The strongest bounds on active-sterile neutrino mixing after Planck data

Light sterile neutrinos can be excited by oscillations with active neutrinos in the early universe. Their properties can be constrained by their contribution as extra-radiation, parameterized in terms of the effective number of neutrino species N_ eff, and to the universe energy density today \Omega_\nu h^2. Both these parameters have been measured to quite a good precision by the Planck satellite experiment. We use this result to update the bounds on the parameter space of (3+1) sterile neutrino scenarios, with an active-sterile neutrino mass squared splitting in the range (10^{-5} - 10^2 ) eV^2. We consider both normal and inverted mass orderings for the active and sterile states. For the first time we take into account the possibility of two non-vanishing active-sterile mixing angles. We find that the bounds are more stringent than those obtained in laboratory experiments. This leads to a strong tension with the short-baseline hints of light sterile neutrinos. In order to relieve this disagreement, modifications of the standard cosmological scenario, e.g. large primordial neutrino asymmetries, are required.Comment: v2 (9 pages, 10 eps figures) revised version. Discussion enlarged. Included bounds from the Planck limit on the sterile neutrino mass. References update

Modelling magnetic flux emergence in the solar convection zone

[Abridged] Bipolar magnetic regions are formed when loops of magnetic flux emerge at the solar photosphere. Our aim is to investigate the flux emergence process in a simulation of granular convection. In particular we aim to determine the circumstances under which magnetic buoyancy enhances the flux emergence rate (which is otherwise driven solely by the convective upflows). We use three-dimensional numerical simulations, solving the equations of compressible magnetohydrodynamics in a horizontally-periodic Cartesian domain. A horizontal magnetic flux tube is inserted into fully developed hydrodynamic convection. We systematically vary the initial field strength, the tube thickness, the initial entropy distribution along the tube axis and the magnetic Reynolds number. Focusing upon the low magnetic Prandtl number regime (Pm<1) at moderate magnetic Reynolds number, we find that the flux tube is always susceptible to convective disruption to some extent. However, stronger flux tubes tend to maintain their structure more effectively than weaker ones. Magnetic buoyancy does enhance the flux emergence rates in the strongest initial field cases, and this enhancement becomes more pronounced when we increase the width of the flux tube. This is also the case at higher magnetic Reynolds numbers, although the flux emergence rates are generally lower in these less dissipative simulations because the convective disruption of the flux tube is much more effective in these cases. These simulations seem to be relatively insensitive to the precise choice of initial conditions: for a given flow, the evolution of the flux tube is determined primarily by the initial magnetic field distribution and the magnetic Reynolds number.Comment: 12 pages, 15 figures, 2 tables. Accepted for publication in Astronomy and Astrophysic

The Lifetimes of Phases in High-Mass Star-Forming Regions

High-mass stars form within star clusters from dense, molecular regions, but is the process of cluster formation slow and hydrostatic or quick and dynamic? We link the physical properties of high-mass star-forming regions with their evolutionary stage in a systematic way, using Herschel and Spitzer data. In order to produce a robust estimate of the relative lifetimes of these regions, we compare the fraction of dense, molecular regions above a column density associated with high-mass star formation, N(H2) > 0.4-2.5 x 10^22 cm^-2, in the 'starless (no signature of stars > 10 Msun forming) and star-forming phases in a 2x2 degree region of the Galactic Plane centered at l=30deg. Of regions capable of forming high-mass stars on ~1 pc scales, the starless (or embedded beyond detection) phase occupies about 60-70% of the dense, molecular region lifetime and the star-forming phase occupies about 30-40%. These relative lifetimes are robust over a wide range of thresholds. We outline a method by which relative lifetimes can be anchored to absolute lifetimes from large-scale surveys of methanol masers and UCHII regions. A simplistic application of this method estimates the absolute lifetimes of the starless phase to be 0.2-1.7 Myr (about 0.6-4.1 fiducial cloud free-fall times) and the star-forming phase to be 0.1-0.7 Myr (about 0.4-2.4 free-fall times), but these are highly uncertain. This work uniquely investigates the star-forming nature of high-column density gas pixel-by-pixel and our results demonstrate that the majority of high-column density gas is in a starless or embedded phase.Comment: 10 pages, accepted to Ap

On the Nature of Small Planets around the Coolest Kepler Stars

We constrain the densities of Earth- to Neptune-size planets around very cool (Te =3660-4660K) Kepler stars by comparing 1202 Keck/HIRES radial velocity measurements of 150 nearby stars to a model based on Kepler candidate planet radii and a power-law mass-radius relation. Our analysis is based on the presumption that the planet populations around the two sets of stars are the same. The model can reproduce the observed distribution of radial velocity variation over a range of parameter values, but, for the expected level of Doppler systematic error, the highest Kolmogorov-Smirnov probabilities occur for a power-law index alpha ~ 4, indicating that rocky-metal planets dominate the planet population in this size range. A single population of gas-rich, low-density planets with alpha = 2 is ruled out unless our Doppler errors are >= 5m/s, i.e., much larger than expected based on observations and stellar chromospheric emission. If small planets are a mix of gamma rocky planets (alpha = 3.85) and 1-gamma gas-rich planets (alpha = 2), then gamma > 0.5 unless Doppler errors are >=4 m/s. Our comparison also suggests that Kepler's detection efficiency relative to ideal calculations is less than unity. One possible source of incompleteness is target stars that are misclassified subgiants or giants, for which the transits of small planets would be impossible to detect. Our results are robust to systematic effects, and plausible errors in the estimated radii of Kepler stars have only moderate impact.Comment: Accepted to the Astrophysical Journa