1,438 research outputs found
From Young and Hot to Old and Cold: Comparing White Dwarf Cooling Theory to Main Sequence Stellar Evolution in Open Clusters
I explore the current ability of both white dwarf cooling theory and main
sequence stellar evolution theory to accurately determine stellar population
ages by comparing ages derived using both techniques for open clusters ranging
from 0.1 to 4 Gyr. I find good agreement between white dwarf and main sequence
evolutionary ages over the entire age range currently available for study. I
also find that directly comparing main sequence turn-off ages to white dwarf
ages is only weakly sensitive to realistic levels of errors in cluster
distance, metallicity, and reddening. Additional detailed comparisons between
white dwarf and main sequence ages have tremendous potential to refine and
calibrate both of these important clocks, and I present new simulations of
promising open cluster targets. The most demanding requirement for these white
dwarf studies are very deep (V > 25-28) cluster observations made necessary by
the faintness of the oldest white dwarfs.Comment: 25 pages, incl. 10 figures, ApJ accepted for April, 200
Mesoscopic Model for Free Energy Landscape Analysis of DNA sequences
A mesoscopic model which allows us to identify and quantify the strength of
binding sites in DNA sequences is proposed. The model is based on the
Peyrard-Bishop-Dauxois model for the DNA chain coupled to a Brownian particle
which explores the sequence interacting more importantly with open base pairs
of the DNA chain. We apply the model to promoter sequences of different
organisms. The free energy landscape obtained for these promoters shows a
complex structure that is strongly connected to their biological behavior. The
analysis method used is able to quantify free energy differences of sites
within genome sequences.Comment: 7 pages, 5 figures, 1 tabl
Spectral modeling of gaseous metal disks around DAZ white dwarfs
We report on our attempt for the first non-LTE modeling of gaseous metal
disks around single DAZ white dwarfs recently discovered by Gaensicke et al.
and thought to originate from a disrupted asteroid. We assume a Keplerian
rotating viscous disk ring composed of calcium and hydrogen and compute the
detailed vertical structure and emergent spectrum. We find that the observed
infrared CaII emission triplet can be modeled with a hydrogen-deficient gas
ring located at R=1.2 R_sun, inside of the tidal disruption radius, with Teff
about 6000 K and a low surface mass density of about 0.3 g/cm**2. A disk having
this density and reaching from the central white dwarf out to R=1.2 R_sun would
have a total mass of 7 10**21 g, corresponding to an asteroid with about 160 km
diameter.Comment: Proceedings, 16th European White Dwarf Workshop, Barcelona, 200
The dimerized phase of ionic Hubbard models
We derive an effective Hamiltonian for the ionic Hubbard model at half
filling, extended to include nearest-neighbor repulsion. Using a spin-particle
transformation, the effective model is mapped onto simple spin-1 models in two
particular cases. Using another spin-particle transformation, a slightly
modified model is mapped into an SU(3) antiferromagnetic Heisenberg model whose
exact ground state is known to be spontaneously dimerized. From the effective
models several properties of the dimerized phase are discussed, like
ferroelectricity and fractional charge excitations. Using bosonization and
recent developments in the theory of macroscopic polarization, we show that the
polarization is proportional to the charge of the elementary excitations
Deep Photometry of the Globular Cluster M5: Distance Estimates from White Dwarf and Main Sequence Stars
We present deep VI photometry of stars in the globular cluster M5 (NGC 5904)
based on images taken with the Hubble Space Telescope. The resulting
color-magnitude diagram reaches below V ~ 27 mag, revealing the upper 2-3
magnitudes of the white dwarf cooling sequence, and main sequence stars eight
magnitudes and more below the turn-off. We fit the main sequence to subdwarfs
of known parallax to obtain a true distance modulus of (m-M)_0 = 14.45 +/- 0.11
mag. A second distance estimate based on fitting the cluster white dwarf
sequence to field white dwarfs with known parallax yielded (m-M)_0 = 14.67 +/-
0.18 mag. We couple our distance estimates with extensive photometry of the
cluster's RR Lyrae variables to provide a calibration of the RR Lyrae absolute
magnitude yielding M_V(RR) = 0.42 +/- 0.10 mag at [Fe/H] = -1.11 dex. We
provide another luminosity calibration in the form of reddening-free Wasenheit
functions. Comparison of our calibrations with predictions based on recent
models combining stellar evolution and pulsation theories shows encouraging
agreement. (Abridged)Comment: AASTeX, 29 pages including 5 figures. Complete photometry data and
FITS-format images are available at
http://physics.bgsu.edu/~layden/ASTRO/PUBL/published.html . Accepted for
publication in the Astrophysical Journal, 2005 October 20. Replaced errant
wording in last sentence of paragraph 4 of conclusion
Evolution of white dwarf stars with high-metallicity progenitors: the role of 22Ne diffusion
Motivated by the strong discrepancy between the main sequence turn-off age
and the white dwarf cooling age in the metal-rich open cluster NGC 6791, we
compute a grid of white dwarf evolutionary sequences that incorporates for the
first time the energy released by the processes of 22Ne sedimentation and of
carbon/oxygen phase separation upon crystallization. The grid covers the mass
range from 0.52 to 1.0 Msun, and it is appropriate for the study of white
dwarfs in metal-rich clusters. The evolutionary calculations are based on a
detailed and self-consistent treatment of the energy released from these two
processes, as well as on the employment of realistic carbon/oxygen profiles, of
relevance for an accurate evaluation of the energy released by carbon/oxygen
phase separation. We find that 22Ne sedimentation strongly delays the cooling
rate of white dwarfs stemming from progenitors with high metallicities at
moderate luminosities, whilst carbon/oxygen phase separation adds considerable
delays at low luminosities. Cooling times are sensitive to possible
uncertainties in the actual value of the diffusion coefficient of 22Ne.
Changing the diffusion coefficient by a factor of 2, leads to maximum age
differences of approx. 8-20% depending on the stellar mass. We find that the
magnitude of the delays resulting from chemical changes in the core is
consistent with the slow down in the white dwarf cooling rate that is required
to solve the age discrepancy in NGC 6791.Comment: 10 pages, 6 figures, to be published in The Astrophysical Journa
Boxed pervasive games: an experience with user-created pervasive games
Pervasive games are rapidly maturing - from early research experiments with locative games we now start to see a range of commercial projects using locative and pervasive technology to create technology-supported pervasive games. In this paper we report on our experiences in transferring the successful involvement of players in computer games to modding for pervasive games. We present the design process, the enabling tools and two sample games provided in boxes to end users. Finally we discuss how our findings inform the design of modding tools for a pervasive game community of the future
WIYN Open Cluster Study XI: WIYN 3.5m Deep Photometry of M35 (NGC 2168)
We present deep BVI observations of the core of M35 and a nearby comparison
field obtained at the WIYN 3.5m telescope under excellent seeing. These
observations display the lower main sequence in BV and VI CMDs down to V = 23.3
and 24.6, respectively. At these faint magnitudes background field stars are
far more numerous than the cluster stars, yet by using a smoothing technique
and CMD density distribution subtraction we recover the cluster fiducial main
sequence and luminosity function to V = 24.6. We find the location of the main
sequence in these CMDs to be consistent with earlier work on other open
clusters, specifically NGC 188, NGC 2420, and NGC 2477. We compare these open
cluster fiducial sequences to stellar models by Baraffe et al. (1998), Siess et
al. (2000), Girardi et al. (2000), and Yi et al. (2001) and find that the
models are too blue in both B-V and V-I for stars below ~0.4 Mo. M35 contains
stars to the limit of the extracted main sequence, at M ~ 0.10-0.15 Mo,
suggesting that M35 may harbor a large number of brown dwarfs, which should be
easy targets for near-IR instrumentation on 8-10m telescopes. We also identify
a new candidate white dwarf in M35 at V = 21.36 +- 0.01. Depending on which WD
models are used to interpret this cluster candidate, it is either a very high
mass WD (1.05 +- 0.05 Mo) somewhat older (0.19-0.26 Gyr, 3-4 sigma) than our
best isochrone age (150 Myr), or it is a modestly massive WD (0.67-0.78 Mo)
much too old (0.42-0.83 Gyr) to belong to the cluster.Comment: 28 pages + 24 figures; to be published in the Sept, 2002 A
Detection of a Stellar Stream Behind Open Cluster NGC 188: Another Part of the Monoceros Stream
We present results from a WIYN/OPTIC photometric and astrometric survey of
the field of the open cluster NGC 188 ((l,b) = (122.8\arcdeg, 22.5\arcdeg)). We
combine these results with the proper-motion and photometry catalog of Platais
et al. and demonstrate the existence of a stellar overdensity in the background
of NGC 188. The theoretical isochrone fits to the color-magnitude diagram of
the overdensity are consistent with an age between 6 and 10 Gyr and an
intermediately metal poor population ([Fe/H] = -0.5 to -1.0). The distance to
the overdensity is estimated to be between 10.0 and 12.6 kpc. The
proper-motions indicate that the stellar population of the overdensity is
kinematically cold.
The distance estimate and the absolute proper motion of the overdensity agree
reasonably well with the predictions of the Pe\~{n}arrubia et al. model of the
formation of the Monoceros stream. Orbits for this material constructed with
plausible radial-velocity values, indicate that dynamically, this material is
unlikely to belong to the thick disk. Taken together, this evidence suggests
that the newly-found overdensity is part of the Monoceros stream.Comment: accepted by A
Classes of fast and specific search mechanisms for proteins on DNA
Problems of search and recognition appear over different scales in biological
systems. In this review we focus on the challenges posed by interactions
between proteins, in particular transcription factors, and DNA and possible
mechanisms which allow for a fast and selective target location. Initially we
argue that DNA-binding proteins can be classified, broadly, into three distinct
classes which we illustrate using experimental data. Each class calls for a
different search process and we discuss the possible application of different
search mechanisms proposed over the years to each class. The main thrust of
this review is a new mechanism which is based on barrier discrimination. We
introduce the model and analyze in detail its consequences. It is shown that
this mechanism applies to all classes of transcription factors and can lead to
a fast and specific search. Moreover, it is shown that the mechanism has
interesting transient features which allow for stability at the target despite
rapid binding and unbinding of the transcription factor from the target.Comment: 65 pages, 23 figure
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