2,675 research outputs found
Spitzer White Dwarf Planet Limits
We present preliminary limits on the presence of planets around white dwarf
stars using the IRAC photometer on the Spitzer space telescope. Planets emit
strongly in the mid-infrared which allows their presence to be detected as an
excess at these wavelengths. We place limits of for 8 stars assuming
ages of , and for 23 stars.We describe our survey, present our
results and comment on approaches to improve our methodology.Comment: 4 pages, 3 figures, to appear in Proceedings of 15th European White
Dwarf Worksho
An Independent Calibration of Stellar Ages: HST Observations of White Dwarfs at V=25
The white dwarf luminosity function of a stellar cluster will have a sharp
truncation at a luminosity which is determined by the time since formation of
the first white dwarfs in that cluster. Calculation of the dependence of this
limiting luminosity on age requires relatively well-understood physics and is
independent of stellar evolutionary models. Thus, measurement of the
termination of the white dwarf luminosity function provides an independent
method to determine the age of a cluster, and thereby to calibrate stellar
evolutionary ages. We have obtained HST WFPC2 data in two open clusters,
identified the white dwarf sequence, and proved the feasibility of this
approach, by detecting white dwarfs to V=25. Much deeper data are feasible.
From our present limited data, we show that degenerate cooling ages are not
consistent with some published isochrone ages for clusters with ages of order
1Gyr.Comment: 5 pages plus 3 figures ps format, paper in press in MNRAS: previous
attempt lost the tex
Stellar Populations and the White Dwarf Mass Function: Connections To Supernova Ia Luminosities
We discuss the luminosity function of SNe Ia under the assumption that recent
evidence for dispersion in this standard candle is related to variations in the
white dwarf mass function (WDMF) in the host galaxies. We develop a simple
parameterization of the WDMF as a function of age of a stellar population and
apply this to galaxies of different morphological types. We show that this
simplified model is consistent with the observed WDMF of Bergeron et al. (1992)
for the solar neighborhood. Our simple models predict that WDMF variations can
produce a range of more than 1.8 mag in M(SN Ia), which is comparable to
the observed value using the data of Phillips (1993) and van den Bergh (1996).
We also predict a galaxy type dependence of M(SN Ia) under standard
assumptions of the star formation history in these galaxies and show that
M(SN Ia) can evolve with redshift. In principle both evolutionary and
galaxy type corrections should be applied to recover the intrinsic range of
M(SN Ia) from the observed values. Our current inadequate knowledge of the
star formation history of galaxies coupled with poor physical understanding of
the SN Ia mechanism makes the reliable estimation of these corrections both
difficult and controversial. The predictions of our models combined with the
observed galaxy and redshift correlations may have the power to discriminate
between the Chandrasekhar and the sub-Chandrasekhar progenitor scenarios for
SNe Ia.Comment: 20 pages, latex + 7 postscript figures, to be published in AJ,
September 199
The Shape of Covariantly Smeared Sources in Lattice QCD
Covariantly smeared sources are commonly used in lattice QCD to enhance the
projection onto the ground state. Here we investigate the dependence of their
shape on the gauge field background and find that the presence of localized
concentrations of magnetic field can lead to strong distortions which reduce
the smearing radii achievable by iterative smearing prescriptions. In
particular, as , iterative procedures like Jacobi smearing require
increasingly large iteration counts in order to reach physically-sized smearing
radii 0.5 fm, and the resulting sources are strongly distorted. To
bypass this issue, we propose a covariant smearing procedure (``free-form
smearing'') that allows us to create arbitrarily shaped sources, including in
particular Gaussians of arbitrary radius.Comment: 1+15 pages, 7 figures (24 pdf images
Precise Ages of Field Stars from White Dwarf Companions
Observational tests of stellar and Galactic chemical evolution call for the
joint knowledge of a star's physical parameters, detailed element abundances,
and precise age. For cool main-sequence (MS) stars the abundances of many
elements can be measured from spectroscopy, but ages are very hard to
determine. The situation is different if the MS star has a white dwarf (WD)
companion and a known distance, as the age of such a binary system can then be
determined precisely from the photometric properties of the cooling WD. As a
pilot study for obtaining precise age determinations of field MS stars, we
identify nearly one hundred candidates for such wide binary systems: a faint WD
whose GPS1 proper motion matches that of a brighter MS star in Gaia/TGAS with a
good parallax (). We model the WD's multi-band
photometry with the BASE-9 code using this precise distance (assumed to be
common for the pair) and infer ages for each binary system. The resulting age
estimates are precise to () for () MS-WD systems.
Our analysis more than doubles the number of MS-WD systems with precise
distances known to date, and it boosts the number of such systems with precise
age determination by an order of magnitude. With the advent of the Gaia DR2
data, this approach will be applicable to a far larger sample, providing ages
for many MS stars (that can yield detailed abundances for over 20 elements),
especially in the age range 2 to 8\,\Gyr, where there are only few known star
clusters.Comment: 9 pages, 5 figures, 1 catalog; Submitted to Ap
Outlaw Community Innovations
Recent studies of outlaw communities provide qualitative evidence of their existence and the organisation of the underlying innovation processes. We provide descriptive results from a large scale survey of two online outlaw communities focussing on Microsoft's XBox. In line with previous findings, we identify two types of participants in outlaw communities - user innovators and adopters. Based on 2,256 responses, we find that users modify their XBox mainly to be able to increase the set of available functions of their XBox. Users are also motivated to modify their XBox for the sake of having fun and to conduct pirate behaviour. Finally, the results from our survey suggest that user innovators are largely intrinsically motivated by fun and the intellectual stimulation of writing code for homebrew software
DNA-Protein Binding Rates: Bending Fluctuation and Hydrodynamic Coupling Effects
We investigate diffusion-limited reactions between a diffusing particle and a
target site on a semiflexible polymer, a key factor determining the kinetics of
DNA-protein binding and polymerization of cytoskeletal filaments. Our theory
focuses on two competing effects: polymer shape fluctuations, which speed up
association, and the hydrodynamic coupling between the diffusing particle and
the chain, which slows down association. Polymer bending fluctuations are
described using a mean field dynamical theory, while the hydrodynamic coupling
between polymer and particle is incorporated through a simple heuristic
approximation. Both of these we validate through comparison with Brownian
dynamics simulations. Neither of the effects has been fully considered before
in the biophysical context, and we show they are necessary to form accurate
estimates of reaction processes. The association rate depends on the stiffness
of the polymer and the particle size, exhibiting a maximum for intermediate
persistence length and a minimum for intermediate particle radius. In the
parameter range relevant to DNA-protein binding, the rate increase is up to
100% compared to the Smoluchowski result for simple center-of-mass motion. The
quantitative predictions made by the theory can be tested experimentally.Comment: 21 pages, 11 figures, 1 tabl
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