1,193 research outputs found
r-process Abundance Patterns in the Globular Cluster M92
Whereas light element abundance variations are a hallmark of globular
clusters, there is little evidence for variation in neutron-capture elements. A
significant exception is M15, which shows a star-to-star dispersion in
neutron-capture abundances of at least one order of magnitude. The literature
contains evidence both for and against a neutron-capture dispersion in M92. We
conducted an analysis of archival Keck/HIRES spectra of 35 stars in M92, 29 of
which are giants, which we use exclusively for our conclusions. M92 conforms to
the light element abundance variations typical of massive clusters. Like other
globular clusters, its neutron-capture abundances were generated by the
r-process. We confirm a star-to-star dispersion in the r-process. Unlike M15,
the dispersion is limited to "first-generation" (low Na, high Mg) stars, and
the dispersion is smaller for Sr, Y, and Zr than for Ba and the lanthanides.
This is the first detection of a relation between light element and
neutron-capture abundances in a globular cluster. We propose that a source of
the main r-process polluted the cluster shortly before or concurrently with the
first generation of star formation. The heavier r-process abundances were
inhomogeneously distributed while the first-generation stars were forming. The
second-generation stars formed after several crossing times (~0.8 Myr); hence,
the second generation shows no r-process dispersion. This scenario imposes a
minimum temporal separation of 0.8 Myr between the first and second
generations.Comment: 35 pages, 12 figures, accepted for publication in Ap
Materials of the final reports on the joint Soviet-American experiment on the Kosmos-936 biosatellite
Biological experiments onboard the Kosmos-936 investigated the effect of weightlessness on the basic components of cells, the genetic structure and energy apparatus. Genetic studies were made on the Drosophila melanogaster. Experiments were made on higher vegetation and fungi as well. The results indicate that weightlessness cannot be the principal barrier for normal development. An experiment with ectopic osteogenesis in weightlessness was carried out. Measurements were made of cosmic radiation inside and outside the biosatellite
Development and Study of Hard-Facing Materials on the Base of Heat-Resisting High-Hardness Steels for Plasma-Jet Hard- Facing in Shielding-Doping Nitrogen Atmosphere
The authors develop hard-facing materials on the base of heat-resisting highhardness steels for plasma-jet hard-facing in nitrogen atmosphere for manufacturing parts of mining and metallurgic equipment which significantly simplify the production process and effect a saving when producing bimetallic parts and tools
Developing and Studying the Methods of Hard-Facing with Heat-Resisting High-Hardness Steels
The authors develop the methods of hard-facing of mining-metallurgic equipment parts with heat-resisting high-hardness steels on the base of plasma-jet hard-facing in the shielding-alloying nitrogen atmosphere
Sub-milliarcsecond Imaging of Quasars and Active Galactic Nuclei. IV. Fine Scale Structure
We have used VLBA fringe visibility data obtained at 15 GHz to examine the
compact structure in 250 extragalactic radio sources. For 171 sources in our
sample, more than half of the total flux density seen by the VLBA remains
unresolved on the longest baselines. There are 163 sources in our list with a
median correlated flux density at 15 GHz in excess of 0.5 Jy on the longest
baselines. For about 60% of the sources, we have at least one observation in
which the core component appears unresolved (generally smaller than 0.05 mas)
in one direction, usually transverse to the direction into which the jet
extends. BL Lacs are on average more compact than quasars, while active
galaxies are on average less compact. Also, in an active galaxy the
sub-milliarcsecond core component tends to be less dominant. IDV sources
typically have a more compact, more core-dominated structure on
sub-milliarcsecond scales than non-IDV sources, and sources with a greater
amplitude of intra-day variations tend to have a greater unresolved VLBA flux
density. The objects known to be GeV gamma-ray loud appear to have a more
compact VLBA structure than the other sources in our sample. This suggests that
the mechanisms for the production of gamma-ray emission and for the generation
of compact radio synchrotron emitting features are related. The brightness
temperature estimates and lower limits for the cores in our sample typically
range between 10^11 and 10^13 K, but they extend up to 5x10^13 K, apparently in
excess of the equipartition brightness temperature, or the inverse Compton
limit for stationary synchrotron sources. The largest component speeds are
observed in radio sources with high observed brightness temperatures, as would
be expected from relativistic beaming (abridged).Comment: 31 pages, 13 figures, 4 tables, accepted for publication in the
Astronomical Journal; minor changes to the text are mad
Properties of the jet in M87 revealed by its helical structure imaged with the VLBA at 8 and 15 GHz
We present full-track high-resolution radio observations of the jet of the
galaxy M87 at 8 and 15 GHz. These observations were taken over three
consecutive days in May 2009 using the Very Long Baseline Array (VLBA), one
antenna of the Very Large Array (VLA), and the Effelsberg 100 m telescope. Our
produced images have dynamic ranges exceeding 20,000:1 and resolve linear
scales down to approximately 100 Schwarzschild radii, revealing a
limb-brightened jet and a faint, steep spectrum counter-jet. We performed
jet-to-counter-jet analysis, which helped estimate the physical parameters of
the flow. The rich internal structure of the jet is dominated by three helical
threads, likely produced by the Kelvin-Helmholtz (KH) instability developing in
a supersonic flow with a Mach number of approximately 20 and an enthalpy ratio
of around 0.3. We produce a CLEAN imaging bias-corrected 8-15GHz spectral index
image, which shows spectrum flattening in regions of helical thread
intersections. This further supports the KH origin of the observed internal
structure of the jet. We detect polarised emission in the jet at distances of
approximately 20 milliarcseconds from the core and find Faraday rotation which
follows a transverse gradient across the jet. We apply Faraday rotation
correction to the polarisation position angle and find that the position angle
changes as a function of distance from the jet axis, which suggests the
presence of a helical magnetic field.Comment: 15 pages, 10 figures, 1 table, Submitted to MNRAS on 25th March 202
The core shift effect in the blazar 3C 454.3
Opacity-driven shifts of the apparent VLBI core position with frequency (the
"core shift" effect) probe physical conditions in the innermost parts of jets
in active galactic nuclei. We present the first detailed investigation of this
effect in the brightest gamma-ray blazar 3C454.3 using direct measurements from
simultaneous 4.6-43 GHz VLBA observations, and a time lag analysis of 4.8-37
GHz lightcurves from the UMRAO, CrAO, and Metsahovi observations in 2007-2009.
The results support the standard Konigl model of jet physics in the VLBI core
region. The distance of the core from the jet origin r_c(nu), the core size
W(nu), and the lightcurve time lag DT(nu) all depend on the observing frequency
nu as r_c(nu)~W(nu)~ DT(nu)~nu^-1/k. The obtained range of k=0.6-0.8 is
consistent with the synchrotron self-absorption being the dominating opacity
mechanism in the jet. The similar frequency dependence of r_c(nu) and W(nu)
suggests that the external pressure gradient does not dictate the jet geometry
in the cm-band core region. Assuming equipartition, the magnetic field strength
scales with distance r as B = 0.4(r/1pc)^-0.8 G. The total kinetic power of
electron/positron jet is about 10^44 ergs/s.Comment: Accepted for publication in MNRAS; 10 pages, 6 figure
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