Astrophysically robust systematics removal using variational inference: Application to the first month of kepler data
Monthly Notices of the Royal Astronomical Society 435:4 (2013) 3639-3653
Abstract:
Space-based transit search missions such as Kepler are collecting large numbers of stellar light curves of unprecedented photometric precision and time coverage. However, before this scientific goldmine can be exploited fully, the data must be cleaned of instrumental artefacts. We present a new method to correct common-mode systematics in large ensembles of very high precision light curves. It is based on a Bayesian linear basis model and uses shrinkage priors for robustness, variational inference for speed and a de-noising step based on empirical mode decomposition to prevent the introduction of spurious noise into the corrected light curves. After demonstrating the performance of our method on a synthetic data set, we apply it to the first month of Kepler data. We compare the results, which are publicly available, to the output of the Kepler pipeline's pre-search data conditioning, and show that the two generally give similar results, but the light curves corrected using our approach have lower scatter, on average, on both long and short time-scales. We finish by discussing some limitations of our method and outlining some avenues for further development. The trend-corrected data produced by our approach are publicly available. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.Constraining the atmosphere of GJ 1214b using an optimal estimation technique
Monthly Notices of the Royal Astronomical Society 434:3 (2013) 2616-2628
Abstract:
We explore cloudy, extended H2-He atmosphere scenarios for the warm super-Earth GJ 1214b using an optimal estimation retrieval technique. This planet, orbiting an M4.5 star only 13 pc from the Earth, is of particular interest because it lies between the Earth and Neptune in size and may be a member of a new class of planet that is neither terrestrial nor gas giant. Its relatively flat transmission spectrum has so far made atmospheric characterization difficult. The Non-linear optimal Estimator for MultivariateE spectral analySIS (NEMESIS) algorithm is used to explore the degenerate model parameter space for a cloudy, H2-He-dominated atmosphere scenario. Optimal estimation is a data-led approach that allows solutions beyond the range permitted by ab initio equilibrium model atmosphere calculations, and as such prevents restriction from prior expectations. We show that optimal estimation retrieval is a powerful tool for this kind of study, and present an exploration of the degenerate atmospheric scenarios for GJ 1214b. Whilst we find a family of solutions that provide a very good fit to the data, the quality and coverage of these data are insufficient for us to more precisely determine the abundances of cloud and trace gases given an H2-He atmosphere, and we also cannot rule out the possibility of a high molecular weight atmosphere. Future ground- and space-based observations will provide the opportunity to confirm or rule out an extended H2-He atmosphere, but more precise constraints will be limited by intrinsic degeneracies in the retrieval problem, such as variations in cloud top pressure and temperature. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.From spectra to atmospheres: Solving the underconstrained retrieval problem for exoplanets
Proceedings of the International Astronomical Union 8:S299 (2013) 275-276
Abstract:
Spectroscopic observations of transiting exoplanets have provided the first indications of their atmospheric structure and composition. Optimal estimation retrievals have been successfully applied to solar system planets to determine the temperature, composition and aerosol properties of their atmospheres, and have recently been applied to exoplanets. We show the effectiveness of the technique when combined with simulated observations from the proposed space telescope EChO, and also discuss the difficulty of constraining a complex system with sparse data and large uncertainties, using the super-Earth GJ 1214b as an example. Copyright © 2013, International Astronomical Union.Hst hot jupiter transmission spectral survey: Detection of water in HAT-P-1b from WFC3 near-IR spatial scan observations
Monthly Notices of the Royal Astronomical Society 435:4 (2013) 3481-3493
Abstract:
We present Hubble Space Telescope near-infrared transmission spectroscopy of the transiting hot-Jupiter HAT-P-1b. We observed one transit withWide Field Camera 3 using the G141 lowresolution grism to cover thewavelength range 1.087-1.678μm. These time series observations were taken with the newly available spatial-scan mode that increases the duty cycle by nearly a factor of 2, thus improving the resulting photometric precision of the data. We measure a planetto-star radius ratio of Rp/R* = 0.117 09 ± 0.000 38 in the white light curve with the centre of transit occurring at 245 6114.345 ± 0.000 133 (JD). We achieve S/N levels per exposure of 1840 (0.061 per cent) at a resolution of δλ = 19.2 nm (R ~ 70) in the 1.1173-1.6549 μm spectral region, providing the precision necessary to probe the transmission spectrum of the planet at close to the resolution limit of the instrument. We compute the transmission spectrum using both single target and differential photometry with similar results. The resultant transmission spectrum shows a significant absorption above the 5s level matching the 1.4 μm water absorption band. In solar composition models, the water absorption is sensitive to the ~1 m bar pressure levels at the terminator. The detected absorption agrees with that predicted by a 1000K isothermal model, as well as with that predicted by a planetary-averaged temperature model. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.Hubble space telescope hot jupiter transmission spectral survey: A detection of na and strong optical absorption in HAT-P-1b
Monthly Notices of the Royal Astronomical Society 437:1 (2013) 46-66