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Nikolajuk M.,Observatoire de Geneva | Nikolajuk M.,University of Bialystok | Walter R.,Observatoire de Geneva
Astronomy and Astrophysics | Year: 2013

Aims. A strong, hard X-ray flare was discovered (IGR J12580+0134) by INTEGRAL in 2011, and is associated to NGC 4845, a Seyfert 2 galaxy never detected at high-energy previously. To understand what happened we observed this event in the X-ray band on several occasions. Methods. Follow-up observations with XMM-Newton, Swift, and MAXI are presented together with the INTEGRAL data. Long and short term variability are analysed and the event wide band spectral shape modelled. Results. The spectrum of the source can be described with an absorbed (NH ∼ 7 × 1022 cm-2) power law (Γ a‰ 2.2), characteristic of an accreting source, plus a soft X-ray excess, likely to be of diffuse nature. The hard X-ray flux increased to maximum in a few weeks and decreased over a year, with the evolution expected for a tidal disruption event. The fast variations observed near the flare maximum allowed us to estimate the mass of the central black hole in NGC 4845 as ∼3 × 105 M⊙. The observed flare corresponds to the disruption of about 10% of an object with a mass of 14-30 Jupiter. The hard X-ray emission should come from a corona forming around the accretion flow close to the black hole. This is the first tidal event where such a corona has been observed. © 2013 ESO.

Eggenberger P.,Observatoire de Geneva | Montalban J.,Luniversite Of Liege | Miglio A.,University of Birmingham
Astronomy and Astrophysics | Year: 2012

Context. Recent asteroseismic observations have led to the determination of rotational frequency splittings for ℓ = 1 mixed modes in red giants. Aims. We investigate how these observed splittings can constrain the modelling of the physical processes transporting angular momentum in stellar interiors. Methods. We first compare models including a comprehensive treatment of shellular rotation only, with the rotational splittings observed for the red giant KIC 8366239. We then study how these asteroseismic constraints can give us information about the efficiency of an additional mechanism for the internal transport of angular momentum. This is done by computing rotating models of KIC 8366239 that include a constant viscosity corresponding to this physical process, in addition to the treatment of shellular rotation. Results. We find that models of red giant stars including shellular rotation only predict steep rotation profiles, which are incompatible with the measurements of rotational splittings in the red giant KIC 8366239. Meridional circulation and shear mixing alone are found to produce an insufficient internal coupling so that an additional mechanism for the internal transport of angular momentum is needed during the post-main sequence evolution. We show that the viscosity ν add corresponding to this mechanism is strongly constrained to be ν add = 3 × 10 4 cm 2 s -1 thanks to the observed ratio of the splittings for modes in the wings to those at the centre of the dipole forests. Such a value of viscosity may suggest that the same unknown physical process is at work during the main sequence and the post-main sequence evolution. © 2012 ESO.

Martin D.V.,Observatoire de Geneva | Triaud A.H.M.J.,Massachusetts Institute of Technology
Astronomy and Astrophysics | Year: 2014

The majority of binary stars do not eclipse. Current searches for transiting circumbinary planets concentrate on eclipsing binaries, and are therefore restricted to a small fraction of potential hosts. We investigate the concept of finding planets transiting non-eclipsing binaries, whose geometry would require mutually inclined planes. Using an N-body code we explore how the number and sequence of transits vary as functions of observing time and orbital parameters. The concept is then generalised thanks to a suite of simulated circumbinary systems. Binaries are constructed from radial-velocity surveys of the solar neighbourhood. They are then populated with orbiting gas giants, drawn from a range of distributions. The binary population is shown to be compatible with the Kepler eclipsing binary catalogue, indicating that the properties of binaries may be as universal as the initial mass function. These synthetic systems produce transiting circumbinary planets occurring on both eclipsing and non-eclipsing binaries. Simulated planets transiting eclipsing binaries are compared with published Kepler detections. We find 1) that planets transiting non-eclipsing binaries are probably present in the Kepler data; 2) that observational biases alone cannot account for the observed over-density of circumbinary planets near the stability limit, which implies a physical pile-up; and 3) that the distributions of gas giants orbiting single and binary stars are likely different. Estimating the frequency of circumbinary planets is degenerate with the spread in mutual inclination. Only a minimum occurrence rate can be produced, which we find to be compatible with 9%. Searching for inclined circumbinary planets may significantly increase the population of known objects and will test our conclusions. Their presence, or absence, will reveal the true occurrence rate and help develop circumbinary planet formation theories. © 2014 ESO.

Anderson R.I.,Observatoire de Geneva | Eyer L.,Observatoire de Geneva | Mowlavi N.,Observatoire de Geneva
Monthly Notices of the Royal Astronomical Society | Year: 2013

Cepheids in Galactic open clusters (CCs) are of great importance as zero-point calibrators of the Galactic Cepheid period-luminosity relationship (PLR). We perform an eight-dimensional all-sky census that aims to identify new bona fide CCs and provides a ranking of membership confidence for known CC candidates according to membership probabilities. The probabilities are computed for combinations of known Galactic open clusters and classical Cepheid candidates, based on spatial, kinematic and population-specific membership constraints. Data employed in this analysis are taken largely from published literature and supplemented by a year-round observing programme on both hemispheres dedicated to determining systemic radial velocities of Cepheids. In total, we find 23 bona fide CCs, 5 of which are candidates identified for the first time, including an overtone-Cepheid member in NGC 129. We discuss a subset of CC candidates in detail, some of which have been previously mentioned in the literature. Our results indicate unlikely membership for seven Cepheids that have been previously discussed in terms of cluster membership. We furthermore revisit the Galactic PLR using our bona fide CC sample and obtain a result consistent with the recent calibration by Turner. However, our calibration remains limited mainly by cluster uncertainties and the small number of long-period calibrators. In the near future, Gaia will enable our study to be carried out in much greater detail and accuracy, thanks to data homogeneity and greater levels of completeness. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Durrer R.,University of Geneva | Neronov A.,Observatoire de Geneva
Astronomy and Astrophysics Review | Year: 2013

We review the possible mechanisms for the generation of cosmological magnetic fields, discuss their evolution in an expanding Universe filled with the cosmic plasma and provide a critical review of the literature on the subject. We put special emphasis on the prospects for observational tests of the proposed cosmological magnetogenesis scenarios using radio and gamma-ray astronomy and ultra-high-energy cosmic rays. We argue that primordial magnetic fields are observationally testable. They lead to magnetic fields in the intergalactic medium with magnetic field strength and correlation length in a well defined range. We also state the unsolved questions in this fascinating open problem of cosmology and propose future observations to address them. © 2013 Springer-Verlag Berlin Heidelberg.

Dumusque X.,Harvard - Smithsonian Center for Astrophysics | Pepe F.,Observatoire de Geneva | Lovis C.,Observatoire de Geneva | Latham D.W.,Harvard - Smithsonian Center for Astrophysics
Astrophysical Journal | Year: 2015

The HARPS spectrograph is showing an extreme stability, close to the m s-1 level, over more than 10 years of data. However, the radial velocities of some stars are contaminated by a spurious one-year signal with an amplitude that can be as high as a few m s-1. This signal is in opposition of phase with the revolution of Earth around the Sun and can be explained by the deformation of spectral lines crossing block stitchings of the CCD when the spectrum of an observed star is alternatively blueshifted and redshifted due to the motion of Earth around the Sun. This annual perturbation can be suppressed by either removing those affected spectral lines from the correlation mask used by the cross-correlation technique to derive precise radial velocities, or by simply fitting a yearly sinusoid to the radial velocity data. This is mandatory if we want to detect long-period low-amplitude signals in the HARPS radial velocities of quiet solar-type stars. © 2015. The American Astronomical Society. All rights reserved.

Triaud A.H.M.J.,Observatoire de Geneva
Astronomy and Astrophysics | Year: 2011

Via the Rossiter-McLaughlin effect, it is possible to measure the sky-projected angle between the stellar spin and a planet's orbital spin. Observed orbital inclinations have been found to range over all possible angles. A tentative detection of a correlation between the dispersion in spin/orbit angle and the youth of the system is revealed, using spin/orbit measurements for hot Jupiters around stars with masses ≥ 1.2 M⊙ for which age estimates are more accurately determined. The probability of this pattern arising by chance has been computed to be 7%. This appears in accordance with tidal dissipation, where non-coplanar hot Jupiters' orbits tidally realign. The results suggest they realign within about 2.5 Gyr. For the sample considered, the results imply that hot Jupiters are placed on noncoplanar orbits early in their history rather than late. The events producing these orbits could involve strong planet-planet scattering. © 2011 ESO.

Girardi L.,National institute for astrophysics | Eggenberger P.,Observatoire de Geneva | Miglio A.,Institute Dastrophysique Et Of Geophysique Of Luniversite Of Liege
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2011

Many intermediate-age star clusters in the Magellanic Clouds present multiple main-sequence turn-offs (MMSTOs), which challenge the classical idea that star formation in such objects took place over short time-scales. It has been recently suggested that the presence of fast rotators among main-sequence stars could be the cause of such features, hence relaxing the need for extended periods of star formation. In this Letter, we compute evolutionary tracks and isochrones of models with and without rotation. We find that, for the same age and input physics, both kinds of models present turn-offs with an almost identical position in the colour-magnitude diagrams (CMDs). As a consequence, a dispersion of rotational velocities in coeval ensembles of stars could not explain the presence of MMSTOs. We construct several synthetic CMDs for the different kinds of tracks and combinations of them. The models that best reproduce the morphology of observed MMSTOs are clearly those assuming a significant spread in the stellar ages - as long as ~400 Myr - added to a moderate amount of convective core overshooting. Only these models produce the detailed 'golf club' shape of observed MMSTOs. A spread in rotational velocities alone cannot do anything similar. We also discuss models involving a mixture of stars with and without overshooting, as an additional scenario to producing MMSTOs with coeval populations. We find that they produce turn-offs with a varying extension in the CMD direction perpendicular to the lower main sequence, which are clearly not present in observed MMSTOs. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Halbwachs J.-L.,11 Rue Of Luniversite | Mayor M.,Observatoire de Geneva | Udry S.,Observatoire de Geneva
Monthly Notices of the Royal Astronomical Society | Year: 2012

Wide binaries are tracers of the gravity field of the Galaxy, but their study requires some caution. A large list of common proper motion stars selected from the third Astronomischen Gesellschaft Katalog (AGK3) was monitored with the CORAVEL (for COrrelation RAdial VELocities) spectrovelocimeter, in order to prepare a sample of physical binaries with very wide separations. 66 stars received special attention, since their radial velocities (RV) seemed to be variable. These stars were monitored over several years in order to derive the elements of their spectroscopic orbits. In addition, 10 of them received accurate RV measurements from the SOPHIE spectrograph of the T193 telescope at the Observatory of Haute-Provence. For deriving the orbital elements of double-lined spectroscopic binaries (SB2), a new method was applied, which assumed that the RV of blended measurements are linear combinations of the RV of the components. 13 SB2 orbits were thus calculated. The orbital elements were eventually obtained for 52 spectroscopic binaries (SB), two of them making a triple system. 40 SB received their first orbit and the orbital elements were improved for 10 others. In addition, 11 SB were discovered with very long periods for which the orbital parameters were not found. The median period of the 40 first orbits is 1yr, and several SB should be resolved or should receive an astrometric orbit in future, providing the masses of the components. In addition, it appeared that HD153252 has a close companion, which is a candidate brown dwarf with a minimum mass of 50 Jupiter masses. The final selection of wide binaries and the derivation of their statistical properties will be presented in a second paper. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Leitherer C.,US Space Telescope Science Institute | Ekstrom S.,Observatoire de Geneva | Meynet G.,Observatoire de Geneva | Schaerer D.,Observatoire de Geneva | And 2 more authors.
Astrophysical Journal, Supplement Series | Year: 2014

We present a new set of synthesis models for stellar populations obtained with Starburst99 and based on new stellar evolutionary tracks with rotation. We discuss models with zero rotation velocity and with velocities of 40% of the break-up velocity on the zero-age main-sequence. These values are expected to bracket realistic rotation velocity distributions in stellar populations. The new rotating models for massive stars are more luminous and hotter due to a larger convective core and enhanced surface abundances. This results in pronounced changes in the integrated spectral energy distribution of a population containing massive stars. The changes are most significant at the shortest wavelengths where an increase of the ionizing luminosity by up to a factor of five is predicted. We also show that high equivalent widths of recombination lines may not necessarily indicate a very young age but can be achieved at ages as late as 107 yr. Comparison of these two boundary cases (0% and 40% of the break-up velocity) will allow users to evaluate the effects of rotation and provide guidance for calibrating the stellar evolution models. We also introduce a new theoretical ultraviolet spectral library built from the Potsdam Wolf-Rayet atmospheres. Its purpose is to help identify signatures of Wolf-Rayet stars in the ultraviolet whose strength is sensitive to the particulars of the evolution models. The new models are available for solar and one-seventh solar metallicities. A complete suite of models can be generated on the Starburst99 Web site. The updated Starburst99 package can be retrieved from that Web site as well. © 2014. The American Astronomical Society. All rights reserved..

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