1,424 research outputs found
Initial Magnetization of Galaxies by Exploding, Magnetized Stars
We conduct a series of magnetohydrodynamical (MHD) simulations of magnetized
interstellar medium (ISM) disturbed by exploding stars. Each star deposits a
randomly oriented, dipolar magnetic field into ISM. The simulations are
performed in a Cartesian box, in a reference frame that is corotating with the
galactic disk. The medium is stratified by vertical galactic gravity. The
resulting turbulent state of ISM magnetized by the stellar explosions is
processed with the aid of Fourier analysis. The results leads to the conclusion
that the input of magnetic energy from exploding stars is additionally
multiplied by differential rotation. The resulting magnetic field appears to
grow up in small-scale component, while the total magnetic flux remains
limited. Our results indicate that magnetic field originating from exploding
stars can be a source of initial magnetic fields for a subsequent dynamo
process.Comment: Published in Proceedings of the 14th Young Scientists Conference on
Astronomy and Space Physics, Kyiv, Ukraine, April 23-28, 200
Fine-Grained Complexity of k-OPT in Bounded-Degree Graphs for Solving TSP
The Traveling Salesman Problem asks to find a minimum-weight Hamiltonian cycle in an edge-weighted complete graph. Local search is a widely-employed strategy for finding good solutions to TSP. A popular neighborhood operator for local search is k-opt, which turns a Hamiltonian cycle C into a new Hamiltonian cycle C\u27 by replacing k edges. We analyze the problem of determining whether the weight of a given cycle can be decreased by a k-opt move. Earlier work has shown that (i) assuming the Exponential Time Hypothesis, there is no algorithm that can detect whether or not a given Hamiltonian cycle C in an n-vertex input can be improved by a k-opt move in time f(k) n^o(k / log k) for any function f, while (ii) it is possible to improve on the brute-force running time of O(n^k) and save linear factors in the exponent. Modern TSP heuristics are very successful at identifying the most promising edges to be used in k-opt moves, and experiments show that very good global solutions can already be reached using only the top-O(1) most promising edges incident to each vertex. This leads to the following question: can improving k-opt moves be found efficiently in graphs of bounded degree? We answer this question in various regimes, presenting new algorithms and conditional lower bounds. We show that the aforementioned ETH lower bound also holds for graphs of maximum degree three, but that in bounded-degree graphs the best improving k-move can be found in time O(n^((23/135+epsilon_k)k)), where lim_{k -> infty} epsilon_k = 0. This improves upon the best-known bounds for general graphs. Due to its practical importance, we devote special attention to the range of k in which improving k-moves in bounded-degree graphs can be found in quasi-linear time. For k <= 7, we give quasi-linear time algorithms for general weights. For k=8 we obtain a quasi-linear time algorithm when the weights are bounded by O(polylog n). On the other hand, based on established fine-grained complexity hypotheses about the impossibility of detecting a triangle in edge-linear time, we prove that the k = 9 case does not admit quasi-linear time algorithms. Hence we fully characterize the values of k for which quasi-linear time algorithms exist for polylogarithmic weights on bounded-degree graphs
Single spin optical read-out in CdTe/ZnTe quantum dot studied by photon correlation spectroscopy
Spin dynamics of a single electron and an exciton confined in CdTe/ZnTe
quantum dot is investigated by polarization-resolved correlation spectroscopy.
Spin memory effects extending over at least a few tens of nanoseconds have been
directly observed in magnetic field and described quantitatively in terms of a
simple rate equation model. We demonstrate an effective (68%) all-optical
read-out of the single carrier spin state through probing the degree of
circular polarization of exciton emission after capture of an oppositely
charged carrier. The perturbation introduced by the pulsed optical excitation
serving to study the spin dynamics has been found to be the main source of the
polarization loss in the read-out process. In the limit of low laser power the
read-out efficiency extrapolates to a value close to 100%. The measurements
allowed us as well to determine neutral exciton spin relaxation time ranging
from 3.4 +/- 0.1 ns at B = 0 T to 16 +/- 3 ns at B = 5 T.Comment: to appear in Phys. Rev.
Three red giants with substellar-mass companions
We present three giant stars from the ongoing Penn State-Toru\'n Planet
Search with the Hobby-Eberly Telescope, which exhibit radial velocity
variations that point to a presence of planetary --mass companions around them.
BD+49 828 is a K0 giant with a = minimum mass companion in
AU (d),
orbit. HD 95127, a log/=,
, K0 giant has a = minimum mass companion in
AU (d), orbit.
Finally, HD 216536, is a K0 giant with a minimum mass companion in
AU (d),
orbit. Both, HD 95127 b and HD 216536 b in their
compact orbits, are very close to the engulfment zone and hence prone to
ingestion in the near future. BD+49 828 b is among the longest period planets
detected with the radial velocity technique until now and it will remain
unaffected by stellar evolution up to a very late stage of its host. We discuss
general properties of planetary systems around evolved stars and planet
survivability using existing data on exoplanets in more detail.Comment: 47 pages, 11 figures. Accepted by Ap
Optical alignment and polarization conversion of neutral exciton spin in individual InAs/GaAs quantum dots
We investigate exciton spin memory in individual InAs/GaAs self-assembled
quantum dots via optical alignment and conversion of exciton polarization in a
magnetic field. Quasiresonant phonon-assisted excitation is successfully
employed to define the initial spin polarization of neutral excitons. The
conservation of the linear polarization generated along the bright exciton
eigenaxes of up to 90% and the conversion from circular- to linear polarization
of up to 47% both demonstrate a very long spin relaxation time with respect to
the radiative lifetime. Results are quantitatively compared with a model of
pseudo-spin 1/2 including heavy-to-light hole mixing.Comment: 5 pages, 3 figure
Animal models: A useful tool to unveil metabolic changes in hepatocellular carcinoma
Hepatocellular carcinoma (HCC) is one the most frequent and lethal human cancers. At present, no effective treatment for advanced HCC exist; therefore, the overall prognosis for HCC patients remains dismal. In recent years, a better knowledge of the signaling pathways involved in the regulation of HCC development and progression, has led to the identification of novel potential targets for therapeutic strategies. However, the obtained benefits from current therapeutic options are disappointing. Altered cancer metabolism has become a topic of renewed interest in the last decades, and it has been included among the core hallmarks of cancer. In the light of growing evidence for metabolic reprogramming in cancer, a wide number of experimental animal models have been exploited to study metabolic changes characterizing HCC development and progression and to further expand our knowledge of this tumor. In the present review, we discuss several rodent models of hepatocarcinogenesis, that contributed to elucidate the metabolic profile of HCC and the implications of these changes in modulating the aggressiveness of neoplastic cells. We also highlight the apparently contrasting results stemming from different animal models. Finally, we analyze whether these observations could be exploited to improve current therapeutic strategies for HCC
Tracking Advanced Planetary Systems (TAPAS) with HARPS-N. III. HD 5583 and BD+15 2375 - two cool giants with warm companions
Evolved stars are crucial pieces to understand the dependency of the planet
formation mechanism on the stellar mass and to explore deeper the mechanism
involved in star-planet interactions. Over the past ten years, we have
monitored about 1000 evolved stars for radial velocity variations in search for
low-mass companions under the Penn State - Torun Centre for Astronomy Planet
Search program with the Hobby-Eberly Telescope. Selected prospective candidates
that required higher RV precision measurements have been followed with HARPS-N
at the 3.6 m Telescopio Nazionale Galileo under the TAPAS project.
We aim to detect planetary systems around evolved stars to be able to build
sound statistics on the frequency and intrinsic nature of these systems, and to
deliver in-depth studies of selected planetary systems with evidence of
star-planet interaction processes. For HD 5583 we obtained 14 epochs of precise
RV measurements collected over 2313 days with the Hobby-Eberly Telescope (HET),
and 22 epochs of ultra-precise HARPS-N data collected over 976 days. For BD+15
2375 we collected 24 epochs of HET data over 3286 days and 25 epochs of HARPS-S
data over 902 days.
We report the discovery of two planetary mass objects orbiting two evolved
Red Giant stars: HD~5583 has a m sin i = 5.78 M companion at 0.529~AU in
a nearly circular orbit (e=0.076), the closest companion to a giant star
detected with the RV technique, and BD+15~2735 that with a m sin i= 1.06
M holds the record of the lightest planet found so far orbiting an
evolved star (in a circular e=0.001, 0.576~AU orbit). These are the third and
fourth planets found within the TAPAS project, a HARPS-N monitoring of evolved
planetary systems identified with the Hobby-Eberly Telescope.Comment: 9 pages, 6 figures. Accepted by Astronomy and Astrophysic
TAPAS IV. TYC 3667-1280-1 b - the most massive red giant star hosting a warm Jupiter
We present the latest result of the TAPAS project that is devoted to intense
monitoring of planetary candidates that are identified within the
PennState-Toru\'n planet search.
We aim to detect planetary systems around evolved stars to be able to build
sound statistics on the frequency and intrinsic nature of these systems, and to
deliver in-depth studies of selected planetary systems with evidence of
star-planet interaction processes.
The paper is based on precise radial velocity measurements: 13 epochs
collected over 1920 days with the Hobby-Eberly Telescope and its
High-Resolution Spectrograph, and 22 epochs of ultra-precise HARPS-N data
collected over 961 days.
We present a warm-Jupiter (, 0.4)
companion with an orbital period of 26.468 days in a circular () orbit
around a giant evolved (, ) star
with . This is the most massive and oldest star
found to be hosting a close-in giant planet. Its proximity to its host
() means that the planet has a probability of
transits; this calls for photometric follow-up study.
This massive warm Jupiter with a near circular orbit around an evolved
massive star can help set constraints on general migration mechanisms for warm
Jupiters and, given its high equilibrium temperature, can help test energy
deposition models in hot Jupiters.Comment: 5 pages, 3 figures, accepted by A&
TAPAS - Tracking Advanced Planetary Systems with HARPS-N. II. Super Li-rich giant HD 107028
Lithium rich giant stars are rare objects. For some of them, Li enrichment
exceeds abundance of this element found in solar system meteorites, suggesting
that these stars have gone through a Li enhancement process. We identified a Li
rich giant HD 107028 with A(Li) > 3.3 in a sample of evolved stars observed
within the PennState Torun Planet Search. In this work we study different
enhancement scenarios and we try to identify the one responsible for Li
enrichment for HD 107028. We collected high resolution spectra with three
different instruments, covering different spectral ranges. We determine stellar
parameters and abundances of selected elements with both equivalent width
measurements and analysis, and spectral synthesis. We also collected multi
epoch high precision radial velocities in an attempt to detect a companion.
Collected data show that HD 107028 is a star at the base of Red Giant Branch.
Except for high Li abundance, we have not identified any other anomalies in its
chemical composition, and there is no indication of a low mass or stellar
companion. We exclude Li production at the Luminosity Function Bump on RGB, as
the effective temperature and luminosity suggest that the evolutionary state is
much earlier than RGB Bump. We also cannot confirm the Li enhancement by
contamination, as we do not observe any anomalies that are associated with this
scenario. After evaluating various scenarios of Li enhancement we conclude that
the Li-overabundance of HD 107028 originates from Main Sequence evolution, and
may be caused by diffusion process.Comment: Accepted for publication in A&
MnAs dots grown on GaN(0001)-(1x1) surface
MnAs has been grown by means of MBE on the GaN(0001)-(1x1) surface. Two
options of initiating the crystal growth were applied: (a) a regular MBE
procedure (manganese and arsenic were delivered simultaneously) and (b)
subsequent deposition of manganese and arsenic layers. It was shown that
spontaneous formation of MnAs dots with the surface density of 1
cm and cm, respectively (as observed by AFM),
occurred for the layer thickness higher than 5 ML. Electronic structure of the
MnAs/GaN systems was studied by resonant photoemission spectroscopy. That led
to determination of the Mn 3d - related contribution to the total density of
states (DOS) distribution of MnAs. It has been proven that the electronic
structures of the MnAs dots grown by the two procedures differ markedly. One
corresponds to metallic, ferromagnetic NiAs-type MnAs, the other is similar to
that reported for half-metallic zinc-blende MnAs. Both system behave
superparamagnetically (as revealed by magnetization measurements), but with
both the blocking temperatures and the intra-dot Curie temperatures
substantially different. The intra-dot Curie temperature is about 260 K for the
former system while markedly higher than room temperature for the latter one.
Relations between growth process, electronic structure and other properties of
the studied systems are discussed. Possible mechanisms of half-metallic MnAs
formation on GaN are considered.Comment: 20+ pages, 8 figure
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