6,317 research outputs found
Isochrones and Luminosity Functions for Old White Dwarfs
Using a new grid of models of cooling white dwarfs, we calculate isochrones
and luminosity functions in the Johnson-Kron/Cousins and HST filter sets for
systems containing old white dwarfs. These new models incorporate a non-grey
atmosphere which is necessary to properly describe the effects of molecular
opacity at the cool temperatures of old white dwarfs. The various functions
calculated and extensively tabulated and plotted are meant to be as utilitarian
as possible for observers so all results are listed in quantities that
observers will obtain. The tables and plots developed should eventually prove
critical in interpreting the results of HST's Advanced Camera observations of
the oldest white dwarfs in nearby globular clusters, in understanding the
results of searches for old white dwarfs in the Galactic halo, and in
determining ages for star clusters of all ages using white dwarfs. As a
practical application we demonstrate the use of these results by deriving the
white dwarf cooling age of the old Galactic cluster M67.Comment: 7 pages, 8 tables, accepted for publication in the Astrophysical
Journa
Amnestically induced persistence in random walks
We study how the Hurst exponent depends on the fraction of the
total time remembered by non-Markovian random walkers that recall only the
distant past. We find that otherwise nonpersistent random walkers switch to
persistent behavior when inflicted with significant memory loss. Such memory
losses induce the probability density function of the walker's position to
undergo a transition from Gaussian to non-Gaussian. We interpret these findings
of persistence in terms of a breakdown of self-regulation mechanisms and
discuss their possible relevance to some of the burdensome behavioral and
psychological symptoms of Alzheimer's disease and other dementias.Comment: 4 pages, 3 figs, subm. to Phys. Rev. Let
The Galactic Inner Halo: Searching for White Dwarfs and Measuring the Fundamental Galactic Constant, Vo/Ro
We establish an extragalactic, zero-motion frame of reference within the
deepest optical image of a globular star cluster, an HST 123-orbit exposure of
M4 (GO 8679, cycle 9). The line of sight beyond M4 (l,b (deg) = 351,16)
intersects the inner halo (spheroid) of our Galaxy at a tangent-point distance
of 7.6 kpc (for Ro = 8 kpc). We isolate these spheroid stars from the cluster
based on their proper motions over the 6-year baseline between these and
previous epoch HST data (GO 5461, cycle 4). Distant background galaxies are
also found on the same sight line using image-morphology techniques. This fixed
reference frame allows us to independently determine the fundamental Galactic
constant, Vo/Ro = 25.3 +/- 2.6 km/s/kpc, thus providing a velocity of the Local
Standard of Rest, v = 202.7 +/- 24.7 km/s for Ro = 8.0 +/- 0.5 kpc. Secondly,
the galaxies allow a direct measurement of M4's absolute proper motion,
mu_total = 22.57 +/- 0.76 mas/yr, in excellent agreement with recent studies.
The clear separation of galaxies from stars in these deep data also allow us to
search for inner-halo white dwarfs. We model the conventional Galactic
contributions of white dwarfs along our line of sight and predict 7.9 (thin
disk), 6.3 (thick disk) and 2.2 (spheroid) objects to the limiting magnitude at
which we can clearly delineate stars from galaxies (V = 29). An additional 2.5
objects are expected from a 20% white dwarf dark halo consisting of 0.5 Mo
objects, 70% of which are of the DA type. After considering the kinematics and
morphology of the objects in our data set, we find the number of white dwarfs
to be consistent with the predictions for each of the conventional populations.
However, we do not find any evidence for dark halo white dwarfs.Comment: 31 pages, including 6 diagrams and 2 tables. Accepted for publication
in Ap
Associations of Plasma Nitrite, L-Arginine and Asymmetric Dimethylarginine With Morbidity and Mortality in Patients With Necrotizing Soft Tissue Infections
The White Dwarf Cooling Sequence of the Globular Cluster Messier 4
We present the white dwarf sequence of the globular cluster M4, based on a
123 orbit Hubble Space Telescope exposure, with limiting magnitude V = 30, I =
28. The white dwarf luminosity function rises sharply for I >25.5, consistent
with the behaviour expected for a burst population. The white dwarfs of M4
extend to approximately 2.5 magnitudes fainter than the peak of the local
Galactic disk white dwarf luminosity function. This demonstrates a clear and
significant age difference between the Galactic disk and the halo globular
cluster M4. Using the same standard white dwarf models (Hansen 1999) to fit
each luminosity function yields ages of 7.3 +/- 1.5 Gyr for the disk and 12.7
+/- 0.7 Gyr for M4 (2-sigma statistical errors).Comment: 14 pages, 4 diagrams. Accepted for publication in the Astrophysical
Journal Letter
On the coupling between molecular diffusion and solvation shell exchange
The connection between diffusion and solvent exchanges between first and second solvation shells is studied by means of molecular dynamics simulations and analytic calculations, with detailed illustrations for water exchange for the Li+ and Na+ ions, and for liquid argon. First, two methods are proposed which allow, by means of simulation, to extract the quantitative speed-up in diffusion induced by the exchange events. Second, it is shown by simple kinematic considerations that the instantaneous velocity of the solute conditions to a considerable extent the character of the exchanges. Analytic formulas are derived which quantitatively estimate this effect, and which are of general applicability to molecular diffusion in any thermal fluid. Despite the simplicity of the kinematic considerations, they are shown to well describe many aspects of solvent exchange/diffusion coupling features for nontrivial systems
4-Hydroxy-1,2,5-oxadiazol-3-yl Moiety as Bioisoster of the Carboxy Function. Synthesis, Ionization Constants, and Molecular Pharmacological Characterization at Ionotropic Glutamate Receptors of Compounds Related to Glutamate and Its Homologues
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