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Bensby T.,Lund Observatory | Alves-Brito A.,University of Santiago de Chile | Oey M.S.,University of Michigan | Yong D.,Australian National University | Melendez J.,University of Sao Paulo
Astrophysical Journal Letters

Based on high-resolution spectra obtained with the MIKE spectrograph on the Magellan telescopes, we present detailed elemental abundances for 20 red giant stars in the outer Galactic disk, located at Galactocentric distances between 9 and 13kpc. The outer disk sample is complemented with samples of red giants from the inner Galactic disk and the solar neighborhood, analyzed using identical methods. For Galactocentric distances beyond 10kpc, we only find chemical patterns associated with the local thin disk, even for stars far above the Galactic plane. Our results show that the relative densities of the thick and thin disks are dramatically different from the solar neighborhood, and we therefore suggest that the radial scale length of the thick disk is much shorter than that of the thin disk. We make a first estimate of the thick disk scale length of L thick = 2.0kpc, assuming L thin = 3.8kpc for the thin disk. We suggest that radial migration may explain the lack of radial age, metallicity, and abundance gradients in the thick disk, possibly also explaining the link between the thick disk and the metal-poor bulge. © 2011. The American Astronomical Society. All rights reserved. Source

Davies M.B.,Lund Observatory | Coleman Miller M.,University of Maryland University College | Bellovary J.M.,University of Michigan
Astrophysical Journal Letters

Black holes exceeding a billion solar masses have been detected at redshifts greater than six. The rapid formation of these objects may suggest a massive early seed or a period of growth faster than Eddington. Here we suggest a new mechanism along these lines. We propose that in the process of hierarchical structure assembly, dense star clusters can be contracted on dynamical timescales due to the nearly free-fall inflow of self-gravitating gas with a mass comparable to or larger than that of the clusters. This process increases the velocity dispersion to the point where the few remaining hard binaries can no longer effectively heat the cluster, and the cluster goes into a period of homologous core collapse. The cluster core can then reach a central density high enough for fast mergers of stellar-mass black holes and hence the rapid production of a black hole seed that could be 105 M or larger. © 2011 The American Astronomical Society. All rights reserved. Source

In 2006 ESO Council authorized a Phase B study of a European AO-telescope with a 42 m segmented primary with a 5-mirror design, the E-ELT. Several reports and working groups have already presented science cases for an E-ELT, specifically exploiting the new capabilities of such a large telescope. One of the aims of the design has been to find a balance in the performances between an E-ELT and the James Webb Space Telescope, JWST. Apart from the larger photon-collecting area, the strengths of the former is the higher attainable spatial and spectral resolutions. The E-ELT AO system will have an optimal performance in the near-IR, which makes it specially advantageous. High-resolution spectroscopy in the near-infrared has, however, not been discussed much. This paper aims at filling that gap, by specifically discussing spectroscopy of stellar (mainly red giant), photospheric abundances. Based on studies in the literature of stellar abundances, at the needed medium to high spectral resolutions in the near-infrared (0.8-2.4 μm), I will try to extrapolate published results to the performance of the E-ELT and explore what could be done at the E-ELT in this field. A discussion on what instrument characteristics that would be needed for stellar abundance analyses in the near-IR will be given. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Dittrich K.,Max Planck Institute for Astronomy | Klahr H.,Max Planck Institute for Astronomy | Johansen A.,Lund Observatory
Astrophysical Journal

Recent numerical simulations have shown long-lived axisymmetric sub- and super-Keplerian flows in protoplanetary disks. These zonal flows are found in local as well as global simulations of disks unstable to the magnetorotational instability. This paper covers our study of the strength and lifetime of zonal flows and the resulting long-lived gas over- and underdensities as functions of the azimuthal and radial size of the local shearing box. We further investigate dust particle concentrations without feedback on the gas and without self-gravity. The strength and lifetime of zonal flows increase with the radial extent of the simulation box, but decrease with the azimuthal box size. Our simulations support earlier results that zonal flows have a natural radial length scale of 5-7 gas pressure scale heights. This is the first study that combines three-dimensional MHD simulations of zonal flows and dust particles feeling the gas pressure. The pressure bumps trap particles with St = 1 very efficiently. We show that St = 0.1 particles (of some centimeters in size if at 5 AU in a minimum mass solar nebula) reach a hundred-fold higher density than initially. This opens the path for particles of St = 0.1 and dust-to-gas ratio of 0.01 or for particles of St ≥ 0.5 and dust-to-gas ratio 10-4 to still reach densities that potentially trigger the streaming instability and thus gravoturbulent formation of planetesimals. © 2013. The American Astronomical Society. All rights reserved.. Source

News Article | April 9, 2016
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Planet Nine, as its name suggests, is the ninth planet of the solar system discovered earlier this year by the same scientist who demoted Pluto to a dwarf planet. A new study said, however, that Planet Nine is actually an imposter that lurked near the solar system and got stolen from its host star. In January, Konstantin Batygin and Michael Brown found a never before seen planet around 10 times the mass of Earth lurking at the edge of the solar system. Some scientists hypothesized that the planet could have originated within the solar system and migrated toward its edges. A team of astronomers now suggests the opposite - the planet may have been stolen by the sun from a nearby star. A new study, published in the open journal Arvix, and led by Alexander Mustill from the Lund Observatory, proposes that the Planet Nine might actually be an exoplanet outside the solar system and orbiting its own host star. The chances of this happens in 0.1 to 2 percent, which is very low, but the team believes this is possible. The probability "Although these probabilities seem low, you have to compare them to each other, and not absolutely," said Mustill. "Because ultimately any very specific outcome is very unlikely," he added. One of the biggest mysteries in the solar system is probably the presence of a distant planet travelling around the sun in a 20,000-year orbit. This planet is found far beyond Pluto. It has been previously suggested that the sun could have captured objects from other stars while they are passing nearby, including planets and even comets. Though its presence is deemed mysterious, it could have been forcibly ejected from its own orbit or formed on its own in its location today, considering the complexities of cosmic bodies in the universe. "I think it's premature to say what's most likely," said Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. This discovery will spur new questions about Planet Nine, where it really came from and how it might affect Earth in the future. © 2016 Tech Times, All rights reserved. Do not reproduce without permission.

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