128,694 research outputs found
Experimental constraints on the polarizabilities of the 6s^2 1S0 and 6s6p 3P0 states of Yb
We utilize accurate experimental data available in the literature to yield
bounds on the polarizabilities of the ground and first excited states of atomic
Yb. For the 6s^2 1S0 ground state, we find the polarizability alpha to be
constrained to 134.4<alpha<144.2 in atomic units, while for the 6s6p 3P0
excited state we find 280.1<alpha<289.9. The uncertainty in each of these
values is 1.0. These constraints provide a valuable check for ab initio and
semi-empirical methods used to compute polarizabilities and other related
properties in Yb.Comment: 7 pages, 1 figur
A three-node C deg element for analysis of laminated composite sandwich shells
A three-node flat shell element with C deg rotation fields has been developed for analysis of arbitrary composite shells. The element may consist of any number of orthotropic layers, each layer having different material properties and angular orientation. The formulation includes coupling between bending and extension, which is essential for analysis of unsymmetric laminates. Shearing deflections are included, since laminated and sandwich construction frequently results in shear stiffness much smaller than bending stiffness. Formulation of the element is straightforward, and calculation of its stiffness matrix is simple and fast. Convergence of solutions with mesh refinement is uniform for both thin and thick shells and is insensitive to element shape, although not as rapid as some other elements that lack one or more capabilities of the newly developed element. An experimental verification of the shall element is reported in the appendix
Spin relaxation in diluted magnetic semiconductor quantum dots
Electron spin relaxation induced by phonon-mediated s-d exchange interaction
in a II-VI diluted magnetic semiconductor quantum dot is investigated
theoretically. The electron-acoustic phonon interaction due to piezoelectric
coupling and deformation potential is included. The resulting spin lifetime is
typically on the order of microseconds. The effectiveness of the
phonon-mediated spin-flip mechanism increases with increasing Mn concentration,
electron spin splitting, vertical confining strength and lateral diameter,
while it shows non-monotonic dependence on the magnetic field and temperature.
An interesting finding is that the spin relaxation in a small quantum dot is
suppressed for strong magnetic field and low Mn concentration at low
temperature.Comment: 11 pages, 11 figures, to be published in Phys. Rev.
Red Runaways: Hypervelocity Stars, Hills Ejecta and Other Outliers in the F-M Star Regime
In this paper we analyze a sample of metal-rich (>-0.8 dex) main sequence
stars in the extended solar neighborhood, investigating kinematic outliers from
the background population. The data, which are taken from the Sloan Digital Sky
Survey, are kinematically profiled as a function of distance from the Galactic
plane using full six dimensional phase space information. Each star is examined
in the context of these kinematic profiles and likelihoods are assigned to
quantify whether a star matches the underlying profile. Since some of these
stars are likely to have been ejected from the disc, we trace back their orbits
in order to determine potential ejection radii. We find that objects with low
probability (i.e. `outliers') are typically more metal poor, faster and, most
importantly, have a tendency to originate from the inner Galaxy compared to the
underlying population.
We also compose a sample of stars with velocities exceeding the local escape
velocity. Although we do not discount that our sample could be contaminated by
objects with spurious proper motions, a number of stars appear to have been
ejected from the disc with exceptionally high velocities. Some of these are
consistent with being ejected from the spiral arms and hence are a rich
resource for further study. Finally we look at objects whose orbits are
consistent with them being ejected at high speeds from the Galactic center. Of
these objects we find that one, J135855.65+552538.19, is inconsistent with
halo, bulge and disk kinematics and could plausibly have been ejected from the
Galactic nucleus via a Hills mechanism.Comment: 17 Pages, 12 Figures, Accepted to A
Pan-STARRS1 Discovery of Two Ultraluminous Supernovae at z ≈ 0.9
We present the discovery of two ultraluminous supernovae (SNe) at z ≈ 0.9 with the Pan-STARRS1 Medium Deep Survey. These SNe, PS1-10ky and PS1-10awh, are among the most luminous SNe ever discovered, comparable to the unusual transients SN 2005ap and SCP 06F6. Like SN 2005ap and SCP 06F6, they show characteristic high luminosities (M_(bol) ≈ –22.5 mag), blue spectra with a few broad absorption lines, and no evidence for H or He. We have constructed a full multi-color light curve sensitive to the peak of the spectral energy distribution in the rest-frame ultraviolet, and we have obtained time series spectroscopy for these SNe. Given the similarities between the SNe, we combine their light curves to estimate a total radiated energy over the course of explosion of (0.9-1.4) × 10^(51) erg. We find photospheric velocities of 12,000-19,000 km s^(–1) with no evidence for deceleration measured across ~3 rest-frame weeks around light curve peak, consistent with the expansion of an optically thick massive shell of material. We show that, consistent with findings for other ultraluminous SNe in this class, radioactive decay is not sufficient to power PS1-10ky, and we discuss two plausible origins for these events: the initial spin-down of a newborn magnetar in a core-collapse SN, or SN shock breakout from the dense circumstellar wind surrounding a Wolf-Rayet star
Measurement of energy eigenstates by a slow detector
We propose a method for a weak continuous measurement of the energy
eigenstates of a fast quantum system by means of a "slow" detector. Such a
detector is only sensitive to slowly-changing variables, e. g. energy, while
its back-action can be limited solely to decoherence of the eigenstate
superpositions. We apply this scheme to the problem of detection of quantum
jumps between energy eigenstates in a harmonic oscillator.Comment: 4 page
A Model for Star Formation, Gas Flows and Chemical Evolution in Galaxies at High Redshifts
Motivated by the increasing use of the Kennicutt-Schmidt (K-S) star formation
law to interpret observations of high redshift galaxies, the importance of gas
accretion to galaxy formation, and the recent observations of chemical
abundances in galaxies at z~2-3, I use simple analytical models to assess the
consistency of these processes of galaxy evolution with observations and with
each other. I derive the time dependence of star formation implied by the K-S
law, and show that the sustained high star formation rates observed in galaxies
at z~2-3 require the accretion of additional gas. A model in which the gas
accretion rate is approximately equal to the combined star formation and
outflow rates broadly reproduces the observed trends of star formation rate
with galaxy age. Using an analytical description of chemical evolution, I also
show that this model, further constrained to have an outflow rate roughly equal
to the star formation rate, reproduces the observed mass-metallicity relation
at z~2.Comment: 7 pages, 3 figures. Accepted for publication in Ap
Supernova 2009kf: An Ultraviolet Bright Type IIP Supernova Discovered with Pan-STARRS 1 and GALEX
We present photometric and spectroscopic observations of a luminous Type IIP Supernova (SN) 2009kf discovered by the Pan-STARRS 1 (PS1) survey and also detected by the Galaxy Evolution Explorer. The SN shows a plateau in its optical and bolometric light curves, lasting approximately 70 days in the rest frame, with an absolute magnitude of M_V = -18.4 mag. The P-Cygni profiles of hydrogen indicate expansion velocities of 9000 km s^(-1) at 61 days after discovery which is extremely high for a Type IIP SN. SN 2009kf is also remarkably bright in the near-ultraviolet (NUV) and shows a slow evolution 10-20 days after optical discovery. The NUV and optical luminosity at these epochs can be modeled with a blackbody with a hot effective temperature (T ~ 16,000 K) and a large radius (R ~ 1 × 10^(15) cm). The bright bolometric and NUV luminosity, the light curve peak and plateau duration, the high velocities, and temperatures suggest that 2009kf is a Type IIP SN powered by a larger than normal explosion energy. Recently discovered high-z SNe (0.7 < z < 2.3) have been assumed to be IIn SNe, with the bright UV luminosities due to the interaction of SN ejecta with a dense circumstellar medium. UV-bright SNe similar to SN 2009kf could also account for these high-z events, and its absolute magnitude M_(NUV) = -21.5 ± 0.5 mag suggests such SNe could be discovered out to z ~ 2.5 in the PS1 survey
Amplification by stochastic interference
A new method is introduced to obtain a strong signal by the interference of
weak signals in noisy channels. The method is based on the interference of 1/f
noise from parallel channels. One realization of stochastic interference is the
auditory nervous system. Stochastic interference may have broad potential
applications in the information transmission by parallel noisy channels
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