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- Published on Tuesday, 25 October 2016 08:00
Vol. 594
In section 2. Astrophysical processes
Cosmic-ray energy spectrum and composition up to the ankle: the case for a second Galactic component
A vital piece of the puzzle regarding the origin of Galactic cosmic rays comes from the two breaks in their power-law energy spectrum at 3 x 106 GeV (called the knee) and 3 x 109 GeV (the ankle). These breaks have remained a mystery for decades. It is clear that above about 109 GeV, the particles are not confined to their parent galaxies by interstellar magnetic fields and gas and, therefore, escape to produce a cosmic background. Below the knee, acceleration sites have been associated with supernova remnants and massive stellar wind shocks interacting with the interstellar medium account for the Galactic cosmic rays below a few 106 GeV; the primary cosmic rays are protons, He, and CNO. The models presented here confirm earlier results that other sources are required for the higher energy particles but goes substantially farther. The authors propose two new agents in the second Galactic component: the Galactic wind termination shock and the environment created by the Wolf-Rayet progenitor of core collapse supernovae whose initial composition depends on the stellar wind He/H abundances. Their models yield nuclear compositions at 108 GeV that include a significant contribution from Fe along with CNO, and none at around 109 GeV, while requiring a smaller extragalactic contribution below that energy than previous models. The paper presents an extensive set of model energy distributions for a broad range of nuclei yields (see Table 4) and a detailed comparison with existing measurements from 1-1011 GeV.
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- Published on Friday, 21 October 2016 15:22
Vol. 594
In section 9. The Sun
Solar-cycle variation of the rotational shear near the solar surface
The near-surface shear layer of the Sun's convection zone might play a crucial role for the dynamo process and thus the generation of magnetic field in a star. This paper establishes the variability of the near-surface shear layer and its relation to the solar cycle through helioseismic observations.
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- Published on Tuesday, 18 October 2016 08:00
Vol. 594
In section 2. Astrophysical processes
The effect of pair-instability mass loss on black-hole mergers
Detection of a gravitational wave signal by the LIGO collaboration has tantalised the scientific community. Two black hole-black hole (BH) binary mergers were detected, one with heavy objects (30 times the mass of our Sun) and more with more normal objects (~10 solar masses). More events are anticipated for the next LIGO run, when the VIRGO detector will also start its search for gravitational waves. In this paper, based on detailed stellar population simulations, the authors predict that no binary BHs can be formed that are heavier than ~50 solar masses each. Predictions for the next LIGO-VIRGO O2 run span a range of 2-70 events.
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- Published on Wednesday, 12 October 2016 12:05
Vol. 594
In section 7. Stellar structure and evolution
ξ Tauri: a unique laboratory to study the dynamic interaction in a compact hierarchical quadruple system
This paper presents a comprehensive study of ξ Tauri, which is a unique and rare close-quadruple stellar system, where dynamical interactions among its members occur on human timescales. It has favourable orbital geometry and luminosity ratios between its components that allow the determination of physical properties of the system and its components with high precision. This study presents a large series of spectroscopic, photometric (including space-borne) observations, and long-baseline optical and infrared spectro-interferometric observations. The authors use both a set of observation-specific models and an N-body model to determine the properties of this complex system. Using perturbation theory, the authors show that the prominent secular and periodic dynamical effects are explainable with a quadrupole interaction between the inner and intermediate orbits. They have constrained the basic properties of all components, including the especially challenging members of the inner triple subsystem, and also provided a distance measurement that is independent of photometry. This systems is thus an excellent test bed for models of formation and evolution of hierarchical systems.
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- Published on Friday, 07 October 2016 15:40
Vol. 594
In section 1. Letters to the Editor
Exocomet signatures around the A-shell star φ Leonis?
About thirty years ago, an extended debris disk was seen for the first time around β Pictoris, a magnitude 3.7, A6V type, young star. Ten years later, changes in the calcium and potassium spectral lines of the star were discovered and shown to be evidence for the evaporation of exocomets in that system. This led to the prediction of the existence of a massive planet in that system. In 2010, a giant planet was imaged directly in the disk of β Pic. Eiroa et al. study a very similar bright A7IV star, φ Leo, of magnitude 4.5 that is known to be seen edge-on. φ Leo is, however significantly older (500-900Myr) than β Pic (~20 Myr). They detect a rich variety of comet-like signatures in the Ca and K spectral lines, raising several questions: Why are comets still seen frequently in this relatively old system? What sends them close to the star? It will be extremely interesting to pursue the analysis, both to determine whether an imbalance of blue-shifted to red-shifted signatures may betray the presence of an eccentric planet, and to directly image this system.
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- Published on Thursday, 29 September 2016 14:42
Vol. 593
In section 7. Stellar structure and evolution
Magnetic inhibition of convection and the fundamental properties of low-mass stars. III. A consistent 10 Myr age for the Upper Scorpius OB association
The paper addresses the problem that young stellar associations often show different ages depending on what stellar type is being studied. A pertinent example is the case in which the stars on the main sequence yield one age while the stars still in pre-main sequence contraction yield another. The explanation for this put forward by the author is that magnetic inhibition of convection, preferentially acting on the lower-mass, pre-main sequence stars slows the contraction thus bringing the ages back into agreement.