Astronomy and planetary science

  • Letter |

    Type Ia supernovae form a class of cosmological 'standard candles', a property that led to the discovery of an accelerating Universe, but recent investigations have revealed that they are more complicated in nature. Here the authors report that their observed spectral diversity is a consequence of the random directions from which their theoretically asymmetrical explosions are viewed, and that this diversity is therefore no longer a concern in using them as standard candles.

    • K. Maeda
    • , S. Benetti
    •  & N. Elias-Rosa
  • Letter |

    If the orbital velocity of an extrasolar planet could be determined, the masses of both the planet and its host star could be calculated using Newton's law of gravity. Here, high-dispersion ground-based spectroscopy of a transit of the extrasolar planet HD 209458b is reported. This allowed the radial component of the planet's orbital velocity to be calculated, and thus the masses of star and planet. Moreover, a strong wind flowing from the irradiated dayside to the non-irradiated nightside of the planet is suggested.

    • Ignas A. G. Snellen
    • , Remco J. de Kok
    •  & Simon Albrecht
  • Letter |

    KBO 55636 (2002 TX300) is one of the Kuiper belt objects — specifically, a member of the water-ice-rich Haumea KBO collisional family. Here, observations are reported of a multi-chord stellar occultation by KBO 55636. Calculations show that KBO 55636 is smaller than previously thought and, like its parent body, is highly reflective. The dynamical age implies either that it has an active resurfacing mechanism, or that fresh water-ice in the outer Solar System can persist for gigayear timescales.

    • J. L. Elliot
    • , M. J. Person
    •  & A. Gilmore
  • Letter |

    A population of Saturn's small moons orbiting outside the main rings are less than 107 years old, which is inconsistent with the formation timescale for the regular satellites. They may have accreted at the rings' edge, but hitherto it has been impossible to model the accretion process. Here a simulation is reported in which the viscous spreading of Saturn's rings beyond the Roche limit gives rise to the small moons.

    • Sébastien Charnoz
    • , Julien Salmon
    •  & Aurélien Crida
  • Review Article |

    The relativistic Big Bang theory is a good description of our expanding Universe. But — as discussed in this review article — a still better theory would describe a mechanism by which matter is more rapidly gathered into galaxies and groups of galaxies, better fitting the observations.

    • P. J. E. Peebles
    •  & Adi Nusser
  • Letter |

    A pinwheel array of deep troughs has been one of the most perplexing features of the north polar layered deposits on Mars. Many ideas have been put forward about how it formed, but there is as yet no consensus. Here, penetrating radar has been used to rule out erosional cutting as a mechanism for the formation of the array. Instead, it is concluded that the troughs are largely depositional in origin, and have migrated to the poles and upwards in elevation over the past two million years or so.

    • Isaac B. Smith
    •  & John W. Holt
  • Letter |

    The Chasma Boreale is a large canyon — 500 km long, up to 100 km wide, and nearly 2 km deep — that cuts into the north polar layered deposits on Mars. Quite how it formed has been unclear. However, new penetrating radar imagery has now been used to show that depositional processes, rather than catastrophic events, were responsible.

    • J. W. Holt
    • , K. E. Fishbaugh
    •  & R. J. Phillips
  • Letter |

    The unusual supernova SN 2005E is distinguished from any supernovae hitherto observed by several features, which are claimed to be explained by a helium detonation in a thin surface layer of an accreting white dwarf. The observed properties of SN 2005cz are now shown to resemble those of SN 2005E. It is argued that these properties are best explained by a core-collapse supernova at the low-mass end of the range of massive stars that explode.

    • K. S. Kawabata
    • , K. Maeda
    •  & K. Itagaki
  • Letter |

    Supernovae are thought to arise through one of two processes. Type Ib/c and type II supernovae are produced when the cores of massive, short-lived stars undergo gravitational core collapse and eject a few solar masses. Type Ia supernovae are thought to form by the thermonuclear detonation of a carbon-oxygen white dwarf. Here a faint type Ib supernova, SN 2005E, is reported that seems not to have had a core-collapse origin, but perhaps arose from a low-mass, old progenitor, probably a helium-accreting white dwarf in a binary.

    • H. B. Perets
    • , A. Gal-Yam
    •  & D. Poznanski
  • Letter |

    NGC 6791 is a well studied open cluster that is so close to us that it can be imaged down to very faint luminosities. Two different ages have been proposed for this cluster, one based on the white dwarf luminosity function and one derived from its main-sequence stars. The discrepancy in age is now resolved by the finding that, as theoretically anticipated, physical separation processes occur in the core of white dwarfs.

    • Enrique García-Berro
    • , Santiago Torres
    •  & Jordi Isern
  • Letter |

    Ultraluminous infrared galaxies are among the most luminous objects in the local Universe and are thought to be powered by intense star formation. In these objects, the rotational lines of molecular hydrogen (H2) observed at mid-infrared wavelengths are not affected by dust obscuration, but the source of excitation has been unknown. Here it is found that H2 emission originates outside the obscured regions; it is proposed that H2 emission traces shocks in the surrounding material that are excited by interactions with nearby galaxies.

    • Nadia L. Zakamska
  • Letter |

    Here, the atmosphere of the extrasolar planet GJ 436b is studied during its 'secondary eclipse'. The findings reveal the presence of some H2O and traces of CO2. The best-fit compositional models contain a high CO abundance and a substantial methane deficiency relative to thermochemical equilibrium models for the predicted hydrogen-dominated atmosphere. Disequilibrium processes such as vertical mixing and polymerization of methane may be required to explain this small methane-to-CO ratio.

    • Kevin B. Stevenson
    • , Joseph Harrington
    •  & Nate B. Lust
  • Letter |

    Three exoplanets around the star HR 8799 have recently been discovered by differential imaging with large telescopes. In such cases, bright scattered starlight means that large angular offsets of the star are required for high-contrast imaging of the planets. Imaging at small angles requires a technique for reducing starlight and associated noise while still transmitting light from the planet. Here such a technique is described: all three HR 8799 planets have been detected using a vector vortex coronagraph on a small-aperture system.

    • E. Serabyn
    • , D. Mawet
    •  & R. Burruss
  • Letter |

    ε Aurigae is a bright, eclipsing binary star system but the cause of each 18-month-long eclipse has been unknown for nearly 190 years, because the companion was, until recently, undetectable. The preferred explanation has been a tilted disk of opaque material and here the authors report interferometric images that do indeed show an opaque disk of very low mass, tilted as expected, crossing the disk of the F star.

    • Brian Kloppenborg
    • , Robert Stencel
    •  & Sean M. Carroll
  • Letter |

    It has been suggested that Earth's current supply of water was delivered by asteroids. The presence of water on the surface of some asteroids has been inferred from the comet-like activity of several small asteroids, including two members of the Themis dynamical family, but hitherto has not been measured. Here, infrared spectra of the asteroid 24 Themis are reported; the results show that ice and organic compounds are not only present, but also prevalent, on its surface.

    • Humberto Campins
    • , Kelsey Hargrove
    •  & Julie Ziffer
  • Letter |

    The main observational signature of star-forming galaxies at the highest redshifts is the Lyman-α (Lyα) emission line. But Lyα photons scatter in the neutral interstellar medium of their host galaxies, and may therefore be greatly absorbed by interstellar dust. It is now shown that the average escaping fraction of Lyα photons from star-forming galaxies at redshift z = 2.2 is just 5 per cent. This implies that numerous conclusions based on Lyα-selected samples will require upwards revision by an order of magnitude.

    • Matthew Hayes
    • , Göran Östlin
    •  & Jens Melinder
  • Letter |

    Massive galaxies in the early Universe have been shown to be forming stars at high rates. Probing the properties of individual star-forming regions is beyond the resolution and sensitivity of existing telescopes. Here, however, observations are reported of the galaxy SMMJ2135–0102 at redshift z=2.3259, which has been gravitationally magnified by a factor of 32 by a galaxy cluster lens in the foreground. The physics underlying star formation here is similar to that in local galaxies, but the energetics are very different.

    • A. M. Swinbank
    • , I. Smail
    •  & J. D. Younger
  • Letter |

    The most distant quasars known, at redshifts z ≈ 6, generally have the same properties as lower-redshift quasars, implying that although the Universe was young at z ≈ 6, such quasars are still evolved objects. One z ≈ 6 quasar was shown to have no detectable emission from hot dust, but it was not clear whether it was an outlier. Now, a second quasar without hot-dust emission has been discovered in a sample of 21 z ≈ 6 quasars. Moreover, hot-dust abundance in these quasars builds up as the central black hole grows.

    • Linhua Jiang
    • , Xiaohui Fan
    •  & Fabian Walter
  • Letter |

    Of the more than 400 known exoplanets, about 70 transit their central star, most in small orbits (with periods of around 1 day, for instance). Here, observations are reported of the transit of CoRoT-9b, which orbits with a period of 95.274 days, on a low eccentricity, around a solar-like star. Its relatively large periastron distance yields a 'temperate' photospheric temperature estimated to be between 250 and 430 K, and its interior composition is inferred to be consistent with those of Jupiter and Saturn.

    • H. J. Deeg
    • , C. Moutou
    •  & G. Wuchterl
  • Letter |

    Although general relativity underlies modern cosmology, its applicability on cosmological length scales has yet to be stringently tested. Now, at a length scale of tens of megaparsecs, the quantity EG, which combines measures of large-scale gravitational lensing, galaxy clustering, and the growth rate of structure, has been measured to be 0.39±0.06, in agreement with the general relativistic prediction of about 0.4.

    • Reinabelle Reyes
    • , Rachel Mandelbaum
    •  & Robert E. Smith
  • Letter |

    Measuring the oscillations of a star can allow the various mixing processes in its interior to be disentangled, through the signature they leave on period spacings in the gravity mode spectrum. Here numerous gravity modes in a young star of about seven solar masses are reported: the mean period spacing enables the extent of the convective core to be determined, and the clear periodic deviation from the mean constrains the location of the chemical transition zone — at about 10 per cent of the radius.

    • Pieter Degroote
    • , Conny Aerts
    •  & Eric Michel
  • Letter |

    Current models indicate that the Milky Way's stellar halo was assembled from many smaller systems, and recent studies claimed that the true Galactic building blocks must have been vastly different from the surviving dwarfs. But the overall abundance pattern of elements in S1020549, the most iron-poor star in the Sculptor dwarf galaxies, is now found to follow that seen in low-metallicity halo stars, indicating that the systems destroyed to form the halo billions of years ago were not fundamentally different from the progenitors of present-day dwarfs.

    • Anna Frebel
    • , Evan N. Kirby
    •  & Joshua D. Simon
  • Letter |

    WASP-12b is a planet of 1.4 Jupiter masses that orbits at a mean distance of only 3.1 stellar radii from its star; its orbital period is 1.1 days, and its radius (1.79 times that of Jupiter) is unexpectedly large. An analysis of its properties now reveals that the planet is losing mass to its host star at a rate of ∼10−7 Jupiter masses per year, and that dissipation of the star's tidal perturbation in the planet's convective envelope provides the energy source for its large volume.

    • Shu-lin Li
    • , N. Miller
    •  & Jonathan J. Fortney
  • Letter |

    Type Ia supernovae are thought to be associated with the thermonuclear explosions of white dwarf stars, but the nuclear runaway that leads to the explosion could occur through two different pathways with different X-ray signatures. The X-ray flux from six nearby elliptical galaxies and galaxy bulges is now observed to reveal that it is a factor of about 30–50 less than predicted by the accretion scenario, where a white dwarf accumulates material from a companion star.

    • Marat Gilfanov
    •  & Ákos Bogdán
  • Letter |

    One of the central predictions of general relativity is that a clock in a gravitational potential well runs more slowly than a similar clock outside the well. This effect, known as gravitational redshift, has been measured using clocks on a tower, an aircraft and a rocket, but here, laboratory experiments based on quantum interference of atoms are shown to produce a much more precise measurement.

    • Holger Müller
    • , Achim Peters
    •  & Steven Chu
  • Letter |

    It is widely accepted that strong and variable radiation detected over all accessible energy bands in a number of active galaxies arises from a relativistic, Doppler-boosted jet pointing close to our line of sight. However, the size of the emitting zone and the location of this region relative to the central supermassive black hole are poorly understood. Here, the coincidence of a γ-ray flare with a dramatic change of optical polarization angle is reported, providing evidence for co-spatiality of optical and γ-ray emission regions and indicating a highly ordered jet magnetic field.

    • A. A. Abdo
    • , M. Ackermann
    •  & M. Sikora
  • Letter |

    Stars form from cold molecular interstellar gas, which is relatively rare in the local Universe, such that galaxies like the Milky Way form only a few new stars per year. However, typical massive galaxies in the distant Universe formed stars much more rapidly, suggesting that young galaxies were more rich in molecular gas. The results of a survey of molecular gas in samples of typical massive star-forming galaxies when the Universe was 40% and 24% of its current age now reveal that distant star-forming galaxies were indeed gas rich.

    • L. J. Tacconi
    • , R. Genzel
    •  & B. Weiner
  • Letter |

    Infrared spectroscopy can probe the conditions and compositions of exoplanet atmospheres. Previous results relied on space-based telescopes that do not provide spectroscopic capability in the 2.4–5.2 μm spectral region. Here, ground-based observations of the dayside emission spectrum for HD 189733b are reported between 2.0–2.4 μm and 3.1–4.1 μm; an unexpected feature at around 3.25 μm is found that is difficult to explain with models that assume local thermodynamic equilibrium conditions, and is assigned to methane.

    • Mark R. Swain
    • , Pieter Deroo
    •  & Thomas Henning
  • Letter |

    Long duration γ-ray bursts mark the explosive death of some massive stars and are a rare sub-class of type Ibc supernovae. To date, central-engine-driven supernovae have been discovered exclusively through their γ-ray emission, yet it is expected that a larger population goes undetected. The discovery of luminous radio emission from the seemingly ordinary type Ibc supernova SN 2009bb, which requires a substantial relativistic outflow powered by a central engine, is now reported.

    • A. M. Soderberg
    • , S. Chakraborti
    •  & M. A. P. Torres
  • Letter |

    Type Ic supernovae have drawn attention since 1998 owing to their sparse association with long duration γ-ray bursts (GRBs). Although the GRB central engine generates ultra-relativistic jets, no relativistic outflows have yet been found in type Ib/c supernovae explosions. Here, radio observations reveal a mildly relativistic expansion in a nearby type Ic supernova, SN 2007gr.

    • Z. Paragi
    • , G. B. Taylor
    •  & B. Paczyński
  • Letter |

    Telescopic measurements of asteroids' colours rarely match laboratory reflectance spectra of meteorites owing to a 'space weathering' process that rapidly reddens asteroid surfaces. 'Unweathered' asteroids, however, with spectra matching ordinary chondrite meteorites, are seen only among small bodies with orbits that cross inside the orbits of Mars and Earth. Such unweathered asteroids are now shown to have experienced orbital intersections closer than the Earth–Moon distance within the past half-million years.

    • Richard P. Binzel
    • , Alessandro Morbidelli
    •  & Alan T. Tokunaga
  • Letter |

    The properties of 'dwarf' galaxies have long challenged the cold dark matter (CDM) model of galaxy formation, as the properties of most observed dwarf galaxies contrast with models based on the dominance of CDM. Here, hydrodynamical simulations (assuming the presence of CDM) are reported in which the analogues of dwarf galaxies — bulgeless and with shallow central dark-matter profiles — arise naturally.

    • F. Governato
    • , C. Brook
    •  & P. Madau
  • Letter |

    The close binary Algol system contains a radio-bright KIV sub-giant star in a very close and rapid orbit with a main sequence B8 star. Evidence points to the existence of an extended, complex coronal magnetosphere originating at the cooler K subgiant, but the detailed morphology of the subgiant's corona and its possible interaction with its companion are unknown. Multi-epoch radio imaging of the Algol system now reveals a large coronal loop suggestive of a persistent asymmetric magnetic field structure aligned between the two stars.

    • W. M. Peterson
    • , R. L. Mutel
    •  & W. M. Goss
  • Letter |

    The amplitude of the magnetic field near the Galactic Centre has been uncertain by two orders of magnitude for several decades. A compilation of previous data now reveals a downward break in the region's non-thermal radio spectrum; this requires that the Galactic Centre field be at least ∼50 microgauss on 400 parsec scales, with evidence supporting a field of 100 microgauss. This would imply that over 10% of the Galaxy's magnetic energy is contained in only around 0.05% (or less) of its volume.

    • Roland M. Crocker
    • , David I. Jones
    •  & Raymond J. Protheroe
  • Letter |

    Existing models of type Ia supernovae generally explain their observed properties, with the exception of the sub-luminous 1991bg-like supernovae. It has long been suspected that the merger of two white dwarfs could give rise to a type Ia event, but simulations so far have failed to produce an explosion. Here, a simulation of the merger of two equal-mass white dwarfs is presented that leads to a sub-luminous explosion; it requires a single common-envelope phase and component masses of about 0.9 solar masses.

    • Rüdiger Pakmor
    • , Markus Kromer
    •  & Wolfgang Hillebrandt