Exoplanet transit

The measurement of the brightness of astronomical objects and its variation with time is one of the most basic techniques in astronomy to obtain information on celestial bodies; in addition to positional and spectral information temporal variability is occasionally referred to as the "fourth" dimension of astrophysics. The beauty of this technique is that the technical requirements to obtain photometry are not very high, and also amateur astronomers with small telescopes can contribute important information. One of the most vivid areas of astronomy today, the study of extrasolar planets around other stars, uses photometry to detect these exoplanets with the so-called transit method. For large Jupiter-like planets, this transit method also works with the rather small telescopes of non-professional astronomers.


However, the quality of ground-based photometry depends on the size of the telescope. Large and expensive instruments are usually not feasible for individual persons and require astronomical institutes or even larger organizations. The Hamburger Sternwarte maintains two telescopes capable of carrying out photometry which are available to our students and faculty members. The Oskar-Lühning-Telekop (OLT) is a 1.2 m telescope located at the Hamburger Sternwarte; an example for an observation of TrES-2b with the OLT is shown on the lower right-hand side. The Planetary Transit Search Telescope (PTST) is a new 60 cm telecope for photometric observations which takes advantage of the much better observing conditions in Mallorca (Spain) and is explicitly intended to be used by students to observe exoplanet transits as well as other interesting timing events.


Exoplanet transit lightcurve of TrES-2b

A fundamental limitation to the achievable accuracy of ground-based photometry is scintillation, i.e., the twinkling of stars, which is caused by changes in the refractive index of atmospheric layers. Especially for large telescopes scintillation is the dominant source of error in photometry. Although ground-based telescopes are extremely useful for photometry, many research areas of modern astronomy do require much higher precision than achievable from the ground. Furthermore, ground-based optical observations are by necessity restricted to night time and uninterrupted photometry covering days, or even months or years, is fundamentally impossible. For these reasons the instruments for highest precision photometry are operated in space. The CoRoT and the Kepler satellite are two current space missions delivering ultra-high-precision photometry of thousands of stars for months and years. Their lightcurves are primarily used to detect and analyze exoplanets, but they also contain huge amounts of information on the stars themselves which can be used, e.g., for asteroseismology and stellar activity studies.


Ground- and space-based photometric data has been used extensively in our group to investigate exoplanets and their host-stars. One example involves the planetary system TrES-2, which has been studied using OLT data and, later, Kepler photometry (see figure on right-hand side). It was suspected that the transit shape of the Jupiter-like planet TrES-2b might vary in time indicating changes of the planetary orbit. Using Kepler data we could rule this out; the transit shape does not change over periods of years (see figure on right-hand side). Another object of intensive research is the system CoRoT-2 which has an highly active star transited by a large planet. The lightcurve of this star shows modulations due to stellar rotation and a heavily spotted stellar surface (see picture below). Furthermore, the transits are massively deformed because the planet crosses a surface that has no uniform brightness distribution. Applying a new method, our group used the lightcurve to reconstruct the spot distribution on the surface of the star and its evolution over half a year.


Exoplanet transit


CoRoT and Kepler observe in the optical and, unfortunately, provide only very limited spectral information. For some cases it is extraordinarily interesting to simultanously observe stars in the optical and near-infrared over an uninterrupted period of several hours, which is not possible with any space-based instrument right now. This observational need is fulfilled by the SOFIA observatory (see picture below) which is located on an aircraft and observes during flight from high atmospheric layers; because SOFIA is flying above the main turbulent atmospheric layers, its light curves are virtually free of scintillation. Thus, its quality of quasi-space-based photometry and its ability to simultaneously observe in the optical and in the infrared provides additional information not delivered by Kepler or CoRoT. Observations with this instrument have been granted and we will use the data to further improve our knowledge on planetary systems - both planets and their host-stars.


Another important area of interest are high-speed phenomena in astrophysics. Such high speed phenomena can occur in compact objects like neutron stars and black holes or in non-thermal energy releases in solar and stellar flares. While at X-ray wavelengths one usually employs imaging photon counting detectors, which register the incoming photons with accuracies typically in the millisecond range, at optical wavelengths high speed imaging has only recently become available and affordable. These imaging techniques also allow to either follow and/or compensate the rapidly varying atmospheric perturbations of astronomical signals caused by the processes of scintillation described above and by seeing, by which astronomers describe the image distortions due to atmospheric turbulence. Our group is engaged in high time resolution astrophysics both in terms of determining the shortest time scales of variability as well as developing techniques to correct for the effects of scintillation in photometry.