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Roediger J.C.,Queens University | Roediger J.C.,University of California at Santa Cruz | Courteau S.,Queens University | Graves G.,Peyton Hall | And 2 more authors.
Astrophysical Journal, Supplement Series | Year: 2014

We present an extensive literature compilation of age, metallicity, and chemical abundance pattern information for the 41 Galactic globular clusters (GGCs) studied by Schiavon et al. Our compilation constitutes a notable improvement over previous similar work, particularly in terms of chemical abundances. Its primary purpose is to enable detailed evaluations of and refinements to stellar population synthesis models designed to recover the above information for unresolved stellar systems based on their integrated spectra. However, since the Schiavon sample spans a wide range of the known GGC parameter space, our compilation may also benefit investigations related to a variety of astrophysical endeavors, such as the early formation of the Milky Way, the chemical evolution of GGCs, and stellar evolution and nucleosynthesis. For instance, we confirm with our compiled data that the GGC system has a bimodal metallicity distribution and is uniformly enhanced in the α elements. When paired with the ages of our clusters, we find evidence that supports a scenario whereby the Milky Way obtained its globular clusters through two channels: in situ formation and accretion of satellite galaxies. The distributions of C, N, O, and Na abundances and the dispersions thereof per cluster corroborate the known fact that all GGCs studied so far with respect to multiple stellar populations have been found to harbor them. Finally, using data on individual stars, we verify that stellar atmospheres become progressively polluted by CN(O)-processed material after they leave the main sequence. We also uncover evidence which suggests that the α elements Mg and Ca may originate from more than one nucleosynthetic production site. We estimate that our compilation incorporates all relevant analyses from the literature up to mid-2012. As an aid to investigators in the fields named above, we provide detailed electronic tables of the data upon which our work is based at http://www.astro.queensu.ca/ people/Stephane-Courteau/roediger2013/index.html. © 2014. The American Astronomical Society. All rights reserved.. Source

Johansson P.H.,University of Helsinki | Johansson P.H.,University of Turku | Naab T.,Max Planck Insitut fur Astrophysik | Ostriker J.P.,Peyton Hall
Astrophysical Journal | Year: 2012

We present a sample of nine high-resolution cosmological simulations in the mass range of M vir = 7 × 1011-4 × 10 12 M starting from ΛCDM initial conditions. Our simulations include primordial radiative cooling, photoionization, star formation, supernova II feedback, but exclude supernova-driven winds and active galactic nucleus feedback. The simulated galaxies assemble in two phases, with the initial growth dominated by compact (r < r eff) in situ star formation fueled by cold, low-entropy gas streams resulting in a very similar mean assembly redshift of z f, ins 2.5 for the in situ stellar component in all galaxies. The late growth is dominated by accretion of old stars formed in subunits outside the main galaxy (r > r eff) resulting in an assembly redshift of z f, acc 0.5-1.5 with much larger scatter. We find a positive correlation between the fraction of accreted stars and the final mass of our galaxies. We show that gravitational feedback strongly suppresses late star formation in massive galaxies contributing to the observed galaxy color bimodality. The accretion of stellar material is also responsible for the observed size growth of early-type galaxies. In addition, we find that the dark matter fractions within the stellar half-mass radii continuously increase toward lower redshift from about f DM 0.05 at z 3 to f DM 0.1-0.3 at z = 0. Furthermore, the logarithmic slope of the total density profile is nearly isothermal at the present day (γ′ 1.9-2.2). Finally, the input of gravitational heating lowers the central dark matter densities in the galaxies, with the effect being smaller compared to simulations without supernova feedback. © 2012. The American Astronomical Society. All rights reserved. Source

Bu D.-F.,Chinese Academy of Sciences | Yuan F.,Chinese Academy of Sciences | Stone J.M.,Peyton Hall
Monthly Notices of the Royal Astronomical Society | Year: 2011

In a hot, dilute, magnetized accretion flow, the electron mean-free path can be much greater than the Larmor radius, and thus thermal conduction is anisotropic and along the magnetic field lines. In this case, if the temperature decreases outward, the flow may be subject to a buoyancy instability - the magnetothermal instability (MTI). The MTI amplifies the magnetic field, and aligns the field lines with the radial direction. If the accretion flow is differentially rotating, a magnetorotational instability (MRI) may also be present. Using two-dimensional, time-dependent magnetohydrodynamic simulations, we investigate the interaction between these two instabilities. We use global simulations that span over two orders of magnitude in radius, centred on the region around the Bondi radius where the infall time of gas is longer than the growth time of both the MTI and MRI. A significant amplification of the magnetic field is produced by both instabilities, although we find that the MTI and MRI primarily amplify the radial and toroidal components of the field, respectively. Most importantly, we find that if the MTI and MRI can amplify the magnetic energy by factors of F t and F r, respectively, then when the MTI and MRI are both present, the magnetic energy can be amplified by a factor of F t·F r. Therefore, we conclude that the amplification of the magnetic energy by the MTI and MRI operates independently. We also find that the MTI contributes to the transport of angular momentum, because radial motions induced by the MTI increase the Maxwell (by amplifying the magnetic field) and Reynolds stresses. Finally, we find that thermal conduction decreases the slope of the radial temperature profile. The increased temperature near the Bondi radius decreases the mass accretion rate. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS. Source

Fressin F.,Harvard - Smithsonian Center for Astrophysics | Knutson H.A.,Harvard - Smithsonian Center for Astrophysics | Charbonneau D.,Harvard - Smithsonian Center for Astrophysics | O'Donovan F.T.,Harvard - Smithsonian Center for Astrophysics | And 4 more authors.
Astrophysical Journal | Year: 2010

We use the Spitzer Space Telescope to estimate the dayside thermal emission of the exoplanet TrES-3 integrated in the 3.6, 4.5, 5.8, and 8.0 μm bandpasses of the Infrared Array Camera (IRAC) instrument. We observe two secondary eclipses and find relative eclipse depths of 0.00346 ± 0.00035, 0.00372 ± 0.00054, 0.00449 ± 0.00097, and 0.00475 ± 0.00046, respectively, in the four IRAC bandpasses. We combine our results with the earlier K-band measurement of De Mooij etal., and compare them with models of the planetary emission. We find that the planet does not require the presence of an inversion layer in the high atmosphere. This is the first very strongly irradiated planet that does not have a temperature inversion, which indicates that stellar or planetary characteristics other than temperature have an important impact on temperature inversion. De Mooij & Snellen also detected a possible slight offset in the timing of the secondary eclipse in the K band. However, based on our four Spitzer channels, we place a 3σ upper limit of |ecos(ω)| < 0.0056, where e is the planet's orbital eccentricity and ω is the longitude of the periastron. This result strongly indicates that the orbit is circular, as expected from tidal circularization theory. Source

Bai X.-N.,Peyton Hall | Bai X.-N.,Harvard - Smithsonian Center for Astrophysics | Stone J.M.,Peyton Hall
Proceedings of the International Astronomical Union | Year: 2012

Protoplanetary disks (PPDs) are widely believed to be turbulent as a result of the magnetorotational instability (MRI). We perform magnetohydrodynamical simulations of PPDs that for the first time, take into account both Ohmic resistivity and ambipolar diffusion in a self-consistent manner. We show that in the inner region of PPDs that corresponds the habitable zone, the MRI is completely suppressed due to the interplay between magnetic field and ambipolar diffusion. The gas in this region is laminar throughout the entire vertical extent of the disk. Instead of MRI-driven accretion, a strong magnetocentrifugal wind is launched that efficiently carries away disk angular momentum. A physical wind geometry requires the presence of a strong current layer that is offset from the disk midplane where horizontal magnetic fields flip. We show that the entire accretion flow proceeds through this strong current layer. The non-turbulent nature of the gas flow strongly favors the habitable zone as the site for planetesimal formation, and has important implications for their subsequent growth into terrestrial planets. Copyright © International Astronomical Union 2014. Source

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