| 1. |
- Fynbo, H. O. U., et al.
(författare)
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Revised rates for the stellar triple-alpha process from measurement of C-12 nuclear resonances
- 2005
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687 .- 1476-4679. ; 433:7022, s. 136-139
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Tidskriftsartikel (refereegranskat)abstract
- In the centres of stars where the temperature is high enough, three alpha-particles (helium nuclei) are able to combine to form C-12 because of a resonant reaction leading to a nuclear excited state(1). (Stars with masses greater than similar to0.5 times that of the Sun will at some point in their lives have a central temperature high enough for this reaction to proceed.) Although the reaction rate is of critical significance for determining elemental abundances in the Universe(1), and for determining the size of the iron core of a star just before it goes supernova(2), it has hitherto been insufficiently determined(2). Here we report a measurement of the inverse process, where a C-12 nucleus decays to three alpha-particles. We find a dominant resonance at an energy of similar to11 MeV, but do not confirm the presence of a resonance at 9.1 MeV (ref. 3). We show that interference between two resonances has important effects on our measured spectrum. Using these data, we calculate the triple-a rate for temperatures from 10(7) K to 10(10) K and find significant deviations from the standard rates(3). Our rate below similar to5 x 10(7) K is higher than the previous standard, implying that the critical amounts of carbon that catalysed hydrogen burning in the first stars are produced twice as fast as previously believed(4). At temperatures above 10(9) K, our rate is much less, which modifies predicted nucleosynthesis in supernovae(5,6).
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| 2. |
- Abdo, A. A., et al.
(författare)
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A limit on the variation of the speed of light arising from quantum gravity effects
- 2009
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 462:7271, s. 331-334
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Tidskriftsartikel (refereegranskat)abstract
- A cornerstone of Einstein’s special relativity is Lorentz invariance—the postulate that all observers measure exactly the same speed of light in vacuum, independent of photon-energy. While special relativity assumes that there is no fundamental length-scale associated with such invariance, there is a fundamental scale (the Planck scale, lPlanck~1.62×10-33cm or EPlanck = MPlanckc2~1.22×1019GeV), at which quantum effects are expected to strongly affect the nature of space–time. There is great interest in the (not yet validated) idea that Lorentz invariance might break near the Planck scale. A key test of such violation of Lorentz invariance is a possible variation of photon speed with energy. Even a tiny variation in photon speed, when accumulated over cosmological light-travel times, may be revealed by observing sharp features in γ-ray burst (GRB) light-curves. Here we report the detection of emission up to ~31GeV from the distant and short GRB090510. We find no evidence for the violation of Lorentz invariance, and place a lower limit of 1.2EPlanck on the scale of a linear energy dependence (or an inverse wavelength dependence), subject to reasonable assumptions about the emission (equivalently we have an upper limit of lPlanck/1.2 on the length scale of the effect). Our results disfavour quantum-gravity theories in which the quantum nature of space–time on a very small scale linearly alters the speed of light.
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| 3. |
- Adriani, O., et al.
(författare)
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An anomalous positron abundance in cosmic rays with energies 1.5-100 GeV
- 2009
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 458:7238, s. 607-609
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Tidskriftsartikel (refereegranskat)abstract
- Antiparticles account for a small fraction of cosmic rays and are known to be produced in interactions between cosmic-ray nuclei and atoms in the interstellar medium(1), which is referred to as a 'secondary source'. Positrons might also originate in objects such as pulsars(2) and microquasars(3) or through dark matter annihilation(4), which would be 'primary sources'. Previous statistically limited measurements(5-7) of the ratio of positron and electron fluxes have been interpreted as evidence for a primary source for the positrons, as has an increase in the total electron+positron flux at energies between 300 and 600 GeV (ref. 8). Here we report a measurement of the positron fraction in the energy range 1.5-100 GeV. We find that the positron fraction increases sharply overmuch of that range, in a way that appears to be completely inconsistent with secondary sources. We therefore conclude that a primary source, be it an astrophysical object or dark matter annihilation, is necessary.
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| 4. |
- Ahlberg, Per E., et al.
(författare)
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Palaeontology: A firm step from water to land
- 2006
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 440:7085, s. 747-749
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Tidskriftsartikel (refereegranskat)
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| 5. |
- Ahlberg, Per Erik, 1963-, et al.
(författare)
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Pelvic claspers confirm chondrichthyan-like internal fertilization in arthrodires
- 2009
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Ingår i: Nature. - : Nature Publishing Group. - 0028-0836 .- 1476-4687. ; 460:7257, s. 888-889
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Tidskriftsartikel (refereegranskat)abstract
- Recent finds(1,2) demonstrate that internal fertilization and viviparity (live birth) were more widespread in the Placodermi, an extinct group of armoured fishes, than was previously realized. Placoderms represent the sister group of the crown group jawed vertebrates (Gnathostomata)(3,4), making their mode(s) of reproduction potentially informative about primitive gnathostome conditions. An ossified pelvic fin basipterygium discovered in the arthrodire Incisoscutum ritchiei was hypothesized to be identical in males and females, with males presumed to have an additional cartilaginous element or series forming a clasper. Here we report the discovery of a completely ossified pelvic clasper in Incisoscutum ritchiei (WAM 03.3.28) which shows that this interpretation was incorrect: the basipterygium described previously(1) is in fact unique to females. The male clasper is a slender rod attached to a square basal plate that articulates directly with the pelvis. It carries a small cap of dermal bone covered in denticles and small hooks that may be homologous with the much larger dermal component of the ptyctodont clasper.
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| 6. |
- Ahlberg, Per, et al.
(författare)
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The axial skeleton of the Devonian tetrapod Ichthyostega.
- 2005
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 437:7055, s. 137-140
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Tidskriftsartikel (refereegranskat)abstract
- Ichthyostega was the first Devonian tetrapod to be subject to a whole-body reconstruction and remains, together with Acanthostega one of only two Devonian tetrapods for which near-complete postcranial material is available. It is thus crucially important for our understanding of the earliest stages of tetrapod evolution and terrestrialization. Based on extensive re-examination of original material, augmented by recently collected specimens, we present a new reconstruction of Ichthyostega that differs substantially from those previously published and reveals hitherto unrecognised regionalization in the vertebral column. Ichthyostega is the earliest vertebrate to show obvious adaptations for non-swimming locomotion. Uniquely among early tetrapods, the presacral vertebral column shows pronounced regionalization of neural arch morphology, suggesting that it was adapted for dorsoventral rather than lateral flexion.
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| 7. |
- Ahlberg, Per, 1963-, et al.
(författare)
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Ventastega curonica and the origin of tetrapod morphology
- 2008
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 453:7199, s. 1199-1204
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Tidskriftsartikel (refereegranskat)abstract
- The gap in our understanding of the evolutionary transition from fish to tetrapod is beginning to close thanks to the discovery of new intermediate forms such as Tiktaalik roseae. Here we narrow it further by presenting the skull, exceptionally preserved braincase, shoulder girdle and partial pelvis of Ventastega curonica from the Late Devonian of Latvia, a transitional intermediate form between the 'elpistostegids' Panderichthys and Tiktaalik and the Devonian tetrapods (limbed vertebrates) Acanthostega and Ichthyostega. Ventastega is the most primitive Devonian tetrapod represented by extensive remains, and casts light on a part of the phylogeny otherwise only represented by fragmentary taxa: it illuminates the origin of principal tetrapod structures and the extent of morphological diversity among the transitional forms
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| 8. |
- Alerstam, Thomas
(författare)
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Animal behaviour - The lobster navigators
- 2003
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 421:6918, s. 27-28
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Tidskriftsartikel (refereegranskat)abstract
- When experimentally displaced in geomagnetic space, spiny lobsters act as if to make their way home. This is a fascinating case of navigation by an invertebrate using a magnetic map sense.
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| 9. |
- Alexandersson, Marina, 1972, et al.
(författare)
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Genome sequence of the Brown Norway rat yields insights into mammalian evolution
- 2004
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Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 428:6982, s. 493-521
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Tidskriftsartikel (refereegranskat)abstract
- The laboratory rat (Rattus norvegicus) is an indispensable tool in experimental medicine and drug development, having made inestimable contributions to human health. We report here the genome sequence of the Brown Norway (BN) rat strain. The sequence represents a high-quality ‘draft’ covering over 90% of the genome. The BN rat sequence is the third complete mammalian genome to be deciphered, and three-way comparisons with the human and mouse genomes resolve details of mammalian evolution. This first comprehensive analysis includes genes and proteins and their relation to human disease, repeated sequences, comparative genome-wide studies of mammalian orthologous chromosomal regions and rearrangement breakpoints, reconstruction of ancestral karyotypes and the events leading to existing species, rates of variation, and lineage-specific and lineageindependent evolutionary events such as expansion of gene families, orthology relations and protein evolution.
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| 10. |
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