10,617 research outputs found
Nucleation at the DNA supercoiling transition
Twisting DNA under a constant applied force reveals a thermally activated
transition into a state with a supercoiled structure known as a plectoneme.
Using transition state theory, we predict the rate of this plectoneme
nucleation to be of order 10^4 Hz. We reconcile this with experiments that have
measured hopping rates of order 10 Hz by noting that the viscosity of the bead
used to manipulate the DNA limits the measured rate. We find that the intrinsic
bending caused by disorder in the base-pair sequence is important for
understanding the free energy barrier that governs the transition. Both
analytic and numerical methods are used in the calculations. We provide
extensive details on the numerical methods for simulating the elastic rod model
with and without disorder.Comment: 18 pages, 15 figure
A Universal Temperature Profile for Galaxy Clusters
We investigate the predicted present-day temperature profiles of the hot,
X-ray emitting gas in galaxy clusters for two cosmological models - a current
best-guess LCDM model and standard cold dark matter (SCDM). Our
numerically-simulated "catalogs" of clusters are derived from high-resolution
(15/h kpc) simulations which make use of a sophisticated, Eulerian-based,
Adaptive Mesh-Refinement (AMR) code that faithfully captures the shocks which
are essential for correctly modelling cluster temperatures. We show that the
temperature structure on Mpc-scales is highly complex and non-isothermal.
However, the temperature profiles of the simulated LCDM and SCDM clusters are
remarkably similar and drop-off as
where and . This decrease
is in good agreement with the observational results of Markevitch et al.(1998)
but diverges, primarily in the innermost regions, from their fit which assumes
a polytropic equation of state. Our result is also in good agreement with a
recent sample of clusters observed by BeppoSAX though there is some indication
of missing physics at small radii (). We discuss the
interpretation of our results and make predictions for new x-ray observations
that will extend to larger radii than previously possible. Finally, we show
that, for , our universal temperature profile is consistent with
our most recent simulations which include both radiative cooling and supernovae
feedback.Comment: 8 pages, 6 figures, accepted for publication in ApJ, full-page
version of Fig. 2 at
http://www.cita.utoronto.ca/+AH4-cloken/PAPERS/UTP/f2.ep
Explaining the entropy excess in clusters and groups of galaxies without additional heating
The X-ray luminosity and temperature of clusters and groups of galaxies do
not scale in a self-similar manner. This has often been interpreted as a sign
that the intracluster medium has been substantially heated by non-gravitational
sources. In this paper, we propose a simple model which, instead, uses the
properties of galaxy formation to explain the observations. Drawing on
available observations, we show that there is evidence that the efficiency of
galaxy formation was higher in groups than in clusters. If confirmed, this
would deplete the low-entropy gas in groups, increase their central entropy and
decrease their X-ray luminosity. A simple, empirical, hydrostatic model appears
to match both the luminosity-temperature relation of clusters and properties of
their internal structure as well.Comment: 5 pages, 4 figures, accepted in ApJL; added one reference, otherwise
unchange
A Search for Jovian Planets around Hot White Dwarfs
Current searches for extrasolar planets have concentrated on observing the
reflex Doppler shift of solar-type stars. Little is known, however, about
planetary systems around non-solar-type stars. We suggest a new method to
extend planetary searches to hot white dwarfs. Near a hot white dwarf, the
atmosphere of a Jovian planet will be photoionized and emit hydrogen
recombination lines, which may be detected by high- dispersion spectroscopic
observations. Multi-epoch monitoring can be used to distinguish between non-LTE
stellar emission and planetary emission, and to establish the orbital
parameters of the detected planets. In the future, high-precision astrometric
measurements of the hot white dwarf will allow the masses of the detected
planets to be determined. Searches for Jovian planets around hot white dwarfs
will provide invaluable new insight on the development of planetary systems
around stars more massive than the Sun and on how stellar evolution affects
these systems. We present high-dispersion spectroscopic observations of the
white dwarf Feige 34 to demonstrate the complexity and feasibility of the
search method.Comment: 10 pages, 2 figures, accepted for publication in the ApJ Letter
Uncharted Territory: The Nexus Between Doctoral Education and Community-based Learning
One of the continual challenges facing professors who prepare TK-12 school leaders through the professional educational doctorate (Ed.D. degree), is ensuring a theory-to-practice framework, curriculum, and pedagogy. Furthermore, professors of doctoral programs whose orientation is social justice, often face the dilemma of how to infuse it as a cross-cutting issue with the theory-practice paradigm. Community-based learning (CBL), when embedded with social justice claims, can serve as a bridge between theory and practice, providing doctoral students with opportunities to achieve both goals. This qualitative case study engaged with this dilemma by investigating the efficacy of a community-based learning component that had been infused into a core doctoral leadership course. Through interviews with doctoral students, we sought to understand their perception of its connection to the program’s social justice orientation, as well as the benefits, challenges, and recommendations for the efficacy of community-based learning. Students’ input guided future course revisions that establish a clearer relationship among social justice themes, the program’s learning outcomes, and the theory-praxis paradigm. Â
The Specific Globular Cluster Frequencies of Dwarf Elliptical Galaxies from the Hubble Space Telescope
The specific globular cluster frequencies (S_N) for 24 dwarf elliptical (dE)
galaxies in the Virgo and Fornax Clusters and the Leo Group imaged with the
Hubble Space Telescope are presented. Combining all available data, we find
that for nucleated dEs --- which are spatially distributed like giant
ellipticals in galaxy clusters --- S_N(dE,N)=6.5 +- 1.2 and S_N increases with
M_V, while for non-nucleated dEs --- which are distributed like late-type
galaxies --- S_N(dE,noN)=3.1 +- 0.5 and there is little or no trend with M_V.
The S_N values for dE galaxies are thus on average significantly higher than
those for late-type galaxies, which have S_N < 1. This suggests that dE
galaxies are more akin to giant Es than to late-type galaxies. If there are
dormant or stripped irregulars hiding among the dE population, they are likely
to be among the non-nucleated dEs. Furthermore, the similarities in the
properties of the globular clusters and in the spatial distributions of dE,Ns
and giant Es suggest that neither galaxy mass or galaxy metallicity is
responsible for high values of S_N. Instead, most metal-poor GCs may have
formed in dwarf-sized fragments that merged into larger galaxies.Comment: 12 pages (uses aaspp4.sty), 2 figures, 1 table, to appear in the
Astrophysical Journa
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