A Via Láctea é duas vezes maior do que os cientistas pensavam

quarta-feira, junho 13, 2018

A&A 612, L8 (2018)

Letter to the Editor

Disk stars in the Milky Way detected beyond 25 kpc from its center

M. López-Corredoira1,2, C. Allende Prieto1,2, F. Garzón1,2, H. Wang3,4, C. Liu3,4 and L. Deng3,4

1 Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain 

2 Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain 

3 Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China 

4 University of Chinese Academy of Sciences, Beijing 100012, PR China 

Received: 22 February 2018 Accepted: 5 April 2018

Source/Fonte: NASA

Abstract

Context. The maximum size of the Galactic stellar disk is not yet known. Some studies have suggested an abrupt drop-off of the stellar density of the disk at Galactocentric distances R ≳ 15 kpc, which means that in practice no disk stars or only very few of them should be found beyond this limit. However, stars in the Milky Way plane are detected at larger distances. In addition to the halo component, star counts have placed the end of the disk beyond 20 kpc, although this has not been spectroscopically confirmed so far. Aims. Here, we aim to spectroscopically confirm the presence of the disk stars up to much larger distances.

Methods. With data from the LAMOST and SDSS-APOGEE spectroscopic surveys, we statistically derived the maximum distance at which the metallicity distribution of stars in the Galactic plane is distinct from that of the halo populations.

Results. Our analysis reveals the presence of disk stars at R > 26 kpc (99.7% C.L.) and even at R > 31 kpc (95.4% C.L.).

Key words: Galaxy: structure – Galaxy: disk – Galaxy: abundances

© ESO 2018

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O mais antigo mamífero do Brasil viveu na era dos dinossauros

A Late Cretaceous mammal from Brazil and the first radioisotopic age for the Bauru Group

Mariela C. Castro, Francisco J. Goin, Edgardo Ortiz-Jaureguizar, E. Carolina Vieytes, Kaori Tsukui, Jahandar Ramezani, Alessandro Batezelli, Júlio C. A. Marsola, Max C. Langer

Published 30 May 2018. DOI: 10.1098/rsos.18048


Abstract

In the last three decades, records of tribosphenidan mammals from India, continental Africa, Madagascar and South America have challenged the notion of a strictly Laurasian distribution of the group during the Cretaceous. Here, we describe a lower premolar from the Late Cretaceous Adamantina Formation, São Paulo State, Brazil. It differs from all known fossil mammals, except for a putative eutherian from the same geologic unity and Deccanolestes hislopi, from the Maastrichtian of India. The incompleteness of the material precludes narrowing down its taxonomic attribution further than Tribosphenida, but it is larger than most coeval mammals and shows a thin layer of parallel crystallite enamel. The new taxon helps filling two major gaps in the fossil record: the paucity of Mesozoic mammals in more northern parts of South America and of tribosphenidans in the Cretaceous of that continent. In addition, high-precision U-Pb geochronology provided a post-Turonian maximal age (≤87.8 Ma) for the type stratum, which is overlain by the dinosaur-bearing Marília Formation, constraining the age of the Adamantina Formation at the site to late Coniacian–late Maastrichtian. This represents the first radioisotopic age for the Bauru Group, a key stratigraphic unit for the study of Cretaceous tetrapods in Gondwana.

KEYWORDS

Tribosphenida enamel reduction Bauru Basin South America U-Pb geochronology mesozoic

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O formato da água: a estrutura e ligação de hidrogênio nos limites da estabilidade da água líquida

sábado, junho 02, 2018

Structure and hydrogen bonding at the limits of liquid water stability

Flaviu Cipcigan, Vlad Sokhan, Glenn Martyna & Jason Crain

Scientific Reports volume 8, Article number: 1718 (2018) | Download Citation

How water molecules are arranged in the liquid around a central reference molecule. The white areas show high-density "shells" while the orange area shows regions where no water molecules can reside.

Abstract

Liquid water exhibits unconventional behaviour across its wide range of stability – from its unusually high liquid-vapour critical point down to its melting point and below where it reaches a density maximum and exhibits negative thermal expansion allowing ice to float. Understanding the molecular underpinnings of these anomalies presents a challenge motivating the study of water for well over a century. Here we examine the molecular structure of liquid water across its range of stability, from mild supercooling to the negative pressure and high temperature regimes. We use a recently-developed, electronically-responsive model of water, constructed from gas-phase molecular properties and incorporating many-body, long-range interactions to all orders; as a result the model has been shown to have high transferability from ice to the supercritical regime. We report a link between the anomalous thermal expansion of water and the behaviour of its second coordination shell and an anomaly in hydrogen bonding, which persists throughout liquid water’s range of stability – from the high temperature limit of liquid water to its supercooled regime.

Acknowledgements

This work was supported by the NPL Strategic Research programme and the STFC Hartree Centre’s Innovation Return on Research programme. FSC acknowledges the Scottish Doctoral Training Centre in Condensed Matter Physics, the NPL Postgraduate Institute and EPSRC for funding under an Industrial CASE studentship. We acknowledge use of Hartree Centre, EPCC and NPL computational resources.

Author information

Affiliations

IBM Research UK, Hartree Centre, Daresbury, WA4 4AD, United Kingdom

Flaviu Cipcigan & Jason Crain

STFC Daresbury Laboratory, Daresbury, WA4 4AD, United Kingdom

Vlad Sokhan

IBM T. J. Watson Research Center, Yorktown Heights, New York, 10598, USA

Glenn Martyna

Contributions

F.S.C., V.P.S., G.J.M., J.C. designed research. F.S.C., V.P.S. conducted research. F.S.C., V.P.S., G.J.M., J.C. analysed and interpreted the results. All authors reviewed the manuscript.

Competing Interests

The authors declare that they have no competing interests.

Corresponding author

Correspondence to Flaviu Cipcigan.

About this article

Publication history

Received 13 October 2017 Accepted 14 December 2017

Published 29 January 2018


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A água, quem diria, são dois líquidos!

Diffusive dynamics during the high-to-low density transition in amorphous ice

Fivos Perakis, Katrin Amann-Winkel, Felix Lehmkühler, Michael Sprung, Daniel Mariedahl, Jonas A. Sellberg, Harshad Pathak, Alexander Späh, Filippo Cavalca, Daniel Schlesinger, Alessandro Ricci, Avni Jain, Bernhard Massani, Flora Aubree, Chris J. Benmore, Thomas Loerting, Gerhard Grübel, Lars G. M. Pettersson, and Anders Nilsson

PNAS August 1, 2017. 114 (31) 8193-8198; published ahead of print June 26, 2017. https://doi.org/10.1073/pnas.1705303114

Edited by Pablo G. Debenedetti, Princeton University, Princeton, NJ, and approved May 31, 2017 (received for review March 31, 2017)

Source/Fonte: New Scientist

Significance

The importance of a molecular-level understanding of the properties, structure, and dynamics of liquid water is recognized in many scientific fields. It has been debated whether the observed high- and low-density amorphous ice forms are related to two distinct liquid forms. Here, we study experimentally the structure and dynamics of high-density amorphous ice as it relaxes into the low-density form. The unique aspect of this work is the combination of two X-ray methods, where wide-angle X-ray scattering provides the evidence for the structure at the atomic level and X-ray photon-correlation spectroscopy provides insight about the motion at the nanoscale, respectively. The observed motion appears diffusive, indicating liquid-like dynamics during the relaxation from the high-to low-density form.

Abstract

Water exists in high- and low-density amorphous ice forms (HDA and LDA), which could correspond to the glassy states of high- (HDL) and low-density liquid (LDL) in the metastable part of the phase diagram. However, the nature of both the glass transition and the high-to-low-density transition are debated and new experimental evidence is needed. Here we combine wide-angle X-ray scattering (WAXS) with X-ray photon-correlation spectroscopy (XPCS) in the small-angle X-ray scattering (SAXS) geometry to probe both the structural and dynamical properties during the high-to-low-density transition in amorphous ice at 1 bar. By analyzing the structure factor and the radial distribution function, the coexistence of two structurally distinct domains is observed at T = 125 K. XPCS probes the dynamics in momentum space, which in the SAXS geometry reflects structural relaxation on the nanometer length scale. The dynamics of HDA are characterized by a slow component with a large time constant, arising from viscoelastic relaxation and stress release from nanometer-sized heterogeneities. Above 110 K a faster, strongly temperature-dependent component appears, with momentum transfer dependence pointing toward nanoscale diffusion. This dynamical component slows down after transition into the low-density form at 130 K, but remains diffusive. The diffusive character of both the high- and low-density forms is discussed among different interpretations and the results are most consistent with the hypothesis of a liquid–liquid transition in the ultraviscous regime.

liquid–liquid transitionglass transitionamorphous iceX-ray photon-correlation spectroscopysupercooled water

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