David Bernat

Dynamics of linear perturbations in f(R) gravity

We consider predictions for structure formation from modifications to general relativity in which the Einstein-Hilbert action is replaced by a general function of the Ricci scalar. We work without fixing a gauge, as well as in explicit popular coordinate choices, appropriate for the modification of existing cosmological code. We present the framework in a comprehensive and practical form that can be directly compared to standard perturbation analyses. By considering the full evolution equations, we resolve perceived instabilities previously suggested, and instead find a suppression of perturbations. This result presents significant challenges for agreement with current cosmological structure formation observations. The findings apply to a broad range of forms of

ESTABLISHING α Oph AS A PROTOTYPE ROTATOR: IMPROVED ASTROMETRIC ORBIT

f(R)

A CLOSE COMPANION SEARCH AROUND L DWARFS USING APERTURE MASKING INTERFEROMETRY AND PALOMAR LASER GUIDE STAR ADAPTIVE OPTICS

for which the modification becomes important at low curvatures, disfavoring them in comparison with the

The Use of Spatial Filtering with Aperture Masking Interferometry and Adaptive Optics

\ensuremath{\Lambda}\mathrm{CDM}

Aperture Mask Interferometry with an Integral Field Spectrograph

scenario. As such, these results provide a powerful method to rule out a wide class of modified gravity models aimed at providing an alternative explanation to the dark energy problem.

The Sloan Digital Sky Survey Data Release 7 Spectroscopic M Dwarf Catalog. I. Data

The nearby star α Oph (Ras Alhague) is a rapidly rotating A5IV star spinning at ~ 89% of its breakup velocity. This system has been imaged extensively by interferometric techniques, giving a precise geometric model of the star’s oblateness and the resulting temperature variation on the stellar surface. Fortuitously, α Oph has a previously known stellar companion, and characterization of the orbit provides an independent, dynamically based check of both the host star and the companion mass. Such measurements are crucial to constrain models of such rapidly rotating stars. In this study, we combine eight years of adaptive optics imaging data from the Palomar, AEOS, and CFHT telescopes to derive an improved, astrometric characterization of the companion orbit. We also use photometry from these observations to derive a model-based estimate of the companion mass. A fit was performed on the photocenter motion of this system to extract a component mass ratio. We find masses of 2.40^(+0.23)_(−0.37) M_⊙ and 0.85^(+0.06)_(−0.04) M_⊙ for α Oph A and α Oph B, respectively. Previous orbital studies of this system found a mass too high for this system, inconsistent with stellar evolutionary calculations. Our measurements of the host star mass are more consistent with these evolutionary calculations, but with slightly higher uncertainties. In addition to the dynamically derived masses, we use IJHK photometry to derive a model-based mass for α Oph B, of 0.77 ± 0.05 M_⊙ marginally consistent with the dynamical masses derived from our orbit. Our model fits predict a periastron passage on 2012 April 19, with the two components having a 50 mas separation from 2012 March to May. A modest amount of interferometric and radial velocity data during this period could provide a mass determination of this star at the few percent level.

The Sloan Digital Sky Survey Data Release 7 M Dwarf Spectroscopic Catalog

We present a close companion search around 16 known early L dwarfs using aperture masking interferometry with Palomar laser guide star adaptive optics (LGS AO). The use of aperture masking allows the detection of close binaries, corresponding to projected physical separations of 0.6-10.0 AU for the targets of our survey. This survey achieved median contrast limits of ΔK ~ 2.3 for separations between 1.2λ/D-4λ/D and ΔK ~ 1.4 at 2/3λ/D. We present four candidate binaries detected with moderate-to-high confidence (90%-98%). Two have projected physical separations less than 1.5 AU. This may indicate that tight-separation binaries contribute more significantly to the binary fraction than currently assumed, consistent with spectroscopic and photometric overluminosity studies. Ten targets of this survey have previously been observed with the Hubble Space Telescope as part of companion searches. We use the increased resolution of aperture masking to search for close or dim companions that would be obscured by full aperture imaging, finding two candidate binaries. This survey is the first application of aperture masking with LGS AO at Palomar. Several new techniques for the analysis of aperture masking data in the low signal-to-noise regime are explored.

The Sloan Digital Sky Survey DR7 Spectroscopic M Dwarf Catalog I: Data

Non-redundant aperture masking interferometry with adaptive optics (AO) is a powerful technique for high contrast at the diffraction limit with high-precision astrometry and photometry. A limitation to the achievable contrast can be attributed to spatial fluctuations of the wavefront—those within a sub-aperture and across sub-apertures—and temporal fluctuations within a single exposure. Spatial filtering addresses spatial fluctuations within a sub-aperture. An optimized pinhole in the focal place preceding the aperture mask is one approach for reducing the variation of the wavefront within a sub-aperture. Similarly, a weak spatial filtering effect is shown to be provided by post-processing the images with an apodized window function, typically used to minimize detector read noise and contamination from wide-separated sources. We explore the effects of spatial filtering through calculation, simulation, and observational tests conducted with a pinhole and aperture mask in the PHARO instrument at the Hale 200 �� Telescope at Palomar Observatory. We find that a pinhole decreases stochastic closure phase errors and calibration errors, but that tight restrictions are placed onto the alignment of binary targets within the pinhole. We propose an observation strategy to relax these restrictions. If implemented, the pinhole could potentially yield an increase in achievable contrast by up to 10%‐25% in H and Ks bands, and more at very high Strehl (80%). We also conclude that correcting low-order wavefront modes within the sub-apertures will be key for reaching high contrasts with extreme-AO systems such as the Gemini Planet Imager and PALM3K to search for planets.

THE SLOAN DIGITAL SKY SURVEY DATA RELEASE 7 SPECTROSCOPIC M DWARF CATALOG. I. DATA

A non-redundant pupil mask placed in front of a low-resolution integral field spectrograph (IFS) adds a spectral dimension to high angular resolution imaging behind adaptive optics systems. We demonstrate the first application of this technique, using the spectroscopic binary star system β CrB as our target. The mask and IFS combination enabled us to measure the first low-resolution spectrum of the F3-F5 dwarf secondary component of β CrB, at an angular separation 141 mas from its A5-A7Vp primary star. To record multi-wavelength closure phases, we collected interferograms simultaneously in 23 spectral channels spanning the J and H bands (1.1 μm-1.8 μm), using the Project 1640 IFS behind the 249-channel PalAO adaptive optics system on the Hale telescope at Palomar Observatory. In addition to providing physical information about the source, spectrally resolved mask fringes have the potential to enhance detection limits over single filter observations. While the overall dynamic range of our observation suffers from large systematic calibration errors, the information gleaned from the full channel range improves the dynamic range by a factor of 3 to 4 over the best single channel.