R-process enrichment from a single event in an ancient dwarf galaxy
Nature 531, 7596 (2016). doi:10.1038/nature17425
Authors: Alexander P. Ji, Anna Frebel, Anirudh Chiti & Joshua D. Simon
Elements heavier than zinc are synthesized through the rapid (r) and slow (s) neutron-capture processes. The main site of production of the r-process elements (such as europium) has been debated for nearly 60 years. Initial studies of trends in chemical abundances in old Milky Way halo stars suggested that these elements are produced continually, in sites such as core-collapse supernovae. But evidence from the local Universe favours the idea that r-process production occurs mainly during rare events, such as neutron star mergers. The appearance of a plateau of europium abundance in some dwarf spheroidal galaxies has been suggested as evidence for rare r-process enrichment in the early Universe, but only under the assumption that no gas accretes into those dwarf galaxies; gas accretion favours continual r-process enrichment in these systems. Furthermore, the universal r-process pattern has not been cleanly identified in dwarf spheroidals. The smaller, chemically simpler, and more ancient ultrafaint dwarf galaxies assembled shortly after the first stars formed, and are ideal systems with which to study nucleosynthesis events such as the r-process. Reticulum II is one such galaxy. The abundances of non-neutron-capture elements in this galaxy (and others like it) are similar to those in other old stars. Here, we report that seven of the nine brightest stars in Reticulum II, observed with high-resolution spectroscopy, show strong enhancements in heavy neutron-capture elements, with abundances that follow the universal r-process pattern beyond barium. The enhancement seen in this ‘r-process galaxy’ is two to three orders of magnitude higher than that detected in any other ultrafaint dwarf galaxy. This implies that a single, rare event produced the r-process material in Reticulum II. The r-process yield and event rate are incompatible with the source being ordinary core-collapse supernovae, but consistent with other possible sources, such as neutron star mergers.
Astronomy: Protoplanet imaged in infancy
Nature 531, 7596 (2016). doi:10.1038/531552c
Astronomers have captured images of a young star system in the earliest stages of planet formation.Researchers in 2014 used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to image the star HL Tau and its dusty disk, some 138 parsecs (450 light years) from
An international team of astronomers, led by the University of Cambridge, has obtained the most detailed ‘fingerprint’ of a rocky planet outside our solar system to date, and found a planet of two halves: one that is almost completely molten, and the other which is almost completely solid.
According to the researchers, conditions on the hot side of the planet are so extreme that it may have caused the atmosphere to evaporate, with the result that conditions on the two sides of the planet vary widely: temperatures on the hot side can reach 2500 degrees Celsius, while temperatures on the cool side are around 1100 degrees. The results are reported in the journal Nature.
Using data from NASA’s Spitzer Space Telescope, the researchers examined a planet known as 55 Cancri e, which orbits a sun-like star located 40 light years away in the Cancer constellation, and have mapped how conditions on the planet change throughout a complete orbit, the first time this has been accomplished for such a small planet.
55 Cancri e is a ‘super Earth’: a rocky exoplanet about twice the size and eight times the mass of Earth, and orbits its parent star so closely that a year lasts just 18 hours. The planet is also tidally locked, meaning that it always shows the same face to its parent star, similar to the Moon, so there is a permanent ‘day’ side and a ‘night’ side. Since it is among the nearest super Earths whose composition can be studied, 55 Cancri e is among the best candidates for detailed observations of surface and atmospheric conditions on rocky exoplanets.
Uncovering the characteristics of super Earths is difficult, since they are so small compared to the parent star and their contrast relative to the star is extremely small compared to larger, hotter gas giant planets, the so-called ‘hot Jupiters’.
“We haven’t yet found any other planet that is this small and orbits so close to its parent star, and is relatively close to us, so 55 Cancri e offers lots of possibilities,” said Dr Brice-Olivier Demory of the University’s Cavendish Laboratory, the paper’s lead author. “We still don’t know exactly what this planet is made of – it’s still a riddle. These results are like adding another brick to the wall, but the exact nature of this planet is still not completely understood.”
55 Cancri e has been extensively studied since it was discovered in 2011. Based on readings taken at different points in time, it was thought to be a water world, or even made of diamond, but researchers now believe that it is almost completely covered by lava.
“We have entered a new era of atmospheric remote sensing of rocky exoplanets,” said study co-author Dr Nikku Madhusudhan, from the Institute of Astronomy at Cambridge. “It is incredible that we are now able to measure the large scale temperature distribution on the surface of a rocky exoplanet.”
Based on these new infrared measurements, the ‘day’ side of the planet appears to be almost completely molten, while the ‘night’ side is almost completely solid. The heat from the day side is not efficiently circulated to the night side, however. On Earth, the atmosphere aids in the recirculation of heat, keeping the temperature across the whole planet within a relatively narrow range. But on 55 Cancri e, the hot side stays hot, and the cold side stays cold.
According to Demory, one possibility for this variation could be either a complete lack of atmosphere, or one which has been partially destroyed due to the strong irradiation from the nearby host star. “On the day side, the temperature is around 2500 degrees Celsius, while on the night side it’s about 1100 degrees – that’s a huge difference,” he said. “We think that there could still be an atmosphere on the night side, but temperatures on the day side are so extreme that the atmosphere may have evaporated completely, meaning that heat is not being efficiently transferred, or transferred at all from the day side to the night side.”
Another possibility for the huge discrepancy between the day side and the night side may be that the molten lava on the day side moves heat along the surface, but since lava is mostly solid on the night side, heat is not moved around as efficiently.
What is unclear however, is where exactly the ‘extra’ heat on 55 Cancri e comes from in the first place, since the observations reveal an unknown source of heat that makes the planet hotter than expected solely from the irradiation from the star – but the researchers may have to wait until the next generation of space telescopes are launched to find out.
For Demory, these new readings also show just how difficult it will be to detect a planet that is similar to Earth. The smaller a planet is, the more difficult it is to detect. And once a rocky planet has been found, there is the question of whether it lies in the so-called habitable zone, where life can be supported. “The problem is, people don’t agree on what the habitable zone is,” said Demory. “For example, some studies consider Mars and Venus to be in the habitable zone, but life as we know it is not possible on either of those planets. Understanding the surface and climate properties of these other worlds will eventually allow us to put the Earth’s climate and habitability into context.”
One possibility might be to look at stars which are much cooler and smaller than our sun, such as the M-dwarfs, which would mean that planets could be much closer to their star and still be in the habitable zone. The sizes of such planets relative to their star would be larger, which make them more detectable from Earth.
But for the time being, Demory and his colleagues plan to keep studying 55 Cancri e, in order to see what other secrets it might hold, including the possibility that it might be surrounded by a torus of gas and dust, which could account for some of the variations in the data. And in 2018, the successor to Hubble and Spitzer, the James Webb Space Telescope, will launch, allowing astronomers to look at planets outside our solar system with entirely new levels of precision.
Brice-Olivier Demory et al. ‘A map of the extreme day-night temperature gradient of a super-Earth exoplanet.’ Nature (2016). DOI: 10.1038/nature17169
The most detailed map of a small, rocky ‘super Earth’ to date reveals a planet almost completely covered by lava, with a molten ‘hot’ side and solid ‘cool’ side.We still don’t know exactly what this planet is made of – it’s still a riddle. These results are like adding another brick to the wall, but the exact nature of this planet is still not completely understood.Brice-Olivier Demory NASA/JPL-Caltech/R. HurtIllustration of the hot lava world 55 Cancri e
The text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.
NASA has selected 36 fellows for its prestigious Einstein, Hubble, and Sagan Fellowships. Each postdoctoral fellowship provides three years of support to awardees to pursue independent research in astronomy and astrophysics. The new fellows will begin their programs in the fall of 2016 at a host university or research center of their choosing in the United States.
Lunar true polar wander inferred from polar hydrogen
Nature 531, 7595 (2016). doi:10.1038/nature17166
Authors: M. A. Siegler, R. S. Miller, J. T. Keane, M. Laneuville, D. A. Paige, I. Matsuyama, D. J. Lawrence, A. Crotts & M. J. Poston
The earliest dynamic and thermal history of the Moon is not well understood. The hydrogen content of deposits near the lunar poles may yield insight into this history, because these deposits (which are probably composed of water ice) survive only if they remain in permanent shadow. If the orientation of the Moon has changed, then the locations of the shadowed regions will also have changed. The polar hydrogen deposits have been mapped by orbiting neutron spectrometers, and their observed spatial distribution does not match the expected distribution of water ice inferred from present-day lunar temperatures. This finding is in contrast to the distribution of volatiles observed in similar thermal environments at Mercury’s poles. Here we show that polar hydrogen preserves evidence that the spin axis of the Moon has shifted: the hydrogen deposits are antipodal and displaced equally from each pole along opposite longitudes. From the direction and magnitude of the inferred reorientation, and from analysis of the moments of inertia of the Moon, we hypothesize that this change in the spin axis, known as true polar wander, was caused by a low-density thermal anomaly beneath the Procellarum region. Radiogenic heating within this region resulted in the bulk of lunar mare volcanism and altered the density structure of the Moon, changing its moments of inertia. This resulted in true polar wander consistent with the observed remnant polar hydrogen. This thermal anomaly still exists and, in part, controls the current orientation of the Moon. The Procellarum region was most geologically active early in lunar history, which implies that polar wander initiated billions of years ago and that a large portion of the measured polar hydrogen is ancient, recording early delivery of water to the inner Solar System. Our hypothesis provides an explanation for the antipodal distribution of lunar polar hydrogen, and connects polar volatiles to the geologic and geophysical evolution of the Moon and the bombardment history of the early Solar System.
Acceleration of petaelectronvolt protons in the Galactic Centre
Nature 531, 7595 (2016). doi:10.1038/nature17147
Galactic cosmic rays reach energies of at least a few petaelectronvolts (of the order of 1015 electronvolts). This implies that our Galaxy contains petaelectronvolt accelerators (‘PeVatrons’), but all proposed models of Galactic cosmic-ray accelerators encounter difficulties at exactly these energies. Dozens of Galactic accelerators capable of accelerating particles to energies of tens of teraelectronvolts (of the order of 1013 electronvolts) were inferred from recent γ-ray observations. However, none of the currently known accelerators—not even the handful of shell-type supernova remnants commonly believed to supply most Galactic cosmic rays—has shown the characteristic tracers of petaelectronvolt particles, namely, power-law spectra of γ-rays extending without a cut-off or a spectral break to tens of teraelectronvolts. Here we report deep γ-ray observations with arcminute angular resolution of the region surrounding the Galactic Centre, which show the expected tracer of the presence of petaelectronvolt protons within the central 10 parsecs of the Galaxy. We propose that the supermassive black hole Sagittarius A* is linked to this PeVatron. Sagittarius A* went through active phases in the past, as demonstrated by X-ray outburstsand an outflow from the Galactic Centre. Although its current rate of particle acceleration is not sufficient to provide a substantial contribution to Galactic cosmic rays, Sagittarius A* could have plausibly been more active over the last 106–107 years, and therefore should be considered as a viable alternative to supernova remnants as a source of petaelectronvolt Galactic cosmic rays.
Planetary science: Signs of a wandering Moon
Nature 531, 7595 (2016). doi:10.1038/531455a
Authors: Ian Garrick-Bethell
The presence of ice at two positions on opposite sides of the Moon suggests that the satellite's orientation was once shifted away from its present spin axis — a finding that has implications for the Moon's volcanic history. See Letter p.480
The black-hole collision that reshaped physics
Nature 531, 7595 (2016). http://www.nature.com/doifinder/10.1038/531428a
Author: Davide Castelvecchi
A momentous signal from space has confirmed decades of theorizing on black holes — and launched a new era of gravitational-wave astronomy.
Planetary science: Cassini aids hunt for Planet Nine
Nature 531, 7595 (2016). doi:10.1038/531417e
Researchers using the Cassini spacecraft have narrowed down the search for the Solar System's hypothetical ninth planet.Planet Nine is thought to be orbiting in the far outer Solar System, but has not yet been found. If it exists, its gravity should tug slightly on
Planetary science: A peek at Pluto's rich landscapes
Nature 531, 7595 (2016). doi:10.1038/531416d
Data collected by NASA's New Horizons probe during its Pluto fly-by last year has revealed just how geologically active Pluto is, and that its moon Charon was once active but is now dead.Jeffrey Moore at the NASA Ames Research Center in Moffett Field, California,