1,417 research outputs found
Measuring stellar oscillations using equivalent widths of absorption lines
Kjeldsen et al. (1995, AJ 109, 1313; astro-ph/9411016) have developed a new
technique for measuring stellar oscillations and claimed a detection in the G
subgiant eta Boo. The technique involves monitoring temperature fluctuations in
a star via their effect on the equivalent width of Balmer lines. In this paper
we use synthetic stellar spectra to investigate the temperature dependence of
the Balmer lines, Ca II, Fe I, the Mg b feature and the G~band. We present a
list of target stars likely to show solar-like oscillations and estimate their
expected amplitudes. We also show that centre-to-limb variations in Balmer-line
profiles allow one to detect oscillation modes with l<=4, which accounts for
the detection by Kjeldsen et al. of modes with degree l=3 in integrated
sunlight.Comment: MNRAS (accepted); 7 pages, LaTeX with necessary style file and
PostScript figures in a single uuencoded Z-compressed .tar fil
Algorithmic Interpretations of Fractal Dimension
We study algorithmic problems on subsets of Euclidean space of low fractal dimension. These spaces are the subject of intensive study in various branches of mathematics, including geometry, topology, and measure theory. There are several well-studied notions of fractal dimension for sets and measures in Euclidean space. We consider a definition of fractal dimension for finite metric spaces which agrees with standard notions used to empirically estimate the fractal dimension of various sets. We define the fractal dimension of some metric space to be the infimum delta>0, such that for any eps>0, for any ball B of radius r >= 2eps, and for any eps-net N, we have |B cap N|=O((r/eps)^delta).
Using this definition we obtain faster algorithms for a plethora of classical problems on sets of low fractal dimension in Euclidean space. Our results apply to exact and fixed-parameter algorithms, approximation schemes, and spanner constructions. Interestingly, the dependence of the performance of these algorithms on the fractal dimension nearly matches the currently best-known dependence on the standard Euclidean dimension. Thus, when the fractal dimension is strictly smaller than the ambient dimension, our results yield improved solutions in all of these settings.
We remark that our definition of fractal definition is equivalent up to constant factors to the well-studied notion of doubling dimension.
However, in the problems that we consider, the dimension appears in the exponent of the running time, and doubling dimension is not precise enough for capturing the best possible such exponent for subsets of Euclidean space. Thus our work is orthogonal to previous results on spaces of low doubling dimension; while algorithms on spaces of low doubling dimension seek to extend results from the case of low dimensional Euclidean spaces to more general metric spaces, our goal is to obtain faster algorithms for special pointsets in Euclidean space
Observation of coherent many-body Rabi oscillations
A two-level quantum system coherently driven by a resonant electromagnetic
field oscillates sinusoidally between the two levels at frequency
which is proportional to the field amplitude [1]. This phenomenon, known as the
Rabi oscillation, has been at the heart of atomic, molecular and optical
physics since the seminal work of its namesake and coauthors [2]. Notably, Rabi
oscillations in isolated single atoms or dilute gases form the basis for
metrological applications such as atomic clocks and precision measurements of
physical constants [3]. Both inhomogeneous distribution of coupling strength to
the field and interactions between individual atoms reduce the visibility of
the oscillation and may even suppress it completely. A remarkable
transformation takes place in the limit where only a single excitation can be
present in the sample due to either initial conditions or atomic interactions:
there arises a collective, many-body Rabi oscillation at a frequency
involving all N >> 1 atoms in the sample [4]. This is true even
for inhomogeneous atom-field coupling distributions, where single-atom Rabi
oscillations may be invisible. When one of the two levels is a strongly
interacting Rydberg level, many-body Rabi oscillations emerge as a consequence
of the Rydberg excitation blockade. Lukin and coauthors outlined an approach to
quantum information processing based on this effect [5]. Here we report initial
observations of coherent many-body Rabi oscillations between the ground level
and a Rydberg level using several hundred cold rubidium atoms. The strongly
pronounced oscillations indicate a nearly complete excitation blockade of the
entire mesoscopic ensemble by a single excited atom. The results pave the way
towards quantum computation and simulation using ensembles of atoms
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Spin detection with a micromechanical trampoline: towards magnetic resonance microscopy harnessing cavity optomechanics
We explore the prospects and benefits of combining the techniques of cavity optomechanics with efforts to image spins using magnetic resonance force microscopy (MRFM). In particular, we focus on a common mechanical resonator used in cavity optomechanics—high-stress stoichiometric silicon nitride (Si3N4) membranes. We present experimental work with a 'trampoline' membrane resonator that has a quality factor above 106 and an order of magnitude lower mass than a comparable standard membrane resonators. Such high-stress resonators are on a trajectory to reach 0.1 force sensitivities at MHz frequencies by using techniques such as soft clamping and phononic-crystal control of acoustic radiation in combination with cryogenic cooling. We present a demonstration of force-detected electron spin resonance of an ensemble at room temperature using the trampoline resonators functionalized with a magnetic grain. We discuss prospects for combining such a resonator with an integrated Fabry–Perot cavity readout at cryogenic temperatures, and provide ideas for future impacts of membrane cavity optomechanical devices on MRFM of nuclear spins.
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Observation of mesoscopic crystalline structures in a two-dimensional Rydberg gas
The ability to control and tune interactions in ultracold atomic gases has
paved the way towards the realization of new phases of matter. Whereas
experiments have so far achieved a high degree of control over short-ranged
interactions, the realization of long-range interactions would open up a whole
new realm of many-body physics and has become a central focus of research.
Rydberg atoms are very well-suited to achieve this goal, as the van der Waals
forces between them are many orders of magnitude larger than for ground state
atoms. Consequently, the mere laser excitation of ultracold gases can cause
strongly correlated many-body states to emerge directly when atoms are
transferred to Rydberg states. A key example are quantum crystals, composed of
coherent superpositions of different spatially ordered configurations of
collective excitations. Here we report on the direct measurement of strong
correlations in a laser excited two-dimensional atomic Mott insulator using
high-resolution, in-situ Rydberg atom imaging. The observations reveal the
emergence of spatially ordered excitation patterns in the high-density
components of the prepared many-body state. They have random orientation, but
well defined geometry, forming mesoscopic crystals of collective excitations
delocalised throughout the gas. Our experiment demonstrates the potential of
Rydberg gases to realise exotic phases of matter, thereby laying the basis for
quantum simulations of long-range interacting quantum magnets.Comment: 10 pages, 7 figure
Analysis of stellar spectra with 3D and NLTE models
Models of radiation transport in stellar atmospheres are the hinge of modern
astrophysics. Our knowledge of stars, stellar populations, and galaxies is only
as good as the theoretical models, which are used for the interpretation of
their observed spectra, photometric magnitudes, and spectral energy
distributions. I describe recent advances in the field of stellar atmosphere
modelling for late-type stars. Various aspects of radiation transport with 1D
hydrostatic, LTE, NLTE, and 3D radiative-hydrodynamical models are briefly
reviewed.Comment: 21 pages, accepted for publication as a chapter in "Determination of
Atmospheric Parameters of B, A, F and G Type Stars", Springer (2014), eds. E.
Niemczura, B. Smalley, W. Pyc
Protocol of a randomized, double-blind, placebo-controlled, parallel-group, multicentre study of the efficacy and safety of nicotinamide in patients with Friedreich ataxia (NICOFA)
Introduction: Currently, no treatment that delays with the progression of Friedreich ataxia is available. In the
majority of patients Friedreich ataxia is caused by homozygous pathological expansion of GAA repeats in the first
intron of the FXN gene. Nicotinamide acts as a histone deacetylase inhibitor. Dose escalation studies have shown,
that short term treatment with dosages of up to 4 g/day increase the expression of FXN mRNA and frataxin protein
up to the levels of asymptomatic heterozygous gene carriers. The long-term effects and the effects on clinical
endpoints, activities of daily living and quality of life are unknown.
Methods: The aim of the NICOFA study is to investigate the efficacy and safety of nicotinamide for the treatment of
Friedreich ataxia over 24 months. An open-label dose adjustment wash-in period with nicotinamide (phase A: weeks 1–4)
to the individually highest tolerated dose of 2–4 g nicotinamide/day will be followed by a 2 (nicotinamide group): 1
(placebo group) randomization (phase B: weeks 5–104). In the nicotinamide group, patients will continue with their
individually highest tolerated dose between 2 and 4 g/d per os once daily and the placebo group patients will be
receiving matching placebo. Safety assessments will consist of monitoring and recording of all adverse events and serious
adverse events, regular monitoring of haematology, blood chemistry and urine values, regular measurement of vital signs
and the performance of physical examinations including cardiological signs. The primary outcome is the change in the
Scale for the Assessment and Rating of Ataxia (SARA) over time as compared with placebo in patients with Friedreich
ataxia based on the linear mixed effect model (LMEM) model. Secondary endpoints are measures of quality of life,
functional motor and cognitive measures, clinician’s and patient’s global impression-change scales as well as the upregulation of the frataxin protein level, safety and survival/death.
Perspective: The NICOFA study represents one of the first attempts to assess the clinical efficacy of an epigenetic
therapeutic intervention for this disease and will provide evidence of possible disease modifying effects of nicotinamide
treatment in patients with Friedreich ataxia
Совершенствование конструкции плоскореза глубокорыхлителя КПГ-250 в условиях КФХ "Правда" Беловского района, Кемеровской области
Объектом исследования является плоскорежущая лапа глубокорыхлителя КПГ-250.
Цель работы – повышение качества обработки почвы, с совершенствованием конструкции глубокорыхлителя КПГ-250.
повышение эффективности уборки зерновых культур, с разработкой конструкции
В процессе исследования проводились технологические и конструкторские расчеты
В результате исследования предложены мероприятия по снижению уплотнения почвы, а также конструкторские решения по повышению эффективности обработки почвы культиватором КПГ-250.The object of the study is flat-cutting paw cultivator KPG-250. Purpose – to improve the quality of the soil, by improving the design of the chisel KPG-250. Improving the efficiency of cleaning of grain crops, with the development of the design. In the process of research was conducted technological and design calculations
The study proposed measures to reduce soil compaction, and design solutions for improving the effectiveness of soil treatment with cultivator KPG-250
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