2,306 research outputs found
Eigenoscillations of the Differentially Rotating Sun: I. 22-year, 4000-year, and quasi-biennial modes
Retrograde waves with frequencies much lower than the rotation frequency
become trapped in the solar radiative interior. The eigenfunctions of the
compressible, nonadiabatic, Rossby-like modes (-mechanism and
radiative losses taken into account) are obtained by an asymptotic method
assuming a very small latitudinal gradient of rotation, without an arbitrary
choice of other free parameters. An integral dispersion relation for the
complex eigenfrequencies is derived as a solution of the boundary value
problem. The discovered resonant cavity modes (called R-modes) are
fundamentally different from the known r-modes: their frequencies are functions
of the solar interior structure, and the reason for their existence is not
related to geometrical effects. The most unstable R-modes are those with
periods of 1--3 yr, 18--30 yr, and 1500--20000 yrs; these three separate period
ranges are known from solar and geophysical data. The growing times of those
modes which are unstable with respect to the -mechanism are and years, respectively. The amplitudes of the R-modes are
growing towards the center of the Sun. We discuss some prospects to develop the
theory of R-modes as a driver of the dynamics in the convective zone which
could explain, e.g., observed short-term fluctuations of rotation, a control of
the solar magnetic cycle, and abrupt changes of terrestrial climate in the
past.Comment: 17 pages, 6 figures, To appear in Astronomy and Astrophysic
XMM-Newton observations of EF Eridani: the textbook example of low-accretion rate polars
Archival X-ray observations of EF Eridani obtained in a low state revealed
distinct X-ray detections at a luminosity L_X ~ 2 10^{29} erg/s, three orders
of magnitude below its high state value. The plasma temperature was found to be
as low as kT \loa 2 keV, a factor 10 below the high state. The X-ray/UV/IR
spectral energy distribution suggests faint residual accretion rather than
coronal emission as being responsible for the low-state X-ray emission. EF Eri
thus showed a clear transition from being shock-dominated in the high state to
be cyclotron-dominated in the low state. From the optical/UV spectral energy
distribution we re-determine the photospheric temperature of the white dwarf to
\~10000K. Contrary to earlier claims, WD model atmospheres produce sufficient
UV flux to reproduce the published GALEX flux and orbital modulation.Comment: A&A, in pres
Bidirectional waveguide coupling with plasmonic Fano nanoantennas
We introduce the concept of a bidirectional, compact single-element Fano nanoantenna that allows for directional coupling of light in opposite directions of a high-index dielectric waveguide for two different operation wavelengths. We utilize a Fano resonance to tailor the radiation phases of a gold nanodisk and a nanoslit that is inscribed into the nanodisk to realize bidirectional scattering. We show that this Fano nanoantenna operates as a bidirectional waveguide coupler at telecommunication wavelengths and, thus, is ideally suitable for integrated wavelength-selective light demultiplexing
Indirect imaging of AM Herculis-stars
AM Herculis stars (or 'polars') are close binaries consisting of a white dwarf with a strong magnetic field and a main sequence star (the 'secondary') which is losing mass to the white dwarf. The magnetic field channels the matter to the magnetic poles, where the released potential energy is radiated away - mainly in X-rays and the UV. The spatial structure of these objects cannot be observed directly, because their apparent size is too small. Indirect imaging methods are capable of restoring information which is only contained in observational data as projections onto a sub-space. These tomographic methods are known from medicine, but can also be applied to astronomical data, when a number of measurements can be regarded as views of the same object under different aspect angles. In this work tomographic methods are applied to time-resolved spectroscopic data of polars. Spectral lines seen in the radiation emitted by the constituents of these systems are notably Doppler-shifted. The Doppler-shift is depending on the viewing angle of the system ('inclination') and is varying with orbital phase. Time-series of one-dimensional data (spectra) can be used to create a two-dimensional map of the intensity distribution in velocity-space (Doppler-tomography), or a map of the line-flux on the assumed surface of the mass-losing star (Roche-tomography) from the line profile changes ...thesi
1RXSJ062518.2+733433: A bright, soft intermediate polar
We present the results of 50 hours time-resolved R-band photometry of the
ROSAT all-sky survey source 1RXSJ062518.2+733433. The source was identified by
Wei et al. (1999) as a cataclysmic variable. Our photometry, performed in 10
nights between February 11, 2003, and March 21, 2003, reveals two stable
periodicities at 19.7874 and 283.118 min, which are identified as probable spin
and orbital periods of the binary. We therefore classify 1RXSJ062518.2+733433
as an intermediate polar. Analysis of the RASS X-ray observations reveal a
variability of 100% in the X-ray flux and a likely soft X-ray excess. The new
IP thus joins the rare group of soft IPs with only four members so far.Comment: submitted to A&A, 5 pages, 6 figures of reduced qualit
National Education Systems in the European Union
The European community, under a variety of names, has been expanding both physically and in its areas of responsibility since the end of World War II. What began as an attmept for economic unity among sovereign states has evolved into a complex supranational organization. Education, traditionally part of the domestic domain of nations, has gradually emerged as a concern of the European Union as it endeavors to enhance its viability as an economic leader. This paper examines the growing trend twoard more specific EU education policy in light of the education systems and policies of England and Germany. It specifically presents curriculum and practicies in the area of civics or citizenship education. National education systems could easily be co-opted by education policies and practices of the European Union. If this is the situation and if the suggested curricula are effective, there may be evidence related to civics education, which indicates an emerging European citizen
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