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Cambridge, United Kingdom

Green D.A.,Astrophysics Group
Bulletin of the Astronomical Society of India

A revised catalogue of 294 Galactic supernova remnants (SNRs) is presented, along with some simple statistics. This catalogue has twenty more entries than did the previous version (from 2009), as 21 new remnants have been added, and one object has been removed as it has been identified as an Hii region. © 2014, Astronomical Society of India, Indian Institute of Astrophysics. All rights reserved. Source

Demory B.-O.,Astrophysics Group | Demory B.-O.,Massachusetts Institute of Technology
Astrophysical Journal Letters

Exoplanet research focusing on the characterization of super-Earths is currently limited to the handful of targets orbiting bright stars that are amenable to detailed study. This Letter proposes to look at alternative avenues to probe the surface and atmospheric properties of this category of planets, known to be ubiquitous in our galaxy. I conduct Markov Chain Monte Carlo light-curves analyses for 97 Kepler close-in RP ≲ 2.0 R ⊕ super-Earth candidates with the aim of detecting their occultations at visible wavelengths. Brightness temperatures and geometric albedos in the Kepler bandpass are constrained for 27 super-Earth candidates. A hierarchical Bayesian modeling approach is then employed to characterize the population-level reflective properties of these close-in super-Earths. I find median geometric albedos Ag in the Kepler bandpass ranging between 0.16 and 0.30, once decontaminated from thermal emission. These super-Earth geometric albedos are statistically larger than for hot Jupiters, which have medians Ag ranging between 0.06 and 0.11. A subset of objects, including Kepler-10b, exhibit significantly larger albedos (Ag ≳ 0.4). I argue that a better understanding of the incidence of stellar irradation on planetary surface and atmospheric processes is key to explain the diversity in albedos observed for close-in super-Earths. © 2014. The American Astronomical Society. All rights reserved. Source

Faraoni V.,Bishops University | Zambrano Moreno A.F.,Bishops University | Nandra R.,Astrophysics Group | Nandra R.,Kavli Institute for Cosmology
Physical Review D - Particles, Fields, Gravitation and Cosmology

The bizarre behavior of the apparent (black hole and cosmological) horizons of the McVittie spacetime is discussed using, as an analogy, the Schwarzschild-deSitter-Kottler spacetime (which is a special case of McVittie anyway). For a dust-dominated "background" universe, a black hole cannot exist at early times because its (apparent) horizon would be larger than the cosmological (apparent) horizon. A phantom-dominated background universe causes this situation, and the horizon behavior, to be time-reversed. © 2012 American Physical Society. Source

Feroz F.,Astrophysics Group | Hobson M.P.,Astrophysics Group
Monthly Notices of the Royal Astronomical Society

Weak gravitational lensing studies of galaxy clusters often assume a spherical cluster model to simplify the analysis, but some recent studies have suggested this simplifying assumption may result in large biases in estimated cluster masses and concentration values, since clusters are expected to exhibit triaxiality. Several such analyses have, however, quoted expressions for the spatial derivatives of the lensing potential in triaxial models, which are open to misinterpretation. In this paper, we give a clear description of weak lensing by triaxial Navarro-Frenk-White (NFW) galaxy clusters and also present an efficient and robust method to model these clusters and obtain parameter estimates. By considering four highly triaxial NFW galaxy clusters, we re-examine the impact of the simplifying assumption of sphericity and find that while the concentration estimates are largely unbiased, except in one of our triaxial NFW simulated clusters, the masses are significantly biased, by up to 40 per cent, for all the clusters we analysed. Moreover, we find that erroneously assuming spherical symmetry can lead to the mistaken conclusion that some substructure is present in the galaxy clusters or, even worse, that multiple galaxy clusters are present in the field. Our cluster fitting method also allows one to answer the question of whether a given cluster exhibits triaxiality or a simple spherical model is good enough. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS. Source

Feroz F.,Astrophysics Group | Hobson M.P.,Astrophysics Group
Monthly Notices of the Royal Astronomical Society

GJ667C is the least massive component of a triple star system which lies at a distance of about 6.8 pc (22.1 light-year) from the Earth. GJ667C has received much attention recently due to the claims that it hosts up to seven planets including three super-Earths inside the habitable zone. We present a Bayesian technique for the analysis of radial velocity (RV) data sets in the presence of correlated noise component ('red noise'), with unknown parameters. We also introduce hyper-parameters in our model in order to deal statistically with under- or overestimated error bars on measured RVs as well as inconsistencies between different data sets. By applying this method to the RV data set of GJ667C, we show that this data set contains a significant correlated (red) noise component with correlation time-scale for HARPS data of the order of 9 d. Our analysis shows that the data only provide strong evidence for the presence of two planets: GJ667Cb and c with periods 7.19 and 28.13 d, respectively, with some hints towards the presence of a third signal with period 91 d. The planetary nature of this third signal is not clear and additional RV observations are required for its confirmation. Previous claims of the detection of additional planets in this system are due the erroneous assumption of white noise. Using the standard white noise assumption, our method leads to the detection of up to five signals in this system. We also find that with the red noise model, the measurement uncertainties from HARPS for this system are underestimated at the level of ~50 per cent. ©2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

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