Solar-type dynamo behaviour in fully convective stars without a tachocline
Nature 535, 7613 (2016). doi:10.1038/nature18638
Authors: Nicholas J. Wright & Jeremy J. Drake
In solar-type stars (with radiative cores and convective envelopes like our Sun), the magnetic field powers star spots, flares and other solar phenomena, as well as chromospheric and coronal emission at ultraviolet to X-ray wavelengths. The dynamo responsible for generating the field depends on the shearing of internal magnetic fields by differential rotation. The shearing has long been thought to take place in a boundary layer known as the tachocline between the radiative core and the convective envelope. Fully convective stars do not have a tachocline and their dynamo mechanism is expected to be very different, although its exact form and physical dependencies are not known. Here we report observations of four fully convective stars whose X-ray emission correlates with their rotation periods in the same way as in solar-type stars. As the X-ray activity–rotation relationship is a well-established proxy for the behaviour of the magnetic dynamo, these results imply that fully convective stars also operate a solar-type dynamo. The lack of a tachocline in fully convective stars therefore suggests that this is not a critical ingredient in the solar dynamo and supports models in which the dynamo originates throughout the convection zone.
Suppression of star formation in dwarf galaxies by photoelectric grain heating feedback
Nature 535, 7613 (2016). doi:10.1038/nature18292
Authors: John C. Forbes, Mark R. Krumholz, Nathan J. Goldbaum & Avishai Dekel
Photoelectric heating—heating of dust grains by far-ultraviolet photons—has long been recognized as the primary source of heating for the neutral interstellar medium. Simulations of spiral galaxies have shown some indication that photoelectric heating could suppress star formation; however, simulations that include photoelectric heating have typically shown that it has little effect on the rate of star formation in either spiral galaxies or dwarf galaxies, which suggests that supernovae are responsible for setting the gas depletion time in galaxies. This result is in contrast with recent work indicating that a star formation law that depends on galaxy metallicity—as is expected with photoelectric heating, but not with supernovae—reproduces the present-day galaxy population better than does a metallicity-independent one. Here we report a series of simulations of dwarf galaxies, the class of galaxy in which the effects of both photoelectric heating and supernovae are expected to be strongest. We simultaneously include space- and time-dependent photoelectric heating in our simulations, and we resolve the energy-conserving phase of every supernova blast wave, which allows us to directly measure the relative importance of feedback by supernovae and photoelectric heating in suppressing star formation. We find that supernovae are unable to account for the observed large gas depletion times in dwarf galaxies. Instead, photoelectric heating is the dominant means by which dwarf galaxies regulate their star formation rate at any given time, suppressing the rate by more than an order of magnitude relative to simulations with only supernovae.
Solar physics: Dynamo theory questioned
Nature 535, 7613 (2016). doi:10.1038/535500a
Authors: Paul Charbonneau
Observations of X-ray emission — a diagnostic tool for the mechanisms driving stellar magnetic fields — from four cool stars call into question accepted models of magnetic-field generation in the Sun and stars. See Letter p.526
Planet hunters seek new ways to detect alien life
Nature 535, 7613 (2016). http://www.nature.com/doifinder/10.1038/535474a
Author: Alexandra Witze
Astrobiologists debate which chemical signatures would hint at life on other worlds.
Optics: Human eye sees single photons
Nature 535, 7613 (2016). doi:10.1038/535469b
People can perceive flashes of light as feeble as a single photon.Alipasha Vaziri at the Rockefeller University in New York City and his colleagues asked three volunteers to stare into an optical system in the dark and listen to two sounds, one of which
Planetary science: Revived telescope finds 104 planets
Nature 535, 7613 (2016). doi:10.1038/535468d
Astronomers have spied 104 new worlds in the Milky Way using NASA's Kepler Space Telescope.Part of Kepler broke down in 2013, but engineers managed to repair it and send it on a fresh mission, dubbed K2. This latest discovery, from Ian Crossfield at the
Celebrating its 50th anniversary this year, the TV series "Star Trek" has captured the public's imagination with the signature phrase, "To boldly go where no one has gone before." The Hubble Space Telescope simply orbits Earth and doesn't "boldly go" deep into space. But it looks deeper into the universe than ever before possible to explore the fabric of time and space and find the farthest objects ever seen. This is epitomized in this Hubble image that is part of its Frontier Fields program to probe the far universe. This view of a massive cluster of galaxies unveils a very cluttered-looking universe filled with galaxies near and far. Some are distorted like a funhouse mirror through a warping-of-space phenomenon first predicted by Einstein a century ago.
Relativistic reverberation in the accretion flow of a tidal disruption event
Nature 535, 7612 (2016). doi:10.1038/nature18007
Authors: Erin Kara, Jon M. Miller, Chris Reynolds & Lixin Dai
Our current understanding of the curved space-time around supermassive black holes is based on actively accreting black holes, which make up only ten per cent or less of the overall population. X-ray observations of that small fraction reveal strong gravitational redshifts that indicate that many of these black holes are rapidly rotating; however, selection biases suggest that these results are not necessarily reflective of the majority of black holes in the Universe. Tidal disruption events, where a star orbiting an otherwise dormant black hole gets tidally shredded and accreted onto the black hole, can provide a short, unbiased glimpse at the space-time around the other ninety per cent of black holes. Observations of tidal disruptions have hitherto revealed the formation of an accretion disk and the onset of an accretion-powered jet, but have failed to reveal emission from the inner accretion flow, which enables the measurement of black hole spin. Here we report observations of reverberation arising from gravitationally redshifted iron Kα photons reflected off the inner accretion flow in the tidal disruption event Swift J1644+57. From the reverberation timescale, we estimate the mass of the black hole to be a few million solar masses, suggesting an accretion rate of 100 times the Eddington limit or more. The detection of reverberation from the relativistic depths of this rare super-Eddington event demonstrates that the X-rays do not arise from the relativistically moving regions of a jet, as previously thought.
Origin and implications of non-radial Imbrium Sculpture on the Moon
Nature 535, 7612 (2016). doi:10.1038/nature18278
Authors: Peter H. Schultz & David A. Crawford
Rimmed grooves, lineations and elongate craters around Mare Imbrium shape much of the nearside Moon. This pattern was coined the Imbrium Sculpture, and it was originally argued that it must have been formed by a giant oblique (~30°) impact, a conclusion echoed by later studies. Some investigators, however, noticed that many elements of the Imbrium Sculpture are not radial to Imbrium, thereby implicating an endogenic or structural origin. Here we use these non-radial trends to conclude that the Imbrium impactor was a proto-planet (half the diameter of Vesta), once part of a population of large proto-planets in the asteroid belt. Such independent constraints on the sizes of the Imbrium and other basin-forming impactors markedly increase estimates for the mass in the asteroid belt before depletion caused by the orbital migration of Jupiter and Saturn. Moreover, laboratory impact experiments, shock physics codes and the groove widths indicate that multiple fragments (up to 2% of the initial diameter) from each oblique basin-forming impactor, such as the one that formed Imbrium, should have survived planetary collisions and contributed to the heavy impact bombardment between 4.3 and 3.8 billion years ago.