406 research outputs found
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
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
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
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
Modeling of the Vela complex including the Vela supernova remnant, the binary system gamma2 Velorum, and the Gum nebula
We study the geometry and dynamics of the Vela complex including the Vela
supernova remnant (SNR), the binary system gamma2 Velorum and the Gum nebula.
We show that the Vela SNR belongs to a subclass of non-Sedov adiabatic remnants
in a cloudy interstellar medium (ISM), the dynamics of which is determined by
the heating and evaporation of ISM clouds. We explain observable
characteristics of the Vela SNR with a SN explosion with energy 1.4 x 10^50
ergs near the step-like boundary of the ISM with low intercloud densities (~
10^{-3} cm^{-3}) and with a volume-averaged density of clouds evaporated by
shock in the north-east (NE) part about four times higher than the one in the
south-west (SW) part. The observed asymmetry between the NE and SW parts of the
Vela SNR could be explained by the presence of a stellar wind bubble (SWB)
blown by the nearest-to-the Earth Wolf-Rayet (WR) star in the gamma2 Velorum
system. We show that the size and kinematics of gamma2 Velorum SWB agree with
predictions of numerical calculations for the evolution of the SWB of M_ini =
35M* star. The low initial mass of the WR star in gamma2 Velorum implies that
the luminosity of the nuclear line of 26Al, produced by gamma2 Velorum, is
below the sensitivity of existing gamma-ray telescopes.Comment: 8 pages, 2 figures, accepted for publication in A&
Constructing a Galactic coordinate system based on near-infrared and radio catalogs
The definition of the Galactic coordinate system was announced by the IAU
Sub-Commission 33b on behalf of the IAU in 1958. For more than 50 years the
definition of the Galactic coordinate system has remained unchanged from this
IAU1958 version. On the basis of deep and all-sky catalogs, the position of the
Galactic plane can be revised and updated definitions of the Galactic
coordinate systems can be proposed. We re-determine the position of the
Galactic plane based on modern large catalogs, such as the Two Micron All-Sky
Survey (2MASS) and the SPECFIND v2.0. This paper also aims to propose a
possible definition of the optimal Galactic coordinate system by adopting the
ICRS position of the Sgr A* at the Galactic center. The near-infrared 2MASS
point source catalog and the SPECFIND v2.0 catalog of radio continuum spectra
are used to determine the mean position of the Galactic plane on the celestial
sphere. By fitting the data to an ideal Galactic equator, the parameters
defining the Galactic coordinate system are obtained. We find that the
obliquity of the Galactic equator on the ICRS principal plane is about
(2MASS) and (SPECFIND v2.0) larger than the J2000.0
value, which is widely used in coordinate transformations between the
equatorial and the Galactic . Depending on the
adopted parameters, data, and methods, the largest difference between the
resulting Galactic coordinate systems is several arcminutes. We derive revised
transformation matrices and parameters describing the orientation of the
Galactic coordinate systems in the ICRS at the 1 milli-arcsecond level to match
the precision of modern observations. For practical applications, we propose
that a revised definition of the Galactic coordinate system should be required
in the near future.Comment: 6 pages, 5 figures, accepted by Astronomy and Astrophysic
Recommended from our members
Quantified evaluation for decisions : measuring achievement of environmental goals and analyzing trade-offs
Federal legislation requires that governmental programs be evaluated on a wide set of criteria to provide policymakers with information on the achievement or lack of achievement of goals and/or objectives. A methodology to identify and quantify measures of achievement is described. In addition, the methodology is designed to facilitate the comparison of alternatives in terms of trade-offs among conflicting goals and/or objectives. An example is presented for quantifying and evaluating the achievement of environmental goals and objectives in water quality improvement projects. The trade-offs between environmental quality and economic well-being are discussed and the usefulness of the methodology for providing assistance to policymakers in making such trade-offs described.Published November 1982. Facts and recommendations in this publication may no longer be valid. Please look for up-to-date information in the OSU Extension Catalog: http://extension.oregonstate.edu/catalo
New infrared star clusters in the Northern and Equatorial Milky Way with 2MASS
We carried out a survey of infrared star clusters and stellar groups on the
2MASS J, H and K_s all-sky release Atlas in the Northern and Equatorial Milky
Way (350 < l < 360, 0 < l < 230). The search in this zone complements that in
the Southern Milky Way (Dutra et al. 2003a). The method concentrates efforts on
the directions of known optical and radio nebulae. The present study provides
167 new infrared clusters, stellar groups and candidates. Combining the two
studies for the whole Milky Way, 346 infrared clusters, stellar groups and
candidates were discovered, whereas 315 objects were previously known. They
constitute an important new sample for future detailed studies.Comment: Accepted to Astronomy and Astrophysic
The Structure of the Star-forming Cluster RCW 38
We present a study of the structure of the high mass star-forming region
RCW~38 and the spatial distribution of its young stellar population. Spitzer
IRAC photometry 3-8um are combined with 2MASS near-IR data to identify young
stellar objects by IR-excess emission from their circumstellar material.
Chandra X-ray data are used to identify class III pre-main sequence stars
lacking circumstellar material. We identify 624 YSOs: 23 class 0/I and 90 flat
spectrum protostars, 437 Class II stars, and 74 Class III stars. We also
identify 29 (27 new) O star candidates over the IRAC field. Seventy-two stars
exhibit IR-variability, including seven class 0/I and 12 flat spectrum YSOs. A
further 177 tentative candidates are identified by their location in the IRAC
[3.6] vs. [3.6]-[5.8] cmd. We find strong evidence of subclustering in the
region. Three subclusters were identified surrounding the central cluster, with
massive and variable stars in each subcluster. The central region shows
evidence of distinct spatial distributions of the protostars and pre-main
sequence stars. A previously detected IR cluster, DB2001_Obj36, has been
established as a subcluster of RCW 38. This suggests that star formation in RCW
38 occurs over a more extended area than previously thought. The gas to dust
ratio is examined using the X-ray derived hydrogen column density, N_H and the
K-band extinction, and found to be consistent with the diffuse ISM, in contrast
with Serpens & NGC1333. We posit that the high photoionising flux of massive
stars in RCW 38 affects the agglomeration of the dust grains.Comment: 98 pages, 15 figure
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