Christopher Russell Stagg

A Lack of Planets in 47 Tucanae from a Hubble Space Telescope Search

We report results from a large Hubble Space Telescope project to observe a significant (~34,000) ensemble of main-sequence stars in the globular cluster 47 Tucanae with a goal of defining the frequency of inner orbit, gas giant planets. Simulations based on the characteristics of the 8.3 days of time series data in the F555W and F814W Wide Field Planetary Camera 2 (WFPC2) filters show that ~17 planets should be detected by photometric transit signals if the frequency of hot Jupiters found in the solar neighborhood is assumed to hold for 47 Tuc. The experiment provided high-quality data sufficient to detect planets. A full analysis of these WFPC2 data reveals ~75 variables, but no light curves resulted for which a convincing interpretation as a planet could be made. The planet frequency in 47 Tuc is at least an order of magnitude below that for the solar neighborhood. The cause of the absence of close-in planets in 47 Tuc is not yet known; presumably the low metallicity and/or crowding of 47 Tuc interfered with planet formation, with orbital evolution to close-in positions, or with planet survival.

Modeling of the eclipsing binaries in the globular cluster NGC 5466

The discovery of three eclipsing binaries in the globular cluster NGC 5466 (Mateo M. et al., 1990, AJ, 100) is a boon to studies of both this ancient (18 Gy) cluster and close binaries. Two of the systems discovered by Mateo et al. appear to be contact systems and one appears to be a short-period Algol system. Basic parameters, especially the masses, of the stars in these systems are urgently required for the evaluation of the binary merger hypothesis for the origin of blue stragglers, and as a test of cluster evolution models. Here we present the results of a study of viable models of these interesting systems, based on a simplex-enhanced, Kurucz-atmospheres version of the Wilson-Devinney Program

The origin of short-period comets

Abstract Recent work has shown that if short-period comets predominantly occur as a result of planetary “captures” of long-period comets, the relevant source flux of nearly parabolic orbits must involve comets with perihelia in the range 5–30 AU, significantly beyond the region in which the flux is observationally constrained. The numbers and orbital parameters of short-period comets therefore in principle provide strong constraints on those theories of cometary origin which predict the existence of a massive inner core to the Oort cloud. Short-period comets now play a role in relation to the inner core as important as that formerly played by the observed near-parabolic flux in relation to the Oort cloud beyond r ⋍ 2 × 10 4 AU .

Improvements to the Wilson-Devinney Code on Computer Platforms at the University of Calgary

We describe improvements in light curve modeling over the past five years at the University of Calgary, and the current status of the effort. The work has concentrated on the atmospheres option in the Wilson-Devinney program, the uniqueness determination question, and code portability. We also describe a number of useful tools to facilitate adjustments and the examination of results and consider areas for further development.

HST Photometry of 47 Tucanae: Time Series Analysis and Search for Giant Planets

Gilliland et al. (2000) have reported HST photometric observations of 34000 stars in the globular cluster 47 Tuc, showing an absence of close-in giant planets in that cluster relative to their frequency in the solar neighborhood. Here we describe the methods of time-series analysis that were used to search the 47 Tuc data for transits by giant extrasolar planets, and the means by which these methods were validated.

Simulation and Modeling of Transit Eclipses by Planets

Light curve analysis by MDW of the photometry and RV data accumulated to date on HD 209458 has made use of a simulations database created for an 8-day HST observing project led by RLG to look for transits in 47 Tuc, We report progress in developing a consistent set of parameters obtained with our versions of the Wilson-Devinney program, WD98 and wd98k93, specially

Working Group on Infrared Astronomy: (Groupe de Travail Pour L’Astronomie Infrarouge)

In 1988, a Joint Commission (9 and 25) meeting on the causes of the well-known limitations on the precision of infrared astronomy led to several suggestions to improve matters (see Milone 1989). These included better reporting of the photometric systems in use by practitioners, redesign of the infrared passbands to be more optimally placed inside the atmospheric windows, and development of a method to ascertain the water vapor content of the atmosphere when the astronomical infrared measurements were being made. An Infrared Astronomy Working Group was formed to look into the matter. Advice and suggestions were solicited from the community at large. All who volunteered information became, de facto, members of the Working Group. A small subgroup composed of Andrew Young, Chris Stagg, and Milone set to work on the central of the recommendations: improvement of the passbands. Young, Milone, k Stagg (1994) (hereafter YMS) summarized the work: existing JHKLMN and Q infrared passbands were found to be both far from standardized, and all too frequently defined, to various degrees, by the water vapor and other components of the terrestrial atmosphere. Following extensive numerical simulations with a MODTRAN 3 terrestrial-atmospheres model package, and Kurucz stellar atmospheres, we suggested a set of improved infrared passbands designed explicitly to fit within, and not be defined by, the terrestrial atmospheric windows; however, we sought to optimize them so as to get the maximum throughput consistent with plausible limitations on precision of manufacture of the filters. In 1995 and again in 1997, a number of improvements were made in the code with which the improved passbands were designed. While they do not much affect the optimization trials and thus the passband recommendations, they have been used to extend the modeling.

G-Dwarf Stars in the Galactic Halo

Since G-dwarf stars are unambiguous, long-lived probes of Galactic evolution, we are extending an earlier, spectroscopic study of G-dwarf stars in the Galactic Pole regions (Corbally and Garrison, 1988a, 1988b; Garrison and Corbally, 1993, ‘GC’). Currently, we are in the process of selecting candidate G-dwarf stars from UBVRI photometry in fields at the North Galactic Pole, while again relying on the Hertzsprung gap to filter out giant stars.

New developments in eclipsing binary light curve modeling

The light curve modeling of binary stars has continued to evolve since its founding by Henry Norris Russell (see Russell and Merrill 1952 and citations therein) nearly a century ago, accelerated in the 1950s by Kopals introduction of Roche geometry into models and by the development of synthetic light curve computer code in the 1970s. Improved physics and the use of more kinds of observational input are providing another round of important advances that promise to enlarge our knowledge of both binary stars and ensembles containing them. Here we discuss the newer horizons of light curve modeling and the steps being taken toward them.

On Improving IR Photometric Passbands

In response to recommendations made by a Working Group on Infrared Extinction and Standardisation of IAU Commission 25, we have compared the past and present versions of the passbands in the Johnson JHKLMNQ broadband photometric system used at a number of observatories with the atmospheric window transmissions calculated by MODTRAN. The existing passbands are very diverse; contrary to a widespread misperception, there is no semblance of a standard set in use. We have used a family of solar-composition model stellar fluxes from Kurucz (1991 private communication) to model the atmospheric extinction under different water-vapor, height, and airmass conditions. Thus, we have simulated extinction curves for the infrared passbands used at several observatories