Compared to bright star searches, surveys for transiting planets against fainter (V=12-18) stars have the advantage of much higher sky densities of dwarf star primaries, which afford easier detection of small transiting bodies. Furthermore, deep searches are capable of probing a wider range of stellar environments. On the other hand, for a given spatial resolution and transit depth, deep searches are more prone to confusion from blended eclipsing binaries. We present a powerful mitigation strategy for the blending problem that includes the use of image deconvolution and high resolution imaging. The techniques are illustrated with Lupus-TR-3 and very recent IR imaging with PANIC on Magellan. The results are likely to have implications for the CoRoT and KEPLER missions designed to detect transiting planets of terrestrial size.
Deep Dive into Transits against Fainter Stars: The Power of Image Deconvolution.
Compared to bright star searches, surveys for transiting planets against fainter (V=12-18) stars have the advantage of much higher sky densities of dwarf star primaries, which afford easier detection of small transiting bodies. Furthermore, deep searches are capable of probing a wider range of stellar environments. On the other hand, for a given spatial resolution and transit depth, deep searches are more prone to confusion from blended eclipsing binaries. We present a powerful mitigation strategy for the blending problem that includes the use of image deconvolution and high resolution imaging. The techniques are illustrated with Lupus-TR-3 and very recent IR imaging with PANIC on Magellan. The results are likely to have implications for the CoRoT and KEPLER missions designed to detect transiting planets of terrestrial size.
Most searches for transiting extrasolar transiting planets fall into two broad categories (see, eg, Mazeh 2008): very wide-field searches targeting bright (V < 12) stars, and narrower field, pointed, dense-field observations monitoring fainter (V > 14) stars (Fig. 1). Surveys of brighter stars have the advantage of more efficient spectroscopic follow-up due to the larger fluxes of their candidates. Fainter searches are typically more efficient in the discovery phase, using less telescope time to densely monitor a similar number of dwarf star targets.
The space-based CoRoT (Barge et al. 2008a) and KEPLER (Borucki et al. 2008) missions bridge part of this gap, pointing at particular dense stellar fields at low Galactic latitude, with the expectation that most of their prime target stars will have 12 < V < 14. This middle range is their “sweet spot” because the stellar mass function ensures that most dwarf hosts will lie at the faint end, while the best photometry required to search for small planets is achieved at the bright end. Certainly the transiting planets reported by the CoRoT team to date, all with Jovian sizes, have host stars in this magnitude range (Barge et al. 2008b;Alonso et al. 2008;Aigrain et al. 2008).
Stars of this magnitude and fainter are increasingly more likely to be blended with foreground or background stars of similar brightness. Furthermore, the ability to perform spectroscopic tests to rule out the possibility that a blended eclipsing binary is masquerading as a transiting planet becomes increasingly difficult as the host star becomes fainter. Since Jovian-sized planets generate a ∼1% dip in host brightness when transiting a Sun-like star, a (totally) eclipsing stellar binary (EcB) system can be nearly five magnitudes fainter than a random blended neighbour along the line-of-sight and still generate a Jovian-like transit signal against the bright blended composite. For the same reason, surveys for terrestrial-sized planets must be able to exclude possible blended EcB up to the ten magnitudes fainter than their survey targets. Clearly, this is an issue that the CoRoT and KEPLER teams will have to face in confirming planets whose apparent masses are too small to yield a robust radial velocity signature.
We were led to consider the issue of possible confusion with an EcB in follow-up work directed at the prime planetary transit candidate, Lupus-TR-3b (Weldrake et al. 2008) In our survey, we monitored about 110,000 stars over a 0.66 square degree field in Lupus for 53 nights in June of 2005 and 2006 with the ANU 40-Inch Telescope equipped with a wide field imager at Siding Spring Observatory (SSO) in Australia (Weldrake et al. 2007;Bayliss et al. 2008b). The resulting 1783 exposures produced photometry to a precision of better than 0.025 mag (rms) for ∼16,000 stars. The photometry was produced using an image subtraction technique, followed by SYSREM (Tamuz et al. 2005) to remove systematics (red noise) common to a large number of stars in the field. The BLS detection scheme of Kovács et al. (2002) was then used to identify promising candidates for the host stars of transiting planets. This initial two-year survey is being extended in time to yield the SuperLupus Survey (Bayliss et al. 2008a), will increase the sensitivity to longer period transiting planets.
The initial photometric selection process produced six candidates in the Lupus field, whose basic characteristics are listed in Table 1. More details will be forthcoming in a subsequent publication (Bayliss et al. 2008b). All candidates were detected at a high Notes:
1 The detection S/N is a measure of the significance of the size of collective transit signal.
2 The η diagnostic is unity or below for likely planetary transits (see Tingley & Sackett 2005).
level of significance, with a large number of in-transit photometric measurements, and depth-duration-period properties, as measured by the η diagnostic (Tingley & Sackett 2005), that made them plausible transiting planet signatures.
After further scrutiny and follow-up observations, however, only Lupus-TR-3 remained as a plausible candidate (Bayliss et al, 2008, in preparation). The K1 dwarf exhibits a 1.3% dip of about 2.6 hour duration every P = 3.91405 days. Subsequent radial velocity measurements taken with the MIKE echelle spectrograph on Magellan I displayed a confirming radial velocity signature of K = 114±25 m/s, appropriately in phase with the transit. The planet, Lupus-TR-3b, thus has derived parameters of M p = 0.81 ± 0.18M J , R p = 0.89 ± 0.07R J , yielding a quite Jovian-like density of ρ p = 1.4 ± 0.4 gm/cm 3 (Weldrake et al. 2008).
This might have been the end of the story, had we not simultaneously been pursuing image deconvolution as a method to discover possible close neighbours in the vicinity of promising candidates.
The long train of monitoring images used to discover Lupus-TR-3b were obtained at SSO, a site with only moderate atmospheric seeing con
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