Resolving the Controversy Over the Core Radius of 47 Tucanae (NGC 104)

This paper investigates the discrepancy between recent measurements of the density profile of the globular cluster 47 Tuc that have used HST data sets. Guhathakurta et al. (1992) used pre-refurbishment WFPC1 V-band images to derive r_c = 23" +/- 2". Calzetti et al. (1993) suggested that the density profile is a superposition of two King profiles (r_c = 8" and r_c = 25") based on U-band FOC images. De Marchi et al. (1996) used deep WFPC1 U-band images to derive r_c = 12" +/- 2". Differences in the adopted cluster centers are not the cause of the discrepancy. Our independent analysis of the data used by De Marchi et al. reaches the following conclusions: (1) De Marchi et al.'s r_c ~ 12" value is spuriously low, a result of radially-varying bias in the star counts in a magnitude limited sample -- photometric errors and a steeply rising stellar luminosity function cause more stars to scatter across the limiting magnitude into the sample than out of it, especially near the cluster center where crowding effects are most severe. (2) Changing the limiting magnitude to the main sequence turnoff, away from the steep part of the luminosity function, partially alleviates the problem and results in r_c = 18". (3) Combining such a limiting magnitude with accurate photometry derived from PSF fitting, instead of the less accurate aperture photometry employed by De Marchi et al., results in a reliable measurement of the density profile which is well fit by r_c = 22" +/- 2". Archival WFPC2 data are used to derive a star list with a higher degree of completeness, greater photometric accuracy, and wider areal coverage than the WFPC1 and FOC data sets; the WFPC2-based density profile supports the above conclusions, yielding r_c = 24" +/- 1.9".Comment: 22 pages, 5 figures, 1 table; accepted for publication in PASP; see http://www.ucolick.org/~raja/hgg.tar.gz for full-resolution figure

HST Fine Guidance Sensor Astrometric Parallaxes for Three Dwarf Novae: SS Aurigae, SS Cygni, and U Geminorum

We report astrometric parallaxes for three well known dwarf novae obtained using the Fine Guidance Sensors on the Hubble Space Telescope. We found a parallax for SS Aurigae of Pi = 5.00 +/- 0.64 mas, for SS Cygni we found Pi = 6.02 +/- 0.46 mas, and for U Geminorum we obtained Pi = 10.37 +/- 0.50 mas. These represent the first true trigonometric parallaxes of any dwarf novae. We briefly compare these results with previous distance estimates. This program demonstrates that with a very modest amount of HST observing time, the Fine Guidance Sensors can deliver parallaxes of unrivaled precision.Comment: 15 pages, 2 Table

Detection of an Extrasolar Planet Atmosphere

We report high precision spectrophotometric observations of four planetary transits of HD 209458, in the region of the sodium resonance doublet at 589.3 nm. We find that the photometric dimming during transit in a bandpass centered on the sodium feature is deeper by (2.32 +/- 0.57) x 10^{-4} relative to simultaneous observations of the transit in adjacent bands. We interpret this additional dimming as absorption from sodium in the planetary atmosphere, as recently predicted from several theoretical modeling efforts. Our model for a cloudless planetary atmosphere with a solar abundance of sodium in atomic form predicts more sodium absorption than we observe. There are several possibilities that may account for this reduced amplitude, including reaction of atomic sodium into molecular gases and/or condensates, photoionization of sodium by the stellar flux, a low primordial abundance of sodium, or the presence of clouds high in the atmosphere.Comment: 26 pages, 8 figures, accepted by ApJ 2001 November 1

Kepler Mission Stellar and Instrument Noise Properties

Kepler Mission results are rapidly contributing to fundamentally new discoveries in both the exoplanet and asteroseismology fields. The data returned from Kepler are unique in terms of the number of stars observed, precision of photometry for time series observations, and the temporal extent of high duty cycle observations. As the first mission to provide extensive time series measurements on thousands of stars over months to years at a level hitherto possible only for the Sun, the results from Kepler will vastly increase our knowledge of stellar variability for quiet solar-type stars. Here we report on the stellar noise inferred on the timescale of a few hours of most interest for detection of exoplanets via transits. By design the data from moderately bright Kepler stars are expected to have roughly comparable levels of noise intrinsic to the stars and arising from a combination of fundamental limitations such as Poisson statistics and any instrument noise. The noise levels attained by Kepler on-orbit exceed by some 50% the target levels for solar-type, quiet stars. We provide a decomposition of observed noise for an ensemble of 12th magnitude stars arising from fundamental terms (Poisson and readout noise), added noise due to the instrument and that intrinsic to the stars. The largest factor in the modestly higher than anticipated noise follows from intrinsic stellar noise. We show that using stellar parameters from galactic stellar synthesis models, and projections to stellar rotation, activity and hence noise levels reproduces the primary intrinsic stellar noise features.Comment: Accepted by ApJ; 26 pages, 20 figure

The Kepler Pixel Response Function

Kepler seeks to detect sequences of transits of Earth-size exoplanets orbiting Solar-like stars. Such transit signals are on the order of 100 ppm. The high photometric precision demanded by Kepler requires detailed knowledge of how the Kepler pixels respond to starlight during a nominal observation. This information is provided by the Kepler pixel response function (PRF), defined as the composite of Kepler's optical point spread function, integrated spacecraft pointing jitter during a nominal cadence and other systematic effects. To provide sub-pixel resolution, the PRF is represented as a piecewise-continuous polynomial on a sub-pixel mesh. This continuous representation allows the prediction of a star's flux value on any pixel given the star's pixel position. The advantages and difficulties of this polynomial representation are discussed, including characterization of spatial variation in the PRF and the smoothing of discontinuities between sub-pixel polynomial patches. On-orbit super-resolution measurements of the PRF across the Kepler field of view are described. Two uses of the PRF are presented: the selection of pixels for each star that maximizes the photometric signal to noise ratio for that star, and PRF-fitted centroids which provide robust and accurate stellar positions on the CCD, primarily used for attitude and plate scale tracking. Good knowledge of the PRF has been a critical component for the successful collection of high-precision photometry by Kepler.Comment: 10 pages, 5 figures, accepted by ApJ Letters. Version accepted for publication

Seasonal variation in hospital encounters with hypoglycaemia and hyperglycaemia

Aim To assess whether rates of hospital encounters with hypoglycaemia and hyperglycaemia display seasonal variation. Methods Time series analyses of the monthly rates of hospital encounters (emergency room visits or inpatient admissions) with hypoglycaemia and hyperglycaemia from 2003 to 2012 using linked healthcare databases in Ontario, Canada. Results Over the study period, there were 129 887 hypoglycaemia and 79 773 hyperglycaemia encounters. The characteristics of people at the time of their encounters were similar across the seasons in 2008 (median age 68 years for hypoglycaemia encounters and 53 years for hyperglycaemia encounters; 50% female; 90% with diabetes). We observed moderate seasonality in both types of encounters (R2 autoregression coefficient 0.58 for hypoglycaemia; 0.59 for hyperglycaemia). The rate of hypoglycaemia encounters appeared to peak between April and June, when on average, there was an additional 49 encounters per month (0.36 encounters per 100 000 persons per month) compared with the other calendar months (5% increase). The rate of hyperglycaemia encounters appeared to peak in January, when on average, there was an additional 69 encounters per month (0.50 encounters per 100 000 persons per month) compared with the other calendar months (11% increase). Conclusions In our region, there is seasonal variation in the rate of hospital encounters with hypoglycaemia and hyperglycaemia. Our findings may help to highlight periods of vulnerability for people, may inform future epidemiological studies and may aid in the appropriate planning of healthcare resources

Molting, Staging, and Wintering Locations of Common Eiders Breeding in the Gyrfalcon Archipelago, Ungava Bay

The northern common eider (Somateria mollissima borealis) has become a source of concern because of recent declines and possible overharvest. Birds that breed in the Canadian mid-Arctic winter in both Greenland and Canada, but the wintering grounds of birds that breed farther south are unknown. Our objectives were thus to identify the molting and wintering areas of birds breeding in Ungava Bay and to compare their home-range sizes during the pre-molt, molt, post-molt, and winter periods. Using satellite telemetry, we determined that common eiders breeding in the lower Arctic winter in Greenland (67%) and Canada (33%). These proportions are consistent with the split established for common eiders that breed farther north. Females spent their pre-molt period close to their breeding islands and had the smallest home ranges during the molt period. Winter home ranges were larger in Canada than in Greenland, probably because they contained more ice. Once settled, birds wintered in a relatively small area and did not undertake long movements. Distance between molt and winter sites ranged between 1054 and 2173 km. Duration of migration to or from wintering areas varied among individuals from rapid movement in a few days to slow progress over a month. Fall migration occurred in late October or early November, and spring migration, in early to mid-May. This study highlights the importance of Ungava Bay for northern common eiders and the need for international collaboration to ensure sustainable use of the resource.L’eider à duvet du nord (Somateria mollissima borealis) représente une source de préoccupation en raison de déclins récents et de récoltes susceptibles d’être excessives. Les oiseaux nichant dans la zone arctique moyenne canadienne hivernent au Groenland et au Canada, mais l’aire d’hivernage des oiseaux nichant plus au sud n’est pas connue. Nos objectifs consistaient donc à identifier les aires de mue et d’hivernage des oiseaux nichant dans la baie d’Ungava et de comparer leur aire d’activité lors des périodes de pré-mue, de mue, d’après-mue et d’hiver. Au moyen de la télémétrie satellitaire, nous avons déterminé que l’eider à duvet nichant dans la zone arctique inférieure hiverne au Groenland (67 %) et au Canada (33 %). Ces proportions sont similaires à celles trouvées dans les colonies d’eider à duvet plus nordiques. Les femelles passent la période de la pré-mue près de leurs îles de nidification et sont les plus restreintes en termes d’aire utilisée lors de la période de mue. Les aires d’activité hivernale étaient plus grandes au Canada qu’au Groenland, probablement en raison de la plus grande présence de glaces au Canada. Une fois sur leur site d’hivernage, les oiseaux évoluaient dans une aire relativement petite et n’entreprenaient pas de longs déplacements. La distance entre les sites de mue et d’hivernage variait entre 1 054 et 2 173 kilomètres. La durée de la migration variait d’un individu à l’autre, allant de déplacements rapides sur quelques jours à des déplacements plus lents sur près d’un mois. La migration automnale a débuté à la fin octobre, début novembre et celle du printemps vers le début et la mi-mai. Cette étude met en évidence la grande importance de la baie d’Ungava pour l’eider à duvet du nord de même que le besoin de collaboration internationale pour assurer l’utilisation soutenue de cette ressource