173 research outputs found
Masses and angular momenta of contact binary stars
Results are presented on component masses and system angular momenta for over
a hundred low-temperature contact binaries. It is found that the secondary
components in close binary systems are very similar in mass. Our observational
evidence strongly supports the argument that the evolutionary process goes from
near-contact binaries to A-type contact binaries, without any need of mass loss
from the system. Furthermore, the evolutionary direction of A-type into W-type
systems with a simultaneous mass and angular momentum loss is also discussed.
The opposite direction of evolution seems to be unlikely, since it requires an
increase of the total mass and the angular momentum of the system.Comment: 4 pages, 4 figures, accepted in MNRA
YY CMi: contact or near contact system?
New V photoelectric observations of the eclipsing system YY CMi, obtained at
La Silla, Chile, and Merate Observatory, Italy, are presented. New times of
minima and ephemeris based on our observations are also given. The V light
curve was analysed by using the WD code to derive the geometrical and physical
parameters of the system. Since no spectroscopic mass ratio is available, the
q-search method was applied to yield the preliminary range of the mass ratio in
order to search for the final solution. First the unspotted solution was
carried out by using the unperturbed parts of the light curve and applying the
DC program of the WD code. The solution was performed by assuming contact (mode
3) and semi-detached (mode 4) configuration, since no classification of the
system is possible from the shape of the light curve. The solution in mode 4
does not lead to an acceptable model, since the secondary was found to be
slightly overcontact. Therefore the contact solution was finally adopted.
Moreover the light curve peculiarities (Max II fainter than Max I and excess of
light around the phase 0.32) were explained by assuming a cool and a hot spot
on the surface of the secondary (cooler) component. The degree of contact is
very small (f about 3%) and the thermal contactis poor (T1-T2) about 650 K.
These results together with the high photometric mass ratio q about 0.89
indicate that YY CMi is very probably a system at the beginning or the end of
the contact phase.Comment: 7 pages, 7 ps figures. Accepted for Astronomy and Astrophysics,
Supplement Serie
The problem of the high mass of the hot component in the recurrent nova T Coronae Borealis solved after 38 years
In this paper, we present long-term spectroscopic observations of the recurrent nova (RN) T CrB obtained between the years 1985 and 1996 using 1.02 m and 2.34 m telescopes of the Vainu Bappu Observatory in India and the long-term photoelectric photometry of the object obtained mainly at the Skalnaté Pleso Observatory and at the Hlohovec Observatory in Slovakia. On the basis of our results, we have returned to the re-analysis of the old radial-velocity measurements published by Kraft (1958). The results obtained solve unambiguously the problem of the apparently too high mass of the hot component of T CrB, unacceptable if it has to be a white dwarf (WD), as several independent lines of evidence suggest
CoBiToM Project -- II: Evolution of contact binary systems close to the orbital period cut-off
Ultra-short orbital period contact binaries (Porb < 0.26 d) host some of the
smallest and least massive stars. These systems are faint and rare, and it is
believed that they have reached a contact configuration after several Gyrs of
evolution via angular momentum loss, mass transfer and mass loss through
stellar wind processes. This study is conducted in the frame of Contact
Binaries Towards Merging (CoBiToM) Project and presents the results from light
curve and orbital analysis of 30 ultra-short orbital period contact binaries,
with the aim to investigate the possibility of them being red nova progenitors,
eventually producing merger events. Approximately half of the systems exhibit
orbital period modulations, as a result of mass transfer or mass loss
processes. Although they are in contact, their fill-out factor is low (less
than 30 per cent), while their mass ratio is larger than the one in longer
period contact binaries. The present study investigates the orbital stability
of these systems and examines their physical and orbital parameters in
comparison to those of the entire sample of known and well-studied contact
binaries, based on combined spectroscopic and photometric analysis. It is found
that ultra-short orbital period contact binaries have very stable orbits, while
very often additional components are gravitationally bound in wide orbits
around the central binary system. We confirmed that the evolution of such
systems is very slow, which explains why the components of ultra-short orbital
period systems are still Main Sequence stars after several Gyrs of evolution
Resolving the pulsations of the subdwarf B star KPD 2109+4401
We present the results of extensive time series photometry of the pulsating
subdwarf B star KPD 2109+4401. Our data set consists of 29 data runs with a
total length of 182.6 hours over 31 days, collected at five observatories in
2004. These data are comprised of high signal-to-noise observations acquired
with larger telescopes and wider time coverage observations obtained with
smaller telescopes. They are sufficient to resolve the pulsation structure to
0.4 Hz and are the most extensive data set for this star to date. With
these data, we identify eight pulsation frequencies extending from 4701 to 5481
Hz, corresponding to periods of 182 to 213 s. The pulsation frequencies
and their amplitudes are examined over several time-scales with some
frequencies showing amplitude variability.Comment: 8 pages, 4 figures, 5 tables; Accepted for publication in MNRA
Notes on Tachyon Effective Actions and Veneziano Amplitudes
In a previous paper (hep-th/0304045) it has been argued that tachyonic
Dirac-Born-Infeld (DBI) actions can be obtained from open string theory in a
limit, which generalizes the usual massless DBI limit. In the present note we
review this construction focusing on a key property of the proposed tachyon
effective actions: how they reproduce appropriate Veneziano amplitudes in a
suitably defined kinematical region. Possible extensions and interesting open
problems are briefly discussed.Comment: harvmac, 31 pages, 15 figures; v2 minor text modification
The Topological Cigar Observables
We study the topologically twisted cigar, namely the SL(2,R)/U(1)
superconformal field theory at arbitrary level, and find the BRST cohomology of
the topologically twisted N=2 theory. We find a one to one correspondence
between the spectrum of the twisted coset and singular vectors in the Wakimoto
modules constructed over the SL(2,R) current algebra. The topological cigar
cohomology is the crucial ingredient in calculating the closed string spectrum
of topological strings on non-compact Gepner models.Comment: 28 page
Realization of Brane Descent Relations in Effective Theories
We examine Sen's descent relations among (non-)BPS D-branes by using low
energy effective field theories of DpDpbar system. We find that the fluctuation
around the kink solution reproduces the low energy matter content on a non-BPS
D(p-1)-brane. The effective action for these fluctuation modes turns out to be
a generalization of Minahan-Zwiebach model. In addition, it is shown that the
fluctuations around the vortex solution consist of massless fields on a BPS
D(p-2)-brane and they are subject to Dirac-Born-Infeld action. We find the
universality that the above results do not refer to particular forms of the
effective action.Comment: 24 pages, LaTeX, 1 eps figure; v2:minor correction
Substitutional effects of in by Cu in CeIn2
We have investigated the evolution of the magnetic properties on the Ce(In1'xCux)2 (0 < x ≤ 0.3) series of alloys. The orthorhombic structure of the CeIn2 alloy (Imma) changes into the hexagonal AlB2-type (P6/mmm) for x = 0.05 and, then, into the hexagonal CaIn2-type (P63/mmm) for higher Cu concentrations, up to x = 0.3. The dc (ac) magnetic susceptibility shows an abrupt decrease of the magnetic transition temperature from 22 K to 5.4 K (x = 0.05). The results indicate the influence of the crystallographic type of structure and disorder effects on the magnetic behavior along the series. © Owned by the authors, published by EDP Sciences, 2014
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