859 research outputs found
Effect of angular momentum distribution on gravitational loss-cone instability in stellar clusters around massive BH
Small perturbations in spherical and thin disk stellar clusters surrounding
massive a black hole are studied. Due to the black hole, stars with
sufficiently low angular momentum escape from the system through the loss cone.
We show that stability properties of spherical clusters crucially depend on
whether the distribution of stars is monotonic or non-monotonic in angular
momentum. It turns out that only non-monotonic distributions can be unstable.
At the same time the instability in disk clusters is possible for both types of
distributions.Comment: 14 pages, 7 figures, submitted to MNRA
The structure of cool accretion disc in semidetached binaries
We present the results of qualitative consideration of possible changes
occurring during the transition from the hot accretion disc to the cool one. We
argue the possible existence of one more type of spiral density waves in the
inner part of the disc where gasdynamical perturbations are negligible. The
mechanism of formation of such a wave as well as its parameters are considered.
We also present the results of 3D gasdynamical simulation of cool accretion
discs. These results confirm the hypothesis of possible formation of the spiral
wave of a new, "precessional" type in the inner regions of the disc. Possible
observational manifestations of this wave are discussed.Comment: LaTeX, 16 pages, 8 figures, to be published in Astron. Z
Investigation of the new cataclysmic variable 1RXS J180834.7+101041
We present the results of our photometric and spectroscopic studies of the
new eclipsing cataclysmic variable star 1RXS J180834.7+101041. Its spectrum
exhibits double-peaked hydrogen and helium emission lines. The Doppler maps
constructed from hydrogen lines show a nonuniform distribution of emission in
the disk similar to that observed in IP Peg. This suggests that the object can
be a cataclysmic variable with tidal density waves in the disk. We have
determined the component masses (M_WD =0.8 \pm 0.22 M_sun and M_RD =0.14 \pm
0.02 M_sun) and the binary inclination (i =78 \pm 1.5 deg) based on well-known
relations between parameters for cataclysmic variable stars. We have modeled
the binary light curves and showed that the model of a disk with two spots is
capable of explaining the main observed features of the light curves.Comment: 22 pages, 9 figures, 2 tables, published in Astronomy Letters, 2011,
37, 845-85
Formation Mechanisms for Spirals in Barred Galaxies
We consider a scenario of formation of the spiral structure in barred
galaxies. This scenario includes the new non-resonant mechanism of elongation
of spirals, due to the characteristic behaviour of the gravitational potential
beyond the principal spiral arms
Molecular Mechanisms in Clear Cell Renal Cell Carcinoma: Role of miRNAs and Hypermethylated miRNA Genes in Crucial Oncogenic Pathways and Processes
Clear cell renal cell carcinoma (ccRCC) is the third most common urological cancer, and it has the highest mortality rate. The increasing drug resistance of metastatic ccRCC has resulted in the search for new biomarkers. Epigenetic regulatory mechanisms, such as genome-wide DNA methylation and inhibition of protein translation by interaction of microRNA (miRNA) with its target messenger RNA (mRNA), are deeply involved in the pathogenesis of human cancers, including ccRCC, and may be used in its diagnosis and prognosis. Here, we review oncogenic and oncosuppressive miRNAs, their putative target genes, and the crucial pathways they are involved in. The contradictory behavior of a number of miRNAs, such as suppressive and anti-metastatic miRNAs with oncogenic potential (for example, miR-99a, miR-106a, miR-125b, miR-144, miR-203, miR-378), is examined. miRNAs that contribute mostly to important pathways and processes in ccRCC, for instance, PI3K/AKT/mTOR, Wnt-β, histone modification, and chromatin remodeling, are discussed in detail. We also separately consider their participation in crucial oncogenic processes, such as hypoxia and angiogenesis, metastasis, and epithelial-mesenchymal transition (EMT). The review also considers the interactions of long non-coding RNAs (lncRNAs) and miRNAs of significance in ccRCC. Recent advances in the understanding of the role of hypermethylated miRNA genes in ccRCC and their usefulness as biomarkers are reviewed based on our own data and those available in the literature. Finally, new data and perspectives concerning the clinical applications of miRNAs in the diagnosis, prognosis, and treatment of ccRCC are discussed
Gravitational stability and dynamical overheating of stellar disks of galaxies
We use the marginal stability condition for galactic disks and the stellar
velocity dispersion data published by different authors to place upper limits
on the disk local surface density at two radial scalelengths .
Extrapolating these estimates, we constrain the total mass of the disks and
compare these estimates to those based on the photometry and color of stellar
populations. The comparison reveals that the stellar disks of most of spiral
galaxies in our sample cannot be substantially overheated and are therefore
unlikely to have experienced a significant merging event in their history. The
same conclusion applies to some, but not all of the S0 galaxies we consider.
However, a substantial part of the early type galaxies do show the stellar
velocity dispersion well in excess of the gravitational stability threshold
suggesting a major merger event in the past. We find dynamically overheated
disks among both seemingly isolated galaxies and those forming pairs. The ratio
of the marginal stability disk mass estimate to the total galaxy mass within
four radial scalelengths remains within a range of 0.4---0.8. We see no
evidence for a noticeable running of this ratio with either the morphological
type or color index.Comment: 25 pages, 5 figures, accepted to Astronomy Letter
Gravitational Loss-Cone Instability in Stellar Systems with Retrograde Orbit Precession
We study spherical and disk clusters in a near-Keplerian potential of
galactic centers or massive black holes. In such a potential orbit precession
is commonly retrograde, i.e. direction of the orbit precession is opposite to
the orbital motion. It is assumed that stellar systems consist of nearly radial
orbits. We show that if there is a loss cone at low angular momentum (e.g., due
to consumption of stars by a black hole), an instability similar to loss-cone
instability in plasma may occur. The gravitational loss-cone instability is
expected to enhance black hole feeding rates. For spherical systems, the
instability is possible for the number of spherical harmonics . If
there is some amount of counter-rotating stars in flattened systems, they
generally exhibit the instability independently of azimuthal number . The
results are compared with those obtained recently by Tremaine for distribution
functions monotonically increasing with angular momentum.
The analysis is based on simple characteristic equations describing small
perturbations in a disk or a sphere of stellar orbits highly elongated in
radius. These characteristic equations are derived from the linearized Vlasov
equations (combining the collisionless Boltzmann kinetic equation and the
Poisson equation), using the action-angle variables. We use two techniques for
analyzing the characteristic equations: the first one is based on preliminary
finding of neutral modes, and the second one employs a counterpart of the
plasma Penrose-Nyquist criterion for disk and spherical gravitational systems.Comment: Accepted to Monthly Notices of the Royal Astronomical Society; typos
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