5,393 research outputs found
Decoherence in a dynamical quantum phase transition
Motivated by the similarity between adiabatic quantum algorithms and quantum
phase transitions, we study the impact of decoherence on the sweep through a
second-order quantum phase transition for the prototypical example of the Ising
chain in a transverse field and compare it to the adiabatic version of Grovers
search algorithm, which displays a first order quantum phase transition. For
site-independent and site-dependent coupling strengths as well as different
operator couplings, the results show that (in contrast to first-order
transitions) the impact of decoherence caused by a weak coupling to a rather
general environment increases with system size (i.e., number of spins/qubits).
This might limit the scalability of the corresponding adiabatic quantum
algorithm.Comment: 14 pages, 9 figure
Near-infrared (NIR) spectra of Centaurs and Kuiper belt objects
We present here an extensive survey of near-infrared (NIR) spectra of Kuiper belt objects (KBOs) and Centaurs taken with the Keck I Telescope. We find that most spectra in our sample are well characterized by a combination of water ice and a featureless continuum. A comparative analysis reveals that the NIR spectral properties have little correlation to the visible colors or albedo, with the exception of the fragment KBOs produced from the giant impact on 2003 EL61. The results suggest that the surface composition of KBOs is heterogeneous, though the exposure of water ice may be controlled by geophysical processes. The Centaurs also display diverse spectral properties, but the source of the variability remains unclear. The results for both the KBOs and the Centaurs point to inherent heterogeneity in either the processes acting on these objects or materials from which they formed
Empirical logic of finite automata: microstatements versus macrostatements
We compare the two approaches to the empirical logic of automata. The first,
called partition logic (logic of microstatements), refers to experiments on
individual automata. The second one, the logic of simulation (logic of
macrostatements), deals with ensembles of automata.Comment: late
Logical equivalence between generalized urn models and finite automata
To every generalized urn model there exists a finite (Mealy) automaton with
identical propositional calculus. The converse is true as well.Comment: 9 pages, minor change
Massive stars and globular cluster formation
We first present chemodynamical simulations to investigate how stellar winds
of massive stars influence early dynamical and chemical evolution of forming
globular clusters (GCs). In our numerical models, GCs form in
turbulent,high-density giant molecular clouds (GMCs), which are embedded in a
massive dark matter halo at high redshifts. We show how high-density, compact
stellar systems are formed from GMCs influenced both by physical processes
associated with star formation and by tidal fields of their host halos. We also
show that chemical pollution of GC-forming GMCs by stellar winds from massive
stars can result in star-to-star abundance inhomogeneities among light elements
(e.g., C, N, and O) of stars in GCs. The present model with a canonical initial
mass function (IMF) also shows a C-N anticorrelation that stars with smaller
[C/Fe] have larger [N/Fe] in a GC. Although these results imply that
``self-pollution'' of GC-forming GMCs by stellar winds from massive stars can
cause abundance inhomogeneities of GCs, the present models with different
parameters and canonical IMFs can not show N-rich stars with [N/Fe] ~ 0.8
observed in some GCs (e.g., NGC 6752). We discuss this apparent failure in the
context of massive star formation preceding low-mass one within GC-forming GMCs
(``bimodal star formation scenario''). We also show that although almost all
stars (~97%) show normal He abundances (Y) of ~0.24 some stars later formed in
GMCs can have Y as high as ~0.3 in some models. The number fraction of He-rich
stars with Y >0.26 is however found to be small (~10^-3) for most models.Comment: 10 pages, 8 figures, accepted by Ap
Generalized 2d dilaton gravity with matter fields
We extend the classical integrability of the CGHS model of 2d dilaton gravity
[1] to a larger class of models, allowing the gravitational part of the action
to depend more generally on the dilaton field and, simultaneously, adding
fermion- and U(1)-gauge-fields to the scalar matter. On the other hand we
provide the complete solution of the most general dilaton-dependent 2d gravity
action coupled to chiral fermions. The latter analysis is generalized to a
chiral fermion multiplet with a non-abelian gauge symmetry as well as to the
(anti-)self-dual sector df = *df (df = -*df) of a scalar field f.Comment: 37 pages, Latex; typos and Eqs. (44,45) corrected; paragraph on p.
26, referring to a work of S. Solodukhin, reformulated; references adde
The Core Composition of a White Dwarf in a Close Double Degenerate System
We report the identification of the double degenerate system NLTT 16249 that
comprises a normal, hydrogen-rich (DA) white dwarf and a peculiar,
carbon-polluted white dwarf (DQ) showing photospheric traces of nitrogen. We
disentangled the observed spectra and constrained the properties of both
stellar components. In the evolutionary scenario commonly applied to the
sequence of DQ white dwarfs, both carbon and nitrogen would be dredged up from
the core. The C/N abundance ratio (~ 50) in the atmosphere of this unique DQ
white dwarf suggests the presence of unprocessed material (14N) in the core or
in the envelope. Helium burning in the DQ progenitor may have terminated early
on the red-giant branch after a mass-ejection event leaving unprocessed
material in the core although current mass estimates do not favor the presence
of a low-mass helium core. Alternatively, some nitrogen in the envelope may
have survived an abridged helium-core burning phase prior to climbing the
asymptotic giant-branch. Based on available data, we estimate a relatively
short orbital period (P <~ 13 hrs) and on-going spectroscopic observations will
help determine precise orbital parameters.Comment: Accepted for publication in ApJ Letter
Cosmological implications of dwarf spheroidal chemical evolution
The chemical properties of dwarf spheroidals in the local group are shown to
be inconsistent with star formation being truncated after the reionization
epoch (z~8). Enhanced levels of [Ba/Y] in stars in dwarf spheroidals like
Sculptor indicate strong s-process production from low-mass stars whose
lifetimes are comparable with the duration of the pre-reionization epoch. The
chemical evolution of Sculptor is followed using a model with SNeII and SNeIa
feedback and mass- and metallicity-dependent nucleosynthetic yields for
elements from H to Pb. We are unable to reproduce the Ba/Y ratio unless stars
formed over an interval long enough for the low-mass stars to pollute the
interstellar medium with s-elements. This robust result challenges the
suggestion that most of the local group dwarf spheroidals are fossils of
reionization and supports the case for large initial dark matter halos.Comment: 7 pages, 4 figures. Accepted for publication in ApJ. Minor changes
following referee repor
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