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A significant number of X-ray binaries are now known to exhibit long-term periodicities on timescales of ~10 - 100 days. Several physical mechanisms have been proposed that give rise to such periodicities, one of which is radiation-driven warping and precession of the accretion disk. Recent theoretical work predicts the stability to disk warping as a, function of the mass ratio, binary radius, viscosity and accretion efficiency. We investigate the stability of the superorbital periodicities in the neutron star X-ray binaries Cyg X-2, LMC X-4, SMC X-l and Her X-l, and thereby confront stability predictions with observation. We find that the period and nature of the superorbital variations in these sources is consistent with the predictions of warping theory.
Macroscopically this sherd appears to be of a fine sandy ware, with ripple burnish. In thin section it can be seen to be a ferruginous clay with a large number of lumps of colloidal iron. It containes a high proportion of small grained and proportionately rounded quartz particles, and a small number of large, calcined flint particles. From the ID, T10, L2,
The introduction of new photometric techniques for faint objects has tremendously increased our understanding of the MC clusters, especially at the old end of the age spectrum. Many of the well-studied clusters of the past have gone through several transformations, starting as true globulars in 1960, becoming intermediate-age objects in the 1970's, but now returning to the true globular classification in the 1980's. LW 868 is an example of a cluster with such a history. Better photometric accuracy is also greatly improving our understanding of younger clusters, allowing detailed comparisons with theoretical evolutionary models, and a more reliable determination of the age-abundance relationship. The dynamical history of MC clusters can also be examined from the new data. The cluster formation rate, as well as the destruction rate, can be reconstructed from modern, deep cluster surveys.
Symmetric, two-sided morphology seems to argue against relativistic effects dominating compact radio emission. This kind of structure has been reported for a number of sources (Phillips and Mutel 1982; Pearson 1983) based on maps made at one frequency. Various arguments, all indirect, can be made for these sources being (1) Twin regions formed at the ends of jets which emerge from an invisible core, or (2) misidentified core-jet sources wherein the core and an unusually bright knot are wrongly taken to be a “double.” A telling test of both hypotheses is to map the sources in question over an octave or so of frequency. Proponents of view (1) would predict that the two double components will show nearly identical spectral indices and that weak central cores with flat or rising spectra might even be revealed. Champions of view (2) would predict that one end or the other will dominate at high frequencies (the core!) or that complex bridges of emission (the jet!) will be revealed between the components at low frequencies. We have followed our initial discovery of 5 symmetric compact doubles by (A) attempting to enlarge the sample of symmetric sources available to study, and (B) by investigating at 5 GHz those doubles for which the best maps exist at 1.7 GHz.
Extragalactic HII regions and their ionizing star clusters are the most prominent signposts of recent starbirth activity in galaxies. In this paper, we present optical measurements of nearby extragalactic HII regions with the aim of investigating possible relationships between the newborn ionizing clusters and their dynamical environment. Evidence for variations in the measured HII region properties (size, Hα luminosity, and Hα equivalent width) are presented as a function of galaxy type and of position within individual galaxies. These variations, in turn, are compared with the corresponding kinematic and dynamical variations. The apparent sensitivity of the ionizing activity to the dynamical environment suggests that the total masses and upper stellar mass limits of the ionizing clusters are somehow constrained by the ambient tidal stresses and shear flows within the galaxies. We speculate that regions of high tidal stress and kinematic shearing tend to suppress the formation and maintenance of giant molecular clouds, thereby hindering the creation of giant clusters and massive stars therein.
We present a new method for finding associations of young stars in M31 using broadband WFPC2 data from the HST data archive. Applying our identification method to 13 WFPC2 fields, covering an area of ∼60 arcmin2, has revealed 80 new candidate young star clusters in these portions of the M31 disk. Most of these clusters are small (≳5 pc) young (∼10–200 Myr) star groups located within large OB associations. We have estimated the reddening values and the ages of each candidate individually by fitting isochrones to the stellar photometry. We provide a catalogue of the candidates including rough approximations of their reddenings and ages.
Using new (Cycle 9) HST / WFPC2 data, we have studied a peculiar star forming region in the nearby Sc-type spiral NGC 6946. The region has a bubble-like shape and a diameter of about 600 pc. Near the centre is an extremely luminous young globular cluster with MV = −13.2 mag. The cluster has a compact core with a core radius of about 1.3 pc but is surrounded by an extended halo with a power-law luminosity profile similar to that observed for young LMC clusters. From Keck / HIRES high-dispersion spectroscopy we measure a velocity dispersion of 10 km s−1, leading to a dynamical mass estimate of about 1.7 × 106 solar masses. The dynamical mass estimate is comparable to the expectation from population synthesis modelling under the assumption of a Salpeter IMF extending down to 0.1 M⊙.
We present a photometric study of the stars in ionizing star clusters embedded in several giant H II regions of M33 (CC93, IC 142, NGC 595, MA2, NGC 604 and NGC 588). Our photometry is based on the HST-WFPC2 images of these clusters. Color-magnitude diagrams and color-color diagrams of these clusters are obtained and are used for estimating the reddenings and ages of the clusters. The luminosity functions (LFs) and initial mass functions (IMFs) of the massive stars in these clusters are also derived. The slopes of the IMFs range from Γ = −0.5 to −2.1. Interestingly, it is found that the IMFs get steeper with increasing galactocentric distance and with decreasing [O/H] abundance.
The surface and bottom topography of the central Greenland ice sheet was determined from airborne ice-radar soundings over a 180 km by 180 km grid centered on the 1974 “Summit” site (lat. 72°18′N., long. 37°55′W.), using the Technical University of Denmark 60 MHz ice radar. Over 6100 km of high-quality radar data were obtained, covering over 99'% of the grid, along lines spaced 12.5 km apart in both north-south and east-west directions. Aircraft location was done with an inertial navigation system (INS) and a pressure altimeter, with control provided by periodically flying over a known point at the center of the grid. The ice radar was used to determine ice thickness; the surface topography was determined independently using height-above-terrain measurements from the aircraft’s radar altimeter. The calculated surface topography is accurate to about ±6 m, with this error arising mostly from radar-altimeter errors. The ice thickness and bottom topography are accurate to about ±50 m, with this error dominated by the horizontal navigation uncertainties due to INS drift; this error increases to about ±125 m in areas of rough bottom relief (about 12% of the grid).
The highest point on Greenland is at lat. 72°34′ N., long. 37°38′W., at an altitude of 3233 ± 6 m a.s.l. The ice surface at this point divides into three sectors, one facing north, one east-south-east, and one west-south-west, with each having a roughly uniform slope. The ice divide between the last two sectors is a well-defined ridge running almost due south. The ice is about 3025 m thick at the summit. Excluding the mountainous north-east corner of the grid, where the ice locally reaches a thickness of about 3470 m and the bed dips to about 370 m below sea-level, the maximum ice thickness, approximately 3375 m, occurs about 97 km south-south-west of the summit. The average bed altitude over the entire grid is 180 m and the average ice thickness is 2975 ± 235 m. The ice in most of the south-west quadrant of the grid is over 3200 m thick, and overlies a relatively smooth, flat basin with altitudes mostly below sea-level. There is no predominant direction to the basal topography over most of the grid; it appears to be undulating, rolling terrain with no obvious ridge/valley structure. The summit of the ice sheet is above the eastern end of a relatively large, smooth, flat plateau, about 10–15 km wide and extending about 50 km to the west. If the basal topography were the sole criterion, then a site somewhere on this plateau or in the south-west basin would be suitable for the drilling of a new deep ice core.
A new short-pulse digital profiling radar system that operates at lower frequencies than most ice radars used in polar regions to date has been designed and built by the U.S. Geological Survey. The transmitter is an avalanche transistor pulser which drives a resistively loaded dipole transmitting antenna. A similar, but separate antenna is connected to the receiver. The receiver has adjustable sensitivity time control (STC) of as much as 60 dB to compensate for attenuation and geometric spreading factors. A fiber-optic cable is used to transmit both control signals and data. The data-acquisition and display system incorporates very high-speed digitizing and signal averaging, real-time profile display, and data storage on standard computer nine-track magnetic tape.
The system was successfully used on Ice Stream B in West Antarctica at centre frequencies of 1, 2, 4, 8, and 12.5 MHz. Bottom-return signal-to-noise ratios of more than 40 dB were obtained at 2 MHz through 800 m of ice. Convoluted internal surfaces not related to present bottom topography were resolved within the ice streams and anomalous strong reflections or “bright spots” were identified near the base of the ice. At present, there is no satisfactory glaciological explanation for either of these observations.
Low-frequency surface-based radar-profiling experiments on Ice Streams Β and C, West Antarctica, have yielded high-resolution images which depict folding of the internal layers that can aid in the interpretation of ice-stream dynamics. Unlike folding seen in most earlier radar studies of ice sheets, the present structures have no relationship to bedrock topography and show tilting of their axial fold planes in the flow direction. Rather than being standing waves created by topography or local variations in basal shear stress, the data show that these folds originate upstream of the region of streaming flow and are advected into the ice streams. The mechanism for producing folds is hypothesized to be changes in the basal boundary conditions as the ice makes the transition from inland ice to ice-stream flow. Migration of this transition zone headward can then cause folds in the internal layering to be propagated down the ice streams.
The heavily-reddened Local Group galaxy IC 10 has been studied at narrow-band emission-line wavelengths and in U, B, V, and I with the APO 3.5m telescope. The four-color data are being used to measure and map the foreground and internal reddening. The color-magnitude diagram clearly isolates the main sequence from the red supergiants and from the abundant foreground stars. The absolute magnitudes of the main sequence stars, together with the anomalously-abundant WR stars reported by Massey & Armandroff, indicate that the galaxy is currently undergoing a fairly extensive burst of star-formation.
We present deep HST color-magnitude diagrams of fields centered on the six old LMC globular clusters NGC 1754, NGC 1835, NGC 1898, NGC 1916, NGC 2005, and NGC 2019. Separate cluster and field star CMDs are shown. The time of formation of the LMC is studied from an analysis of the cluster CMDs. Based on a comparison of the CMDs with sequences of the Milky Way clusters M3, M5, and M55, we suggest that the LMC formed its first stars at the same time as the Milky Way to within 1 Gyr. We find additional evidence that these LMC globular clusters are as old as the oldest Milky Way clusters through a comparison of our data with the horizontal branch evolutionary models of Lee, Demarque, & Zinn (1994).
The evolution of the LMC following its formation is studied through an analysis of the field star CMDs. Through an automated comparison with stellar evolution models, we extract the star formation histories implied by the CMDs. Our best-fit star formation histories imply that the LMC has been actively forming stars over the last 4 Gyr, in agreement with previous field star studies. The four fields that lie in the Bar also contain significant numbers of stars with ages of 4–8 Gyr in the best-fit cases. The most notable disagreement between the best-fit models and observed CMDs is in the color of the red giant branch.
Star formation in the outermost regions of the LMC, several kpc from the center, has been dormant for some Gyr. With deep HST and wide-field ground-based imaging surveys, sufficient stars can be measured to accurately delineate the critical turn-off and subgiant branch regions of the CMD, and thus provide a picture of the first 10 Gyr of star formation in the LMC.
Spatial studies of the emission line regions in planetary nebulae (PN) can provide insight into the physical and chemical environments across the nebulae. In a collaborative effort by the coauthors, a K-band Fabry-Perot etalon has been coupled with an advanced 256 × 256 InSb focal plane array at the Wyoming Infrared Observatory 2.3m telescope. This system permits us to obtain spatially resolved, 0.24″/pixel, moderate spectral resolution (R ≈ 800), flux-density IR emission line images of astronomical sources. We obtained continuum-subtracted images of Br γ, HeI 2.06 μm, the 2-μm UIR features, and the 3.3 μm PAH dust feature in the PN NGC 6572, NGC 7027, and NGC 7662. One objective was to determine the spatial morphology of two unidentified emission lines, UIR1−2.199 μm, and UIR2−2.287 μm (Geballe et al. 1991). These UIR lines appear in the spectra of many PN (Hora et al. 1997) and in the Orion Nebula (Luhman & Rieke 1996). Geballe et al. suggested that the UIR lines are most likely forbidden transitions and showed that the parent ion ionization potential is ≈ 30–40 eV, while the ionization potential for the ions themselves is 40–60 eV. Here we directly compare the distribution of the UIR emitters to that of the gas (H+,He+) and dust (PAHs).
The present Report covers the period 1982-1984. As with previous Reports, it has not been possible to cover all publications or even every field of research. Some subsections of this Report, the Magellanic Clouds for example, report a body of work as extensive as that in some commissions.
This paper presents physical properties of interplanetary dust determined by in-situ techniques. It is probable that, like millimeter-sized meteoroids (Jacchia, et al. 1967), most interplanetary dust is cometary matter. Although a cometary origin for interplanetary dust is widely accepted (Whipple 1967) (Millman, 1972) there is currently no unambiguous proof of this hypothesis. The results presented here must be interpreted accordingly. It must also be remembered that even if interplanetary particles are cometary, they might possibly be altered in the interplanetary medium by collisions and by thermal effects during close perihelion passages, so the dust particles may not be representative of unaltered cometary material.
It is not easy to measure distances to galaxies. That we even try is as much a measure of our audacity as it is an indication of our ingenuity. But we do try, and we feel that we must try, because so many of our grandest questions and most basic problems about the universe depend on our knowledge of its size and scale.
This Joint Discussion brings together some of the astronomers who have concerned themselves with this difficult task. They do not all agree among themselves. This is not a topic that has reached a state of selfcomplacent fixity, but rather is still a subject of controversy and doubt. The Joint Discussion was designed by its organizers to expose the problems rather than to solve them, to discover the questions rather than to agree on an answer to them.