Report - School of Physics
Report - School of Physics
Report - School of Physics
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stars, or large telescopes providing deep exposures to increase the number <strong>of</strong> targets<br />
monitored. The current transit search surveys therefore group into two classes,<br />
using small or larger (> 70 cm) telescopes. Support for the implicit hypothesis that<br />
the orbits <strong>of</strong> planetary systems are randomly distributed in the Galaxy comes from<br />
the fact that stellar rotation axes are themselves randomly distributed (obtained<br />
by v sin i distributions, and independently confirmed by magnetic-field orientation<br />
studies).<br />
The wide-angle survey teams follow STARE and Vulcan in using small (10 cm) wideangle<br />
(10 ◦ ) CCD cameras with a pixel size <strong>of</strong> order 1 arcsec or larger, sacrificing<br />
angular resolution to expand the field <strong>of</strong> view. The faint limit, at V ∼ 12−13 reaches<br />
to d ∼ 300 − 500 pc, comparable to the disk scale height, so that target fields cover<br />
the entire sky, which may contain some 1000 hot transiting Jupiters to this limit<br />
(Horne, 2003). The deep surveys use (mosaic) cameras on 1–4 m telescopes, reaching<br />
V ∼ 19 − 21 and d ∼ 4 − 5 kpc, so that Galactic plane and open cluster fields are<br />
primary targets. Horne (2003) predicts up to 200 hot Jupiters per month being<br />
discovered by ongoing ground transit surveys in the future (perhaps by the year<br />
2010).<br />
The size <strong>of</strong> planets detectable from transits with ground-based searches is limited<br />
by the Earth’s atmosphere (Section 1.3). The photometric precision <strong>of</strong> typical<br />
lightcurves is a little under 1%, which corresponds to about Jupiter-sized planets<br />
for solar-type stars. Ground-based surveys are further limited in their time coverage<br />
<strong>of</strong> potential transiting planets by daytime and bad weather periods. The small<br />
number <strong>of</strong> transit surveys operating over more than a few months so far, and the<br />
lack <strong>of</strong> continuous observations on the target fields, are the most likely reason why<br />
only few planets have been discovered through transit detections so far, somewhat<br />
in contrast with the large number <strong>of</strong> surveys listed in Table 2. The situation could<br />
be significantly improved by ground-based networks <strong>of</strong> telescopes spanning a range<br />
<strong>of</strong> longitudes to ensure continuous observational coverage <strong>of</strong> target fields.<br />
The discovery <strong>of</strong> a temporary dimming <strong>of</strong> the stellar lightcurve alone is not sufficient<br />
to secure the detection <strong>of</strong> a transiting planet. Grazing eclipsing binary stars,<br />
background binaries, brown dwarfs and stellar spots can cause lightcurves similar to<br />
transiting planets. Follow-up measurements, in particular radial velocity measurements,<br />
and determination <strong>of</strong> stellar parameters, therefore play an important role in<br />
the detection <strong>of</strong> planetary transits to exclude other causes <strong>of</strong> light dimming.<br />
Up to now, five confirmed planets have been discovered by ground-based transit<br />
searches: four using the 1.3 m OGLE telescope, and one with a 10 cm ground-based<br />
system (TrES-1). The OGLE experiment (Optical Gravitational Lensing Experiment)<br />
uses the 1.3 m Warsaw Telescope at Las Campanas Observatory, Chile. It is<br />
equipped with a mosaic <strong>of</strong> 8 CCDs <strong>of</strong> 2k×4k each, giving a field <strong>of</strong> view <strong>of</strong> 35 arcmin<br />
square with 0.26 arcsec/pixel. The telescope is primarily used to search for<br />
microlensing events by viewing near the Galactic centre, but significant time (more<br />
than three months) was made available for transit searches. It has monitored some<br />
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