Diane Gilmore

What determines the morphological fractions in clusters of galaxies

A reexamination of Dresslers sample of nearly 6000 galaxies in 55 clusters shows that the morphology-clustercentric radius relation is more fundamental than the morphology-local density relation. This conclusion is supported by improved correlations when the clustercentric radius is used as the independent parameter, and by a comparison of galaxies with the same normalized clustercentric radii but different values of the local density. The morphology-radius relation, when normalized by a characteristic cluster radius, R c opt , does not vary as a function of the number density within 0.5 Mpc, N 0.5 , the X-ray luminosity, L X , or the velocity dispersion of the cluster, V disp

On the interpretation of the morphology-density relation for galaxies in clusters

A reexamination of Dresslers sample of nearly 6000 galaxies in 55 clusters shows that galactic morphology correlates as well with the projected distance from the center of the cluster, R(clus), as it does with the projected local density of the 10 nearest galaxies. Most of the variation occurs within 0.5 Mpc of the cluster center. The effect is strongest in the 34 clusters with D galaxies, with the percentage of ellipticals rising from 18 + or - 3 percent at R(clus) = 0.6 Mpc to 65 + or - 7 percent at R(clus) = 0.1 Mpc. Various correlations suggest that the physical mechanisms which result in the population gradients for the spiral and S0 galaxies are different than the mechanism responsible for the preponderance of ellipticals near the cluster centers. Tentative evidence is also found that the fraction of S0 galaxies declines sharply within 0.2 Mpc of the cluster center, especially around central D galaxies. 19 refs.

HD 98800: A unique stellar system of post-T tauri stars

HD 98800 is a system of four stars, and it has a large infrared excess that is thought to be due to a dust disk within the system. In this paper we present new astrometric observations made with Hipparcos, as well as photometry from Hubble Space Telescope WFPC2 images. Combining these observations and reanalyzing previous work allow us to estimate the age and masses of the stars in the system. Uncertainty in these ages and masses results from uncertainty in the temperatures of the stars and any reddening they may have. We find that HD 98800 is most probably about 10 Myr old, although it may be as young as 5 Myr or as old as 20 Myr old. The stars in HD 98800 appear to have metallicities that are about solar. An age of 10 Myr means that HD 98800 is a member of the post T Tauri class of objects, and we argue that the stars in HD 98800 can help us understand why post T Tauris have been so elusive, HD 98800 may have formed in the Centaurus star-forming region, but it is extraordinary in being so young and yet so far from where it was born.

Collision of comet Shoemaker-Levy 9 with Jupiter observed by the NASA infrared telescope facility

The National Aeronautics and Space Administration (NASA) Infrared Telescope Facility was used to investigate the collision of comet Shoemaker-Levy 9 with Jupiter from 12 July to 7 August 1994. Strong thermal infrared emission lasting several minutes was observed after the impacts of fragments C, G, and R. All impacts warmed the stratosphere and some the troposphere up to several degrees. The abundance of stratospheric ammonia increased by more than 50 times. Impact-related particles extended up to a level where the atmospheric pressure measured several millibars. The north polar near-infrared aurora brightened by nearly a factor of 5 a week after the impacts.

High-Precision Stellar Radial Velocities in the Galactic Center*

We present radial velocities for 85 cool stars projected onto the central parsec of the Galaxy. The majority of these velocities have relative errors of ~1 km s-1, or a factor of ~30-100 smaller than those previously obtained with proper-motion or other radial velocity measurements for a similar stellar sample. The error in a typical individual stellar velocity, including all sources of uncertainty, is 1.7 km s-1. Two similar data sets were obtained 1 month apart, and the total error in the relative velocities is 0.80 km s-1 in the case where an object is common to both data sets. The data are used to characterize the velocity distribution of the old population in the Galactic center. We find that the stars have a Gaussian velocity distribution with a mean heliocentric velocity of -10.1 ± 11.0 km s-1 (blueshifted) and a standard deviation of 100.9 ± 7.7 km s-1; the mean velocity of the sample is consistent with no bulk line-of-sight motion with respect to the local standard of rest. At the 1 σ level, the data are consistent with a symmetric velocity distribution about any arbitrary axis in the plane of the sky. We find evidence for a flattening in the distribution of late-type stars within a radius of ~0.4 pc and infer a volume density distribution of r-1/4 in this region. Finally, we establish a first epoch of radial velocity measurements that can be compared with subsequent epochs to measure small accelerations (1 km s-1 yr-1), corresponding to the magnitude expected over a time span of several years for stars nearest to Sgr A*.

Large-Scale Storms in Saturn's Atmosphere During 1994

Large-scale storms are rarely observed in Saturns atmosphere, but their appearance traces the wind velocity field, providing information on the vertical structure of the clouds and on the dynamics of the atmosphere. Two large-scale atmospheric disturbances formed by clouds highly reflective in the visible part of the spectrum were observed on Saturn during 1994. An equatorial disturbance with a longitudinal size of ∼27,000 kilometers drifted in longitude with a velocity of 273.6 meters per second. A second disturbance, a rapidly evolving convective storm with an initial size ∼7000 kilometers, was observed at 56 degrees south, moving with a zonal velocity of 15.5 meters per second.

Radio observations of the remnant of the supernova of A.D. 1006. I - Total intensity observations

VLA observations are presented of the supernova of A.D. 1006 at 1370 and 1665 MHz with 16 x 20 arcsec resolution. A circular shell whose peak brightness varies by a factor of about 11 with azimuth is seen, with two bright regions dominating the emission; these have extremely sharp outer edges. Several discrete filaments can be followed for substantial portions of the remnant diameter. 29 references.

Radio observations of the remnant of the supernova of AD 1006. II - Polarization observations

Polarized flux observations of the SNR of AD 1006 made with the NRAO VLA are presented. The gross morphology in polarized intensity reveals two bright limbs across from each other. The SE quadrant, faintest in total intensity, shows significant polarized flux, while the extension of the NE, fairly bright in total intensity, appears almost unpolarized. Significant polarized flux is also seen in the SNR center. An integrated polarized fraction of about 13 percent is found; ratios as high as 30 percent may occur locally. The observations can be used to infer an intrinsic polarization position angle, and it is deduced that the ordered component of the magnetic field is primarily radial. The polarized fraction upper limits imply that the magnetic field is dominantly disordered, and it is estimated that the radial component contains only about 20 percent of the total magnetic energy density.

Space Telescope Imaging Spectrograph Ultraviolet Spectra of Large Magellanic Cloud Planetary Nebulae: A Study of Carbon Abundances and Stellar Evolution*

We acquired spectra of 24 LMC planetary nebulae (PNs) in the 1150-3000 A range in order to determine carbon and other ionic abundances. The sample more than doubles the number of LMC PNs with high-quality UV spectra in this wavelength range and whose optical images are available in the Hubble Space Telescope archive. The Space Telescope Imaging Spectrograph was used with a very large aperture to obtain virtually slitless spectra; thus, the monochromatic images in the major nebula emission lines are also available. The analysis of the data shows extremely high quality spectra. This paper presents the emission lines identified and measured and the calculation of the ionic abundances of the emitting carbon and other ions, as well as total carbon abundance. P Cygni profiles have been found in a fraction of the nebulae, and the limiting velocities of the stellar winds estimated. The total carbon abundance can be inferred reliably in most nebulae. We found that the average carbon abundance in round and elliptical PNs is one order of magnitude larger than that of the bipolar PNs, while elliptical and round PNs with a bipolar core have a bimodal behavior. This results confirm that bipolarity in LMC PNs is tightly correlated with high-mass progenitors. When compared with predicted yields, we found that the observed abundance ratio shows a shift toward higher carbon abundances, which may be due to initial conditions assumed in the models not appropriate for LMC PNs.

THE SPEED AND ORIENTATION OF THE PARSEC-SCALE JET IN 3C 279

A high degree of relativistic beaming is inferred for the jets of blazars on the basis of several lines of evidence, but the intrinsic speed and angle of the jet to the line of sight for individual sources are difficult to measure. We have calculated inverse Compton Doppler factors for 3C 279 using the collection of VLBI data (including high-resolution space VLBI data at low frequencies) recently published by us (as Wehrle et al. and Piner et al.) and the collection of multiwavelength spectra recently published by Hartman et al. From the Doppler factor and superluminal apparent speed, we then calculate the Lorentz factor and angle to the line of sight of the parsec-scale relativistic jet. We follow the method previously used by Unwin et al. for 3C 345 to model the jet components as homogeneous spheres and the VLBI core as an unresolved inhomogeneous conical jet, using K?nigls formalism. The conical jet model can be made to match both the observed X-ray emission and the VLBI properties of the core with a suitable choice of Doppler factor, implying that the core makes a significant contribution to the X-ray emission, in contrast to the situation for 3C 345, where the jet components dominated the X-ray emission. The parameters of the K?nigl models indicate that the jet is particle dominated at the radii that produce significant emission (from ~5 to 20 pc from the apex of the jet for most models) and is not in equipartition. At the inner radius of the K?nigl jet the magnetic field is of order 0.1 G and the relativistic-particle number density is of order 10 cm-3. The kinetic energy flux in the jet is of order 1046(1 + k) ergs s-1, where k is the ratio of proton to electron energy, which implies a mass accretion rate of order 0.1(1 + k)/? M? yr-1, where ? is the efficiency of conversion of mass to kinetic energy. When all components are included in the calculation, then on average the core produces about half of the X-rays, with the other half being split between the long-lived component C4 and the brightest inner-jet component. We calculate an average speed and angle to the line of sight for the region of the jet interior to 1 mas of v = 0.992c (? = 8) and ? = 4? and an average speed and angle to the line of sight for C4 (at r ? 3 mas) of v = 0.997c (? = 13) and ? = 2?. These values imply average Doppler factors of ? = 12 for the inner jet and ? = 21 for C4.