Dublin Institute of Advanced Studies

Dublin, Ireland

Dublin Institute of Advanced Studies

Dublin, Ireland
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Adetunji A.Q.,University of Manitoba | Ferguson I.J.,University of Manitoba | Jones A.G.,Dublin Institute of Advanced Studies
Journal of Geophysical Research: Solid Earth | Year: 2015

Magnetotelluric (MT) responses at the Proterozoic Grenville Front in Canada have been interpreted as being caused by lithospheric electrical anisotropy, and the area is often noted as a classic example of lithospheric anisotropy. This study reevaluates evidence for the electrical anisotropy using 56 MT stations. The spatially uniform MT responses noted at the Grenville Front extend to ~200 km southeast and for at least 400 km along strike and are associated with rocks at less than 150 km depth. Examination of induction arrows at longer periods shows arrows at high angle to the MT conductive direction consistent with the presence of macroscopic resistivity structures. New 2-D anisotropic inversions show that electrical anisotropy is not required to fit the MT data. The results indicate that in the resistive mantle lithosphere beneath the Grenville Front, and in conductive lithosphere in adjacent areas, the maximum horizontal resistivity anisotropy is <10%, much less than the factor of 15 determined in earlier 1-D studies. The results suggest that the upper lithospheric mantle in the area is devoid of significant electrical anisotropy and that the observed MT response directionality is due to large-scale resistivity structure. We interpret the spatially consistent MT responses observed at the Grenville Front as being associated with the resistive Archean lithosphere extending southeast beneath the Grenville Front. The obliquity between seismic and MT responses arises because the Superior fabric is oblique to the seismic fast direction. If dextral shearing occurred, it appears to have not caused any significant shape preferred electrical anisotropy. ©2015. American Geophysical Union. All Rights Reserved.

Ambrogi L.,Gran Sasso Science Institute | Aharonian F.,Gran Sasso Science Institute | Aharonian F.,Dublin Institute of Advanced Studies | Aharonian F.,Max Planck Institute for Nuclear Physics | De Ona Wilhelmi E.,Institute for Space science CSIC IEEC
Journal of Physics: Conference Series | Year: 2016

The potential of a next-generation ground based gamma-ray telescope array has been investigated. In addition to the ideal Gaussian shaped PSF, more realistic non-Gaussian PSFs with tails have been considered and their impact on the detector performance has been studied. The capability of the instrument to resolve multiple sources has been analyzed and the corresponding detector sensitivity estimated. These scenarios are particularly interesting in the framework of Galactic objects, where the observation of more than one source in the same field of view (FoV) is very likely to happen. © Published under licence by IOP Publishing Ltd.

Padovani M.,Institute Of Ciancies Of Lespai Csicieec | Walmsley C.M.,INAFOsservatorio Astrofisico di Arcetri | Walmsley C.M.,Dublin Institute of Advanced Studies | Tafalla M.,Observatorio Astronmico Nacional IGN | And 3 more authors.
Astronomy and Astrophysics | Year: 2011

Aims. We studied the abundance of HCN, H13CN, and HN 13C in a sample of prestellar cores, in order to search for species associated with high density gas. Methods. We used the IRAM 30 m radiotelescope to observe along the major and the minor axes of L1498, L1521E, and TMC 2, three cores chosen on the basis of their CO depletion properties. We mapped the J = 1 → 0 transition of HCN, H13CN, and HN13C towards the source sample plus the J = 1 → 0 transition of N2H+ and the J = 2 → 1 transition of C18O in TMC 2. We used two different radiative transfer codes, making use of recent collisional rate calculations, in order to determine more accurately the excitation temperature, leading to a more exact evaluation of the column densities and abundances. Results. We find that the optical depths of both H13CN(1-0) and HN13C(1-0) are non-negligible, allowing us to estimate excitation temperatures for these transitions in many positions in the three sources. The observed excitation temperatures are consistent with recent computations of the collisional rates for these species and they correlate with hydrogen column density inferred from dust emission. We conclude that HCN and HNC are relatively abundant in the high density zone, n(H2) ∼ 105 cm-3, where CO is depleted. The relative abundance [HNC]/[HCN] differs from unity by at most 30% consistent with chemical expectations. The three hyperfine satellites of HCN(1-0) are optically thick in the regions mapped, but the profiles become increasingly skewed to the blue (L1498 and TMC 2) or red (L1521E) with increasing optical depth suggesting absorption by foreground layers. © 2011 ESO.

Dolan B.P.,National University of Ireland, Maynooth | Dolan B.P.,Dublin Institute of Advanced Studies | Szabo R.J.,Heriot - Watt University | Szabo R.J.,Maxwell Institute for Mathematical science
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

We study vacuum states and symmetric fermions in the equivariant dimensional reduction of the Yang-Mills-Dirac theory over the six-dimensional homogeneous space SU(3)/U(1)×U(1) endowed with a family of SU(3) structures including a nearly Kähler structure. We derive the fixed tree-level scalar potentials of the induced Yang-Mills-Higgs theory and compute the dynamically generated gauge and Higgs boson masses as functions of the metric moduli of the coset space. We find an integrable subsector of the Higgs field theory that is governed by a sine-Gordon-type model whose topological soliton solutions are determined nonperturbatively by the gauge coupling and that tunnel between families of infinitely degenerate vacua. The reduction of the Dirac action for symmetric fermions yields exactly massless chiral fermions containing subsectors that have fixed tree-level Yukawa interactions. We compute dynamical fermion mass matrices explicitly and compare them at different points of the moduli space, some of which support consistent heterotic flux vacua. © 2013 American Physical Society.

Wienen M.,Max Planck Institute for Radio Astronomy | Wyrowski F.,Max Planck Institute for Radio Astronomy | Schuller F.,Max Planck Institute for Radio Astronomy | Menten K.M.,Max Planck Institute for Radio Astronomy | And 4 more authors.
Astronomy and Astrophysics | Year: 2012

Context. The APEX Telescope Large Area Survey: the GALaxy (ATLASGAL) is an unbiased continuum survey of the inner Galactic disk at 870 μm. It covers ±60° in Galactic longitude and aims to find all massive clumps at various stages of high-mass star formation in the inner Galaxy, particularly the earliest evolutionary phases. Aims. We aim to determine properties such as the gas kinetic temperature and dynamics of new massive cold clumps found by ATLASGAL. Most importantly, we derived their kinematical distances from the measured line velocities. Methods. We observed the ammonia (J,K) = (1, 1) to (3, 3) inversion transitions toward 862 clumps of a flux-limited sample of submm clumps detected by ATLASGAL and extracted 13CO (1-0) spectra from the Galactic Ring Survey (GRS). We determined distances for a subsample located at the tangential points (71 sources) and for 277 clumps whose near/far distance ambiguity is resolved. Results. Most ATLASGAL clumps are cold with rotational temperatures from 10-30 K with a median of 17 K. They have a wide range of NH 3 linewidths (1-7 km s -1) with 1.9 km s -1 as median, which by far exceeds the thermal linewidth, as well as a broad distribution of high column densities from 10 14 to 10 16 cm -2 (median of 2 × 10 15 cm -2) with an NH 3 abundance in the range of 5 to 30 × 10 -8. ATLASGAL sources are massive, ≳ 100 M, and a fraction of clumps with a broad linewidth is in virial equilibrium. We found an enhancement of clumps at Galactocentric radii of 4.5 and 6 kpc. The comparison of the NH 3 lines as high-density probes with the GRS 13CO emission as low-density envelope tracer yields broader linewidths for 13CO than for NH 3. The small differences in derived clump velocities between NH 3 (representing dense core material) and 13CO (representing more diffuse molecular cloud gas) suggests that the cores are essentially at rest relative to the surrounding giant molecular cloud. Conclusions. The high detection rate (87%) confirms ammonia as an excellent probe of the molecular content of the massive, cold clumps revealed by ATLASGAL. A clear trend of increasing rotational temperatures and linewidths with evolutionary stage is seen for source samples ranging from 24 μm dark clumps to clumps with embedded HII regions. The survey provides the largest ammonia sample of high-mass star forming clumps and thus presents an important repository for the characterization of statistical properties of the clumps and the selection of subsamples for detailed, high-resolution follow-up studies. © 2012 ESO.

Merello M.,University of Chile | Merello M.,University of Texas at Austin | Bronfman L.,University of Chile | Garay G.,University of Chile | And 4 more authors.
Astrophysical Journal | Year: 2013

We report molecular line and dust continuum observations toward the high-mass star-forming region G331.5-0.1, one of the most luminous regions of massive star formation in the Milky Way, located at the tangent region of the Norma spiral arm, at a distance of 7.5 kpc. Molecular emission was mapped toward the G331.5-0.1 GMC in the CO(J = 1 → 0) and C18O(J = 1 → 0) lines with NANTEN, while its central region was mapped in CS(J = 2 → 1 and J = 5 → 4) with SEST, and in CS(J = 7 → 6) and 13CO(J = 3 → 2) with ASTE. Continuum emission mapped at 1.2 mm with SIMBA and at 0.87 mm with LABOCA reveal the presence of six compact and luminous dust clumps, making this source one of the most densely populated central regions of a GMC in the Galaxy. The dust clumps are associated with molecular gas and they have the following average properties: size of 1.6 pc, mass of 3.2 × 10 3 M ⊙, molecular hydrogen density of 3.7 × 104 cm-3, dust temperature of 32 K, and integrated luminosity of 5.7 × 105 L⊙, consistent with values found toward other massive star-forming dust clumps. The CS and 13CO spectra show the presence of two velocity components: a high-velocity component at -89 km s-1, seen toward four of the clumps, and a low-velocity component at -101 km s-1 seen toward the other two clumps. Radio continuum emission is present toward four of the molecular clumps, with spectral index estimated for two of them of 0.8 ± 0.2 and 1.2 ± 0.2. A high-velocity molecular outflow is found at the center of the brightest clump, with a line width of 26 km s-1 (FWHM) in CS(J = 7 → 6). Observations of SiO(J = 7 → 6 and J = 8 → 7), and SO(JK = 88 → 77 and JK = 87 → 76) lines provide estimates of the gas rotational temperature toward this outflow >120 K and >75 K, respectively. © 2013. The American Astronomical Society. All rights reserved.

Becker T.W.,University of Southern California | Schaeffer A.J.,Dublin Institute of Advanced Studies | Lebedev S.,Dublin Institute of Advanced Studies | Conrad C.P.,University of Hawaii at Manoa
Geophysical Research Letters | Year: 2015

An absolute plate motion (APM) model is required to address issues such as the thermochemical evolution of Earth's mantle. All APM models have to rely on indirect inferences, including those based on hot spots and seismic anisotropy, each with their own set of uncertainties. Here, we explore a seafloor spreading-aligned reference frame. We show that this reference frame fits azimuthal seismic anisotropy in the uppermost mantle very well. The corresponding Euler pole is close to those of hot spot reference frames, ridge motion minimizing models, and geodynamic estimates of net rotation and predicts clear trench motion patterns. We conclude that a net rotation pole guided by the spreading-aligned model (at 64°E, 61°S, with moderate rotation of ∼ 0.2 ... 0.3°/Myr) could indeed represent a standard, comprehensive reference frame for present-day plate motions with respect to the deep mantle. Key Points A spreading-aligned plate motion reference frame matches seismic anisotropy This defines a comprehensive reference frame matching hot spots Universal reference frame clarifies subduction zone trench migration ©2015. American Geophysical Union. All Rights Reserved.

Ambrogi L.,University of L'Aquila | De Ona Wilhelmi E.,Institute for Space science CSIC IEEC | Aharonian F.,University of L'Aquila | Aharonian F.,Dublin Institute of Advanced Studies | Aharonian F.,Max Planck Institute for Nuclear Physics
Astroparticle Physics | Year: 2016

The potential of an array of imaging atmospheric Cherenkov telescopes to detect gamma-ray sources in complex regions has been investigated. The basic characteristics of the gamma-ray instrument have been parameterized using simple analytic representations. In addition to the ideal (Gaussian form) point spread function (PSF), the impact of more realistic non-Gaussian PSFs with tails has been considered. Simulations of isolated point-like and extended sources have been used as a benchmark to test and understand the response of the instrument. The capability of the instrument to resolve multiple sources has been analyzed and the corresponding instrument sensitivities calculated. The results are of particular interest for weak gamma-ray emitters located in crowded regions of the Galactic plane, where the chance of clustering of two or more gamma-ray sources within 1 deg is high. © 2016 Elsevier B.V. All rights reserved.

Leurini S.,Max Planck Institute for Radio Astronomy | Menten K.M.,Max Planck Institute for Radio Astronomy | Walmsley C.M.,National institute for astrophysics | Walmsley C.M.,Dublin Institute of Advanced Studies
Astronomy and Astrophysics | Year: 2016

Context. Class I methanol masers are thought to be tracers of interstellar shock waves. However, they have received relatively little attention mostly as a consequence of their low luminosities compared to other maser transitions. This situation has changed recently and Class I methanol masers are now routinely used as signposts of outflow activity especially in high extinction regions. The recent detection of polarisation in Class I lines now makes it possible to obtain direct observational information about magnetic fields in interstellar shocks. Aims. We make use of newly calculated collisional rate coefficients for methanol to investigate the excitation of Class I methanol masers and to reconcile the observed Class I methanol maser properties with model results. Methods.We performed large velocity gradient calculations with a plane-parallel slab geometry appropriate for shocks to compute the pump and loss rates which regulate the interactions of the different maser systems with the maser reservoir. We study the dependence of the pump rate coefficient, the maser loss rate, and the inversion efficiency of the pumping scheme of several Class I masers on the physics of the emitting gas. Results. We predict inversion in all transitions where maser emission is observed. Bright Class I methanol masers are mainly hightemperature (>100 K) high-density (n(H2) 107-108 cm-3) structures with methanol maser emission measures, corresponding to high methanol abundances close to the limits set by collisional quenching. Our model predictions reproduce reasonably well most of the observed properties of Class I methanol masers. Class I masers in the 25 GHz series are the most sensitive to the density of the medium and mase at higher densities than other lines. Moreover, even at high density and high methanol abundances, their luminosity is predicted to be lower than that of the 44 GHz and 36 GHz masers. Our model predictions also reflect the observational result that the 44 GHz line is almost always stronger than the 36 GHz maser. By comparison between observed isotropic photon luminosities and our model predictions, we infer maser beam solid angles of roughly 10-3 steradian. Conclusions. We find that the Class I masers can reasonably be separated into three families: the (J + 1)-1 - J0-E type series, the (J + 1)0 - J1-A type, and the J2 - J1-E lines at 25 GHz. The 25 GHz lines behave in a different fashion from the other masers as they are only inverted at high densities above 106 cm-3 in contrast to other Class I masers. Therefore, the detection of maser activity in all three families is a clear indication of high densities. © ESO 2016.

Crocker R.M.,Australian National University | Bicknell G.V.,Australian National University | Taylor A.M.,Dublin Institute of Advanced Studies | Carretti E.,National institute for astrophysics | Carretti E.,CSIRO
Astrophysical Journal | Year: 2015

The Galactic center's giant outflows are manifest in three different, nonthermal phenomena: (1) the hard-spectrum, γ-ray "Fermi bubbles" emanating from the nucleus and extending to |b| ∼ 50°; (2) the hard-spectrum, total-intensity microwave (∼20-40 GHz) "haze" extending to |b| ∼ 35° in the lower reaches of the Fermi bubbles; and (3) the steep-spectrum, polarized, "S-PASS" radio (∼2-20 GHz) lobes that envelop the bubbles and extend to |b|∼ 60°. We find that the nuclear outflows inflate a genuine bubble in each Galactic hemisphere that has the classical structure, working outward, of reverse shock, contact discontinuity (CD), and forward shock. Expanding into the finite pressure of the halo and given appreciable cooling and gravitational losses, the CD of each bubble is now expanding only very slowly. We find observational signatures in both hemispheres of giant, reverse shocks at heights of ∼1 kpc above the nucleus; their presence ultimately explains all three of the nonthermal phenomena mentioned above. Synchrotron emission from shock-reaccelerated cosmic-ray electrons explains the spectrum, morphology, and vertical extent of the microwave haze and the polarized radio lobes. Collisions between shock-reaccelerated hadrons and denser gas in cooling condensations that form inside the CD account for most of the bubbles' γ-ray emissivity. Inverse Compton emission from primary electrons contributes at the 10%-30% level. Our model suggests that the bubbles are signatures of a comparatively weak but sustained nuclear outflow driven by Galactic center star formation over few × 108 yr. © 2015. The American Astronomical Society. All rights reserved.

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