey using SuperWASP N. R. PARLEY, N. MCBRIDE and S. F. GREEN Planetary and Space Sciences Research Institute, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK (E-mail:
[email protected])
C. A. HASWELL, W. I. CLARKSON and A. J. NORTON Department of Physics & Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
D. J. CHRISTIAN, A. FITZSIMMONS, F. P. KEENAN, D. POLLACCO, R. RYANS and R. A. STREET APS Division, Department of Pure & Applied Physics, Queen’s University, University Road, Belfast, BT7 1NN, UK
A. COLLIER-CAMERON, K. HORNE and T. A. LISTER School of Physics & Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife, KY16 9SS, UK
N. A. EVANS and C. HELLIER Astrophysics Group, School of Chemistry & Physics, Keele University, Staffordshire, ST5 5BG, UK
S. T. HODGKIN and J. IRWIN Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
S. R. KANE Department of Physics, University of Florida, Gainesville, FL, 32611-8440, USA
J. P. OSBORNE and R. G. WEST Department of Physics & Astronomy, University of Leicester, Leicester, LE1 7RH, UK
I. SKILLEN Isaac Newton Group of Telescopes, Apartado de correos 321, Santa Cruz de la Palma, Tenerife, E-38700, Spain
P. J. WHEATLEY Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
(Received 7 November 2005; Accepted 22 March 2006)
Abstract. The SuperWASP project is an ultra-wide angle search for extra solar planetary transits. However, it can also serendipitously detect solar system objects, such as asteroids and comets. Each SuperWASP instrument consists of up to eight cameras, combined with high-quality peltier-cooled CCDs, which photometrically survey large numbers of stars in the magnitude range 7–15. Each camera covers a 7.8 · 7.8 degree field of view. Located on La Palma, the SuperWASP-I instrument has been observing the Northern Hemisphere with five cameras since its inauguration in April 2004. The ultra-wide angle field of
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view gives SuperWASP the possibility of discovering new fast moving (near to Earth) asteroids that could have been missed by other instruments. However, it provides an excellent opportunity to produce a magnitude-limited lightcurve survey of known main belt asteroids. As slow moving asteroids stay within a single SuperWASP field for several weeks, and may be seen in many fields, a survey of all objects brighter than magnitude 15 is possible. This will provide a significant increase in the total number of lightcurves available for statistical studies without the inherent bias against longer periods present in the current data sets. We present the methodology used in the automated collection of asteroid data from SuperWASP and some of the first examples of lightcurves from numbered asteroids.
Keywords: Asteroids, minor planets, surveys
1. SuperWASP Specifications The acronym WASP stands for Wide Angle Search for Planets, its main scientific objective being a first alarm search for transiting extrasolar planetary systems. SuperWASP will also be used extensively for the detection and monitoring of all types of variable star and other roles also include the detection of optical transients, including X-ray bursts, and locating NearEarth Objects and Asteroids. However, the main function presented here is the ability of SuperWASP to detect and produce light-curves for a number of main belt asteroids. There are currently two SuperWASP instruments. SuperWASP-North, which started operation in April 2004, is situated on the island of La Palma in the Canary Islands amongst the Isaac Newton Group (ING) of telescopes. SuperWASP-South, which is due to be commissioned at the end of 2005, is situated at the site of the South African Astronomical Observatory (SAAO) near Sutherland, South Africa. The instruments each consist of a rapidslewing fork telescope mount holding 8 cameras. The optics comprise Canon 200 mm f/1.8 lenses, giving each camera a field of view of 7.8 · 7.8. The detectors are Andor e2v 2048 · 2048 back-illuminated which are peltier cooled and have a 4 s readout time. Data in 2004 were taken using no filters on the WASP lenses, although from 2005 both SuperWASP-South and SuperWASP-North will use dichroics that allow transmission between 400 and 750 nm, defining the waveband in the filter and not the lenses or atmosphere. The pipeline software transforms the fluxes to values which are comparable to Tycho-V magnitudes. The SuperWASP observatories are designed to operate completely automatically. Their robotic control system includes procedures for emergency shutdown and instrument stowage that can be triggered by a dedicated weather station. Acquisition of calibration frames and avoidance of the moon and crowded regions is also fully automated. For further information about the current status of the SuperWASP project visit http://www.superwasp.org and refer to Clarkson et al. (2004).
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2. Data Reduction Pipeline It was decided, for the purposes of SuperWASP, that pre-existing photometry software, normally used to interpret this type of data, was inadequate for the wide-angle field of view of the SuperWASP cameras. Hence a custom data reduction pipeline had to be developed and built. The first step in the data reduction pipeline is to produce a statistical frame classification using minimal input assumptions. Master calibration frames are produced on a per-night basis, then an optimal combination of master frames from several nights is created, where each frame is weighted by time interval between each master frame and the data being reduced, as well as by quality. The raw frames are then calibrated using flat, dark and bias master frames produced in this way, and corrections are made for shutter travel time. The next stage of the pipeline is a full astrometric solution of the field of view using the Tycho2 catalogue as a base (for objects of magnitude less than 15). Objects are then identified by using the USNO-B1 catalogue and photometry is performed for 3 separate apertures, allowing the object blending to be measured. Objects that can not be matched, such as transient objects, gamma-ray bursts or asteroids, are labelled as orphans for subsequent examination. Post photometry calibrations software, converting the SuperWASP count rate into visual magnitudes from an iterative fit to a 9 term photometric model, help to reduce any night-to-night offsets in the data. The software applies an iteratively-derived fit to account for the effects of atmospheric extinction, making use of the Tycho-2 catalogue data to determine magnitudes for all stars calibrated approximately to the Tycho-2 V system. The observed instrumental magnitude ms,t for star s at time t is modelled as: ms;t ¼Vs þ zo þ coððB VÞs ðB VÞ0 Þ þ ðk0 þ dk0t ÞXs;t þ k00 ððB VÞs ðB VÞ0 ÞXs;t
ð2:1Þ
where Vs, (B)V)s are the Tycho-2 V and colour index respectively, zo is the magnitude offset, co is the colour transform coefficient, k¢, k¢¢ are the first and second extinction coefficients and Xs,t is the star’s airmass at the given time. (B)V)¢ is defined as a fiducial star colour value for the set of extinction calibration stars used on the night under consideration. The term dk¢t is added to compensate for the variation of extinction from the frame centre to the star’s position. Since asteroids are present only in the orphan data, the B)V colours are not used in the SuperWasp calibration process. The colour terms in (2.1) are not therefore used in production of the asteroid magnitudes. We are in the process of investigating the uncertainties in both the derived absolute magnitudes and the lightcurves, which result from the
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non-application of the colour transformation and secondary extinction coefficient respectively, in Equation 2.1. The end results of the pipeline are finally stored in a custom built MySQL archive at The University of Leicester, enabling powerful SQL querying of the data.
3. Current Asteroid Search Strategy Asteroids are to be found among the objects which the SuperWASP pipeline has labelled as orphans, i.e entities that could not be matched to a catalogued object by the pipeline. The first focus was to find, within the SuperWASP data, asteroids which are already known. The magnitude limit of 15 meant that it was unlikely that SuperWASP will discover any new main belt asteroids, due to the vast number of survey telescopes which are already operating to much deeper magnitudes. To search for known asteroids, the RA and DEC of all the numbered asteroids for the specific time and date of the image frame are first computed. Modern computers, using a 2 body ephemeris program, which gives results within acceptable margins of error for the purpose of this survey, can carry out such a process in a matter of seconds. The results were then filtered for asteroids which would be within SuperWASP’s magnitude limit and which would be visible in the frame being searched. The RA and DEC of any asteroids so determined were then matched with the RA and DEC of the orphans that the pipeline had found in the same frame. A positive match was recorded if the orphan was within one or two pixels (~15–30 arc s) of a known asteroid. On average there are about 200 orphans for each 7.8 · 7.8 frame. The density of orphans thus meant that there were very few false positives. The final linking of the known asteroids to the orphan data was done in the form of a relational MySQL database, which connected the unique identity of each orphan to the asteroid number that matched it. Figure 1 shows the instances of (350) Ornamenta detected over several nights. The crosses show the determined RA and DEC of the asteroid and the smooth curve illustrates the predicted RA and DEC over the same time period.
4. First Results As SuperWASP is not primarily dedicated to finding asteroids, the instrument is not always pointing close to the ecliptic. So far, based on preliminary data taken in May, June and August 2004, 35 asteroids have been identified.
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Figure 1. Locations of orphan object detections (crosses) associated with asteroid (350), overlayed by the asteroid’s predicted position (solid line) as determined by the JPL Horizon System. Shown are 365 associated detections over 15 nights, from the 4th May 2004 to the 26th May 2004 excluding nights when SuperWASP was not operational or it was cloudy.
However, to date, only a limited set of test data, from May 2004, has been processed in full by the pipeline. These data, with the instrument not pointing in the ecliptic, yielded four asteroids. These were identified as follows: (a) (b) (c) (d)
(146), (350), (907), (914),
Lucina: 34 observations over 1 night Ornamenta: 365 observations over 15 nights Rhoda: 92 observations over 3 nights Palisana: 17 observations over 1 night
Figure 2 shows a light-curve for (350) Ornamenta, reduced to unit distance, derived from data spanning 8 nights. On two of the nights (not shown) conditions were not ideal and the data were noisier than under nominal conditions. A period of 9.156±0.01 h is derived using a 10th order Fourier fit (the fit is shown) and is comparable with 9.17 h reported by Schober et al.
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(1993). Currently, the SuperWASP flux processing algorithm is being further developed to reduce night-to-night offsets. At magnitude 14.3, with an amplitude of 0.2, asteroid 350 provides a challenging test of the capabilities of SuperWASP for light-curve derivation. We expect ~1% photometry at 12 mag, ~7% at 14 mag and ~20% at the software limit of 15 mag.
5. Future Work 5.1. AUTOMATED
LIGHTCURVES
There are on average approximately 680 asteroids brighter than magnitude 15 distributed along the ecliptic on any given night. As demonstrated in Figure 3, SuperWASP North covers a minimum of 20% of the ecliptic during its normal planet hunting operations. Therefore, assuming uniform distribution of asteroids along the ecliptic, an approximate estimate of 130 asteroids would be expected to have been observed in the 2004 data. Also, SuperWASP South would be expected to detect a similar number of asteroids as the northern telescope, increasing the number of asteroids in the survey further. From 2005, SuperWASP will also operate a full sky mode, in conjunction with planet hunting fields in Figure 3. This should increase the number of asteroids detected, although those detected using this mode will not have as many light curve points since each field will not be returned to as often. For
Figure 2. Reduced magnitude light-curve of asteroid 350 folded around a period of 9.156±0.01 h.
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Figure 3. Sky location of the SuperWASP fields observed in 2004 (boxes) compared with the ecliptic (solid line). The 2004 dataset arises from 30 s exposures taken in a cycle among 8 fields, returning to each field every 10 min. Fields with RA less than 150 degrees, close to the ecliptic, were not part of the first test datasets.
these low coverage asteroids SuperWASP data could act as a test bed for lightcurve reconstruction techniques that will need to be employed in surveys such as Pan-STARRS and the LSST. As SuperWASP detects more asteroids, it will become necessary to automate other tasks such as determining the period of asteroids where this is not known. Eventually a catalogue of asteroids, detected with SuperWASP, will be produced containing photometric information. 5.2. MOVING
OBJECT DETECTION
Due to the large field of view of SuperWASP, it is very likely that it will have observed other moving objects, such as Near Earth Objects, as they travel close to the Earth. The next task is to produce software to search automatically the orphan list for these moving objects in the nightly data, with a view to matching these objects over several nights.
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Acknowledgements The WASP Consortium consists of representatives from the University of Cambridge (Wide Field Astronomy Unit), Instituto de Astrofisca de Canarias, Isaac Newton Group (La Palma), University of Keele, University of Leicester, The Open University, Queen’s University Belfast and University of St Andrews. The SuperWASP and WASP-S Cameras were constructed and operated with funds made available from Consortium Universities and PPARC.
References Schober, H. J., Erikson, A., Hahn, G., Lagerkvist, C. I., and Oja, T.: 1993, A&AS 101, 499 . Clarkson, W. I., Christian, D. C., and Collier-Cameron, A. et al.: 2004, AAS 205, 171.01 .
