arXiv subjects

Ben R. Oppenheimer

Publications and source records attributed to Ben R. Oppenheimer.

26 records · Page 2Linked to original sources

arXiv2006

Temporal Evolution of Coronagraphic Dynamic Range, and Constraints on Companions to Vega

The major obstacle to the direct detection of companions to nearby stars is the overwhelming brightness of the host star. Current instruments employing the combination of adaptive optics (AO) and coronagraphy can typically detect objects within 2'' of the star that are 10^{4-5} times fainter. Correlated speckle noise is one of the biggest obstacles limiting such high-contrast imaging. We have obtained a series of 284 8 s, AO-corrected, coronagraphically occulted H-band images of the star Vega at the 3.63 m AEOS telescope located on Haleakala, Hawaii. This dataset is unique for studying the temporal behavior of speckle noise, and represents the first time such a study on highly corrected _coronagraphic_ AO images has been carried out in a quantitative way. We find the speckle pattern to be highly stable in both position and time in our data. This is due to the fact that the AO system corrects disturbances to the stellar wave front at the level where the instrumental wave front errors dominate. Because of this, we find that our detection limit is not significantly improved simply with increased exposure time alone. However, we are able to improve our dynamic range by 1.5-2 magnitudes through subtraction of static/quasi-static speckles in two rotating frames: the telescope pupil frame and the deformable mirror frame. Furthermore, from our data, we are able to constrain the mass of any purported companion to Vega to be less than 45 M_J at 8 AU and less than 30 M_J at 16 AU, radii not previously probed at these sensitivities.

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arXiv2006

Astrometry and Photometry with Coronagraphs

We propose a solution to the problem of astrometric and photometric calibration of coronagraphic images with a simple optical device which, in theory, is easy to use. Our design uses the Fraunhofer approximation of Fourier optics. Placing a periodic grid of wires (we use a square grid) with known width and spacing in a pupil plane in front of the occulting coronagraphic focal plane mask produces fiducial images of the obscured star at known locations relative to the star. We also derive the intensity of these fiducial images in the coronagraphic image. These calibrator images can be used for precise relative astrometry, to establish companionship of other objects in the field of view through measurement of common proper motion or common parallax, to determine orbits, and to observe disk structure around the star quantitatively. The calibrator spots also have known brightness, selectable by the coronagraph designer, permitting accurate relative photometry in the coronagraphic image. This technique, which enables precision exoplanetary science, is relevant to future coronagraphic instruments, and is particularly useful for `extreme' adaptive optics and space-based coronagraphy.

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arXiv2005

Low Order Aberrations in Band-Limited Lyot Coronagraphs

We study the way Lyot coronagraphs with unapodized entrance pupils respond to small, low order phase aberrations. This study is applicable to ground-based adaptive optics coronagraphs operating at 90% and higher Strehl ratios, as well as to some space-based coronagraphs with intrinsically higher Strehl ratio imaging. We utilize a second order expansion of the monochromatic point-spread function (written as a power spectrum of a power series in the phase aberration over clear aperture) to derive analytical expressions for the response of a `band-limited' Lyot coronagraph (BLC) to small, low order, phase aberrations. The BLC possesses a focal plane mask with an occulting spot whose opacity profile is a spatially band-limited function rather than a hard-edged, opaque disk. The BLC is, to first order, insensitive to tilt and astigmatism. Undersizing the stop in the re-imaged pupil plane (the Lyot plane) following the focal plane mask can alleviate second order effects of astigmatism, at the expense of system throughput and angular resolution. The optimal degree of such undersizing depends on individual instrument designs and goals. Our analytical work engenders physical insight, and complements existing numerical work on this subject. Our methods can be extended to treat the passage of higher order aberrations through band-limited Lyot coronagraphs, by using our polynomial decomposition or an analogous Fourier approach.

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arXiv2003

Cool White Dwarfs Revisited -- New Spectroscopy and Photometry

In this paper we present new and improved data on 38 cool white dwarfs identified by Oppenheimer et al. 2001 (OHDHS) as candidate dark halo objects. Using the high-res spectra obtained with LRIS, we measure radial velocities for 13 WDs that show an H alpha line. We show that the knowledge of RVs decreases the UV-plane velocities by only 6%. The radial velocity sample has a W-velocity dispersion of sig_W = 59 km/s--in between the values associated with the thick disk and the stellar halo. We also see indications for the presence of two populations by analyzing the velocities in the UV plane. In addition, we present CCD photometry for half of the sample, and with it recalibrate the photographic photometry of the remaining WDs. Using the new photometry in standard bands, and by applying the appropriate color-magnitude relations for H and He atmospheres, we obtain new distance estimates. New distances of the WDs that were not originally selected as halo candidates yield 13 new candidates. On average, new distances produce velocities in the UV plane that are larger by 10%, with already fast objects gaining more. Using the new data, while applying the same UV-velocity cut (94 km/s) as in OHDHS, we find a density of cool WDs of 1.7e-4 pc^-3, confirming the value of OHDHS. In addition, we derive the density as a function of the UV-velocity cutoff. The density (corrected for losses due to higher UV cuts) starts to flatten out at 150 km/s (0.4e-4 pc^-3), and is minimized (thus minimizing a possible non-halo contamination) at 190 km/s (0.3e-4 pc^-3). These densities are in a rough agreement with the estimates for the stellar halo WDs, corresponding to a factor of 1.9 and 1.4 higher values.

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arXiv2003

Polarization Effects in Reflecting Coronagraphs for White Light Applications in Astronomy

The properties of metal thin films have been largely overlooked in discussions of the technical limitations and problems that arise in the field of direct detection of exoplanets. Here, polarization properties and anisotropy properties of highly reflecting thin metal films are examined within the context of the requirements for the ultra-low scattered-light system performance of coronagraphs applied to space and ground-based high-contrast, white-light astronomy. Wavelength-dependent optical constants for highly reflecting thin metal films, taken from the literature are used to calculate the polarization-dependent transmissivity of a typical coronagraph. The effects of degraded performance on the astronomical science are examined. Suggestions are made for future work.

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arXiv2003

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.

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arXiv2002

The Potential of Differential Astrometric Interferometry from the High Antarctic Plateau

The low infrared background and high atmospheric transparency are the principal advantages of Antarctic Plateau sites for astronomy. However, the poor seeing (between one and three arcseconds) negates much of the sensitivity improvements that the Antarctic atmosphere offers, compared to mid-latitude sites such as Mauna Kea or Cerro Paranal. The seeing at mid-latitude sites, though smaller in amplitude, is dominated by turbulence at altitudes of 10 to 20 km. Over the Antarctic plateau, virtually no high altitude turbulence is present in the winter. The mean square error for an astrometric measurement with a dual-beam, differential astrometric interferometer in the very narrow angle regime is proportional to the integral of $h^2 C_N^2(h)$. Therefore, sites at which the turbulence occurs only at low altitudes offer large gains in astrometric precision. We show that a modest interferometer at the South Pole can achieve 10 $μ$as differential astrometry 300 times faster than a comparable interferometer at a good mid-latitude site, in median conditions. Science programs that would benefit from such an instrument include planet detection and orbit determination and astrometric observation of stars microlensed by dark matter candidates.

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arXiv1998

Brown Dwarfs

After a discussion of the physical processes in brown dwarfs, we present a complete, precise definition of brown dwarfs and of planets inspired by the internal physics of objects between 0.1 and 0.001 M_sun. We discuss observational techniques for characterizing low-luminosity objects as brown dwarfs, including the use of the lithium test and cooling curves. A brief history of the search for brown dwarfs leads to a detailed review of known isolated brown dwarfs with emphasis on those in the Pleiades star cluster. We also discuss brown dwarf companions to nearby stars, paying particular attention to Gliese 229B, the only known cool brown dwarf.

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