Astrometric Monitoring of the HR 8799 Planets: Orbit Constraints from Self-Consistent Measurements

We present new astrometric measurements from our ongoing monitoring campaign of the HR 8799 directly imaged planetary system. These new data points were obtained with NIRC2 on the W.M. Keck II 10 meter telescope between 2009 and 2014. In addition, we present updated astrometry from previously published observations in 2007 and 2008. All data were reduced using the SOSIE algorithm, which accounts for systematic biases present in previously published observations. This allows us to construct a self-consistent data set derived entirely from NIRC2 data alone. From this dataset, we detect acceleration for two of the planets (HR 8799b and e) at $>$3$\sigma$. We also assess possible orbital parameters for each of the four planets independently. We find no statistically significant difference in the allowed inclinations of the planets. Fitting the astrometry while forcing coplanarity also returns $\chi^2$ consistent to within 1$\sigma$ of the best fit values, suggesting that if inclination offsets of $\lesssim$20$^{o}$ are present, they are not detectable with current data. Our orbital fits also favor low eccentricities, consistent with predictions from dynamical modeling. We also find period distributions consistent to within 1$\sigma$ with a 1:2:4:8 resonance between all planets. This analysis demonstrates the importance of minimizing astrometric systematics when fitting for solutions to highly undersampled orbits.Comment: 18 pages, 11 figures. Accepted for publication in A

The Structure of High Strehl Ratio Point-Spread Functions

We describe the symmetries present in the point-spread function (PSF) of an optical system either located in space or corrected by an adaptive o to Strehl ratios of about 70% and higher. We present a formalism for expanding the PSF to arbitrary order in terms of powers of the Fourier transform of the residual phase error, over an arbitrarily shaped and apodized entrance aperture. For traditional unapodized apertures at high Strehl ratios, bright speckles pinned to the bright Airy rings are part of an antisymmetric perturbation of the perfect PSF, arising from the term that is first order in the residual phase error. There are two symmetric second degree terms. One is negative at the center, and, like the first order term, is modulated by the perfect image's field strength -- it reduces to the Marechal approximation at the center of the PSF. The other is non-negative everywhere, zero at the image center, and can be responsible for an extended halo -- which limits the dynamic range of faint companion detection in the darkest portions of the image. In regimes where one or the other term dominates the speckles in an image, the symmetry of the dominant term can be exploited to reduce the effect of those speckles, potentially by an order of magnitude or more. We demonstrate the effects of both secondary obscuration and pupil apodization on the structure of residual speckles, and discuss how these symmetries can be exploited by appropriate telescope and instrument design, observing strategies, and filter bandwidths to improve the dynamic range of high dynamic range AO and space-based observations. Finally, we show that our analysis is relevant to high dynamic range coronagraphy.Comment: Accepted for publication in ApJ; 20 pages, 4 figure

Speckle Control with a remapped-pupil PIAA-coronagraph

The PIAA is a now well demonstrated high contrast technique that uses an intermediate remapping of the pupil for high contrast coronagraphy (apodization), before restoring it to recover classical imaging capabilities. This paper presents the first demonstration of complete speckle control loop with one such PIAA coronagraph. We show the presence of a complete set of remapping optics (the so-called PIAA and matching inverse PIAA) is transparent to the wavefront control algorithm. Simple focal plane based wavefront control algorithms can thus be employed, without the need to model remapping effects. Using the Subaru Coronagraphic Extreme AO (SCExAO) instrument built for the Subaru Telescope, we show that a complete PIAA-coronagraph is compatible with a simple implementation of a speckle nulling technique, and demonstrate the benefit of the PIAA for high contrast imaging at small angular separation.Comment: 6 figures, submitted to PAS

A new 1.6-micron map of Titan’s surface

We present a new map of Titan's surface obtained in the spectral 'window' at ∼1.6 μm between strong methane absorption. This pre-Cassini view of Titan's surface was created from images obtained using adaptive optics on the W.M. Keck II telescope and is the highest resolution map yet made of Titan's surface. Numerous surface features down to the limits of the spatial resolution (∼200–300 km) are apparent. No features are easily identifiable in terms of their geologic origin, although several are likely craters

Measuring the Mass of a Pre-Main Sequence Binary Star Through the Orbit of TWA 5A

We present the results of a five year monitoring campaign of the close binary TWA 5Aab in the TW Hydrae association, using speckle and adaptive optics on the W.M. Keck 10 m telescopes. These measurements were taken as part of our ongoing monitoring of pre-main sequence (PMS) binaries in an effort to increase the number of dynamically determined PMS masses and thereby calibrate the theoretical PMS evolutionary tracks. Our observations have allowed us to obtain the first determination of this system's astrometric orbit. We find an orbital period of 5.94 +- 0.09 years and a semi-major axis of 0.066" +- 0.005". Combining these results with a kinematic distance, we calculate a total mass of 0.71 +- 0.14 M_sun (D/44 pc)^3. for this system. This mass measurement, as well as the estimated age of this system, are consistent to within 2$\sigma$ of all theoretical models considered. In this analysis, we properly account for correlated uncertainties, and show that while these correlations are generally ignored, they increase the formal uncertainties by up to a factor of five and therefore are important to incorporate. With only a few more years of observation, this type of measurement will allow the theoretical models to be distinguished

Mid Infrared Observations of Van Maanen 2: No Substellar Companion

The results of a comprehensive infrared imaging search for the putative 0.06 solar mass astrometric companion to the 4.4 pc white dwarf van Mannen 2 are reported. Adaptive optics images acquired at 3.8 microns reveal a diffraction limited core of 0.09" and no direct evidence of a secondary. Models predict that at 5 Gyr, a 50 jupiter mass brown dwarf would be only 1 magnitude fainter than van Maanen 2 at this wavelength and the astrometric analysis suggested a separation of 0.2". In the case of a chance alignment along the line of sight, a 0.4 mag excess should be measured. An independent photometric observation at the same wavelength reveals no excess. In addition, there exist published ISO observations of van Maanen 2 at 6.8 and 15.0 microns which are consistent with photospheric flux of a 6750 K white dwarf. If recent brown dwarf models are correct, there is no substellar companion with T > 500 K.Comment: 11 pages, 3 figures, 1 table, accepted to ApJ Letter

Speckle noise and dynamic range in coronagraphic images

This paper is concerned with the theoretical properties of high contrast coronagraphic images in the context of exoplanet searches. We derive and analyze the statistical properties of the residual starlight in coronagraphic images, and describe the effect of a coronagraph on the speckle and photon noise. Current observations with coronagraphic instruments have shown that the main limitations to high contrast imaging are due to residual quasi-static speckles. We tackle this problem in this paper, and propose a generalization of our statistical model to include the description of static, quasi-static and fast residual atmospheric speckles. The results provide insight into the effects on the dynamic range of wavefront control, coronagraphy, active speckle reduction, and differential speckle calibration. The study is focused on ground-based imaging with extreme adaptive optics, but the approach is general enough to be applicable to space, with different parameters.Comment: 31 pages, 18 figure