54,687 research outputs found
Information Flow in an R and D Laboratory
Statistical analysis of hypotheses concerning roles of technological gatekeeper and primary groups in flow of information in small research and development laborator
The Dynamics of Stellar Coronae Harboring Hot-jupiters II. A Space Weather Event on A Hot-jupiter
We carry out a numerical simulation depicting the effects of a Coronal Mass
Ejection (CME) event on a close-in giant planet in an extrasolar system. We
drive the CME in a similar manner as in simulations of space weather events on
Earth. The simulation includes the planetary orbital motion, which leads to the
forming of a comet-like planetary magnetotail which is oriented almost
perpendicular to the direction of propagation of the CME. The combination of
this feature and the fact that the CME does not expand much by the time it
reaches the planet leads to a unique CME-magnetosphere interaction, where the
CME itself is highly affected by the presence of the planetary magnetosphere.
We find that the planet is well-shielded from CME penetration, even for a
relatively weak internal magnetic field. The planetary angular momentum loss
associated with such an event is negligible compared to the total planetary
angular momentum. We also find that the energy which is deposited in the
magnetosphere is much higher than in the case of the Earth, and our simulation
suggests there is a large-scale change in the orientation of the
magnetosphere-ionosphere current system during the CME event.Comment: 16 pages, 10 figures, accepted to Ap
An Update on the 0Z Project
We give an update on our 0Z Survey to find more extremely metal poor (EMP)
stars with [Fe/H] < -3 dex through mining the database of the Hamburg/ESO
Survey. We present the most extreme such stars we have found from ~1550
moderate resolution follow up spectra. One of these, HE1424-0241, has highly
anomalous abundance ratios not seen in any previously known halo giant, with
very deficient Si, moderately deficient Ca and Ti, highly enhanced Mn and Co,
and low C, all with respect to Fe. We suggest a SNII where the nucleosynthetic
yield for explosive alpha-burning nuclei was very low compared to that for the
hydrostatic alpha-burning element Mg, which is normal in this star relative to
Fe. A second, less extreme, outlier star with high [Sc/Fe] has also been found.
We examine the extremely metal-poor tail of the HES metallicity distribution
function (MDF). We suggest on the basis of comparison of our high resolution
detailed abundance analyses with [Fe/H](HES) for stars in our sample that the
MDF inferred from follow up spectra of the HES sample of candidate EMP stars is
heavily contaminated for [Fe/H](HES) < -3 dex; many of the supposed EMP stars
below that metallicity are of substantially higher Fe-metallicity, including
most of the very C-rich stars, or are spurious objects.Comment: to appear in conference proceedings "First Stars III", ed. B. O'Shea,
A. Heger & T.Abel, 4 pages, 2 figure
Lower Bounds in the Preprocessing and Query Phases of Routing Algorithms
In the last decade, there has been a substantial amount of research in
finding routing algorithms designed specifically to run on real-world graphs.
In 2010, Abraham et al. showed upper bounds on the query time in terms of a
graph's highway dimension and diameter for the current fastest routing
algorithms, including contraction hierarchies, transit node routing, and hub
labeling. In this paper, we show corresponding lower bounds for the same three
algorithms. We also show how to improve a result by Milosavljevic which lower
bounds the number of shortcuts added in the preprocessing stage for contraction
hierarchies. We relax the assumption of an optimal contraction order (which is
NP-hard to compute), allowing the result to be applicable to real-world
instances. Finally, we give a proof that optimal preprocessing for hub labeling
is NP-hard. Hardness of optimal preprocessing is known for most routing
algorithms, and was suspected to be true for hub labeling
Femtosecond transparency in the extreme ultraviolet
Electromagnetically induced transparency-like behavior in the extreme
ultraviolet (XUV) is studied theoretically, including the effect of intense 800
nm laser dressing of He 2s2p (1Po) and 2p^2 (1Se) autoionizing states. We
present an ab initio solution of the time-dependent Schrodinger equation (TDSE)
in an LS-coupling configuration interaction basis set. The method enables a
rigorous treatment of optical field ionization of these coupled autoionizing
states into the N = 2 continuum in addition to N = 1. Our calculated transient
absorption spectra show encouraging agreement with experiment.Comment: 25 pages, 7 figures, 1 tabl
The Interaction of Venus-like, M-dwarf Planets with the Stellar Wind of Their Host Star
We study the interaction between the atmospheres of Venus-like,
non-magnetized exoplanets orbiting an M-dwarf star, and the stellar wind using
a multi-species Magnetohydrodynaic (MHD) model. We focus our investigation on
the effect of enhanced stellar wind and enhanced EUV flux as the planetary
distance from the star decreases. Our simulations reveal different topologies
of the planetary space environment for sub- and super-Alfvenic stellar wind
conditions, which could lead to dynamic energy deposition in to the atmosphere
during the transition along the planetary orbit. We find that the stellar wind
penetration for non-magnetized planets is very deep, up to a few hundreds of
kilometers. We estimate a lower limit for the atmospheric mass-loss rate and
find that it is insignificant over the lifetime of the planet. However, we
predict that when accounting for atmospheric ion acceleration, a significant
amount of the planetary atmosphere could be eroded over the course of a billion
years.Comment: 13 pages, 7 figures, accepted to Ap
The Coronal Structure of AB Doradus
We perform a numerical simulation of the corona of the young, rapidly
rotating K0 dwarf AB Doradus using a global MHD model. The model is driven by a
surface map of the radial magnetic field constructed using Zeeman-Doppler
Imaging. We find that the global structure of the stellar corona is dominated
by strong azimuthal tangling of the magnetic field due to the rapid rotation.
The MHD solution enables us to calculate realistic Alfv\'en surfaces and we can
therefore estimate the stellar mass loss rate and angular momentum loss rate
without making undue theoretical simplifications. We consider three cases,
parametrized by the base density of the corona, that span the range of possible
solutions for the system. We find that overall, the mass and angular-momentum
loss rates are higher than in the solar case; the mass loss rates are 10 to 500
times higher, and the angular momentum loss rate can be up to
higher than present day solar values. Our simulations show that this model can
be use to constrain the wide parameter space of stellar systems. It also shows
that an MHD approach can provide more information about the physical system
over the commonly used potential field extrapolation.Comment: 13 pages, 7 figure
Effects of Preference for Attachment to Low-degree Nodes on the Degree Distributions of a Growing Directed Network and a Simple Food-Web Model
We study the growth of a directed network, in which the growth is constrained
by the cost of adding links to the existing nodes. We propose a new
preferential-attachment scheme, in which a new node attaches to an existing
node i with probability proportional to 1/k_i, where k_i is the number of
outgoing links at i. We calculate the degree distribution for the outgoing
links in the asymptotic regime (t->infinity), both analytically and by Monte
Carlo simulations. The distribution decays like k c^k/Gamma(k) for large k,
where c is a constant. We investigate the effect of this
preferential-attachment scheme, by comparing the results to an equivalent
growth model with a degree-independent probability of attachment, which gives
an exponential outdegree distribution. Also, we relate this mechanism to simple
food-web models by implementing it in the cascade model. We show that the
low-degree preferential-attachment mechanism breaks the symmetry between in-
and outdegree distributions in the cascade model. It also causes a faster decay
in the tails of the outdegree distributions for both our network growth model
and the cascade model.Comment: 10 pages, 7 figures. A new figure added. Minor modifications made in
the tex
Model Independent Extraction of Without Heavy Quark Symmetry
A new method to extract is proposed based on a sum--rule for
semileptonic decays of the meson. The method relies on much weaker
assumptions than previous approaches which are based on heavy--quark symmetry.
This sum--rule only relies on the assumption that the virtual
pair content of the meson can be neglected. The extraction of the CKM
matrix element also requires that the sum--rule saturates in the kinematically
accessible region.Comment: 10 pages revtex3 manuscript. No figures, U. of MD PP #94--086. With
our apologies, some innocuous errors corrected and some references added that
had been brought to our attentio
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