Exoplanets and Stellar Physics

Abstract:

A number of space missions dedicated to the search for exoplanets via the transit method, such as COROT, Eddington and Kepler, are planned for launch over the next few years. They will need to address problems associated with the automated and efficient detection of planetary transits in light curves affected by a variety of noise sources, including stellar variability. To maximize the scientific return of these missions, it is important to develop and test appropriate algorithms in advance of their launch dates. Starting from a general-purpose maximum-likelihood approach we discuss the links between a variety of period- and transit-finding methods. The natural endpoint of this hierarchy of methods is shown to be a fast, robust and statistically efficient least-squares algorithm based on box-shaped transits. This approach is predicated on the assumption of periodic transits hidden in random noise, usually assumed to be superposed on a flat continuum with regular continuous sampling. We next show how to generalize the transit-finding method to the more realistic scenario where complex stellar (micro) variability, irregular sampling and long gaps in the data are all present. Tests of this methodology on simulated Eddington light curves, including realistic stellar microvariability, irregular sampling and gaps in the data record, are used to quantify the performance. Visually, these systematic effects can completely overwhelm the underlying signal of interest. However, in the case where transit durations are short compared to the dominant time-scales for stellar variability and data record segments, it is possible to decouple the transit signal from the remainder. We conclude that even with realistic contamination from stellar variability, irregular sampling, and gaps in the data record, it is still possible to detect transiting planets with an efficiency close to the idealized theoretical bound. In particular, space missions have the potential to approach the regime of detecting Earth-like planets around G2V-type stars.

Abstract:

Hydrothermal plumes may be responsible for transmitting radiogenic or tidally generated heat from Europa's rocky interior through a liquid ocean to the base of its ice shell. This process has been implicated in the formation of chaos regions and lenticulae by melting or exciting convection in the ice layer. In contrast to earlier work, we argue that Europa's ocean should be treated as an unstratified fluid. We have adapted and expanded upon existing work describing buoyant plumes in a rotating, unstratified environment. We discuss the scaling laws governing the flow and geometry of plumes on Europa and perform a laboratory experiment to obtain scaling constants and to visualize plume behavior in a Europa‐like parameter regime. We predict that hydrothermal plumes on Europa are of a lateral scale (at least 25–50 km) comparable to large chaos regions; they are too broad to be responsible for the formation of individual lenticulae. Plume heat fluxes (0.1–10 W/m2) are too weak to allow complete melt‐through of the ice layer. Current speeds in the plume (3–8 mm/s) are much slower than indicated by previous studies. The observed movement of ice blocks in the Conamara Chaos region is unlikely to be driven by such weak flow.