4,035 research outputs found
From cellular properties to population asymptotics in the Population Balance Equation
Proliferating cell populations at steady state growth often exhibit broad
protein distributions with exponential tails. The sources of this variation and
its universality are of much theoretical interest. Here we address the problem
by asymptotic analysis of the Population Balance Equation. We show that the
steady state distribution tail is determined by a combination of protein
production and cell division and is insensitive to other model details. Under
general conditions this tail is exponential with a dependence on parameters
consistent with experiment. We discuss the conditions for this effect to be
dominant over other sources of variation and the relation to experiments.Comment: Exact solution of Eq. 9 is adde
Pioneer Mars 1979 mission options
A preliminary investigation of lower cost Mars missions which perform useful exploration objectives after the Viking/75 mission was conducted. As a study guideline, it was assumed that significant cost savings would be realized by utilizing Pioneer hardware currently being developed for a pair of 1978 Venus missions. This in turn led to the additional constraint of a 1979 launch with the Atlas/Centaur launch vehicle which has been designated for the Pioneer Venus missions. Two concepts, using an orbiter bus platform, were identified which have both good science potential and mission simplicity indicative of lower cost. These are: (1) an aeronomy/geology orbiter, and (2) a remote sensing orbiter with a number of deployable surface penetrometers
Trajectory and propulsion characteristics of comet rendezvous opportunities
Trajectory and propulsion characteristics of spacecraft rendezvous mission opportunities to comets during 1975 to 199
Celebrities, credibility, and complementary frames: raising the agenda of sustainable and other ‘inconvenient’ food issues in social media campaigning
© 2018, © 2018 Australian and New Zealand Communication Association. In a rapidly changing and crowded media landscape, food sustainability advocates face new challenges in engaging the public. Participants in digital networks often reside in social media communities that support their own views. This action-research study, which investigates international meat reduction social media campaigns, indicates that certain digital media advocacy strategies can assist in engaging a broader base and raising the agenda of issues surrounding environmental and other impacts of meat. Social media processes and frameworks such as agenda melding and connective action facilitate connections in digital networks and offer potential to build and broaden communities. Other strategies identified include: featuring the environment as one of a suite of complementary frames; and utilising high-profile experts or celebrities who promote or are associated with complementary frames, are seen to be credible, and embrace bigger-than-self intrinsic values. The findings have implications for media advocates who wish to break through digital echo chambers
Intrinsic limits to gene regulation by global crosstalk
Gene regulation relies on the specificity of transcription factor (TF) - DNA
interactions. In equilibrium, limited specificity may lead to crosstalk: a
regulatory state in which a gene is either incorrectly activated due to
noncognate TF-DNA interactions or remains erroneously inactive. We present a
tractable biophysical model of global crosstalk, where many genes are
simultaneously regulated by many TFs. We show that in the simplest regulatory
scenario, a lower bound on crosstalk severity can be analytically derived
solely from the number of (co)regulated genes and a suitable parameter that
describes binding site similarity. Estimates show that crosstalk could present
a significant challenge for organisms with low-specificity TFs, such as
metazoans, unless they use appropriate regulation schemes. Strong cooperativity
substantially decreases crosstalk, while joint regulation by activators and
repressors, surprisingly, does not; moreover, certain microscopic details about
promoter architecture emerge as globally important determinants of crosstalk
strength. Our results suggest that crosstalk imposes a new type of global
constraint on the functioning and evolution of regulatory networks, which is
qualitatively distinct from the known constraints acting at the level of
individual gene regulatory elements
Linear instability criteria for ideal fluid flows subject to two subclasses of perturbations
In this paper we examine the linear stability of equilibrium solutions to
incompressible Euler's equation in 2- and 3-dimensions. The space of
perturbations is split into two classes - those that preserve the topology of
vortex lines and those in the corresponding factor space. This classification
of perturbations arises naturally from the geometric structure of
hydrodynamics; our first class of perturbations is the tangent space to the
co-adjoint orbit. Instability criteria for equilibrium solutions are
established in the form of lower bounds for the essential spectral radius of
the linear evolution operator restricted to each class of perturbation.Comment: 29 page
On the spectrum of the Laplace operator of metric graphs attached at a vertex -- Spectral determinant approach
We consider a metric graph made of two graphs
and attached at one point. We derive a formula relating the
spectral determinant of the Laplace operator
in terms of the spectral
determinants of the two subgraphs. The result is generalized to describe the
attachment of graphs. The formulae are also valid for the spectral
determinant of the Schr\"odinger operator .Comment: LaTeX, 8 pages, 7 eps figures, v2: new appendix, v3: discussions and
ref adde
Supermassive black hole seeds from sub-keV dark matter
Quasars observed at redshifts are powered by supermassive black
holes which are too large to have grown from early stellar remnants without
efficient super-Eddington accretion. A proposal for alleviating this tension is
for dust and metal-free gas clouds to have undergone a process of direct
collapse, producing black hole seeds of mass
around redshift . For direct collapse to occur, a large flux of UV
photons must exist to photodissociate molecular hydrogen, allowing the gas to
cool slowly and avoid fragmentation. We investigate the possibility of sub-keV
mass dark matter decaying or annihilating to produce the UV flux needed to
cause direct collapse. We find that annihilating dark matter with a mass in the
range of can produce the
required flux while avoiding existing constraints. A non-thermally produced
dark matter particle which comprises the entire dark matter abundance requires
a thermally averaged cross section of
cms. Alternatively, the flux could originate from a thermal relic which
comprises only a fraction of the total dark matter density.
Decaying dark matter models which are unconstrained by independent
astrophysical observations are unable to sufficiently suppress molecular
hydrogen, except in gas clouds embedded in dark matter halos which are larger,
cuspier, or more concentrated than current simulations predict. Lastly, we
explore how our results could change with the inclusion of full
three-dimensional effects. Notably, we demonstrate that if the
self-shielding is less than the conservative estimate used in this work, the
range of both annihilating and decaying dark matter models which can cause
direct collapse is significantly increased.Comment: 24 pages, 9 figures. Updated to match published versio
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