- Published on Friday, 13 January 2017 08:00
In section 2. Astrophysical processes
Power requirements for cosmic ray propagation models involving diffusive reacceleration; estimates and implications for the damping of interstellar turbulence
This paper presents a surprisingly efficient physical process for diffusively accelerating cosmic ray particles . In this process, interstellar turbulence suffices to produce a substantial fraction of the cosmic ray energy in the Galaxy. Under very reasonable physical conditions, this acceleration may also be a previously unconsidered mechanism for damping the same turbulence on scales well above the diffusive subscale. This proposal is tested using GALPROP modeling, and can be easily incorporated in other particle propagation codes.
- Published on Friday, 13 January 2017 00:00
In section 1. Letters
A non-glitch speed-up event in the Crab Pulsar
The Crab pulsar is one of the first radio pulsars ever detected, and the first one to be clearly linked to a supernova remnant. The neutron star spins at 33 ms and its rotation can be monitored both with radio telescopes and X-ray instruments, as the Crab is also a strong X-ray pulsar. The Crab pulsar is a precise clock. It is monitored monthly by radio telescope, and often by several X-ray instruments in orbit. Its spin period is known to an accuracy about 10 significant digits. As any other radio pulsar, the Crab pulsar slowly spins down due to rotational energy losses, which power the observed emission at all wavelengths. Sometimes it also shows glitches - sudden (less than a minute) increases of the spin frequency caused by some internal processes within the pulsar. In this Letter the discovery of a new type of speed-up event is reported. It is not impulsive, like a glitch, but it lasted for over two years. It is not easy to explain this new type of event. One possibility suggested by the author is in terms of an extra source of heat generated within the neutron star, resulting in a slowing down of the superfluid interior and speeding up the outer crust.
- Published on Thursday, 12 January 2017 14:40
In section 6. Interstellar and circumstellar matter
Anatomy of the internal bow shocks in the IRAS 04166+2706 protostellar jet
Highly-collimated jets and wide-angle outflows are two related components of the mass-ejection activity that are associated with stellar birth. The authors use the ALMA interferometer to study the relation between the jet and the outflow in the IRAS 04166+2706 protostar, which drives a molecular jet that contains multiple emission peaks symmetrically located from the central source, as well as a wide-angle outflow consisting of two conical shells. They find that the emission peaks in the IRAS 04166+2706 jet likely represent internal bow shocks where material is being ejected laterally away from the jet axis. While the linear momentum of the ejected gas is dominated by the component in the jet direction, the sideways component is not negligible, and could potentially affect the distribution of gas in the surrounding outflow and core.
- Published on Thursday, 12 January 2017 14:24
In section 7. Stellar structure and evolution
The 2008 outburst in the young stellar system Z CMa. III. Multi-epoch high-angular resolution images and spectra of the components in near-infrared
Z CMa is a complex pre-main sequence binary with a current separation of 110 mas, known to consist of a FU Orionis star (SE component) and an embedded Herbig Be star (NW component). This paper presents a wealth of new observations, both imaging and spectroscopic, of each component of the Z CMa system during its most recent 2008-2009 outburst (as well as an extra observation once the system was back in its quiescent state). The authors confirm that the NW star dominates the system flux over the 1.1-3.8 mum range and is responsible for the photometric outburst. From extracted medium-resolution (R~2000-4000) near-infrared (1.1-2.4 mum) spectra of the individual components, the authors show that the SE component has a spectrum typical of FU Orionis objects, and that the NW component spectrum is characteristic of embedded out-bursting protostars and EX Or objects. The blueing of the continuum of the NW component along with the absolute flux and color-variation of the system during the outburst suggests that the outburst was caused by a complex interplay between (a) a variation of the extinction in the line of sight of the NW component, and (b) the emission of shocked regions close to the NW component.
- Published on Monday, 09 January 2017 12:18
In section 1. Letters
Clear widens the field for observations of the Sun with multi-conjugate adaptive optics
Adaptive optics has revitalized ground-based solar astronomy, by providing diffraction-limited resolution for large solar telescopes. The thickness of the tropospheric turbulent layers, however, limits the diffraction-limited field of view to approximately ten arcseconds when using a single deformable mirror and wavefront sensor. Multi Conjugate Adaptive Optics (MCAO) uses multiple deformable mirrors and wavefront sensors, each conjugated to a different atmospheric height, to produce wider diffraction-limited field of views, but has up to now been demonstrated only on stellar fields. Schmidt et al. obtained the first MCAO-corrected observations of the Sun with the Clear MCAO instrument on the New Solar Telescope, which also is the first three-layer MCAO system on any telescope. They demonstrate a well corrected field of view that is much widened, and therefore demonstrate that MCAO will be a critical component of future 4m-class solar telescopes.
- Published on Thursday, 22 December 2016 14:17
In section 10. Planets and planetary systems
How primordial is the structure of comet 67P/C-G? Combined collisional and dynamical models suggest a late formation
Formation of bi-lobed shapes by sub-catastrophic collisions. A late origin of comet 67P’s structure
Comets are generally thought of as being primordial objects. With its high porosity and low internal strength, comet 67P/Churmyov-Gerasimenko seemed to confirm this widely held opinion: how could such a fragile object have suffered collisions? In two papers, M. Jutzi and collaborators combine models of the dynamical evolution of the solar system and SPH collision simulations to address this problem. They show that objects such as 67P may have experienced many shape-changing collisions over the lifetime of the Solar System. If this is true, these comets may not be primordial. While the collisions lead to compaction of part of the material, some material is ejected and reaccumulated, leading to additional macroporosity. They also show that the formation of bi-lobed structures such as 67P/C-G is a natural outcome of low energy, sub-catastrophic collisions involving elongated, rotating bodies. This study sheds new light on comets and on the interpretation of the measurements and results of the Rosetta mission.