Intro : Papers

Introduction

With the closure of the Sleuth transit survey, Sherlock is now offline.

(Edited extract from Sherlock: An Automated Follow-Up Telescope for Wide-Field Transit Searches.)

We have assembled a telescope named Sherlock to examine transit candidates from Sleuth, as well as a number of other transit surveys which monitor stars brighter than V=13. Highly automated and observing in custom RGB filters with a better angular resolution (1.7 arcsec/pixel) than Sleuth (10 arcsec/pixel), Sherlock is able to reject most of the contaminants from these transit surveys. In so doing, Sherlock will greatly reduce the rate of false positives, bringing the number of transit candidates to a manageable level.

These two figures show an example of how blends of eclipsing binaries can be separated from transiting planets. PSST identified a star in Auriga undergoing 2.5% deep eclipses with a period of 2.4 days (figure on left). Examination of the Digitized Sky Survey images (figure on right) revealed two sources within the PSF of the PSST instrument. Photometry of the field during a subsequent eclipse with a 14-inch telescope on the rooftop of the Caltech Astronomy building demonstrated that the fainter star was undergoing deep (14%) eclipses. When the light from both stars was summed within a single photometric aperture, the transit shape and depth as observed by the PSST was reproduced (figure on right).

Measurement of the color dependence of the transit depth should remove both grazing incidence binaries (due to the effect of limb-darkening on the eclipse depth), and blends of eclipsing binaries (due the change in relative brightness of the blended and occulted stars as a function of color). In contrast to this, planetary transits should be nearly constant with color. Furthermore, the increased angular resolution will separate the light from physically unassociated blended stars, and subsequent photometry will reveal which object is undergoing eclipses. Sherlock will not be able to reject all sources of false detection. Central eclipses by very dim stellar objects (notably M dwarfs) will not show detectable variation with color. In addition, blends wherein the occulted and blending star have the same color will not be distinguished. Even with these exceptions, however, Sherlock is expected to reduce greatly the ratio of false positives from these surveys. Multi-epoch spectra of viable candidates will be gathered with high-resolution spectrographs on 1-2 m class telescopes, which should rule out the presence of stellar or brown dwarf companions. Surviving candidates will be monitored with Keck HIRES to determine the radial velocity orbit induced by the planetary companion.

The table below lists the specifications for Sherlock. The system is located in the same clamshell enclosure as Sleuth, our primary transit search instrument. Weather decisions are made by the on-site 200-inch telescope night assistant, with additional protection provided by a weather station capable of closing the clamshell roof. Sherlock is completely automated, calculating future times of eclipse for all active candidates, and observing the highest priority object in eclipse each night. The automated nature of the system is an advantage over comparatively labor- and resource-intensive multi-epoch spectroscopic follow-up. In addition, this dome hosts the new all-sky camera Snoop, which provides weather monitoring for the observatory. Given that observing time on this system should be plentiful, we invite teams conducting wide-field transit surveys to contact us regarding follow-up of their candidates.