9,920 research outputs found
The role of turbulent pressure as a coherent pulsational driving mechanism: the case of the delta Scuti star HD 187547
HD 187547 was the first candidate that led to the suggestion that solar-like
oscillations are present in delta Scuti stars. Longer observations, however,
show that the modes interpreted as solar-like oscillations have either very
long mode lifetimes, longer than 960 days, or are coherent. These results are
incompatible with the nature of `pure' stochastic excitation as observed in
solar-like stars. Nonetheless, one point is certain: the opacity mechanism
alone cannot explain the oscillation spectrum of HD 187547. Here we present new
theoretical investigations showing that convection dynamics can intrinsically
excite coherent pulsations in the chemically peculiar delta Scuti star HD
187547. More precisely, it is the perturbations of the mean Reynold stresses
(turbulent pressure) that drives the pulsations and the excitation takes place
predominantly in the hydrogen ionization zone.Comment: 8 pages, 4 figures, accepted to Ap
Spin-polarized transport in ferromagnetic multilayered semiconductor nanostructures
The occurrence of inhomogeneous spin-density distribution in multilayered
ferromagnetic diluted magnetic semiconductor nanostructures leads to strong
dependence of the spin-polarized transport properties on these systems. The
spin-dependent mobility, conductivity and resistivity in
(Ga,Mn)As/GaAs,(Ga,Mn)N/GaN, and (Si,Mn)/Si multilayers are calculated as a
function of temperature, scaled by the average magnetization of the diluted
magnetic semiconductor layers. An increase of the resistivity near the
transition temperature is obtained. We observed that the spin-polarized
transport properties changes strongly among the three materials.Comment: 3 pages, 4 figure
A thorough analysis of the short- and mid-term activity-related variations in the solar acoustic frequencies
The frequencies of the solar acoustic oscillations vary over the activity
cycle. The variations in other activity proxies are found to be well correlated
with the variations in the acoustic frequencies. However, each proxy has a
slightly different time behaviour. Our goal is to characterize the differences
between the time behaviour of the frequency shifts and of two other activity
proxies, namely, the area covered by sunspots and the 10.7cm flux. We define a
new observable that is particularly sensitive to the short-term frequency
variations. We then compare the observable when computed from model frequency
shifts and from observed frequency shifts obtained with the Global Oscillation
Network Group (GONG) for cycle 23. Our analysis shows that on the shortest
time-scales the variations in the frequency shifts seen in the GONG
observations are strongly correlated with the variations in the area covered by
sunspots. However, a significant loss of correlation is still found. We verify
that the times when the frequency shifts and the sunspot area do not vary in a
similar way tend to coincide with the times of the maxima of the quasi-biennial
variations seen in the solar seismic data. A similar analysis of the relation
between the 10.7cm flux and the frequency shifts reveals that the short-time
variations in the frequency shifts follow even more closely those of the 10.7cm
flux than those of the sunspot area. However, a loss of correlation between
frequency shifts and 10.7cm flux variations is still found around the same
times.Comment: 7 pages, 6 figures, accepted for publication in MNRA
On the relation between activity-related frequency shifts and the sunspot distribution over the solar cycle 23
The activity-related variations in the solar acoustic frequencies have been
known for 30 years. However, the importance of the different contributions is
still not well established. With this in mind, we developed an empirical model
to estimate the spot-induced frequency shifts, which takes into account the
sunspot properties, such as area and latitude. The comparison between the model
frequency shifts obtained from the daily sunspot records and those observed
suggests that the contribution from a stochastic component to the total
frequency shifts is about 30%. The remaining 70% is related to a global,
long-term variation. We also propose a new observable to investigate the short-
and mid-term variations of the frequency shifts, which is insensitive to the
long-term variations contained in the data. On the shortest time scales the
variations in the frequency shifts are strongly correlated with the variations
in the total area covered by sunspots. However, a significant loss of
correlation is still found, which cannot be fully explained by ignoring the
invisible side of the Sun when accounting for the total sunspot area. We also
verify that the times when the frequency shifts and the sunspot areas do not
vary in a similar way tend to coincide with the times of the maximum amplitude
of the quasi-biennial variations found in the seismic data.Comment: 4 pages, 2 figures, proceedings of the Joint TASC2 - KASC9 Workshop -
SPACEINN - HELAS8 Conference "Seismology of the Sun and the Distant Stars
2016: Using Today's Successes to Prepare the Future". To be published by the
EPJ Web of Conference
Electron transfer driven decomposition of adenine and selected analogs as probed by experimental and theoretical methods
We report on a combined experimental and theoretical study of electron transfer induced decomposition of adenine and a selection of analogue molecules in collisions with potassium atoms (K). Time-of-flight negative ion mass spectra have been obtained in a wide collision energy range (6–68 eV in the centre-of-mass frame), providing a comprehensive investigation of the fragmentation patterns of purine, adenine, 9-methyl adenine, 6-dimethyl adenine and 2-D adenine. Following our recent communication about selective hydrogen loss from the transient negative ions (TNI) produced in these collisions [T. Dunha et al. J. Chem. Phys. 148, 021101 (2018)], this work focuses on the production of smaller fragment anions. In the low-energy part of the present range, several dissociation channels that are accessible in free electron attachment experiments are absent from the present mass spectra, notably NH2 loss from adenine and 9-methyl adenine. This can be understood in terms of a relatively long transit time of the K+ cation in the vicinity of the TNI tending to enhance the likelihood of intramolecular electron transfer. In this case, the excess energy can be redistributed through the available degrees of freedom inhibiting fragmentation pathways. Ab initio theoretical calculations were performed for 9-methyl adenine (9-mAd) and adenine (Ad) in the presence of a potassium atom and provided a strong basis for the assignment the lowest unoccupied molecular orbitals accessed in the collision process
Radial abundance gradients in the outer Galactic disk as traced by main-sequence OB stars
Using a sample of 31 main-sequence OB stars located between galactocentric
distances 8.4 - 15.6 kpc, we aim to probe the present-day radial abundance
gradients of the Galactic disk. The analysis is based on high-resolution
spectra obtained with the MIKE spectrograph on the Magellan Clay 6.5-m
telescope on Las Campanas. We used a non-NLTE analysis in a self-consistent
semi-automatic routine based on TLUSTY and SYNSPEC to determine atmospheric
parameters and chemical abundances. Stellar parameters (effective temperature,
surface gravity, projected rotational velocity, microturbulence, and
macroturbulence) and silicon and oxygen abundances are presented for 28 stars
located beyond 9 kpc from the Galactic centre plus three stars in the solar
neighborhood. The stars of our sample are mostly on the main-sequence, with
effective temperatures between 20800 - 31300 K, and surface gravities between
3.23 - 4.45 dex. The radial oxygen and silicon abundance gradients are negative
and have slopes of -0.07 dex/kpc and -0.09 dex/kpc, respectively, in the region
\,kpc. The obtained gradients are compatible with the
present-day oxygen and silicon abundances measured in the solar neighborhood
and are consistent with radial metallicity gradients predicted by
chemodynamical models of Galaxy Evolution for a subsample of young stars
located close to the Galactic plane.Comment: Accepted for publication in the A&
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