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J. R. P. Angel

Publications and source records attributed to J. R. P. Angel.

8 recordsLinked to original sources
arXiv2006

Thermal Infrared Constraint to a Planetary Companion of Vega with the MMT Adaptive Optics System

Vega may have a massive companion in a wide orbit, as evidenced by structure in its cold dust debris. We have tested this hypothesis by direct imaging with adaptive optics in the M band. The observations were made with a newly commissioned thermal infrared camera, Clio, on the 6.5 MMT AO system with low-background deformable secondary. The observations constrain a planet to be less than 7 M$_J$ at the approximate position angle expected from the dust structure and at a radius $>$ 20AU (2.5 arcsec) . This result is more stringent than similar previous near-infrared observations of Vega, that achieve limits of 20 and 10 M$_J$ at separations of 7 arcsec. The higher sensitivity is due both to the more favorable contrast of gas giant planets at M band and to the higher Strehl and more stable point spread function at longer wavelengths. Future L' or M band observations could provide a powerful approach for wide separation planet detection, especially for cooler, and thus older or less massive planets. The natural best targets are nearby stars where planets in the range of 5-15 M$_J$ and as old as several Gyr are expected to be detectable with this technique.

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arXiv2005

First tests of wavefront sensing with a constellation of laser guide beacons

Adaptive optics to correct current telescopes over wide fields, or future very large telescopes over even narrow fields, will require real-time wavefront measurements made with a constellation of laser beacons. Here we report the first such measurements, made at the 6.5 m MMT with five Rayleigh beacons in a 2 arcmin pentagon. Each beacon is made with a pulsed beam at 532 nm, of 4 W at the exit pupil of the projector. The return is range-gated from 20-29 km and recorded at 53 Hz by a 36-element Shack-Hartmann sensor. Wavefronts derived from the beacons are compared with simultaneous wavefronts obtained for individual natural stars within or near the constellation. Observations were made in seeing averaging 1.0 arcsec with 2/3 of the aberration measured to be from a ground layer of mean height 380 m. Under these conditions, subtraction of the simple instantaneous average of the five beacon wavefronts from the stellar wavefronts yielded a 40% rms reduction in the measured modes of the distortion over a 2 arcmin field. We discuss the use of multiple Rayleigh beacons as an alternative to single sodium beacons on 8 m telescopes, and the impact of the new work on the design of a multi-sodium beacon system for the 25 m Giant Magellan Telescope.

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arXiv2004

Adaptive Optics Nulling Interferometric Constraints on the Mid-Infrared Exozodiacal Dust Emission around Vega

We present the results of mid-infrared nulling interferometric observations of the main-sequence star alpha Lyr (Vega) using the 6.5 m MMT with its adaptive secondary mirror. From the observations at 10.6 microns, we find that there is no resolved emission from the circumstellar environment (at separations greater than 0.8 AU) above 2.1% (3 sigma limit) of the level of the stellar photospheric emission. Thus, we are able to place an upper limit on the density of dust in the inner system of 650 times that of our own solar system's zodiacal cloud. This limit is roughly 2.8 times better than those determined with photometric excess observations such as those by IRAS. Comparison with far-infrared observations by IRAS shows that the density of warm dust in the inner system (< 30 AU) is significantly lower than cold dust at larger separations. We consider two scenarios for grain removal, the sublimation of ice grains and the presence of a planetary mass "sweeper." We find that if sublimation of ice grains is the only removal process, a large fraction (> 80%) of the material in the outer system is ice.

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arXiv2002

An Optical Ultra-High-Resolution Cross-dispersed Echelle Spectrograph with Adaptive Optics

A prototype cross-dispersed optical echelle spectrograph of very high resolution has been designed and built at Steward Observatory and tested at the Starfire Optical Range (SOR) 1.5 m telescope. It is the first spectrograph to take advantage of diffraction-limited images provided by adaptive optics in order to achieve a potential resolving power of $R \sim 600,000$. The wavelength coverage in a single exposure is about 300 Å, which is approximately 100 times that of conventional spectrographs operating at comparable resolution. This was achieved by recording 60 cross-dispersed orders across the $18\times 18$ mm$^2$ area of the CCD detector. The total efficiency of the system, including the sky and telescope transmission, spectrograph and CCD detector, is measured to be 1.3% at peak, much higher than that of other ultra-high-resolution spectrographs. Sample stellar spectra with $R \sim 250,000$ are presented.

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arXiv2000

The Dark Matter Telescope

Weak gravitational lensing enables direct reconstruction of dark matter maps over cosmologically significant volumes. This research is currently telescope-limited. The Dark Matter Telescope (DMT) is a proposed 8.4 m telescope with a 3 degree field of view, with an etendue of 260 $(m. degree)^2$, ten times greater than any other current or planned telescope. With its large etendue and dedicated observational mode, the DMT fills a nearly unexplored region of parameter space and enables projects that would take decades on current facilities. The DMT will be able to reach 10-sigma limiting magnitudes of 27-28 magnitude in the wavelength range .3 - 1 um over a 7 square degree field in 3 nights of dark time. Here we review its unique weak lensing cosmology capabilities and the design that enables those capabilities.

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arXiv1998

The range of masses and periods explored by radial velocity searches for planetary companions

In this paper we develop an analytical expression relating the sensitivity to a periodic signal to the duration and accuracy of a given set of data. The effects of windowing are explored, and also the sensitivity to periods longer than the total length of observations. As specific examples, we outline the performance of our technique on large amplitude and large eccentricity radial velocity signals recently discussed in the literature and we delineate the region explored by the measurements of 14 single stars made over a twelve year period by Walker et al. (1995). Had any of these stars shown motion like that caused by the exo-planets recently detected, it would have been easily detected. Finally, we suggest an observing strategy for future large radial velocity surveys which, if implemented, will allow coverage of the largest range of parameter space with the smallest amount of observing time per star. We suggest that about 10-15 measurements be made of each star in the first two years of the survey, then 2--3 measurements per year thereafter, provided no (or slow) variation is observed. More frequent observations would of course be indicated if such variations were present.

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arXiv1997

Mesosphere Sodium Column Density and the Sodium Laser Guide Star Brightness

The first time simultaneous measurements of sodium column density and the absolute flux from a sodium laser guide star, created by a monochromatic 3 W cw laser, tuned to the peak of the sodium D2 hyperfine structure, were conducted at the MMT and CFA 60 inch telescope in 1997. The results show that linearly and circularly polarized laser returns are proportional to the simultaneous sodium column density. Moreover, circularly polarized laser provides about 30% increase in fluorescent return over linearly polarized laser. A laser guide star with R = 10.3 mag. or absolute flux of 8.4x10^5 photons/s/m^2, could be formed from a 1 watt projected circularly polarized sodium laser beam when sodium layer abundance N(Na) = 3.7x10^9 /cm^2. Together with the distributed column density measurements (e.g. seasonal and diurnal variations), we can project laser power requirements for any specified guide star brightness. The mesosphere sodium column density variation was measured above Tucson sky throughout the year, through sodium absorption line measurements in stellar and solar spectra. Previous measurements, e.g. Papen et al, 1996, have not been made at this latitude (32 degrees). Further, our absorption method is more direct and may be more accurate than the lidar methods normally used. The seasonal variation amplitude is smaller than that at higher latitudes. While the annual mean sodium column density tends to be lower than at higher latitudes. Diurnal sodium column density tends to vary by as much as a factor of two within an hour.

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