3,097 research outputs found
First-principles study of the energetics of charge and cation mixing in U_{1-x} Ce_x O_2
The formalism of electronic density-functional-theory, with Hubbard-U
corrections (DFT+U), is employed in a computational study of the energetics of
U_{1-x} Ce_x O_2 mixtures. The computational approach makes use of a procedure
which facilitates convergence of the calculations to multiple self-consistent
DFT+U solutions for a given cation arrangement, corresponding to different
charge states for the U and Ce ions in several prototypical cation
arrangements. Results indicate a significant dependence of the structural and
energetic properties on the nature of both charge and cation ordering. With the
effective Hubbard-U parameters that reproduce well the measured
oxidation-reduction energies for urania and ceria, we find that charge transfer
between U(IV) and Ce(IV) ions, leading to the formation of U(V) and Ce(III),
gives rise to an increase in the mixing energy in the range of 4-14 kJ/mol of
formula unit, depending on the nature of the cation ordering. The results
suggest that although charge transfer between uranium and cerium ions is
disfavored energetically, it is likely to be entropically stabilized at the
high temperatures relevant to the processing and service of urania-based solid
solutions.Comment: 8 pages, 6 figure
Optical observations of the luminous Type IIn Supernova 2010jl for over 900 days
The luminous Type IIn Supernova (SN) 2010jl shows strong evidence for the
interaction of the SN ejecta with dense circumstellar material (CSM). We
present observations of SN 2010jl for d after its earliest
detection, including a sequence of optical spectra ranging from to
d. We also supplement our late time spectra and the photometric
measurements in the literature with an additional epoch of new, late time
photometry. Combining available photometric and spectroscopic data, we
derive a semi-bolometric optical light curve and calculate a total radiated
energy in the optical for SN 2010jl of erg. We also
examine the evolution of the H emission line profile in detail and find
evidence for asymmetry in the profile for d that is not easily
explained by any of the proposed scenarios for this fascinating event. Finally,
we discuss the interpretations from the literature of the optical and
near-infrared light curves, and propose that the most likely explanation of
their evolution is the formation of new dust in the dense, pre-existing CSM
wind after d.Comment: 14 pages, 10 figures, 5 tables. Full version of Table 3 is included
as an ancillary fil
The Cepheid distance to the maser-host galaxy NGC 4258: Studying systematics with the Large Binocular Telescope
We identify and phase a sample of 81 Cepheids in the maser-host galaxy NGC
4258 using the Large Binocular Telescope (LBT), and obtain calibrated mean
magnitudes in up to 4 filters for a subset of 43 Cepheids using archival HST
data. We employ 3 models to study the systematic effects of extinction, the
assumed extinction law, and metallicity on the Cepheid distance to NGC 4258. We
find a correction to the Cepheid colors consistent with a grayer extinction law
in NGC 4258 compared to the Milky Way (), although we believe this is
indicative of other systematic effects. If we combine our Cepheid sample with
previously known Cepheids, we find a significant metallicity adjustment to the
distance modulus of mag/dex, for the Zaritsky et
al. (1994) metallicity scale, as well as a weak trend of Cepheid colors with
metallicity. Conclusions about the absolute effect of metallicity on Cepheid
mean magnitudes appear to be limited by the available data on the metallicity
gradient in NGC 4258, but our Cepheid data require at least some metallicity
adjustment to make the Cepheid distance consistent with independent distances
to the LMC and NGC 4258. From our ensemble of models and the geometric maser
distance of NGC 4258 ( mag), we estimate
mag ( kpc).Comment: Accepted for publication in MNRAS. 28 pages, 13 figures, 11 tables. A
brief video summarizing the key results of this paper can be found at
http://youtu.be/ICTTNyxZ89
RJK Observations of the Optical Afterglow of GRB 991216
We present near-infrared and optical observations of the afterglow to the
Gamma-Ray Burst (GRB) 991216 obtained with the F. L. Whipple Observatory 1.2-m
telescope and the University of Hawaii 2.2-m telescope. The observations range
from 15 hours to 3.8 days after the burst. The temporal behavior of the data is
well described by a single power-law decay with index -1.36 +/-0.04,
independent of wavelength. The optical spectral energy distribution, corrected
for significant Galactic reddening of E(B-V)=0.626, is well fitted by a single
power-law with index -0.58 +/- 0.08. Combining the IR/optical observations with
a Chandra X-ray measurement gives a spectral index of -0.8 +/- 0.1 in the
synchrotron cooling regime. A comparison between the spectral and temporal
power-law indices suggest that a jet is a better match to the observations than
a simple spherical shock.Comment: Accepted to the Astrophysical Journal, 12 pages, 4 postscript figure
The search for failed supernovae with the Large Binocular Telescope: constraints from 7 yr of data
We report updated results for the first 7 yr of our programme to monitor 27 galaxies within 10 Mpc using the Large Binocular Telescope to search for failed supernovae (SNe) – core collapses of massive stars that form black holes without luminous SNe. In the new data, we identify no new compelling candidates and confirm the existing candidate. Given the six successful core-collapse SNe in the sample and one likely failed SN, the implied fraction of core collapses that result in failed SNe is f=0.14^(+0.33)_(−0.10) at 90 per cent confidence. If the current candidate is a failed SN, the fraction of failed SN naturally explains the missing high-mass red supergiants SN progenitors and the black hole mass function. If the current candidate is ultimately rejected, the data imply a 90 per cent confidence upper limit on the failed SN fraction of f < 0.35
Parametrization of LSDA+ for noncollinear magnetic configurations: Multipolar magnetism in UO
To explore the formation of noncollinear magnetic configurations in materials
with strongly correlated electrons, we derive a noncollinear LSDA+ model
involving only one parameter , as opposed to the difference between the
Hubbard and Stoner parameters . Computing in the constrained random
phase approximation, we investigate noncollinear magnetism of uranium dioxide
UO and find that the spin-orbit coupling (SOC) stabilizes the 3
ordered magnetic ground state. The estimated SOC strength in UO is as large
as 0.73 eV per uranium atom, making spin and orbital degrees of freedom
virtually inseparable. Using a multipolar pseudospin Hamiltonian, we show how
octupolar and dipole-dipole exchange coupling help establish the 3
magnetic ground state with canted ordering of uranium -orbitals. The
cooperative Jahn-Teller effect does not appear to play a significant part in
stabilizing the noncollinear 3 state, which has the lowest energy
even in an undistorted lattice. The choice of parameter in the LSDA+
model has a notable quantitative effect on the predicted properties of UO,
in particular on the magnetic exchange interaction and, perhaps trivially, on
the band gap: The value of eV computed fully delivers
the band gap of 2.11~eV in good agreement with experiment, and a balanced
account of other pertinent energy scales.Comment: 14 pages, 9 figure
Nucleosynthesis of Nickel-56 from Gamma-Ray Burst Accretion Disks
We examine the prospects for producing Nickel-56 from black hole accretion
disks, by examining a range of steady state disk models. We focus on relatively
slowly accreting disks in the range of 0.05 - 1 solar masses per second, as are
thought to be appropriate for the central engines of long-duration gamma-ray
bursts. We find that significant amounts of Nickel-56 are produced over a wide
range of parameter space. We discuss the influence of entropy, outflow
timescale and initial disk position on mass fraction of Nickel-56 which is
produced. We keep careful track of the weak interactions to ensure reliable
calculations of the electron fraction, and discuss the role of the neutrinos.Comment: 10 pages, 9 figure
The search for failed supernovae with the Large Binocular Telescope: confirmation of a disappearing star
We present Hubble Space Telescope imaging confirming the optical disappearance of the failed supernova (SN) candidate identified by Gerke, Kochanek & Stanek. This ∼25 M⊙ red supergiant experienced a weak ∼10^6 L⊙ optical outburst in 2009 and is now at least 5 mag fainter than the progenitor in the optical. The mid-IR flux has slowly decreased to the lowest levels since the first measurements in 2004. There is faint (2000–3000 L⊙) near-IR emission likely associated with the source. We find the late-time evolution of the source to be inconsistent with obscuration from an ejected, dusty shell. Models of the spectral energy distribution indicate that the remaining bolometric luminosity is >6 times fainter than that of the progenitor and is decreasing as ∼t^(−4/3). We conclude that the transient is unlikely to be an SN impostor or stellar merger. The event is consistent with the ejection of the envelope of a red supergiant in a failed SN and the late-time emission could be powered by fallback accretion on to a newly formed black hole. Future IR and X-ray observations are needed to confirm this interpretation of the fate for the star
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