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Bykov A.,RAS Ioffe Physical - Technical Institute | Gehrels N.,NASA | Krawczynski H.,Washington University in St. Louis | Lemoine M.,CNRS Paris Institute of Astrophysics | And 3 more authors.
Space Science Reviews | Year: 2012

In this review we confront the current theoretical understanding of particle acceleration at relativistic outflows with recent observational results on various source classes thought to involve such outflows, e.g. gamma-ray bursts, active galactic nuclei, and pulsar wind nebulae. We highlight the possible contributions of these sources to ultra-high-energy cosmic rays. © 2012 Springer Science+Business Media B.V.


Belehaki A.,Institute of Astronomy | Lilensten J.,Institute Of Planetologie Et Dastrophysique Of Grenoble
Journal of Space Weather and Space Climate | Year: 2013

Reaching the end of the second year of the Journal of Space Weather and Space Climate, we present an overview of important issues that resulted in the clarification of our editorial policy, mainly triggered by the response of the scientific communities related to SWSC. We conclude this editorial with future perspectives concerning the impact factor and the charging policy. © A. Belehaki et al., Published by EDP Sciences 2013.


Maschberger T.,Institute Of Planetologie Et Dastrophysique Of Grenoble | Maschberger T.,Argelander Institute For Astronomie | Maschberger T.,Institute of Astronomy | Clarke C.J.,Institute of Astronomy
Monthly Notices of the Royal Astronomical Society | Year: 2011

Recent analyses of mass segregation diagnostics in star-forming regions invite a comparison with the output of hydrodynamic simulations of star formation. In this work we investigate the state of mass segregation of 'stars' (i.e. sink particles in the simulations) in the case of hydrodynamical simulations which omit feedback. We first discuss methods to quantify mass segregation in substructured regions, either based on the minimum spanning tree (Allison's Λ), or through analysis of correlations between stellar mass and local stellar surface number densities. We find that the presence of even a single 'outlier' (i.e. a massive object far from other stars) can cause the Allison Λ method to describe the system as inversely mass segregated, even where in reality the most massive sink particles are overwhelmingly in the centres of the subclusters. We demonstrate that a variant of the Λ method is less susceptible to this tendency but also argue for an alternative representation of the data in the plane of stellar mass versus local surface number density. The hydrodynamical simulations show global mass segregation from very early times which continues throughout the simulation, being only mildly influenced during subcluster merging. We find that up to ≈2-3 per cent of the 'massive' sink particles (m > 2.5M⊙) are in relative isolation because they have formed there, although other sink particles can form later in their vicinity. Ejections of massive sinks from subclusters do not contribute to the number of isolated massive sink particles, as the gravitational softening in the calculation suppresses this process. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.


Parker R.J.,ETH Zurich | Andersen M.,Institute Of Planetologie Et Dastrophysique Of Grenoble
Monthly Notices of the Royal Astronomical Society | Year: 2014

We use N-body simulations to compare the evolution of spatial distributions of stars and brown dwarfs in young star-forming regions. We use three different diagnostics: the ratio of stars to brown dwarfs as a function of distance from the region's centre, RSSR, the local surface density of stars compared to brown dwarfs, ΣLDR, and we compare the global spatial distributions using the ΛMSR method. From a suite of 20 initially statistically identical simulations, 6/20 attain RSSR ≪ 1 and ΣLDR ≪ 1 and ΛMSR ≪ 1, indicating that dynamical interactions could be responsible for observed differences in the spatial distributions of stars and brown dwarfs in star-forming regions. However, many simulations also display apparently contradictory results - for example, in some cases the brown dwarfs have much lower local densities than stars (ΣLDR ≪ 1), but their global spatial distributions are indistinguishable (ΛMSR = 1) and the relative proportion of stars and brown dwarfs remains constant across the region (RSSR = 1). Our results suggest that extreme caution should be exercised when interpreting any observed difference in the spatial distribution of stars and brown dwarfs, and that a much larger observational sample of regions/clusters (with complete mass functions) is necessary to investigate whether or not brown dwarfs form through similar mechanisms to stars. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Sakai N.,University of Tokyo | Ceccarelli C.,Institute Of Planetologie Et Dastrophysique Of Grenoble | Bottinelli S.,French National Center for Scientific Research | Sakai T.,University of Tokyo | Yamamoto S.,University of Tokyo
Astrophysical Journal | Year: 2012

Distribution of the CH3OH (JK = 2K-1 K, 96.7GHz) emission has been investigated toward NGC1333 IRAS4B, a low-mass Class 0 protostar which harbors a hot corino, with Nobeyama Millimeter Array. The CH3OH emission is found to be prominent in the shocked region caused by an impact of the molecular outflow from the protostars. The direction of the outflow which is responsible for the shock seems to be opposite to that of a compact outflow known previously in the CO (J = 2-1), HCN (J = 1-0), H2CO (312-211), and CH3OH (JK = 7K-6K) emissions, whereas it is the same as that of the faint second outflow found in the H2CO emission. This double outflow structure can be interpreted most naturally by the existence of more than two protostars in IRAS4B. On the other hand, a centrally condensed component associated apparently with IRAS4B cannot be recognized in our CH 3OH observation. Our observation suggests that, in this source, the CH3OH (JK = 2K-1K) emission preferentially traces the shocked regions rather than the hot corino around the protostar. © 2012. The American Astronomical Society. All rights reserved..


Parker R.J.,ETH Zurich | Maschberger T.,Institute Of Planetologie Et Dastrophysique Of Grenoble | Alves de Oliveira C.,European Space Agency
Monthly Notices of the Royal Astronomical Society | Year: 2012

We apply two different algorithms to search for mass segregation to a recent observational census of the ρ Ophiuchi star-forming region. First, we apply the Λ MSR method, which compares the minimum spanning tree (MST) of a chosen subset of stars to MSTs of random subsets of stars in the cluster, and determine the mass segregation ratio, Λ MSR. Secondly, we apply the m-Σ method, which calculates the local stellar surface density around each star and determines the statistical significance of the average surface density for a chosen mass bin compared to the average surface density in the whole cluster. Using both methods, we find no indication of mass segregation (normal or inverse) in the spatial distribution of stars and brown dwarfs in ρ Ophiuchi. Although ρ Ophiuchi suffers from high visual extinction, we show that a significant mass segregation signature would be detectable, albeit slightly diluted, despite dust obscuration of centrally located massive stars. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Vincent A.C.,McGill University | Martin P.,Institute Of Planetologie Et Dastrophysique Of Grenoble | Cline J.M.,McGill University
Journal of Cosmology and Astroparticle Physics | Year: 2012

We compare the full-sky morphology of the 511 keV gamma ray excess measured by the INTEGRAL/SPI experiment to predictions of models based on dark matter (DM) scatterings that produce low-energy positrons: either MeV-scale DM that annihilates directly into e +e - pairs, or heavy DM that inelastically scatters into an excited state (XDM) followed by decay into e +e - and the ground state. By direct comparison to the data, we find that such explanations are consistent with dark matter halo profiles predicted by numerical many-body simulations for a Milky Way-like galaxy. Our results favor an Einasto profile over the cuspier NFW distribution and exclude decaying dark matter scenarios whose predicted spatial distribution is too broad. We obtain a good fit to the shape of the signal using six fewer degrees of freedom than previous empirical fits to the 511 keV data. We find that the ratio of flux at Earth from the galactic bulge to that of the disk is between 1.9 and 2.4, taking into account that 73% of the disk contribution may be attributed to the beta decay of radioactive 26Al. © 2012 IOP Publishing Ltd and Sissa Medialab srl.


Lazareff B.,Institute Of Planetologie Et Dastrophysique Of Grenoble | Le Bouquin J.-B.,Institute Of Planetologie Et Dastrophysique Of Grenoble | Berger J.-P.,European Southern Observatory
Astronomy and Astrophysics | Year: 2012

Context. Optical interferometers are subject to many atmospheric and instrumental artifacts that contribute to the degradation of their instrumental contrast, hence their performances. The differential birefringence is, among these effects, one of the trickiest to control, in particular for instrument using fibers, where it can be far larger than the one arising in the optical mirror trains. Several solutions have been tested in the past, ranging from polarization splitting to fiber tweaking. We adopt a new solution for the PIONIER instrument, a four-telescope (4T) combiner at the Very Large Telescope Interferometer (VLTI). Aims. We present a method to cancel the instrumental birefringence in an optical interferometer, allowing the joint detection of the fringe patterns of both polarizations, and substantial gains to be made in both signal-to-noise ratio and readout speed. Methods. A thin (2 mm) plate of birefringent material (LiNbO 3) is inserted in each of the four beams. The incidence angle of each plate is adjustable. This allows us to introduce a controlled amount of birefringence in each beam and to cancel the instrumental differential birefringence. We present our derivation of the induced birefringence versus incidence angle and discuss the design choices. Results. Our proposed solution is implemented in the Pionier instrument. Before correction, the instrumental birefringence was of order 5 μm (path length). The adjustment takes about one hour, results in a birefringence of less than 0.1 μm, and is stable for at least the duration of an observing run (several days). Conclusions. We demonstrate on an operational near-infrared interferometer a novel, simple, low-cost, and effective technique to control the differential birefringence. The predictability and stability of the correction make this technique ideal for an automated correction in the VLTI second generation instruments. © 2012 ESO.


Maschberger T.,Institute Of Planetologie Et Dastrophysique Of Grenoble
Astrophysics and Space Science Proceedings | Year: 2014

Star formation occurs in flocculent, chaotic environments. Therefore the accretion rates of proto-stars moving through a cloud will not be completely smooth and contains a random noise term. Stochastic accretion can be described with a non-linear stochastic differential equation for the accretion rate, for example dm = m2(dt+dW) where dW describes the fluctuations. A sample of seeds growing in this model develops a mass function that follows the whole shape of the initial mass function. Both the log-normal low-mass part and the high-mass power law are a consequence of the same stochastic growth equation. © Springer International Publishing Switzerland 2014.


Simon Wedlund C.,Belgian Institute for Space Aeronomy | Gronoff G.,NASA | Lilensten J.,Institute Of Planetologie Et Dastrophysique Of Grenoble | Menager H.,Institute Of Planetologie Et Dastrophysique Of Grenoble | Barthelemy M.,Institute Of Planetologie Et Dastrophysique Of Grenoble
Annales Geophysicae | Year: 2011

The mean energy W expended in a collision of electrons with atmospheric gases is a useful parameter for fast aeronomy computations. Computing this parameter in transport kinetic models with experimental values can tell us more about the number of processes that have to be taken into account and the uncertainties of the models. We present here computations for several atmospheric gases of planetological interest (CO2, CO, N2, O2, O, CH4, H, He) using a family of multi-stream kinetic transport codes. Results for complete atmospheres for Venus, Earth, Mars, Jupiter and Titan are also shown for the first time. A simple method is derived to calculate W of gas mixtures from single-component gases and is conclusively checked against the W values of these planetary atmospheres. Discrepancies between experimental and theoretical values show where improvements can be made in the measurement of excitation and dissociation cross-sections of specific neutral species, such as CO2 and CO. © Author(s) 2011.

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