Hubble Space Telescope Observations of the Oldest Star Clusters in the LMC

We present V, V-I color-magnitude diagrams (CMDs) for three old star clusters in the Large Magellanic Cloud (LMC): NGC 1466, NGC 2257 and Hodge 11. Our data extend about 3 magnitudes below the main-sequence turnoff, allowing us to determine accurate relative ages and the blue straggler frequencies. Based on a differential comparison of the CMDs, any age difference between the three LMC clusters is less than 1.5 Gyr. Comparing their CMDs to those of M 92 and M 3, the LMC clusters, unless their published metallicities are significantly in error, are the same age as the old Galactic globulars. The similar ages to Galactic globulars are shown to be consistent with hierarchial clustering models of galaxy formation. The blue straggler frequencies are also similar to those of Galactic globular clusters. We derive a true distance modulus to the LMC of (m-M)=18.46 +/- 0.09 (assuming (m-M)=14.61 for M 92) using these three LMC clusters.Comment: 22 pages; to be published in Ap

A two-dimensional Kolmogorov-Smirnov test for crowded field source detection: ROSAT sources in NGC 6397

We present a two-dimensional version of the classical one-dimensional Kolmogorov-Smirnov (K-S) test, extending an earlier idea due to Peacock (1983) and an implementation proposed by Fasano & Franceschini (1987). The two-dimensional K-S test is used to optimise the goodness of fit in an iterative source-detection scheme for astronomical images. The method is applied to a ROSAT/HRI x-ray image of the post core-collapse globular cluster NGC 6397 to determine the most probable source distribution in the cluster core. Comparisons to other widely-used source detection methods, and to a Chandra image of the same field, show that our iteration scheme is superior in measuring statistics-limited sources in severely crowded fields.Comment: 12 pages, 6 figures, 6 tables. Accepted by MNRA

Deep HST Imaging in NGC 6397: Stellar Dynamics

Multi-epoch observations with ACS on HST provide a unique and comprehensive probe of stellar dynamics within NGC 6397. We are able to confront analytic models of the globular cluster with the observed stellar proper motions. The measured proper motions probe well along the main sequence from 0.8 to below 0.1 M$_\odot$ as well as white dwarfs younger than one gigayear. The observed field lies just beyond the half-light radius where standard models of globular cluster dynamics (e.g. based on a lowered Maxwellian phase-space distribution) make very robust predictions for the stellar proper motions as a function of mass. The observed proper motions show no evidence for anisotropy in the velocity distribution; furthermore, the observations agree in detail with a straightforward model of the stellar distribution function. We do not find any evidence that the young white dwarfs have received a natal kick in contradiction with earlier results. Using the observed proper motions of the main-sequence stars, we obtain a kinematic estimate of the distance to NGC 6397 of $2.2^{+0.5}_{-0.7}$ kpc and a mass of the cluster of $1.1 \pm 0.1 \times 10^5 \mathrm{M}_\odot$ at the photometric distance of 2.53 kpc. One of the main-sequence stars appears to travel on a trajectory that will escape the cluster, yielding an estimate of the evaporation timescale, over which the number of stars in the cluster decreases by a factor of e, of about 3 Gyr. The proper motions of the youngest white dwarfs appear to resemble those of the most massive main-sequence stars, providing the first direct constraint on the relaxation time of the stars in a globular cluster of greater than or about 0.7 Gyr.Comment: 25 pages, 20 figures, accepted for publication in Astrophysical Journa

Oscillations in β Ursae Minoris

Aims. From observations of the K4III star β UMi we attempt to determine whether oscillations or any other form of variability is present. Methods. A high-quality photometric time series of ≈1000 days in length obtained from the SMEI instrument on the Coriolis satellite is analysed. Various statistical tests were performed to determine the significance of features seen in the power density spectrum of the light curve. Results. Two oscillations with frequencies 2.44 and 2.92 μHz have been identified. We interpret these oscillations as consecutive overtones of an acoustic spectrum, implying a large frequency spacing of 0.48 μHz. Using derived asteroseismic parameters in combination with known astrophysical parameters, we estimate the mass of β UMi to be 1.3 ± 0.3 M. Peaks of the oscillations in the power density spectrum show width, implying that modes are stochastically excited and damped by convection. The mode lifetime is estimated at 18 ± 9 days

Cassini: Mission to Saturn and Titan

The Cassini Mission to Saturn and Titan represents an important step into the exploration of the outerplanets. It will expand on the flyby encounters of Pioneer and Voyager and parallel the detailed exploration of the Jupiter system to be accomplished by the Galileo Mission. By continuing the study of the two giant planets and enabling detailed comparisons of their structure and behavior, Cassini will provide a tremendous insight into the formation and evolution of the solar system. In addition, by virtue of its focus on the Saturnian satellite Titan, Cassini will return detailed data on an environment whose atmospheric chemistry may resemble that of the primitive Earth. The scientific objectives can be divided into five categories: Titan, Saturn, rings, icy satellites, and magnetospheres. The key area of interest to exobiologists is Titan; the other four scientific categories will be discussed briefly to provide a comprehensive overview of the Cassini Mission

Structure and Dynamics of the Globular Cluster Palomar 13

We present Keck/DEIMOS spectroscopy and Canada-France-Hawaii Telescope/MegaCam photometry for the Milky Way globular cluster Palomar 13. We triple the number of spectroscopically confirmed members, including many repeat velocity measurements. Palomar 13 is the only known globular cluster with possible evidence for dark matter, based on a Keck/High Resolution Echelle Spectrometer 21 star velocity dispersion of σ = 2.2 ± 0.4 km s^(–1). We reproduce this measurement, but demonstrate that it is inflated by unresolved binary stars. For our sample of 61 stars, the velocity dispersion is σ = 0.7^(+0.6)_(–0.5) km s^(–1). Combining our DEIMOS data with literature values, our final velocity dispersion is σ = 0.4^(+0.4)_( –0.3) km s^(–1). We determine a spectroscopic metallicity of [Fe/H] = –1.6 ± 0.1 dex, placing a 1σ upper limit of σ_([Fe/H]) ~ 0.2 dex on any internal metallicity spread. We determine Palomar 13's total luminosity to be M_V = –2.8 ± 0.4, making it among the least luminous known globular clusters. The photometric isophotes are regular out to the half-light radius and mildly irregular outside this radius. The outer surface brightness profile slope is shallower than typical globular clusters (Σ α r^η, η = –2.8 ± 0.3). Thus at large radius, tidal debris is likely affecting the appearance of Palomar 13. Combining our luminosity with the intrinsic velocity dispersion, we find a dynamical mass of M_(1/2) = 1.3^(+2:7)_(–1.3) × 10^3 M_☉ and a mass-to-light ratio of M/L_V = 2.4^(+5.0)_(–2.4) M_☉/L_☉. Within our measurement errors, the mass-to-light ratio agrees with the theoretical predictions for a single stellar population. We conclude that, while there is some evidence for tidal stripping at large radius, the dynamical mass of Palomar 13 is consistent with its stellar mass and neither significant dark matter, nor extreme tidal heating, is required to explain the cluster dynamics

An Empirical Measure of the Rate of White Dwarf Cooling in 47 Tucanae

We present an empirical determination of the white dwarf cooling sequence in the globular cluster 47 Tucanae. Using spectral models, we determine temperatures for 887 objects from Wide Field Camera 3 data, as well as 292 objects from data taken with the Advanced Camera for Surveys. We make the assumption that the rate of white dwarf formation in the cluster is constant. Stellar evolution models are then used to determine the rate at which objects are leaving the main sequence, which must be the same as the rate at which objects are arriving on the white dwarf sequence in our field. The result is an empirically derived relation between temperature ($T_{eff}$) and time ($t$) on the white dwarf cooling sequence. Comparing this result to theoretical cooling models, we find general agreement with the expected slopes between 20,000K and 30,000K and between 6,000K and 20,000K, but the transition to the Mestel cooling rate of $T_{eff} \propto t^{-0.4}$ is found to occur at hotter temperatures, and more abruptly than is predicted by any of these models.Comment: 10 pages, 16 figures, accepted for publication in Ap

Optimization of Turbine Rim Seals

Experiments are being conducted to gain an understanding of the physics of rim scale cavity ingestion in a turbine stage with the high-work, single-stage characteristics envisioned for Advanced Subsonic Transport (AST) aircraft gas turbine engines fo the early 21st century. Initial experimental measurements to be presented include time-averaged turbine rim cavity and main gas path static pressure measurements for rim seal coolant to main gas path mass flow ratios between 0 and 0.02. The ultimate objective of this work is develop improved rim seal design concepts for use in modern high-work, single sage turbines n order to minimize the use of secondary coolant flow. Toward this objective the time averaged and unsteady data to be obtained in these experiments will be used to 1) Quantify the impact of the rim cavity cooling air on the ingestion process. 2) Quantify the film cooling benefits of the rim cavity purge flow in the main gas path. 3) Quantify the impact of the cooling air on turbine efficiency. 4) Develop/evaluate both 3D CFD and analytical models of the ingestion/cooling process