4,062 research outputs found
Tuning terahertz transitions in a double-gated quantum ring
We theoretically investigate the optical functionality of a semiconducting
quantum ring manipulated by two electrostatic lateral gates used to induce a
double quantum well along the ring. The well parameters and corresponding
inter-level spacings, which lie in the THz range, are highly sensitive to the
gate voltages. Our analysis shows that selection rules for inter-level dipole
transitions, caused by linearly polarized excitations, depend on the
polarization angle with respect to the gates. In striking difference from the
conventional symmetric double well potential, the ring geometry permits
polarization-dependent transitions between the ground and second excited
states, allowing the use of this structure in a three-level lasing scheme.Comment: 7 pages, 6 figure
Estimating the masses of extra-solar planets
All extra-solar planet masses that have been derived spectroscopically are
lower limits since the inclination of the orbit to our line-of-sight is unknown
except for transiting systems. It is, however, possible to determine the
inclination angle, i, between the rotation axis of a star and an observer's
line-of-sight from measurements of the projected equatorial velocity (v sin i),
the stellar rotation period (P_rot) and the stellar radius (R_star). This
allows the removal of the sin i dependency of spectroscopically derived
extra-solar planet masses under the assumption that the planetary orbits lie
perpendicular to the stellar rotation axis. We have carried out an extensive
literature search and present a catalogue of v sin i, P_rot, and R_star
estimates for exoplanet host stars. In addition, we have used Hipparcos
parallaxes and the Barnes-Evans relationship to further supplement the R_star
estimates obtained from the literature. Using this catalogue, we have obtained
sin i estimates using a Markov-chain Monte Carlo analysis. This allows proper
1-sigma two-tailed confidence limits to be placed on the derived sin i's along
with the transit probability for each planet to be determined. While a small
proportion of systems yield sin i's significantly greater than 1, most likely
due to poor P_rot estimations, the large majority are acceptable. We are
further encouraged by the cases where we have data on transiting systems, as
the technique indicates inclinations of ~90 degrees and high transit
probabilities. In total, we estimate the true masses of 133 extra-solar
planets. Of these, only 6 have revised masses that place them above the 13
Jupiter mass deuterium burning limit. Our work reveals a population of
high-mass planets with low eccentricities and we speculate that these may
represent the signature of different planetary formation mechanisms at work.Comment: 40 pages, 6 tables, 2 figures. Accepted for publication in the
Monthly Notices of the Royal Astronomical Society after editing of Tables 1 &
6 for electronic publication. Html abstract shortened for astro-ph submissio
Hubble Space Telescope Observations of SV Cam: II. First Derivative Lightcurve Modelling using PHOENIX and ATLAS Model Atmospheres
The variation of the specific intensity across the stellar disc is essential
input parameter in surface brightness reconstruction techniques such as Doppler
imaging, where the relative intensity contributions of different surface
elements are important in detecting starspots. We use PHOENIX and ATLAS model
atmospheres to model lightcurves derived from high precision (S/N ~ 5000) HST
data of the eclipsing binary SV Cam (F9V + K4V), where the variation of
specific intensity across the stellar disc will determine the contact points of
the binary system lightcurve. For the first time we use chi^2 comparison fits
to the first derivative profiles to determine the best-fitting model
atmosphere. We show the wavelength dependence of the limb darkening and that
the first derivative profile is sensitive to the limb-darkening profile very
close to the limb of the primary star. It is concluded that there is only a
marginal difference (< 1sigma) between the chi^2 comparison fits of the two
model atmospheres to the HST lightcurve at all wavelengths. The usefulness of
the second derivative of the light-curve for measuring the sharpness of the
primary's limb is investigated, but we find that the data are too noisy to
permit a quantitative analysis.Comment: 10 pages, 20 figures, accepted by MNRA
Fast spectroscopic variations on rapidly-rotating, cool dwarfs. 3: Masses of circumstellar absorbing clouds on AB Doradus
New time-resolved H alpha, Ca II H and K and Mg II h and k spectra of the rapidly-rotating K0 dwarf star AB Doradus (= HD 36705). The transient absorption features seen in the H alpha line are also present in the Ca II and Mg II resonance lines. New techniques are developed for measuring the average strength of the line absorption along lines of sight intersecting the cloud. These techniques also give a measure of the projected cloud area. The strength of the resonance line absorption provides useful new constraints on the column densities, projected surface areas, temperatures and internal turbulent velocity dispersions of the circumstellar clouds producing the absorption features. At any given time the star appears to be surrounded by at least 6 to 10 clouds with masses in the range 2 to 6 x 10(exp 17) g. The clouds appear to have turbulent internal velocity dispersions of order 3 to 20 km/s, comparable with the random velocities of discrete filamentary structures in solar quiescent prominences. Night-to-night changes in the amount of Ca II resonance line absorption can be explained by changes in the amplitude of turbulent motions in the clouds. The corresponding changes in the total energy of the internal motions are of order 10(exp 29) erg per cloud. Changes of this magnitude could easily be activated by the frequent energetic (approximately 10(exp 34) erg) x ray flares seen on this star
Understanding and informing decisions on Sustainable Agricultural Intensification in Sub-Saharan Africa
Sustainable Agricultural Intensification (SAI) was initially defined as increasing agricultural production without adverse environmental impacts and without increasing the area under agriculture. Overtime the concept has been broadened to integrate social, economic, and environmental components of sustainability, each of which covers multiple facets or indicators of performance. It is recognized however that it may not be possible to optimise all these aspects of sustainability simultaneously and that trade-offs between them are likely to occur, although synergies are also possible. There has been disagreement over how to achieve SAI, with some proposing that only an “agroecological” intensification pathway delivers sustainability. Others take a broader perspective arguing that all aspects of ecological, genetic, and socio-economic intensification need to be considered, but then assessed in terms of the sustainability of the outcomes. A major concern is that intensification that focuses on agricultural technology can lead to inequitable outcomes for women and poorer households, while agroecological intensification building upon local capitals is generally considered more equitable. Understanding the potential outcomes and inherent trade-offs of different approaches requires interdisciplinary research, evidence and decision-making tools, some examples of which are presented in this Special Issue
Force-Free Models of Magnetically Linked Star-Disk Systems
Disk accretion onto a magnetized star occurs in a variety of astrophysical
contexts, from young stars to X-ray pulsars. The magnetohydrodynamic
interaction between the stellar field and the accreting matter can have a
strong effect on the disk structure, the transfer of mass and angular momentum
between the disk and the star, and the production of bipolar outflows, e.g.,
plasma jets. We study a key element of this interaction - the time evolution of
the magnetic field configuration brought about by the relative rotation between
the disk and the star - using simplified, largely semianalytic, models. We
first discuss the rapid inflation and opening up of the magnetic field lines in
the corona above the accretion disk, which is caused by the differential
rotation twisting. Then we consider additional physical effects that tend to
limit this expansion, such as the effect of plasma inertia and the possibility
of reconnection in the disk's corona, the latter possibly leading to repeated
cycles in the evolution. We also derive the condition for the existence of a
steady state for a resistive disk and conclude that a steady state
configuration is not realistically possible. Finally, we generalize our
analysis of the opening of magnetic field lines by using a non-self-similar
numerical model that applies to an arbitrarily rotating (e.g. keplerian) disk.Comment: 75 pages, 22 figures, 2 tables. Submitted to Astrophysical Journa
Self-consistent multi-mode lasing theory for complex or random lasing media
A semiclassical theory of single and multi-mode lasing is derived for open
complex or random media using a self-consistent linear response formulation.
Unlike standard approaches which use closed cavity solutions to describe the
lasing modes, we introduce an appropriate discrete basis of functions which
describe also the intensity and angular emission pattern outside the cavity.
This constant flux (CF) basis is dictated by the Green function which arises
when formulating the steady state Maxwell-Bloch equations as a self-consistent
linear response problem. This basis is similar to the quasi-bound state basis
which is familiar in resonator theory and it obeys biorthogonality relations
with a set of dual functions. Within a single-pole approximation for the Green
function the lasing modes are proportional to these CF states and their
intensities and lasing frequencies are determined by a set of non-linear
equations. When a near threshold approximation is made to these equations a
generalized version of the Haken-Sauermann equations for multi-mode lasing is
obtained, appropriate for open cavities. Illustrative results from these
equations are given for single and few mode lasing states, for the case of
dielectric cavity lasers. The standard near threshold approximation is found to
be unreliable. Applications to wave-chaotic cavities and random lasers are
discussed.Comment: 18 pages, 9 figure
Gravitationally lensed QSOs in the ISSIS/WSO-UV era
Gravitationally lensed QSOs (GLQs) at redshift z = 1-2 play a key role in
understanding the cosmic evolution of the innermost parts of active galaxies
(black holes, accretion disks, coronas and internal jets), as well as the
structure of galaxies at intermediate redshifts. With respect to studies of
normal QSOs, GLQ programmes have several advantages. For example, a monitoring
of GLQs may lead to unambiguous detections of intrinsic and extrinsic
variations. Both kinds of variations can be used to discuss central engines in
distant QSOs, and mass distributions and compositions of lensing galaxies. In
this context, UV data are of particular interest, since they correspond to
emissions from the immediate surroundings of the supermassive black hole. We
describe some observation strategies to analyse optically bright GLQs at z of
about 1.5, using ISSIS (CfS) on board World Space Observatory-Ultraviolet.Comment: 7 pages, 4 figures, Accepted for publication in Astrophysics & Space
Scienc
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