4,856 research outputs found
The Minimum-Mass Extrasolar Nebula: In-Situ Formation of Close-In Super-Earths
Close-in super-Earths, with radii R = 2-5 R_Earth and orbital periods P < 100
days, orbit more than half, and perhaps nearly all Sun-like stars in the
universe. We use this omnipresent population to construct the minimum-mass
extrasolar nebula (MMEN), the circumstellar disk of solar-composition solids
and gas from which such planets formed, if they formed near their current
locations and did not migrate. In a series of back-of-the-envelope
calculations, we demonstrate how in-situ formation in the MMEN is fast,
efficient, and can reproduce many of the observed properties of close-in
super-Earths, including their gas-to-rock fractions. Testable predictions are
discussed.Comment: Accepted to MNRAS 2013 March
Short-Term Dynamical Interactions Among Extrasolar Planets
We show that short-term perturbations among massive planets in multiple
planet systems can result in radial velocity variations of the central star
which differ substantially from velocity variations derived assuming the
planets are executing independent Keplerian motions. We discuss two alternate
fitting methods which can lead to an improved dynamical description of multiple
planet systems. In the first method, the osculating orbital elements are
determined via a Levenberg-Marquardt minimization scheme driving an N-body
integrator. The second method is an improved analytic model in which orbital
elements are allowed to vary according to a simple model for resonant
interactions between the planets. Both of these methods can determine the true
masses for the planets by eliminating the sin(i) degeneracy inherent in fits
that assume independent Keplerian motions. We apply our fitting methods to the
GJ876 radial velocity data (Marcy et al. 2001), and argue that the mass factors
for the two planets are likely in the 1.25-2.0 rangeComment: 13 pages, including 4 figures and 3 tables Accepted by Astrophyiscal
Journal Letter
Action potential energy efficiency varies among neuron types in vertebrates and invertebrates.
The initiation and propagation of action potentials (APs) places high demands on the energetic resources of neural tissue. Each AP forces ATP-driven ion pumps to work harder to restore the ionic concentration gradients, thus consuming more energy. Here, we ask whether the ionic currents underlying the AP can be predicted theoretically from the principle of minimum energy consumption. A long-held supposition that APs are energetically wasteful, based on theoretical analysis of the squid giant axon AP, has recently been overturned by studies that measured the currents contributing to the AP in several mammalian neurons. In the single compartment models studied here, AP energy consumption varies greatly among vertebrate and invertebrate neurons, with several mammalian neuron models using close to the capacitive minimum of energy needed. Strikingly, energy consumption can increase by more than ten-fold simply by changing the overlap of the Na+ and K+ currents during the AP without changing the APs shape. As a consequence, the height and width of the AP are poor predictors of energy consumption. In the Hodgkin–Huxley model of the squid axon, optimizing the kinetics or number of Na+ and K+ channels can whittle down the number of ATP molecules needed for each AP by a factor of four. In contrast to the squid AP, the temporal profile of the currents underlying APs of some mammalian neurons are nearly perfectly matched to the optimized properties of ionic conductances so as to minimize the ATP cost
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