217 research outputs found
Periodic Pattern in the Residual-Velocity Field of OB Associations
An analysis of the residual-velocity field of OB associations within 3 kpc of
the Sun has revealed periodic variations in the radial residual velocities
along the Galactic radius vector with a typical scale length of
lambda=2.0(+/-0.2) kpc and a mean amplitude of fR=7(+/-1) km/s. The fact that
the radial residual velocities of almost all OB-associations in rich
stellar-gas complexes are directed toward the Galactic center suggests that the
solar neighborhood under consideration is within the corotation radius. The
azimuthal-velocity field exhibits a distinct periodic pattern in the region
0<l<180 degrees, where the mean azimuthal-velocity amplitude is ft=6(+/-2)
km/s. There is no periodic pattern of the azimuthal-velocity field in the
region 180<l<360 degrees. The locations of the Cygnus arm, as well as the
Perseus arm, inferred from an analysis of the radial- and azimuthal-velocity
fields coincide. The periodic patterns of the residual-velocity fields of
Cepheids and OB associations share many common features.Comment: 21 page
Peculiar Features of the Velocity Field of OB Associations and the Spiral Structure of the Galaxy
Some of the peculiar features of the periodic velocity-field structure for OB
associations can be explained by using the model of Roberts and Hausman (1984),
in which the behavior of a system of dense clouds is considered in a perturbed
potential. The absence of statistically significant variations in the azimuthal
velocity across the Carina arm, probably, results from its sharp increase
behind the shock front, which is easily blurred by distance errors. The
existence of a shock wave in the spiral arms and, at the same time, the
virtually free motion of OB associations in epicycles can be reconciled in the
model of particle clouds with a mean free path of 0.2-2 kpc. The velocity field
of OB associations exhibits two appreciable nonrandom deviations from an ideal
spiral pattern: a 0.5-kpc displacement of the Cygnus- and Carina-arm fragments
from one another and a weakening of the Perseus arm in quadrant III. However,
the identified fragments of the Carina, Cygnus, and Perseus arms do not belong
to any of the known types of spurs.Comment: 14 pages, 3 postscript figures, to be published in Astronomy Letter
The mitochondrial gene order and CYTB gene evolution in insects
Over millions of years of evolution, the genomes of modern insects have accumulated a significant number of mutations, which often can lead up a blind alley when carrying out phylogenetic research. Genomic differences between some representatives belonging to the same family or group are often so great that they demand using nonconventional methods of the phylogenetic analysis. It is known that molecular evolution goes by the way of not only single nucleotide substitutions, but also by larger genomic reorganizations, such as insertion or deletion of large genome fragments, and even changing the order of genes. Mitochondrial DNA genes (mtDNA) are quite often used as markers for phylogenetic research into many organisms including arthropods, because mtDNA is multicopied, is inherited maternally, does not undergo recombination and accumulates mutations quickly enough (relative to the nuclear genome). To date, a large number of full nucleotide sequences of mitogenomes (thousands of organisms) has been deposited in public databases; however, their phylogenetic analysis has obstacles, especially for representatives of the insects (Insecta), whose evolution takes a considerable part of geological time. In this work we describe the application and a comparison of two ways of the phylogenetic analysis for different groups of insects. The first method uses the variability of the nucleotide sequence of mtDNA, and the second one analyses the order of genes in full mitochondrial genomes of insects that can be used as an additional marker in phylogenetic research into representatives of the order Hymenoptera
Globular cluster system and Milky Way properties revisited
Updated data of the 153 Galactic globular clusters are used to readdress
fundamental parameters of the Milky Way. We build a reduced sample,
decontaminated of the clusters younger than 10Gyr, those with retrograde orbits
and/or evidence of relation to dwarf galaxies. The 33 metal-rich globular
clusters of the reduced sample extend basically to the Solar circle and
distribute over a region with projected axial-ratios typical of an oblate
spheroidal, . The 81
metal-poor globular clusters span a nearly spherical region of axial-ratios
extending from the central parts to the outer halo. A new
estimate of the Sun's distance to the Galactic center is provided, . The metal-rich and metal-poor radial-density distributions
flatten for and are well represented both by a power-law
with a core-like term and S\'ersic's law; at large distances they fall off as
. Both metallicity components appear to have a common origin,
which is different from that of the dark matter halo. Structural similarities
of the metal-rich and metal-poor radial distributions with the stellar halo are
consistent with a scenario where part of the reduced sample was formed in the
primordial collapse, and part was accreted in an early period of merging. This
applies to the bulge as well, suggesting an early merger affecting the central
parts of the Galaxy. We estimate that the present globular cluster population
corresponds to of the original one. The fact that the
volume-density radial distributions of the metal-rich and metal-poor globular
clusters of the reduced sample follow both a core-like power-law and S\'ersic's
law indicates that we are dealing with spheroidal subsystems in all scales.Comment: 14 pages and 6 figures. Astronomy & Astrophysics, accepted on NOv. 2
The SEGUE Stellar Parameter Pipeline. II. Validation with Galactic Globular and Open Clusters
We validate the performance and accuracy of the current SEGUE (Sloan
Extension for Galactic Understanding and Exploration) Stellar Parameter
Pipeline (SSPP), which determines stellar atmospheric parameters (effective
temperature, surface gravity, and metallicity) by comparing derived overall
metallicities and radial velocities from selected likely members of three
globular clusters (M 13, M 15, and M 2) and two open clusters (NGC 2420 and M
67) to the literature values. Spectroscopic and photometric data obtained
during the course of the original Sloan Digital Sky Survey (SDSS-I) and its
first extension (SDSS-II/SEGUE) are used to determine stellar radial velocities
and atmospheric parameter estimates for stars in these clusters. Based on the
scatter in the metallicities derived for the members of each cluster, we
quantify the typical uncertainty of the SSPP values, sigma([Fe/H]) = 0.13 dex
for stars in the range of 4500 K < Teff < 7500 K and 2.0 < log g < 5.0, at
least over the metallicity interval spanned by the clusters studied (-2.3 <
[Fe/H] < 0). The surface gravities and effective temperatures derived by the
SSPP are also compared with those estimated from the comparison of the
color-magnitude diagrams with stellar evolution models; we find satisfactory
agreement. At present, the SSPP underestimates [Fe/H] for
near-solar-metallicity stars, represented by members of M 67 in this study, by
about 0.3 dex.Comment: 56 pages, 8 Tables, 15 figures, submitted to the Astronomical Journa
Using Cepheids to determine the galactic abundance gradient I. The solar neighbourhood
A number of studies of abundance gradients in the galactic disk have been
performed in recent years. The results obtained are rather disparate: from no
detectable gradient to a rather significant slope of about -0.1 dex kpc -1. The
present study concerns the abundance gradient based on the spectroscopic
analysis of a sample of classical Cepheids. These stars enable one to obtain
reliable abundances of a variety of chemical elements. Additionally, they have
well determined distances which allow an accurate determination of abundance
distributions in the galactic disc. Using 236 high resolution spectra of 77
galactic Cepheids, the radial elemental distribution in the galactic disc
between galactocentric distances in the range 6-11 kpc has been investigated.
Gradients for 25 chemical elements (from carbon to gadolinium) are derived...Comment: 28 pages, 14 postscript figures, LaTeX, uses Astronomy and
Astrophysics macro aa.cls, graphicx package, to be published in Astronomy and
Astrophysics (2002) also available at
http://www.iagusp.usp.br/~maciel/index.htm
Photometric study of open clusters Berkeley 96, Berkeley 97, King 12, NGC 7261, NGC 7296, and NGC 7788
The BVRcIc CCD photometry in the fields of six open clusters toward the
Perseus spiral arm is presented. These data along with JHKs magnitudes taken
from 2MASS catalog, have been used to determine cluster's ages, distances and
color excesses. In addition, the gaps in mass function of Be 97, King 12 and
NGC 7788 clusters have been revealed in mass intervals of 1.3-1.5, 1.4-1.6 and
1.5-1.7 solar masses, respectively.Comment: 11 pages, 12 figure
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