10,761 research outputs found
Turbulent Disks are Never Stable: Fragmentation and Turbulence-Promoted Planet Formation
A fundamental assumption in our understanding of disks is that when the
Toomre Q>>1, the disk is stable against fragmentation into self-gravitating
objects (and so cannot form planets via direct collapse). But if disks are
turbulent, this neglects a spectrum of stochastic density fluctuations that can
produce rare, high-density mass concentrations. Here, we use a
recently-developed analytic framework to predict the statistics of these
fluctuations, i.e. the rate of fragmentation and mass spectrum of fragments
formed in a turbulent Keplerian disk. Turbulent disks are never completely
stable: we calculate the (always finite) probability of forming
self-gravitating structures via stochastic turbulent density fluctuations in
such disks. Modest sub-sonic turbulence above Mach number ~0.1 can produce a
few stochastic fragmentation or 'direct collapse' events over ~Myr timescales,
even if Q>>1 and cooling is slow (t_cool>>t_orbit). In trans-sonic turbulence
this extends to Q~100. We derive the true Q-criterion needed to suppress such
events, which scales exponentially with Mach number. We specify to turbulence
driven by MRI, convection, or spiral waves, and derive equivalent criteria in
terms of Q and the cooling time. Cooling times >~50*t_dyn may be required to
completely suppress fragmentation. These gravoturbulent events produce mass
spectra peaked near ~M_disk*(Q*M_disk/M_star)^2 (rocky-to-giant planet masses,
increasing with distance from the star). We apply this to protoplanetary disk
models and show that even minimum mass solar nebulae could experience
stochastic collapse events, provided a source of turbulence.Comment: 15 pages, 5 figures (+appendix), accepted to ApJ (added
clarifications and discussion to match accepted version
Discovery of TUG-770: a highly potent free fatty acid receptor 1 (FFA1/GPR40) agonist for treatment of type 2 diabetes
Free fatty acid receptor 1 (FFA1 or GPR40) enhances glucose-stimulated insulin secretion from pancreatic β-cells and currently attracts high interest as a new target for the treatment of type 2 diabetes. We here report the discovery of a highly potent FFA1 agonist with favorable physicochemical and pharmacokinetic properties. The compound efficiently normalizes glucose tolerance in diet-induced obese mice, an effect that is fully sustained after 29 days of chronic dosing
Curvature-induced symmetry breaking in nonlinear Schrodinger models
We consider a curved chain of nonlinear oscillators and show that the
interplay of curvature and nonlinearity leads to a symmetry breaking when an
asymmetric stationary state becomes energetically more favorable than a
symmetric stationary state. We show that the energy of localized states
decreases with increasing curvature, i.e. bending is a trap for nonlinear
excitations. A violation of the Vakhitov-Kolokolov stability criterium is found
in the case where the instability is due to the softening of the Peierls
internal mode.Comment: 4 pages (LaTex) with 6 figures (EPS
Comparison of Two Plans for Administration Organization of the School Districts of Sanpete County
Educational administration exists to develop and guide the educational program. Every local public school system is a legal agency of the state for the conduct of public education. State laws usually set up the administrative units for school districts and places upon local boards of education the responsibility for the management of the local schools. Such boards represent the people of the state, and the district, and they perform a large number of legally designated function
Models for energy and charge transport and storage in biomolecules
Two models for energy and charge transport and storage in biomolecules are
considered. A model based on the discrete nonlinear Schrodinger equation with
long-range dispersive interactions (LRI's) between base pairs of DNA is offered
for the description of nonlinear dynamics of the DNA molecule. We show that
LRI's are responsible for the existence of an interval of bistability where two
stable stationary states, a narrow, pinned state and a broad, mobile state,
coexist at each value of the total energy. The possibility of controlled
switching between pinned and mobile states is demonstrated. The mechanism could
be important for controlling energy storage and transport in DNA molecules.
Another model is offered for the description of nonlinear excitations in
proteins and other anharmonic biomolecules. We show that in the highly
anharmonic systems a bound state of Davydov and Boussinesq solitons can exist.Comment: 12 pages (latex), 12 figures (ps
The extra-tropical Northern Hemisphere temperature in the last two millennia: reconstructions of low-frequency variability
We present two new multi-proxy reconstructions of the extra-tropical Northern Hemisphere (30–90° N) mean temperature: a two-millennia long reconstruction reaching back to 1 AD and a 500-yr long reconstruction reaching back to 1500 AD. The reconstructions are based on compilations of 32 and 91 proxies, respectively, of which only little more than half pass a screening procedure and are included in the actual reconstructions. The proxies are of different types and of different resolutions (annual, annual-to-decadal, and decadal) but all have previously been shown to relate to local or regional temperature. We use a reconstruction method, LOCal (LOC), that recently has been shown to confidently reproduce low-frequency variability. Confidence intervals are obtained by an ensemble pseudo-proxy method that both estimates the variance and the bias of the reconstructions. The two-millennia long reconstruction shows a well defined Medieval Warm Period, with a peak warming ca. 950–1050 AD reaching 0.6 °C relative to the reference period 1880–1960 AD. The 500-yr long reconstruction confirms previous results obtained with the LOC method applied to a smaller proxy compilation; in particular it shows the Little Ice Age cumulating in 1580–1720 AD with a temperature minimum of −1.0 °C below the reference period. The reconstructed local temperatures, the magnitude of which are subject to wide confidence intervals, show a rather geographically homogeneous Little Ice Age, while more geographical inhomogeneities are found for the Medieval Warm Period. Reconstructions based on different subsets of proxies show only small differences, suggesting that LOC reconstructs 50-yr smoothed extra-tropical NH mean temperatures well and that low-frequency noise in the proxies is a relatively small problem
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