Observatoire Astronomique de Strasbourg

Strasbourg, France

Observatoire Astronomique de Strasbourg

Strasbourg, France
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Boffin H.M.J.,ESO | Miszalski B.,South African Astronomical Observatory | Frew D.J.,Macquarie University | Acker A.,Observatoire Astronomique de Strasbourg | And 6 more authors.
Proceedings of the International Astronomical Union | Year: 2011

We have found the central star of Abell 70 (PN G038.1-25.4, hereafter A 70) to be a binary consisting of a G8 IV-V secondary and a hot white dwarf. The secondary shows enhanced Ba II and Sr II features, firmly classifying it as a barium star. The nebula is found to have Type-I chemical abundances with helium and nitrogen enrichment, which combined with future abundance studies of the central star, will establish A 70 as a unique laboratory for studying s-process AGB nucleosynthesis. © 2012 International Astronomical Union.

Crouseilles N.,French Institute for Research in Computer Science and Automation | Glanc P. P.,French Institute for Research in Computer Science and Automation | Hirstoaga S.A.,French Institute for Research in Computer Science and Automation | Hirstoaga S.A.,Max Planck Institute for Plasma Physics (Garching) | And 3 more authors.
European Physical Journal D | Year: 2014

While developing a new semi-Lagrangian solver, the gap between a linear Landau run in 1D × 1D and a 5D gyrokinetic simulation in toroidal geometry is quite huge. Intermediate test cases are welcome for testing the code. A new fully two-dimensional conservative semi-Lagrangian (CSL) method is presented here and is validated on 2D polar geometries. We consider here as building block, a 2D guiding-center type equation on an annulus and apply it on two test cases. First, we revisit a 2D test case previously done with a PIC approach [J. Pétri, A&A 503, 1 (2009)] and detail the boundary conditions. Second, we consider a 4D drift-kinetic slab simulation (see [V. Grandgirard, M. Brunetti, P. Bertrand, N. Besse, X. Garbet, P. Ghendrih, G. Manfredi, Y. Sarazin, O. Sauter, E. Sonnendrücker, J. Vaclavik, L. Villard, J. Comput. Phys. 217, 395 (2006)]). In both cases, the new method appears to be a good alternative to deal with this type of models since it improves the lack of mass conservation of the standard semi-Lagrangian (BSL) method. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag.

Pechacek T.,Czech Republic Astronomical Institute | Goosmann R.W.,Observatoire Astronomique de Strasbourg | Karas V.,Czech Republic Astronomical Institute | Czerny B.,Copernicus Astronomical Center | Dovciak M.,Czech Republic Astronomical Institute
Astronomy and Astrophysics | Year: 2013

Context. We study some general properties of accretion disc variability in the context of stationary random processes. In particular, we are interested in mathematical constraints that can be imposed on the functional form of the Fourier power-spectrum density (PSD) that exhibits a multiply broken shape and several local maxima. Aims. We develop a methodology for determining the regions of the model parameter space that can in principle reproduce a PSD shape with a given number and position of local peaks and breaks of the PSD slope. Given the vast space of possible parameters, it is an important requirement that the method is fast in estimating the PSD shape for a given parameter set of the model. Methods. We generated and discuss the theoretical PSD profiles of a shot-noise-type random process with exponentially decaying flares. Then we determined conditions under which one, two, or more breaks or local maxima occur in the PSD. We calculated positions of these features and determined the changing slope of the model PSD. Furthermore, we considered the influence of the modulation by the orbital motion for a variability pattern assumed to result from an orbiting-spot model. Results. We suggest that our general methodology can be useful for describing non-monotonic PSD profiles (such as the trend seen, on different scales, in exemplary cases of the high-mass X-ray binary Cygnus X-1 and the narrow-line Seyfert galaxy Ark 564). We adopt a model where these power spectra are reproduced as a superposition of several Lorentzians with varying amplitudes in the X-ray-band light curve. Our general approach can help in constraining the model parameters and in determining which parts of the parameter space are accessible under various circumstances. © 2013 ESO.

Trzesniewski T.,Jagiellonian University | Czerny B.,Copernicus Astronomical Center | Karas V.,Czech Republic Astronomical Institute | Pechaccek T.,Czech Republic Astronomical Institute | And 3 more authors.
Astronomy and Astrophysics | Year: 2011

Aims. We study the variability mechanism of active galactic nuclei (AGN) within the framework of the flare model. We examine the case of Seyfert/LINER galaxy NGC 4258, which is observed at high inclination angle and exhibits rapid fluctuations in its X-ray light curve. Methods. We construct a model light curve based on the assumption of magnetic flares localized in the equatorial plane and orbiting with Keplerian speed at each given radius. We calculate the level of variability as a function of the inclination of an observer, taking into account all effects of general relativity near a rotating supermassive black hole. Results. The variability level is a monotonic function of the source inclination. It rises more rapidly for larger values of the black hole spin (Kerr parameter a) and for steeper emissivity (index β of the radial profile). We compare the expected level of variability for the viewing angle 81.6 deg, as inferred for NGC 4258, with the case of moderate viewing angles of about 30 deg, which are typical of Seyfert type-1 galaxies. Conclusions. Highly inclined sources such as this one are particularly suitable to test the flare model because the orbital motion, Doppler boosting, and light bending are all expected to have maximum effect when the accretion disk is seen almost edge-on. The model is consistent with the NGC 4258 variability, where the obscuring material is thought to be localized mainly toward the equatorial plane rather than forming a geometrically thick torus. Once the intrinsic timescales of the flare duration are determined with higher precision, this kind of highly inclined objects with a precisely known mass of the black hole can be used to set independent constraints on the spin parameter. © 2011 ESO.

Lefevre C.,LERMA | Pagani L.,LERMA | Min M.,University of Amsterdam | Poteet C.,Stanford University | And 2 more authors.
EAS Publications Series | Year: 2016

Dust grains evolve during the life cycle of the interstellar matter. From their birth places to dense molecular clouds, they grow by coagulation and acquire ice mantles, mainly composed of water. These morphological changes affect their optical properties. However, it remains a highly degenerate issue to determine their composition, size distribution, and shape from observations. In particular, using wavelengths associated to dust emission alone is not sufficient to investigate dense cold cores. Fortunately, scattering has turned out to be a powerful tool to investigate molecular clouds from the outer regions to the core. In particular, it is possible to quantify the amount of dust aggregates needed to reproduce observations from 1.25 to 8 μm. © 2016 EAS, EDP Sciences.

Barker M.K.,University of Edinburgh | Ferguson A.M.N.,University of Edinburgh | Cole A.A.,University of Tasmania | Ibata R.,Observatoire Astronomique de Strasbourg | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

The outer regions of disc galaxies are becoming increasingly recognized as key testing sites for models of disc assembly and evolution. Important issues are the epoch at which the bulk of the stars in these regions formed and how discs grow radially over time. To address these issues, we use Hubble Space Telescope Advanced Camera for Surveys imaging to study the star formation history (SFH) of two fields at 9.1 and 11.6 kpc along M33's northern major axis. These fields lie at ~ 4 and 5 V-band disc scalelengths and straddle the break in M33's surface brightness profile. The colour-magnitude diagrams (CMDs) reach the ancient main-sequence turn-off with a signal-to-noise ratio of ~ 5. From detailed modelling of the CMDs, we find that the majority of stars in both fields combined formed at z < 1. The mean age in the inner field, S1, is ~ 3 ± 1 Gyr and the mean metallicity is [M/H]~- 0.5 ± 0.2 dex. The SFH of S1 unambiguously reveals how the inside-out growth previously measured for M33's inner disc out to extends out to the disc edge at In comparison, the outer field, S2, is older (mean age ~ 7 ± 2 Gyr), more metal-poor (mean [M/H]~- 0.8 ± 0.3 dex), and contains ~ 30 times less stellar mass. These results provide the most compelling evidence yet that M33's age gradient reverses at large radii near the disc break and that this reversal is accompanied by a break in stellar mass surface density. We discuss several possible interpretations of this behaviour including radial stellar mixing, warping of the gaseous disc, a change in star formation efficiency and a transition to another structural component. These results offer one of the most detailed views yet of the peripheral regions of any disc galaxy and provide a much needed observational constraint on the last major epoch of star formation in the outer disc. © 2010 The Authors. Journal compilation © 2010 RAS.

Pineda J.L.,Jet Propulsion Laboratory | Goldsmith P.F.,Jet Propulsion Laboratory | Chapman N.,Jet Propulsion Laboratory | Snell R.L.,University of Massachusetts Amherst | And 3 more authors.
Astrophysical Journal | Year: 2010

We report a study of the relation between dust and gas over a 100 deg 2 area in the Taurus molecular cloud. We compare the H2 column density derived from dust extinction with the CO column density derived from the 12CO and 13CO J = 1 → 0 lines. We derive the visual extinction from reddening determined from 2MASS data. The comparison is done at an angular size of 200″ corresponding to 0.14 pc at a distance of 140 pc. We find that the relation between visual extinction AV and N(CO) is linear between AV ≃ 3 and 10 mag in the region associated with the B213-L1495 filament. In other regions, the linear relation is flattened for AV ≳ 4 mag. We find that the presence of temperature gradients in the molecular gas affects the determination of N(CO) by ∼30%-70% with the largest difference occurring at large column densities. Adding a correction for this effect and accounting for the observed relation between the column density of CO and CO2 ices and AV, we find a linear relationship between the column of carbon monoxide and dust for observed visual extinctions up to the maximum value in our data ≃ 23 mag. We have used these data to study a sample of dense cores in Taurus. Fitting an analytical column density profile to these cores we derive an average volume density of about 1.4 × 104 cm-3 and a CO depletion age of about 4.2 × 105 yr. At visual extinctions smaller than ∼3 mag, we find that the CO fractional abundance is reduced by up to two orders of magnitude. The data show a large scatter suggesting a range of physical conditions of the gas. We estimate the H2 mass of Taurus to be about 1.5 × 104 M⊙, independently derived from the AV and N(CO) maps.We derive a CO integrated intensity to H 2 conversion factor of about 2.1×1020 cm -2 (K km s-1)-1, which applies even in the region where the [CO]/[H2] ratio is reduced by up to two orders of magnitude. The distribution of column densities in our Taurus maps resembles a log-normal function but shows tails at large and low column densities. The length scale at which the high column density tail starts to be noticeable is about 0.4 pc.

Vollmer B.,Observatoire Astronomique de Strasbourg | Nehlig F.,Observatoire Astronomique de Strasbourg | Ibata R.,Observatoire Astronomique de Strasbourg
Astronomy and Astrophysics | Year: 2016

New deep VLA D array Hi observations of the highly inclined nearby spiral galaxy NGC 2683 are presented. Archival C array data were processed and added to the new observations. To investigate the 3D structure of the atomic gas disk, we made different 3D models for which we produced model Hi data cubes. The main ingredients of our best-fit model are (i) a thin disk inclined by 80.; (ii) a crude approximation of a spiral and/or bar structure by an elliptical surface density distribution of the gas disk; (iii) a slight warp in inclination between 10 kpc . R . 20 kpc (decreasing by 10.); (iv) an exponential flare that rises from 0.5 kpc at R = 9 kpc to 4 kpc at R = 15 kpc, stays constant until R = 22 kpc, and decreases its height for R 22 kpc; and (v) a low surface-density gas ring with a vertical offset of 1.3 kpc. The slope of NGC 2683fs flare is comparable, but somewhat steeper than those of other spiral galaxies. NGC 2683fs maximum height of the flare is also comparable to those of other galaxies. On the other hand, a saturation of the flare is only observed in NGC 2683. Based on the comparison between the high resolution model and observations, we exclude the existence of an extended atomic gas halo around the optical and thin gas disk. Under the assumption of vertical hydrostatic equilibrium we derive the vertical velocity dispersion of the gas. The high turbulent velocity dispersion in the flare can be explained by energy injection by (i) supernovae; (ii) magneto-rotational instabilities; (iii) interstellar medium stirring by dark matter substructure; or (iv) external gas accretion. The existence of the complex large-scale warping and asymmetries favors external gas accretion as one of the major energy sources that drives turbulence in the outer gas disk. We propose a scenario where this external accretion leads to turbulent adiabatic compression that enhances the turbulent velocity dispersion and might quench star formation in the outer gas disk of NGC 2683. © 2016 ESO.

Acker A.,Observatoire Astronomique de Strasbourg
EAS Publications Series | Year: 2015

Since 2012, French amateurs in astrophotography have been equipped with CCD imagers with high sensitivity and ∼1 arcsec resolution, allowing them to obtain images with high resolution and contrast, and with unprecedented depth, using narrow-band filters. On the other hand, they have been methodically scanning the digital sky survey (DSS), thereby visually identifying dozens of faint PN candidates in the Galaxy. These two methods lead to the discovery of 88 possible/probable PN. The large bipolar object Ou4 is one of the most interesting PN known. © 2015 EAS, EDP Sciences.

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