150 research outputs found
Exoplanet HD209458b: inflated hydrogen atmosphere but no sign of evaporation
Many extrasolar planets orbit closely to their parent star. Their existence
raises the fundamental problem of loss and gain in their mass. For exoplanet
HD209458b, reports on an unusually extended hydrogen corona and a hot layer in
the lower atmosphere seem to support the scenario of atmospheric inflation by
the strong stellar irradiation. However, difficulties in reconciling
evaporation models with observations call for a reassessment of the problem.
Here, we use HST archive data to report a new absorption rate of ~8.9% +/- 2.1%
by atomic hydrogen during the HD209458b transit, and show that no sign of
evaporation could be detected for the exoplanet. We also report evidence of
time variability in the HD209458 Lyman-a flux, a variability that was not
accounted for in previous studies, which corrupted their diagnostics. Mass loss
rates thus far proposed in the literature in the range 5x(10^{10}-10^{11} g
s^{-1}) must induce a spectral signature in the Lyman-a line profile of
HD209458 that cannot be found in the present analysis. Either an unknown
compensation effect is hiding the expected spectral feature or else the mass
loss rate of neutrals from HD209458 is modest.Comment: corrected for typos. Published 2007 December 10 in Apj
Addition-Deletion Networks
We study structural properties of growing networks where both addition and
deletion of nodes are possible. Our model network evolves via two independent
processes. With rate r, a node is added to the system and this node links to a
randomly selected existing node. With rate 1, a randomly selected node is
deleted, and its parent node inherits the links of its immediate descendants.
We show that the in-component size distribution decays algebraically, c_k ~
k^{-beta}, as k-->infty. The exponent beta=2+1/(r-1) varies continuously with
the addition rate r. Structural properties of the network including the height
distribution, the diameter of the network, the average distance between two
nodes, and the fraction of dangling nodes are also obtained analytically.
Interestingly, the deletion process leads to a giant hub, a single node with a
macroscopic degree whereas all other nodes have a microscopic degree.Comment: 8 pages, 5 figure
Genetic Correlations in Mutation Processes
We study the role of phylogenetic trees on correlations in mutation
processes. Generally, correlations decay exponentially with the generation
number. We find that two distinct regimes of behavior exist. For mutation rates
smaller than a critical rate, the underlying tree morphology is almost
irrelevant, while mutation rates higher than this critical rate lead to strong
tree-dependent correlations. We show analytically that identical critical
behavior underlies all multiple point correlations. This behavior generally
characterizes branching processes undergoing mutation.Comment: revtex, 8 pages, 2 fig
Use of an aptitude test in University entrance: a validity study
In 2005, the National Foundation for Educational Research (NFER) was commissioned
to evaluate the potential value of using an aptitude test (the SAT Reasoning TestTM) as
an additional tool in the selection of candidates for admission to higher education (HE).
This five-year study was co-funded by the Department for Business, Innovation, and
Skills (BIS), the NFER, the Sutton Trust and the College Board. This report presents
findings from the final phase of the project, relating the prior attainment and SAT®
scores of participating students who graduated in 2006 to their degree outcomes. It also
summarises findings from the study as a whole, and cross references where
appropriate to the various interim reports
Rank Statistics in Biological Evolution
We present a statistical analysis of biological evolution processes.
Specifically, we study the stochastic replication-mutation-death model where
the population of a species may grow or shrink by birth or death, respectively,
and additionally, mutations lead to the creation of new species. We rank the
various species by the chronological order by which they originate. The average
population N_k of the kth species decays algebraically with rank, N_k ~ M^{mu}
k^{-mu}, where M is the average total population. The characteristic exponent
mu=(alpha-gamma)/(alpha+beta-gamma)$ depends on alpha, beta, and gamma, the
replication, mutation, and death rates. Furthermore, the average population P_k
of all descendants of the kth species has a universal algebraic behavior, P_k ~
M/k.Comment: 4 pages, 3 figure
Acid sensing by the Drosophila olfactory system.
The odour of acids has a distinct quality that is perceived as sharp, pungent and often irritating. How acidity is sensed and translated into an appropriate behavioural response is poorly understood. Here we describe a functionally segregated population of olfactory sensory neurons in the fruitfly, Drosophila melanogaster, that are highly selective for acidity. These olfactory sensory neurons express IR64a, a member of the recently identified ionotropic receptor (IR) family of putative olfactory receptors. In vivo calcium imaging showed that IR64a+ neurons projecting to the DC4 glomerulus in the antennal lobe are specifically activated by acids. Flies in which the function of IR64a+ neurons or the IR64a gene is disrupted had defects in acid-evoked physiological and behavioural responses, but their responses to non-acidic odorants remained unaffected. Furthermore, artificial stimulation of IR64a+ neurons elicited avoidance responses. Taken together, these results identify cellular and molecular substrates for acid detection in the Drosophila olfactory system and support a labelled-line mode of acidity coding at the periphery
The Astropy Problem
The Astropy Project (http://astropy.org) is, in its own words, "a community
effort to develop a single core package for Astronomy in Python and foster
interoperability between Python astronomy packages." For five years this
project has been managed, written, and operated as a grassroots,
self-organized, almost entirely volunteer effort while the software is used by
the majority of the astronomical community. Despite this, the project has
always been and remains to this day effectively unfunded. Further, contributors
receive little or no formal recognition for creating and supporting what is now
critical software. This paper explores the problem in detail, outlines possible
solutions to correct this, and presents a few suggestions on how to address the
sustainability of general purpose astronomical software
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