56,639 research outputs found
Acoustic power absorption and enhancement generated by slow and fast MHD waves
We used long duration, high quality, unresolved (Sun-as-a star) observations
collected by the ground based network BiSON and by the instruments GOLF and
VIRGO on board the ESA/NASA SOHO satellite to search for solar-cycle-related
changes in mode characteristics in velocity and continuum intensity for the
frequency range between 2.5mHz < nu < 6.8mHz. Over the ascending phase of solar
cycle 23 we found a suppression in the p-mode amplitudes both in the velocity
and intensity data between 2.5mHz <nu< 4.5mHz with a maximum suppression for
frequencies in the range between 2.5mHz <nu< 3.5mHz. The size of the amplitude
suppression is 13+-2 per cent for the velocity and 9+-2 per cent for the
intensity observations. Over the range 4.5mHz <nu< 5.5mHz the findings hint
within the errors to a null change both in the velocity and intensity
amplitudes. At still higher frequencies, in the so called High-frequency
Interference Peaks (HIPs) between 5.8mHz <nu < 6.8mHz, we found an enhancement
in the velocity amplitudes with the maximum 36+-7 per cent occurring for 6.3mHz
<nu< 6.8mHz. However, in intensity observations we found a rather smaller
enhancement of about 5+-2 per cent in the same interval. There is evidence that
the frequency dependence of solar-cycle velocity amplitude changes is
consistent with the theory behind the mode conversion of acoustic waves in a
non-vertical magnetic field, but there are some problems with the intensity
data, which may be due to the height in the solar atmosphere at which the VIRGO
data are taken.Comment: Accepted for publication in A&A. 10 pages, 9 figures
Spectroscopic Observations of Planetary Nebulae in the Northern Spur of M31
We present spectroscopy of three planetary nebulae (PNe) in the Northern Spur
of the Andromeda Galaxy (M31) obtained with the Double Spectrograph on the 5.1
m Hale Telescope at the Palomar Observatory. The samples are selected from the
observations of Merrett et al. Our purpose is to investigate formation of the
substructures of M31 using PNe as a tracer of chemical abundances. The [O III]
4363 auroral line is detected in the spectra of two objects, enabling
temperature determinations. Ionic abundances are derived from the observed
collisionally excited lines, and elemental abundances of nitrogen, oxygen, and
neon as well as sulphur and argon are estimated. Correlations between oxygen
and the alpha-element abundance ratios are studied, using our sample and the
M31 disk and bulge PNe from the literature. In one of the three PNe, we
observed relatively higher oxygen abundance compared to the disk sample in M31
at similar galactocentric distances. The results of at least one of the three
Northern Spur PNe might be in line with the proposed possible origin of the
Northern Spur substructure of M31, i.e. the Northern Spur is connected to the
Southern Stream and both substructures comprise the tidal debris of the
satellite galaxies of M31.Comment: 5 tables, 17 figures; accepted for publication in Ap
Design And Optimization Of Pv/diesel Hybrid Power System In A Hotel
Electric power is a necessity for the development of a society, without it would be impossible to see the world as we actually know. Its use is closely related to modern life and human development. Renewable energy is a good alternative for energy supply in regions where conventional power networks are absent or service quality is intermittent. The importance of these renewable energy sources lies in their lower emissions of carbon dioxide (CO2) to atmosphere and the reduced dependency on raw materials for importation and transport for power generation. Renewable energy is becoming economically competitive in the short to medium term. Hybrid power systems is a novel approach recently gaining popularity, since they combine multiple sources of renewable energy such as solar, wind, mini hydro-power, and also could include conventional generators as support. This combination allows the optimization of the power generation system by reducing emissions from the petroleum and coal based energy sources. This paper presents a pilot hybrid power system design for the power supply in the hotel ‘La mansion’ in the town of Acacias, Meta (Colombia). Simulation and optimization were based on a computer program known as HOGA educational version 2.2, which initially considered economic parameters associated to photovoltaic system such as: overall power, replacement cost and operation and maintenance costs, as well as all powers to consider in the simulation. The main purpose is to present an alternative power source in the hotel located in an area with high rates of power outages, and gradually include photovoltaic solar power combined with conventional energy supply, to meet the energy demand when the peak consumption periods occurs during power outages from the power grid. This paper confirmed that hybrid power systems are a good energy supply alternative as auxiliary small power systems because they are capable of supplying the power demands during power grid outages or even in total absence of the power grid. Simulations showed that the combination of diesel and solar photovoltaics is a good alternative to meet lighting power demand in the hotel ‘La Mansion’, even in night hours thanks to the battery storag
Signatures of magnetic activity in the seismic data of solar-type stars observed by Kepler
In the Sun, the frequencies of the acoustic modes are observed to vary in
phase with the magnetic activity level. These frequency variations are expected
to be common in solar-type stars and contain information about the
activity-related changes that take place in their interiors. The unprecedented
duration of Kepler photometric time-series provides a unique opportunity to
detect and characterize stellar magnetic cycles through asteroseismology. In
this work, we analyze a sample of 87 solar-type stars, measuring their temporal
frequency shifts over segments of length 90 days. For each segment, the
individual frequencies are obtained through a Bayesian peak-bagging tool. The
mean frequency shifts are then computed and compared with: 1) those obtained
from a cross-correlation method; 2) the variation in the mode heights; 3) a
photometric activity proxy; and 4) the characteristic timescale of the
granulation. For each star and 90-d sub-series, we provide mean frequency
shifts, mode heights, and characteristic timescales of the granulation.
Interestingly, more than 60% of the stars show evidence for (quasi-)periodic
variations in the frequency shifts. In the majority of the cases, these
variations are accompanied by variations in other activity proxies. About 20%
of the stars show mode frequencies and heights varying approximately in phase,
in opposition to what is observed for the Sun.Comment: Accepted for publication in ApJS, 19(+86) pages, 11(+89) figures,
2(+87) table
Seismic signatures of magnetic activity in solar-type stars observed by Kepler
The properties of the acoustic modes are sensitive to magnetic activity. The
unprecedented long-term Kepler photometry, thus, allows stellar magnetic cycles
to be studied through asteroseismology. We search for signatures of magnetic
cycles in the seismic data of Kepler solar-type stars. We find evidence for
periodic variations in the acoustic properties of about half of the 87 analysed
stars. In these proceedings, we highlight the results obtained for two such
stars, namely KIC 8006161 and KIC 5184732.Comment: 4 pages, 1 figure, to appear in the Proceedings of the IAUS34
Dynamics of bright matter wave solitons in a quasi 1D Bose-Einstein condensate with a rapidly varying trap
The dynamics of a bright matter wave soliton in a quasi 1D Bose-Einstein
condensate with periodically rapidly varying trap is considered. The governing
equation is derived based on averaging over fast modulations of the
Gross-Pitaevskii (GP) equation. This equation has the form of GP equation with
effective potential of more complicated structure than unperturbed trap. For
the case of inverted (expulsive) quadratic trap corresponding to unstable GP
equation, the effective potential can be stable. For the bounded in space trap
potential it is showed that the bifurcation exists, i.e.,the single well
potential bifurcates to the triple well effective potential. Stabilization of
BEC cloud on-site state in the temporary modulated optical lattice is found.
(analogous to the Kapitza stabilization of the pendulum). The predictions of
the averaged GP equation are confirmed by the numerical simulations of GP
equation with rapid perturbations.Comment: 15 pages, 4 figure
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