The spatial distribution of O-B5 stars in the solar neighborhood as measured by Hipparcos

We have developed a method to calculate the fundamental parameters of the vertical structure of the Galaxy in the solar neighborhood from trigonometric parallaxes alone. The method takes into account Lutz-Kelker-type biases in a self-consistent way and has been applied to a sample of O-B5 stars obtained from the Hipparcos catalog. We find that the Sun is located 24.2 +/- 1.7 (random) +/- 0.4 (systematic) pc above the galactic plane and that the disk O-B5 stellar population is distributed with a scale height of 34.2 +/- 0.8 (random) +/- 2.5 (systematic) pc and an integrated surface density of (1.62 +/- 0.04 (random) +/- 0.14 (systematic)) 10^{-3} stars pc^{-2}. A halo component is also detected in the distribution and constitutes at least ~5% of the total O-B5 population. The O-B5 stellar population within ~100 pc of the Sun has an anomalous spatial distribution, with a less-than-average number density. This local disturbance is probably associated with the expansion of Gould's belt.Comment: 14 pages, 3 figures, to appear in the May 2001 issue of the Astronomical Journa

Conceptual mechanization studies for a horizon definition spacecraft electrical power subsystem

Solar cell-battery electrical power subsystem for horizon definition spacecraf

Reconsidering the galactic coordinate system

Initially defined by the IAU in 1958, the galactic coordinate system was thereafter in 1984 transformed from the B1950.0 FK4-based system to the J2000.0 FK5-based system. In 1994, the IAU recommended that the dynamical reference system FK5 be replaced by the ICRS, which is a kinematical non-rotating system defined by a set of remote radio sources. However the definition of the galactic coordinate system was not updated. We consider that the present galactic coordinates may be problematic due to the unrigorous transformation method from the FK4 to the FK5, and due to the non-inertiality of the FK5 system with respect to the ICRS. This has led to some confusions in applications of the galactic coordinates. We tried to find the transformation matrix in the framework of the ICRS after carefully investigating the definition of the galactic coordinate system and transformation procedures, however we could not find a satisfactory galactic coordinate system that is connected steadily to the ICRS. To avoid unnecessary misunderstandings, we suggest to re-consider the definition of the galactic coordinate system which should be directly connected with the ICRS for high precise observation at micro-arcsecond level.Comment: 10 pages, 3 figures, accepted for publication in A&

Optical Technologies for UV Remote Sensing Instruments

Over the last decade significant advances in technology have made possible development of instruments with substantially improved efficiency in the UV spectral region. In the area of optical coatings and materials, the importance of recent developments in chemical vapor deposited (CVD) silicon carbide (SiC) mirrors, SiC films, and multilayer coatings in the context of ultraviolet instrumentation design are discussed. For example, the development of chemically vapor deposited (CVD) silicon carbide (SiC) mirrors, with high ultraviolet (UV) reflectance and low scatter surfaces, provides the opportunity to extend higher spectral/spatial resolution capability into the 50-nm region. Optical coatings for normal incidence diffraction gratings are particularly important for the evolution of efficient extreme ultraviolet (EUV) spectrographs. SiC films are important for optimizing the spectrograph performance in the 90 nm spectral region. The performance evaluation of the flight optical components for the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument, a spectroscopic instrument to fly aboard the Solar and Heliospheric Observatory (SOHO) mission, designed to study dynamic processes, temperatures, and densities in the plasma of the upper atmosphere of the Sun in the wavelength range from 50 nm to 160 nm, is discussed. The optical components were evaluated for imaging and scatter in the UV. The performance evaluation of SOHO/CDS (Coronal Diagnostic Spectrometer) flight gratings tested for spectral resolution and scatter in the DGEF is reviewed and preliminary results on resolution and scatter testing of Space Telescope Imaging Spectrograph (STIS) technology development diffraction gratings are presented

Detection of a Corrugated Velocity Pattern in the Spiral Galaxy NGC 5427

Here we report the detection, in Halpha emission, of a radial corrugation in the velocity field of the spiral galaxy NGC 5427. The central velocity of the Halpha line displays coherent, wavy-like variations in the vicinity of the spiral arms. The spectra along three different arm segments show that the maximum amplitude of the sinusoidal line variations are displaced some 500 pc from the central part of the spiral arms. The peak blueshifted velocities appear some 500 pc upstream the arm, whereas the peak redshifted velocities are located some 500 pc downstream the arm. This kinematical behavior is similar to the one expected in a galactic bore generated by the interaction of a spiral density wave with a thick gaseous disk, as recently modeled by Martos & Cox (1998).Comment: Accepted for publication in Ap

An X-ray Imaging Study of the Stellar Population in RCW49

We present the results of a high-resolution X-ray imaging study of the stellar population in the Galactic massive star-forming region RCW49 and its central OB association Westerlund 2. We obtained a 40 ks X-ray image of a 17'x17' field using the Chandra X-ray Observatory and deep NIR images using the Infrared Survey Facility in a concentric 8'3x8'3 region. We detected 468 X-ray sources and identified optical, NIR, and Spitzer Space Telescope MIR counterparts for 379 of them. The unprecedented spatial resolution and sensitivity of the X-ray image, enhanced by optical and infrared imaging data, yielded the following results: (1) The central OB association Westerlund 2 is resolved for the first time in the X-ray band. X-ray emission is detected from all spectroscopically-identified early-type stars in this region. (2) Most (86%) X-ray sources with optical or infrared identifications are cluster members in comparison with a control field in the Galactic Plane. (3) A loose constraint (2--5 kpc) for the distance to RCW49 is derived from the mean X-ray luminosity of T Tauri stars. (4) The cluster X-ray population consists of low-mass pre--main-sequence and early-type stars as obtained from X-ray and NIR photometry. About 30 new OB star candidates are identified. (5) We estimate a cluster radius of 6'--7' based on the X-ray surface number density profiles. (6) A large fraction (90%) of cluster members are identified individually using complimentary X-ray and MIR excess emission. (7) The brightest five X-ray sources, two Wolf-Rayet stars and three O stars, have hard thermal spectra.Comment: 19 pages, 17 figures, 4 tables. ApJ in pres

A high-resolution radio survey of the Vela supernova remnant

This paper presents a high-resolution radio continuum (843 MHz) survey of the Vela supernova remnant. The contrast between the structures in the central pulsar-powered nebula of the remnant and the synchrotron radiation shell allows the remnant to be identified morphologically as a member of the composite class. The data are the first of a composite remnant at spatial scales comparable with those available for the Cygnus Loop and the Crab Nebula, and make possible a comparison of radio, optical and soft X-ray emission from the resolved shell filaments. The survey, made with the Molonglo Observatory Synthesis Telescope, covers an area of 50 square degrees at a resolution of 43'' x 60'', while imaging structures on scales up to 30'.Comment: 18 pages, 7 jpg figures (version with ps figures at http://astro.berkeley.edu/~dbock/papers/); AJ, in pres

Genetic differentiation of European grayling (Thymallus thymallus) populations in Serbia, based on mitochondrial and nuclear DNA analyses

<p>Abstract</p> <p>Background</p> <p>The structure and diversity of grayling (<it>Thymallus thymallus</it>) populations have been well studied in most of its native habitat; however the southernmost populations of the Balkan Peninsula remain largely unexplored. The purpose of this study was to assess the genetic diversity of Serbian grayling populations, detect the impact of stocking and provide guidelines for conservation and management.</p> <p>Methods</p> <p>Eighty grayling individuals were collected from four rivers (Ibar, Lim, Drina and Rzav). The mitochondrial DNA control region (CR; 595 bp of the 3'end and 74 bp of flanking tRNA) and the ATP6 gene (630 bp fragment) were sequenced for 20 individuals (five from each locality). In addition, all individuals were genotyped with 12 microsatellite loci. The diversity and structure of the populations as well as the recent and ancient population declines were studied using specialized software.</p> <p>Results</p> <p>We detected three new haplotypes in the mtDNA CR and four haplotypes in the ATP6 gene of which three had not been described before. Previously, one CR haplotype and two ATP6 gene haplotypes had been identified as allochthonous, originating from Slovenia. Reconstruction of phylogenetic relations placed the remaining two CR haplotypes from the River Danube drainage of Serbia into a new clade, which is related to the previously described sister Slovenian clade. These two clades form a new Balkan clade. Microsatellite marker analysis showed that all four populations are genetically distinct from each other without any sign of intra-population structure, although stocking of the most diverse population (Drina River) was confirmed by mtDNA analysis. Recent and historical population declines of Serbian grayling do not differ from those of other European populations.</p> <p>Conclusions</p> <p>Our study shows that (1) the Ibar, Lim and Drina Rivers grayling populations are genetically distinct from populations outside of Serbia and thus should be managed as native populations in spite of some introgression in the Drina River population and (2) the Rzav River population is not appropriate for further stocking activities since it originates from stocked Slovenian grayling. However, the Rzav River population does not represent an immediate danger to other populations because it is physically isolated from these.</p