1,897 research outputs found
Combining symmetry collective states with coupled cluster theory: Lessons from the Agassi model Hamiltonian
The failures of single-reference coupled cluster for strongly correlated
many-body systems is flagged at the mean-field level by the spontaneous
breaking of one or more physical symmetries of the Hamiltonian. Restoring the
symmetry of the mean-field determinant by projection reveals that coupled
cluster fails because it factorizes high-order excitation amplitudes
incorrectly. However, symmetry-projected mean-field wave functions do not
account sufficiently for dynamic (or weak) correlation. Here we pursue a merger
of symmetry projection and coupled cluster theory, following previous work
along these lines that utilized the simple Lipkin model system as a testbed [J.
Chem. Phys. 146, 054110 (2017)]. We generalize the concept of a
symmetry-projected mean-field wave function to the concept of a symmetry
projected state, in which the factorization of high-order excitation amplitudes
in terms of low-order ones is guided by symmetry projection and is not
exponential, and combine them with coupled cluster theory in order to model the
ground state of the Agassi Hamiltonian. This model has two separate channels of
correlation and two separate physical symmetries which are broken under strong
correlation. We show how the combination of symmetry collective states and
coupled cluster is effective in obtaining correlation energies and order
parameters of the Agassi model throughout its phase diagram
Reply to Comment on "Reevaluation of the parton distribution of strange quarks in the nucleon"
A Comment on the recently published reevaluation of the polarization-averaged
parton distribution of strange quarks in the nucleon using final data on the
multiplicities of charged kaons in semi-inclusive deep-inelastic scattering is
reviewed. Important features of the comparison of one-dimensional projections
of the multidimensional HERMES data are pointed out. A test of the
leading-order extraction of xS(x) using the difference between charged-kaon
multiplicities is repeated. The results are consistent with leading-order
predictions within the uncertainties in the input data, and do not invalidate
the earlier extraction of xS(x).Comment: Reply Comment to arXiv:1407.372
A New Timescale for Period Change in the Pulsating DA White Dwarf WD 0111+0018
We report the most rapid rate of period change measured to date for a
pulsating DA (hydrogen atmosphere) white dwarf (WD), observed in the 292.9 s
mode of WD 0111+0018. The observed period change, faster than 10^{-12} s/s,
exceeds by more than two orders of magnitude the expected rate from cooling
alone for this class of slow and simply evolving pulsating WDs. This result
indicates the presence of an additional timescale for period evolution in these
pulsating objects. We also measure the rates of period change of nonlinear
combination frequencies and show that they share the evolutionary
characteristics of their parent modes, confirming that these combination
frequencies are not independent modes but rather artifacts of some nonlinear
distortion in the outer layers of the star.Comment: 10 pages, 6 figures, accepted for publication in The Astrophysical
Journa
3D Model Atmospheres for Extremely Low-Mass White Dwarfs
We present an extended grid of mean three-dimensional (3D) spectra for
low-mass, pure-hydrogen atmosphere DA white dwarfs (WDs). We use CO5BOLD
radiation-hydrodynamics 3D simulations covering Teff = 6000-11,500 K and logg =
5-6.5 (cgs units) to derive analytical functions to convert spectroscopically
determined 1D temperatures and surface gravities to 3D atmospheric parameters.
Along with the previously published 3D models, the 1D to 3D corrections are now
available for essentially all known convective DA WDs (i.e., logg = 5-9). For
low-mass WDs, the correction in temperature is relatively small (a few per cent
at the most), but the surface gravities measured from the 3D models are lower
by as much as 0.35 dex. We revisit the spectroscopic analysis of the extremely
low-mass (ELM) WDs, and demonstrate that the 3D models largely resolve the
discrepancies seen in the radius and mass measurements for relatively cool ELM
WDs in eclipsing double WD and WD + milli-second pulsar binary systems. We also
use the 3D corrections to revise the boundaries of the ZZ Ceti instability
strip, including the recently found ELM pulsators.Comment: 11 pages, 8 figures, accepted for publication in the Astrophysical
Journa
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A Status Report On A Planet Search Around White Dwarf Stars
We have continued monitoring a pilot sample of 15 isolated, pulsating DA white dwarfs for center-of-mass motion caused by a planetary companion. Roughly 7 years into our survey, we have preliminary evidence for periodic variations in pulse arrival times for at least two white dwarfs in our sample. The variations in these systems are unlikely to be caused by secular evolution and are possibly the result of motion of the white dwarf around a center of mass. We have yet to claim confirmation of a planet. GD66 is a previously published candidate system, with a modulation in pulse arrival times that could be caused by a 2.0 M-J sin i planetary companion with an 8.3 year orbital period. Another candidate system, WD1354+0108, has a phase modulation consistent with a 0.7 M-J sin i planet at 2.3 AU (a 4.5 year orbit). We see similar behavior in two independent frequencies within this star, and while a sinusoid is currently a marginally better fit to the data than a straight line (as we might expect from cooling alone in a DAV), we are hesitant to over-interpret our results. Finally, we have a third system, WD0018+0031, that shows a change in pulse arrival times inconsistent with cooling alone; a 2.7 M-J planet at an orbit of about 5 AU could cause the observed trend. Observations of these candidate systems are ongoing in order to constrain any planetary companions that may be present.Astronom
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