40 research outputs found
Evolution of the progenitor binary of V1309 Scorpii before merger
It was recently demonstrated that the eruption of V1309 Sco was a result of a
merger of the components of a cool contact binary. We computed a set of
evolutionary models of the detached binaries with different initial parameters
to compare it with pre-burst observations of V1309 Sco. The models are based on
our recently developed evolutionary model of the formation of cool contact
binaries. The best agreement with observations was obtained for binaries with
initial masses of 1.8-2.0 solar masses and initial periods of 2.5-3.1 d. The
evolution of these binaries consists of three phases: at first the binary is
detached and both components lose mass and angular momentum through a
magnetized wind. This takes almost two thirds of the total evolutionary
lifetime. The remaining third is spent in a semi-detached configuration of the
Algol-type, following the Roche-lobe overflow by the initially more massive
component. When the other component leaves the main sequence and moves toward
the giant branch, a contact configuration is formed for a short time, followed
by the coalescence of both components.Comment: 5 pages, 1 figure, Astronomy and Astrophysics, in prin
Effective temperatures of magnetic CP stars from full spectral energy distributions
New determinations of effective temperatures of 23 magnetic, chemically
peculiar (mCP) stars were obtained from a fit of metal enhanced model
atmospheres to the observed spectral energy distributions (SED) from UV to red.
The root-mean-square (RMS) method was used to fit the theoretical SED to the
observations corrected for reddening if necessary, with metallicity and
effective temperature as the fitting parameters. Gravity was assumed to be
equal to log g = 4 for main sequence stars and to log g = 3 for two giants in
the considered sample. Equal weights were given to the UV part and visual part
of SED. Independently of the formal quality of fit resulting from the RMS
method applied to the whole SED, the quality of fit was additionally checked
for each star by determination of the temperature from the best fitting model
atmosphere to the UV part and the visual part of SED separately. Both
temperatures should be close to one another if the global best fitting model
satisfactorily describes the full observed SED. This is the case for about a
half of the investigated stars but the difference exceeds 750 K for the
remaining stars with the extreme values above 2000 K. Possible reasons for such
discrepancies are discussed. New, revised calibrations of effective temperature
and bolometric corrections of mCP stars in terms of reddening free Stromgren
indices are given.Comment: 13 pages, 5 figures, accepted to MNRA
The Araucaria Project: accurate stellar parameters and distance to evolved eclipsing binary ASAS J180057-2333.8 in Sagittarius Arm
We have analyzed the double-lined eclipsing binary system ASAS J180057-2333.8 from the All Sky Automated Survey (ASAS) catalogue. We measure absolute physical and orbital parameters for this system based on archival V-band and I-band ASAS photometry, as well as on high-resolution spectroscopic data obtained with ESO 3.6m/HARPS and CORALIE spectrographs. The physical and orbital parameters of the system were derived with an accuracy of about 0.5-3 percent. The system is a very rare configuration of two bright well-detached giants of spectral types K1 and K4 and luminosity class II. The radii of the stars are R1 = 52.12 ± 1.38 and R2 = 67.63 ± 1.40 R⊙ and their masses are M1 = 4.914 ± 0.021 and M2 = 4.875 ± 0.021M⊙. The exquisite accuracy of 0.5 percent obtained for the masses of the components is one of the best mass determinations for giants. We derived a precise distance to the system of 2.14 ± 0.06kpc (stat.) ± 0.05 (syst.) which places the star in the Sagittarius-Carina arm. The Galactic rotational velocity of the star is Θs = 258 ± 26kms−1 assuming Θ0 = 238kms−1. A comparison with parsec isochrones places the system at the early phase of core helium burning with an age of slightly larger than 100 million years. The effect of overshooting on stellar evolutionary tracks was explored using the mesa star cod
Basal Chromospheric Flux and Maunder Minimum-type Stars: The quiet-Sun Chromosphere as a Universal Phenomenon
Aims: We demonstrate the universal character of the quiet-Sun chromosphere
among inactive stars (solar-type and giants). By assessing the main physical
processes, we shed new light on some common observational phenomena. Methods:
We discuss measurements of the solar Mt. Wilson S-index, obtained by the
Hamburg Robotic Telescope around the extreme minimum year 2009, and compare the
established chromospheric basal Ca II K line flux to the Mt. Wilson S-index
data of inactive ("flat activity") stars, including giants. Results: During the
unusually deep and extended activity minimum of 2009, the Sun reached S-index
values considerably lower than in any of its previously observed minima. In
several brief periods, the Sun coincided exactly with the S-indices of inactive
("flat", presumed Maunder Minimum-type) solar analogues of the Mt. Wilson
sample; at the same time, the solar visible surface was also free of any plages
or remaining weak activity regions. The corresponding minimum Ca II K flux of
the quiet Sun and of the presumed Maunder Minimum-type stars in the Mt. Wilson
sample are found to be identical to the corresponding Ca II K chromospheric
basal flux limit. Conclusions: We conclude that the quiet-Sun chromosphere is a
universal phenomenon among inactive stars. Its mixed-polarity magnetic field,
generated by a local, "fast" turbulent dynamo finally provides a natural
explanation for the minimal soft X-ray emission observed for inactive stars.
Given such a local dynamo also works for giant chromospheres, albeit on larger
length scales, i.e., l ~ R/g, with R and g as stellar radius and surface
gravity, respectively, the existence of giant spicular phenomena and the
guidance of mechanical energy toward the acceleration zone of cool stellar
winds along flux-tubes have now become traceable.Comment: 6 pages, 4 figures; Astronomy & Astrophysics (Research Note), in
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Feasibility study of the positronium imaging with the J-PET tomograph
A detection system of the conventional PET tomograph is set-up to record data
from e+ e- annihilation into two photons with energy of 511 keV, and it gives
information on the density distribution of a radiopharmaceutical in the body of
the object. In this paper we explore the possibility of performing the three
gamma photons imaging based on ortho-positronium annihilation, as well as the
possibility of positronium mean lifetime imaging with the J-PET tomograph
constructed from plastic scintillators. For this purposes simulations of the
ortho-positronium formation and its annihilation into three photons were
performed taking into account distributions of photons' momenta as predicted by
the theory of quantum electrodynamics and the response of the J-PET tomograph.
In order to test the proposed ortho-positronium lifetime image reconstruction
method, we concentrate on the decay of the ortho-positronium into three photons
and applications of radiopharmaceuticals labeled with isotopes emitting a
prompt gamma quantum. The proposed method of imaging is based on the
determination of hit-times and hit-positions of registered photons which
enables the reconstruction of the time and position of the annihilation point
as well as the lifetime of the ortho-positronium on an event-by-event basis. We
have simulated the production of the positronium in a cylindrical phantom
composed of a set of different materials in which the ortho-positronium
lifetime varied from 2.0 ns to 3.0 ns, as expected for ortho-positronium
created in the human body. The presented reconstruction method for total-body
J-PET like detector allows to achieve a mean lifetime resolution of about 40
ps. Recent Positron Annihilation Lifetime Spectroscopy measurements of
cancerous and healthy uterine tissues show that this sensitivity may allow to
study the morphological changes in cell structures.Comment: accepted in PMB
(http://iopscience.iop.org/article/10.1088/1361-6560/aafe20
Study of the Sigma-nucleus potential by the (pi^-,K^+) reaction on medium-to-heavy nuclear targets
In order to study the Sigma-nucleus optical potential, we measured inclusive
(pi^-,K^+) spectra on medium-to-heavy nuclear targets: CH_2, Si, Ni, In and Bi.
The CH_2 target was used to calibrate the excitation energy scale by using the
elementary process p + pi^- -> K^+ + Sigma^-, where the C spectrum was also
extracted. The calibration was done with +-0.1 MeV precision. The angular
distribution of the elementary cross section was measured, and agreed well with
the previous bubble chamber data, but with better statistics, and the
magnitudes of the cross sections of the measured inclusive (pi^-,K^+) spectra
were also well calibrated. All of the inclusive spectra were found to be
similar in shape at a region near to the Sigma^- binding energy threshold,
showing a weak mass-number dependence on the magnitude of the cross section.
The measured spectra were compared with a theoretical calculation performed
within the framework of the Distorted Wave Impulse Approximation (DWIA). It has
been demonstrated that a strongly repulsive \sig-nucleus potential with a
non-zero size of the imaginary part is required to reproduce the shape of the
measured spectra.Comment: 21 pages, 24 figures, submitted to PR
Nonlinearity and Topology
The interplay of nonlinearity and topology results in many novel and emergent
properties across a number of physical systems such as chiral magnets, nematic
liquid crystals, Bose-Einstein condensates, photonics, high energy physics,
etc. It also results in a wide variety of topological defects such as solitons,
vortices, skyrmions, merons, hopfions, monopoles to name just a few.
Interaction among and collision of these nontrivial defects itself is a topic
of great interest. Curvature and underlying geometry also affect the shape,
interaction and behavior of these defects. Such properties can be studied using
techniques such as, e.g. the Bogomolnyi decomposition. Some applications of
this interplay, e.g. in nonreciprocal photonics as well as topological
materials such as Dirac and Weyl semimetals, are also elucidated