Fission and reconfiguration of bilobate comets as revealed by 67P/Churyumov–Gerasimenko
Nature 534, 7607 (2016). doi:10.1038/nature17670
Authors: Masatoshi Hirabayashi, Daniel J. Scheeres, Steven R. Chesley, Simone Marchi, Jay W. McMahon, Jordan Steckloff, Stefano Mottola, Shantanu P. Naidu & Timothy Bowling
The solid, central part of a comet—its nucleus—is subject to destructive processes, which cause nuclei to split at a rate of about 0.01 per year per comet. These destructive events are due to a range of possible thermophysical effects; however, the geophysical expressions of these effects are unknown. Separately, over two-thirds of comet nuclei that have been imaged at high resolution show bilobate shapes, including the nucleus of comet 67P/Churyumov–Gerasimenko (67P), visited by the Rosetta spacecraft. Analysis of the Rosetta observations suggests that 67P’s components were brought together at low speed after their separate formation. Here, we study the structure and dynamics of 67P’s nucleus. We find that sublimation torques have caused the nucleus to spin up in the past to form the large cracks observed on its neck. However, the chaotic evolution of its spin state has so far forestalled its splitting, although it should eventually reach a rapid enough spin rate to do so. Once this occurs, the separated components will be unable to escape each other; they will orbit each other for a time, ultimately undergoing a low-speed merger that will result in a new bilobate configuration. The components of four other imaged bilobate nuclei have volume ratios that are consistent with a similar reconfiguration cycle, pointing to such cycles as a fundamental process in the evolution of short-period comet nuclei. It has been shown that comets were not strong contributors to the so-called late heavy bombardment about 4 billion years ago. The reconfiguration process suggested here would preferentially decimate comet nuclei during migration to the inner solar system, perhaps explaining this lack of a substantial cometary flux.
France launches massive meteor-spotting network
Nature 534, 7607 (2016). http://www.nature.com/doifinder/10.1038/nature.2016.20070
Author: Traci Watson
Tracking space rocks that reach Earth will give insight into the early Solar System.
In 1936, astronomers observed signs that the young star FU Orionis had begun gobbling material from its surrounding disk of gas and dust with a sudden voraciousness. During a three-month binge, as matter turned into energy, the star became 100 times brighter, heating the disk around it to temperatures of up to 12,000 degrees Fahrenheit. The brightening is the most extreme event of its kind that has been confirmed around a star the size of the sun, and may have implications for how stars and planets form. The intense baking of the star's surrounding disk likely changed its chemistry, permanently altering material that could one day turn into planets. FU Orionis is still devouring gas to this day, although not as quickly.
Water is a hot topic in the study of exoplanets, including "hot Jupiters," whose masses are similar to that of Jupiter, but lie much closer to their parent star than Jupiter is to the sun. They are estimated to be a scorching 2,000 degrees Fahrenheit, meaning any water they host would take the form of water vapor.
Cold, clumpy accretion onto an active supermassive black hole
Nature 534, 7606 (2016). doi:10.1038/nature17969
Authors: Grant R. Tremblay, J. B. Raymond Oonk, Françoise Combes, Philippe Salomé, Christopher, P. O’Dea, Stefi A. Baum, G. Mark Voit, Megan Donahue, Brian R. McNamara, Timothy A. Davis, Michael A. McDonald, Alastair C. Edge, Tracy E. Clarke, Roberto Galván-Madrid, Malcolm N. Bremer, Louise O. V. Edwards, Andrew C. Fabian, Stephen Hamer, Yuan Li, Anaëlle Maury, Helen R. Russell, Alice C. Quillen, C. Megan Urry, Jeremy S. Sanders & Michael W. Wise
Supermassive black holes in galaxy centres can grow by the accretion of gas, liberating energy that might regulate star formation on galaxy-wide scales. The nature of the gaseous fuel reservoirs that power black hole growth is nevertheless largely unconstrained by observations, and is instead routinely simplified as a smooth, spherical inflow of very hot gas. Recent theory and simulations instead predict that accretion can be dominated by a stochastic, clumpy distribution of very cold molecular clouds—a departure from the ‘hot mode’ accretion model—although unambiguous observational support for this prediction remains elusive. Here we report observations that reveal a cold, clumpy accretion flow towards a supermassive black hole fuel reservoir in the nucleus of the Abell 2597 Brightest Cluster Galaxy (BCG), a nearby (redshift z = 0.0821) giant elliptical galaxy surrounded by a dense halo of hot plasma. Under the right conditions, thermal instabilities produce a rain of cold clouds that fall towards the galaxy’s centre, sustaining star formation amid a kiloparsec-scale molecular nebula that is found at its core. The observations show that these cold clouds also fuel black hole accretion, revealing ‘shadows’ cast by the molecular clouds as they move inward at about 300 kilometres per second towards the active supermassive black hole, which serves as a bright backlight. Corroborating evidence from prior observations of warmer atomic gas at extremely high spatial resolution, along with simple arguments based on geometry and probability, indicate that these clouds are within the innermost hundred parsecs of the black hole, and falling closer towards it.
Astrophysics: Relativity passes black-hole test
Nature 534, 7606 (2016). doi:10.1038/534154e
General relativity holds true, even under the extreme conditions of colliding black holes.In 2015, the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) saw the first evidence of gravitational waves, which had been created by two merging black holes. Walter Del Pozzo at the University of