Cohesive forces prevent the rotational breakup of rubble-pile asteroid (29075) 1950 DA
Nature 512, 7513 (2014). doi:10.1038/nature13632
Authors: Ben Rozitis, Eric MacLennan & Joshua P. Emery
Space missions and ground-based observations have shown that some asteroids are loose collections of rubble rather than solid bodies. The physical behaviour of such ‘rubble-pile’ asteroids has been traditionally described using only gravitational and frictional forces within a granular material. Cohesive forces in the form of small van der Waals forces between constituent grains have recently been predicted to be important for small rubble piles (ten kilometres across or less), and could potentially explain fast rotation rates in the small-asteroid population. The strongest evidence so far has come from an analysis of the rotational breakup of the main-belt comet P/2013 R3 (ref. 7), although that was indirect and poorly constrained by observations. Here we report that the kilometre-sized asteroid (29075) 1950 DA (ref. 8) is a rubble pile that is rotating faster than is allowed by gravity and friction. We find that cohesive forces are required to prevent surface mass shedding and structural failure, and that the strengths of the forces are comparable to, though somewhat less than, the forces found between the grains of lunar regolith.
Astronomical instrumentation: Atmospheric blurring has a new enemy
Nature 512, 7513 (2014). doi:10.1038/512144a
Authors: Brent Ellerbroek
A fully automated optics system that corrects atmospheric blurring of celestial objects has imaged 715 star systems thought to harbour planets, completing each observation in less time than it takes to read this article.
Interacting supernovae from photoionization-confined shells around red supergiant stars
Nature 512, 7514 (2014). doi:10.1038/nature13522
Authors: Jonathan Mackey, Shazrene Mohamed, Vasilii V. Gvaramadze, Rubina Kotak, Norbert Langer, Dominique M.-A. Meyer, Takashi J. Moriya & Hilding R. Neilson
Betelgeuse, a nearby red supergiant, is a fast-moving star with a powerful stellar wind that drives a bow shock into its surroundings. This picture has been challenged by the discovery of a dense and almost static shell that is three times closer to the star than the bow shock and has been decelerated by some external force. The two physically distinct structures cannot both be formed by the hydrodynamic interaction of the wind with the interstellar medium. Here we report that a model in which Betelgeuse’s wind is photoionized by radiation from external sources can explain the static shell without requiring a new understanding of the bow shock. Pressure from the photoionized wind generates a standing shock in the neutral part of the wind and forms an almost static, photoionization-confined shell. Other red supergiants should have much more massive shells than Betelgeuse, because the photoionization-confined shell traps up to 35 per cent of all mass lost during the red supergiant phase, confining this gas close to the star until it explodes. After the supernova explosion, massive shells dramatically affect the supernova light curve, providing a natural explanation for the many supernovae that have signatures of circumstellar interaction.
Supernovae are the most powerful stellar explosions in the universe. Some of them are produced by the detonation of a white dwarf, the stripped-down core of an ordinary star at the end of its life. But 12 years ago, astronomers began noticing weak stellar blasts, a kind of mini-supernova. When one such explosion occurred in the galaxy NGC 1309, astronomers looking through Hubble archival images found for the first time the star system that produced the supernova blast of a white dwarf.
Astronomy: Fresh look at Galactic rim
Nature 512, 7512 (2014). doi:10.1038/512008b
A survey has provided the most detailed look yet at a mysterious ring of stars at the fringes of the Milky Way.Using data from the Pan-STARRS1 telescope in Hawaii, Colin Slater and Eric Bell at the University of Michigan in Ann Arbor and their
Astrophysics: Best gauge of exoplanet size
Nature 512, 7512 (2014). doi:10.1038/512009b
Astronomers have made the most precise measurement so far of an exoplanet's size — for Kepler-93b, which orbits a star around 100 parsecs away.Sarah Ballard at the University of Washington in Seattle and her colleagues estimated the planet's diameter at about 18,800 kilometres (1.48
Astrophysics: Novae join the γ-ray generators
Nature 512, 7512 (2014). doi:10.1038/512009c
Astronomers have identified a previously unknown source of cosmic γ-radiation.High-energy γ-rays are released in extremely energetic events such as pulsars and supernovae. But they were thought to be unlikely products of classical novae: explosions that occur on the surfaces of compact, burnt-out stars called
Astrophysics: Portrait of a doomed star
Nature 512, 7512 (2014). doi:10.1038/512034a
Authors: Stephen Justham
Some stars explode in thermonuclear supernovae, but understanding of why this occurs comes mainly from indirect clues. Now, the progenitor of a member of a strange class of such explosions may have been detected directly. See Letterp.54
A luminous, blue progenitor system for the type Iax supernova 2012Z
Nature 512, 7512 (2014). doi:10.1038/nature13615
Authors: Curtis McCully, Saurabh W. Jha, Ryan J. Foley, Lars Bildsten, Wen-fai Fong, Robert P. Kirshner, G. H. Marion, Adam G. Riess & Maximilian D. Stritzinger
Type Iax supernovae are stellar explosions that are spectroscopically similar to some type Ia supernovae at the time of maximum light emission, except with lower ejecta velocities. They are also distinguished by lower luminosities. At late times, their spectroscopic properties diverge from those of other supernovae, but their composition (dominated by iron-group and intermediate-mass elements) suggests a physical connection to normal type Ia supernovae. Supernovae of type Iax are not rare; they occur at a rate between 5 and 30 per cent of the normal type Ia rate. The leading models for type Iax supernovae are thermonuclear explosions of accreting carbon–oxygen white dwarfs that do not completely unbind the star, implying that they are ‘less successful’ versions of normal type Ia supernovae, where complete stellar disruption is observed. Here we report the detection of the luminous, blue progenitor system of the type Iax SN 2012Z in deep pre-explosion imaging. The progenitor system's luminosity, colours, environment and similarity to the progenitor of the Galactic helium nova V445 Puppis suggest that SN 2012Z was the explosion of a white dwarf accreting material from a helium-star companion. Observations over the next few years, after SN 2012Z has faded, will either confirm this hypothesis or perhaps show that this supernova was actually the explosive death of a massive star.