85 research outputs found
A model of superoutbursts in binaries of SU UMa type
A new mechanism explaining superoutbursts in binaries of SU UMa type is
proposed. In the framework of this mechanism the accretion rate increase
leading to the superoutburst is associated with formation of a spiral wave of a
new "precessional" type in inner gasdynamically unperturbed parts of the
accretion disc. The possibility of existence of this type of waves was
suggested in our previous work (astro-ph/0403053). The features of the
"precessional" spiral wave allow explaining both the energy release during the
outburst and all its observational manifestations. The distinctive
characteristic of a superoutburst in a SU UMa type star is the appearance of
the superhump on the light curve. The proposed model reproduces well the
formation of the superhump as well as its observational features, such as the
period that is 3-7% longer than the orbital one and the detectability of
superhumps regardless of the binary inclination.Comment: LaTeX, 20 pages, 4 figures, to be published in Astron. Z
Atmospheric parameters of 82 red giants in the Kepler field
Context: Accurate fundamental parameters of stars are essential for the
asteroseismic analysis of data from the NASA Kepler mission. Aims: We aim at
determining accurate atmospheric parameters and the abundance pattern for a
sample of 82 red giants that are targets for the Kepler mission. Methods: We
have used high-resolution, high signal-to-noise spectra from three different
spectrographs. We used the iterative spectral synthesis method VWA to derive
the fundamental parameters from carefully selected high-quality iron lines.
After determination of the fundamental parameters, abundances of 13 elements
were measured using equivalent widths of the spectral lines. Results: We
identify discrepancies in log g and [Fe/H], compared to the parameters based on
photometric indices in the Kepler Input Catalogue (larger than 2.0 dex for log
g and [Fe/H] for individual stars). The Teff found from spectroscopy and
photometry shows good agreement within the uncertainties. We find good
agreement between the spectroscopic log g and the log g derived from
asteroseismology. Also, we see indications of a potential metallicity effect on
the stellar oscillations. Conclusions: We have determined the fundamental
parameters and element abundances of 82 red giants. The large discrepancies
between the spectroscopic log g and [Fe/H] and values in the Kepler Input
Catalogue emphasize the need for further detailed spectroscopic follow-up of
the Kepler targets in order to produce reliable results from the asteroseismic
analysis.Comment: 16 Pages, 12 Figures, accepted for publication in A&
The hydrogen and helium lines of the symbiotic binary Z And during its brightening at the end of 2002
High resolution observations in the region of the lines Halpha, He II 4686
and Hgamma of the spectrum of the symbiotic binary Z And were performed during
its small-amplitude brightening at the end of 2002. The profiles of the
hydrogen lines were double-peaked. These profiles give a reason to suppose that
the lines can be emitted mainly by an optically thin accretion disc. The Halpha
line is strongly contaminated by the emission of the envelope, therefore for
consideration of accretion disc properties we use the Hgamma line. The Halpha
line had broad wings which are supposed to be determined mostly from radiation
damping but high velocity stellar wind from the compact object in the system
can also contribute to their appearance. The Hgamma line had a broad emission
component which is assumed to be emitted mainly from the inner part of the
accretion disc. The line He II 4686 had a broad emission component too, but it
is supposed to appear in a region of a high velocity stellar wind. The outer
radius of the accretion disc can be calculated from the shift between the
peaks. Assuming, that the orbit inclination can ranges from 47 to
76, we estimate the outer radius as 20 - 50 R_sun. The behaviour of the
observed lines can be considered in the framework of the model proposed for
interpretation of the line spectrum during the major 2000 - 2002 brightening of
this binary.Comment: 19 pages, 5 figures. Accepted for publication in Astronomy Report
UV and EUV Instruments
We describe telescopes and instruments that were developed and used for
astronomical research in the ultraviolet (UV) and extreme ultraviolet (EUV)
regions of the electromagnetic spectrum. The wavelength ranges covered by these
bands are not uniquely defined. We use the following convention here: The EUV
and UV span the regions ~100-912 and 912-3000 Angstroem respectively. The
limitation between both ranges is a natural choice, because the hydrogen Lyman
absorption edge is located at 912 Angstroem. At smaller wavelengths,
astronomical sources are strongly absorbed by the interstellar medium. It also
marks a technical limit, because telescopes and instruments are of different
design. In the EUV range, the technology is strongly related to that utilized
in X-ray astronomy, while in the UV range the instruments in many cases have
their roots in optical astronomy. We will, therefore, describe the UV and EUV
instruments in appropriate conciseness and refer to the respective chapters of
this volume for more technical details.Comment: To appear in: Landolt-Boernstein, New Series VI/4A, Astronomy,
Astrophysics, and Cosmology; Instruments and Methods, ed. J.E. Truemper,
Springer-Verlag, Berlin, 201
3D Gasdynamic Modelling of the Changes in the Flow Structure During Transition From Quiescent to Active State in Symbiotic Stars
The results of 3D modelling of the flow structure in the classical symbiotic
system Z~Andromedae are presented. Outbursts in systems of this type occur when
the accretion rate exceeds the upper limit of the steady burning range.
Therefore, in order to realize the transition from a quiescent to an active
state it is necessary to find a mechanism able to sufficiently increase the
accretion rate on a time scale typical to the duration of outburst development.
Our calculations have confirmed the transition mechanism from quiescence to
outburst in classic symbiotic systems suggested earlier on the basis of 2D
calculations (Bisikalo et al, 2002). The analysis of our results have shown
that for wind velocity of 20 km/s an accretion disc forms in the system. The
accretion rate for the solution with the disc is ~22.5-25% of the mass loss
rate of the donor, that is, ~4.5-5*10^(-8)Msun/yr for Z And. This value is in
agreement with the steady burning range for white dwarf masses typically
accepted for this system. When the wind velocity increases from 20 to 30 km/s
the accretion disc is destroyed and the matter of the disc falls onto the
accretor's surface. This process is followed by an approximately twofold
accretion rate jump. The resulting accretion rate growth is sufficient for
passing the upper limit of the steady burning range, thereby bringing the
system into an active state. The time during which the accretion rate is above
the steady burning value is in a very good agreement with observations.
The analysis of the results presented here allows us to conclude that small
variations in the donor's wind velocity can lead to the transition from the
disc accretion to the wind accretion and, as a consequence, to the transition
from quiescent to active state in classic symbiotic stars.Comment: 21 pages, 7 figure
The GAMMA-400 space observatory: status and perspectives
The present design of the new space observatory GAMMA-400 is presented in
this paper. The instrument has been designed for the optimal detection of gamma
rays in a broad energy range (from ~100 MeV up to 3 TeV), with excellent
angular and energy resolution. The observatory will also allow precise and high
statistic studies of the electron component in the cosmic rays up to the multi
TeV region, as well as protons and nuclei spectra up to the knee region. The
GAMMA-400 observatory will allow to address a broad range of science topics,
like search for signatures of dark matter, studies of Galactic and
extragalactic gamma-ray sources, Galactic and extragalactic diffuse emission,
gamma-ray bursts and charged cosmic rays acceleration and diffusion mechanism
up to the knee
E2F1 and KIAA0191 expression predicts breast cancer patient survival
<p>Abstract</p> <p>Background</p> <p>Gene expression profiling of human breast tumors has uncovered several molecular signatures that can divide breast cancer patients into good and poor outcome groups. However, these signatures typically comprise many genes (~50-100), and the prognostic tests associated with identifying these signatures in patient tumor specimens require complicated methods, which are not routinely available in most hospital pathology laboratories, thus limiting their use. Hence, there is a need for more practical methods to predict patient survival.</p> <p>Methods</p> <p>We modified a feature selection algorithm and used survival analysis to derive a 2-gene signature that accurately predicts breast cancer patient survival.</p> <p>Results</p> <p>We developed a tree based decision method that segregated patients into various risk groups using <it>KIAA0191 </it>expression in the context of <it>E2F1 </it>expression levels. This approach led to highly accurate survival predictions in a large cohort of breast cancer patients using only a 2-gene signature.</p> <p>Conclusions</p> <p>Our observations suggest a possible relationship between <it>E2F1 </it>and <it>KIAA0191 </it>expression that is relevant to the pathogenesis of breast cancer. Furthermore, our findings raise the prospect that the practicality of patient prognosis methods may be improved by reducing the number of genes required for analysis. Indeed, our <it>E2F1/KIAA0191 </it>2-gene signature would be highly amenable for an immunohistochemistry based test, which is commonly used in hospital laboratories.</p
The evolution of rotating stars
First, we review the main physical effects to be considered in the building
of evolutionary models of rotating stars on the Upper Main-Sequence (MS). The
internal rotation law evolves as a result of contraction and expansion,
meridional circulation, diffusion processes and mass loss. In turn,
differential rotation and mixing exert a feedback on circulation and diffusion,
so that a consistent treatment is necessary.
We review recent results on the evolution of internal rotation and the
surface rotational velocities for stars on the Upper MS, for red giants,
supergiants and W-R stars. A fast rotation is enhancing the mass loss by
stellar winds and reciprocally high mass loss is removing a lot of angular
momentum. The problem of the ``break-up'' or -limit is critically
examined in connection with the origin of Be and LBV stars. The effects of
rotation on the tracks in the HR diagram, the lifetimes, the isochrones, the
blue to red supergiant ratios, the formation of W-R stars, the chemical
abundances in massive stars as well as in red giants and AGB stars, are
reviewed in relation to recent observations for stars in the Galaxy and
Magellanic Clouds. The effects of rotation on the final stages and on the
chemical yields are examined, as well as the constraints placed by the periods
of pulsars. On the whole, this review points out that stellar evolution is not
only a function of mass M and metallicity Z, but of angular velocity
as well.Comment: 78 pages, 7 figures, review for Annual Review of Astronomy and
Astrophysics, vol. 38 (2000
Gravitating discs around black holes
Fluid discs and tori around black holes are discussed within different
approaches and with the emphasis on the role of disc gravity. First reviewed
are the prospects of investigating the gravitational field of a black
hole--disc system by analytical solutions of stationary, axially symmetric
Einstein's equations. Then, more detailed considerations are focused to middle
and outer parts of extended disc-like configurations where relativistic effects
are small and the Newtonian description is adequate.
Within general relativity, only a static case has been analysed in detail.
Results are often very inspiring, however, simplifying assumptions must be
imposed: ad hoc profiles of the disc density are commonly assumed and the
effects of frame-dragging and completely lacking. Astrophysical discs (e.g.
accretion discs in active galactic nuclei) typically extend far beyond the
relativistic domain and are fairly diluted. However, self-gravity is still
essential for their structure and evolution, as well as for their radiation
emission and the impact on the environment around. For example, a nuclear star
cluster in a galactic centre may bear various imprints of mutual star--disc
interactions, which can be recognised in observational properties, such as the
relation between the central mass and stellar velocity dispersion.Comment: Accepted for publication in CQG; high-resolution figures will be
available from http://www.iop.org/EJ/journal/CQ
A Study of an Outburst in the Classical Symbiotic Star Z And in a Colliding-Wind Model
Two-dimensional gas-dynamical modeling of the mass-flow structure is used to
study the outburst development in the classical symbiotic star Z And. The
stage-by-stage rise of the light during the outburst can be explained in the
framework of the colliding winds model. We suggest a scenario for the
development of the outburst and study the possible influence of the changes of
the flow structure on the light of the system. The model variations of the
luminosity due to the formation of a system of shocks are in good agreement
with the observed light variations
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