Abhijith Rajan

Parameters and Predictions for the Long-Period Transiting Planet HD 17156b

We report high-cadence time series photometry of the recently discovered transiting exoplanet system HD 17156, spanning the time of transit on UT 2007 October 1, from three separate observatories. We present a joint analysis of our photometry, previously published radial velocity measurements, and times of transit center for three additional events. Adopting the spectroscopically determined values and uncertainties for the stellar mass and radius, we estimate a planet radius of -->Rp = 1.01 ± 0.09 RJup and an inclination of -->i = 86.5+ 1.1−0.7 deg. We find a time of transit center of -->Tc = 2,454,374.8338 ± 0.0020 HJD and an orbital period of -->P = 21.21691 ± 0.00071 days and note that the four transits reported to date show no sign of timing variations that would indicate the presence of a third body in the system. Our results do not preclude the existence of a secondary eclipse, but imply that there is only a 9.2% chance for this to be present and an even lower probability (6.9%) that the secondary eclipse would be a nongrazing event. Due to its eccentric orbit and long period, HD 17156b is a fascinating object for the study of the dynamics of exoplanet atmospheres. To aid such future studies, we present theoretical light curves for the variable infrared emission from the visible hemisphere of the planet throughout its orbit.

Updated optical modeling of JWST coronagraph performance contrast, stability, and strategies

We update performance simulations and contrast predictions for JWSTs coronagraphs based on the latest infor- mation on the as-built telescope and instrument properties, including both static and dynamic contributions to wavefront error. By combining optical modeling of the telescope, instruments and coronagraph optics along with STScIs rigorously-validated exposure time calculation engine, we develop updated contrast models including contributions from effects such as target acquisition residuals, stellar color differences, etc. We present assessments of the impact of wavefront error changes over time between science and PSF reference stars, using modeled wavefront drifts on various timescales based on available observatory structural/thermal/optical modeling and tested performance during the OTIS cryo test, extrapolated to on-orbit conditions. For NIRCam we explore tradeoffs between different occulting masks at a given wavelength. Between now and the start of Cycle 1 science, these and other updated simulations will enable the science community to prepare analysis tools and PSF subtraction software to hit the ground running with JWST coronagraphic observations.

Making good use of JWST's coronagraphs: tools and strategies from a user's perspective

The James Webb Space Telescope (JWST) and its suite of instruments, modes and high contrast capabilities will enable imaging and characterization of faint and dusty astrophysical sources1-3 (exoplanets, proto-planetary and debris disks, dust shells, etc.) in the vicinity of hosts (stars of all sorts, active galactic nuclei, etc.) with an unprecedented combination of sensitivity and angular resolution at wavelengths beyond 2 μm. Two of its four instruments, NIRCam4, 5 and MIRI,6 feature coronagraphs7, 8 for wavelengths from 2 to 23 μm. JWST will stretch the current parameter space (contrast at a given separation) towards the infrared with respect to the Hubble Space Telescope (HST) and in sensitivity with respect to what is currently achievable from the ground with the best adaptive optics (AO) facilities. The Coronagraphs Working Group at the Space Telescope Science Institute (STScI) along with the Instruments Teams and internal/external partners coordinates efforts to provide the community with the best possible preparation tools, documentation, pipelines, etc. Here we give an update on user support and operational aspects related to coronagraphy. We aim at demonstrating an end to end observing strategy and data management chain for a few science use cases involving coronagraphs. This includes the choice of instrument modes as well as the observing and point-spread function (PSF) subtraction strategies (e.g. visibility, reference stars selection tools, small grid dithers), the design of the proposal with the Exposure Time Calculator (ETC), and the Astronomers Proposal Tool (APT), the generation of realistic simulated data at small working angles and the generation of high level, science-grade data products enabling calibration and state of the art data-processing.