17,459 research outputs found
The frequency of muscle protein polymorphism in Menidia menidia (Atherinidae) along the Atlantic coast
(PDF has 6 pages.
High-resolution simulations of the final assembly of Earth-like planets 2: water delivery and planetary habitability
The water content and habitability of terrestrial planets are determined
during their final assembly, from perhaps a hundred 1000-km "planetary embryos"
and a swarm of billions of 1-10 km "planetesimals." During this process, we
assume that water-rich material is accreted by terrestrial planets via impacts
of water-rich bodies that originate in the outer asteroid region. We present
analysis of water delivery and planetary habitability in five high-resolution
simulations containing about ten times more particles than in previous
simulations (Raymond et al 2006a, Icarus, 183, 265-282). These simulations
formed 15 terrestrial planets from 0.4 to 2.6 Earth masses, including five
planets in the habitable zone. Every planet from each simulation accreted at
least the Earth's current water budget; most accreted several times that amount
(assuming no impact depletion). Each planet accreted at least five water-rich
embryos and planetesimals from past 2.5 AU; most accreted 10-20 water-rich
bodies.
We present a new model for water delivery to terrestrial planets in
dynamically calm systems, with low-eccentricity or low-mass giant planets --
such systems may be very common in the Galaxy. We suggest that water is
accreted in comparable amounts from a few planetary embryos in a "hit or miss"
way and from millions of planetesimals in a statistically robust process.
Variations in water content are likely to be caused by fluctuations in the
number of water-rich embryos accreted, as well as from systematic effects such
as planetary mass and location, and giant planet properties.Comment: Astrobiology, in pres
Neutrino masses in quartification schemes
The idea of quark-lepton universality at high energies has recently been
explored in unified theories based upon the quartification gauge group SU(3)^4.
These schemes encompass a quark-lepton exchange symmetry that results upon the
introduction of leptonic colour. It has been demonstrated that in models in
which the quartification gauge symmetry is broken down to the standard model
gauge group, gauge coupling constant unification can be achieved, and there is
no unique scenario. The same is also true when the leptonic colour gauge group
is only partially broken, leaving a remnant SU(2)_\ell symmetry at the standard
model level. Here we perform an analysis of the neutrino mass spectrum of such
models. We show that these models do not naturally generate small Majorana
neutrino masses, thus correcting an error in our earlier quartification paper,
but with the addition of one singlet neutral fermion per family there is a
realisation of see-saw suppressed masses for the neutrinos. We also show that
these schemes are consistent with proton decay.Comment: 12 pages, minor changes. To appear in Phys. Rev.
Giving and Receiving Peer Advice in an Online Breast Cancer Support Group
People have access to experiential information and advice about health online. The types of advice exchanged affect the nature of online communities and potentially patient decision making. The aim of this study was to examine the ways in which peers exchange advice within an online health forum in order to better understand online groups as a resource for decision making. Messages collected over a one-month period from an online breast cancer support forum were analyzed for examples of advice exchange. The majority of the messages solicited advice through problem disclosure or requests for information and opinion. A novel form of advice solicitationââanyone in the same boat as meââwas noted as was the use of personal experience as a form of advice giving. Women construct their advice requests to target like-minded people. The implications in terms of decision making and support are discussed
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