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Scientific Frontiers in Research on Extrasolar Planets ASP Conference Series, Vol. 28?, 2003 D. Deming et al.

An Automated Search for Extrasolar Planet Transits
M. G. Hidas, J. K. Webb, M. C. B. Ashley, C. H. Lineweaver University of New South Wales, Sydney 2052 NSW, Australia J. Anderson University of California, Berkeley, USA

arXiv:astro-ph/0209388 v1 19 Sep 2002

M. Irwin Institute of Astronomy, Cambridge, UK Abstract. We are setting up a new search for transiting extra-solar planets using the 0.5m Automated Patrol Telescop e at Siding Spring Observatory, Australia. We will b egin regular observations in Septemb er 2002. We exp ect to find 7 new planets p er year.

1.

Intro duction

An increasing numb er of teams are searching for extra-solar planets using the transit method (see Horne 2002 for a review). Although the probability of observing a transit for any given star is small, using a wide-field telescop e a large numb er of stars can b e monitored, p otentially yielding a higher detection rate than the radial velocity surveys (e.g. Marcy et al. 2002; Mayor et al. 2002). Furthermore, for a transiting planet orbiting a sufficiently bright star, the planet's size, as well as its actual mass and orbital characteristics (from follow-up sp ectroscopy) can b e determined, constraining models of its structure and formation. 2. The Automated Patrol Telescop e

The Automated Patrol Telescop e (APT) is a 0.5m telescop e of Baker-Nunn design, modified for use with a CCD. It is owned and op erated by the University of New South Wales, and located at Siding Spring Observatory, Australia. The current CCD camera images a 2 в3 field with 9.4 pixels. An upgrade is planned (for early 2003) to a pair of 3kв6k CCDs with 4.2 pixels, covering the entire useful field of view of the telescop e ( 6 in diameter). The telescop e and dome are computer-controlled, with the p ossibility of remote or fully automated observation. Ab out 50% of the observing time on the APT is reserved for this planet search. A significant drawback of our current system is that the images are undersampled (the FWHM of the p oint spread-function is 1.3 pixels). Combined with 1


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Hidas et al.

Figure 1. Photometric precision for a series of 150 second exp osures taken with the APT. The field is centered on the op en cluster NGC6633, close to the Galactic plane. The limits due to photon shot noise in the star and sky flux are shown. Readout noise is negligible. Systematic errors limit the precision for bright stars. Stars brighter than V 10.5 are saturated.


An Automated Search for Extrasolar Planet Transits

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intra-pixel sensitivity variations on the CCD, this causes photometric errors of 3% when using standard ap erture photometry. Anderson and King (2000) have develop ed software which deals with the undersampling problem. We are using a modified version of this software. It builds up a model of the "effective" p oint-spread function, which is the instrumental p oint-spread function convolved with the sensitivity of a single pixel, and measures stars by fitting this model to the image. We can now measure stars down to V 14 with a relative precision of 1% (Fig 1) in a single 150 second exp osure. For the brightest stars, systematic errors limit the precision. We are working on identifying and eliminating these errors. We are ab out to b egin regular observations. We select four dense stellar fields near the Galactic plane and cycle b etween them, taking exp osures through a V filter. This way we obtain 5 images of each field p er hour, for 6­8 hours p er clear night. We observe each field for 2­3 months. 3. Estimated detection rate

Kovґcs, Zucker, & Mazeh (2002) show that their b ox-fitting algorithm for dea tecting transit signals in stellar lightcurves requires an effective signal-to-noise ratio (for the combined measurements during the transit) of 6 for a significant detection. For a typical 1% deep, 3 hour long transit, we can achieve this by observing just 3 transits. This is true for 1000 stars with V < 14 in one low Galactic latitude APT field. To a magnitude limit of V = 14, near the Galactic plane, F, G, K and M typ e main sequence stars constitute 60% of the stars in the field. Upp er main sequence stars are too large for a Jupiter to cause a 1% transit. Thus, with the 5 times larger field of view of our new CCD system, we will measure 3000 stars p er field with sufficient precision to detect an orbiting Jupiter. Due to observational constraints (weather, lunar and diurnal cycle, other pro jects), we estimate that we will detect 10% of transits which occur in our fields. We are most likely to detect close-orbiting planets with p eriods of a few days. Almost 1% of nearby stars host one of these "Hot Jupiters" (Butler et al. 2001). Their orbits have a 10% probability of b eing edge-on. We therefore exp ect to find one planet p er 104 stars measured with sufficient precision. We plan to observe 24 fields p er year, yielding approximately 7 new planets each year. References Anderson, J., & King, I. R. 2000, PASP, 112, 1360 Butler, R. P. et al. 2000, http://exoplanets.org/stats2000.ps Horne K. 2002, in ASP Conf. Ser. Vol. 28?, Scientific Frontiers in Research on Extrasolar Planets, ed. D. Deming et al., in press Kovґcs, G., Zucker, S., & Mazeh, T. 2002, A&A, 391, 369 a Marcy, G. W. et al. 2002, http://exoplanets.org Mayor, M. et al. 2002, http://obswww.unige.chudry/planet/planet.html