A model for the kHz QPO in neutron star binaries

SF2A-2008: Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics Eds.: C. Charbonnel, F. Combes and R. Samadi. Available online at http://proc.sf2a.asso.fr, p.287International audienceTwin quasi-periodic oscillations have been observed in the emission spectrum of neutron star binaries. The models that have been proposed do not succeed to explain the frequency difference between the two kHz QPO, which is close but distinct from the rotation frequency of the neutron star. I will present a new model based on the dynamics of the gas trapped in the neutron star magnetosphere

Dynamical signatures of Rossby vortices in cavity-hosting disks

International audienceContext. Planets are formed amidst young circumstellar disks of gas and dust. The latter is traced by thermal radiation, where strong asymmetric clumps have been observed in a handful of cases. These dust traps could be key to understanding the early stages of planet formation, when solids grow from micron-size to planetesimals.Aims. Vortices are among the few known asymmetric dust trapping scenarios. The present work aims to predict their characteristics in a complementary observable. Namely, line-of-sight velocities are well suited to trace the presence of a vortex. Moreover, the dynamics of disks is subject to recent developments.Methods. Two-dimensional hydro simulations were performed in which a vortex forms at the edge of a gas-depleted region. We derived idealized line-of-sight velocity maps, varying disk temperature and orientation relative to the observer. The signal of interest, as a small perturbation to the dominant axisymmetric component in velocity, may be isolated in observational data using a proxy for the dominant quasi-Keplerian velocity. We propose that the velocity curve on the observational major axis be such a proxy.Results. Applying our method to the disk around HD 142527 as a study case, we predict that line-of-sight velocities are barely detectable by currently available facilities, depending on disk temperature. We show that corresponding spirals patterns can also be detected with similar spectral resolutions, which will help to test against alternative explanations

The Rossby wave instability and planet formation: 3D numerical simulations

SF2A-2008: Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics Eds.: C. Charbonnel, F. Combes and R. Samadi. Available online at http://proc.sf2a.asso.frInternational audienceModels of planet formation do not explain yet the growth of planetesimals as in certain ranges of grain size collisions are too slow compared to estimated planet formation time. The Rossby wave instability (RWI) may solve this problem by the formation of Rossby vortices in the accretion disc, speeding up the accumulation of grains in their centre ( te{Peggy} ). Up to now, only two dimensions numerical studies of the RWI have been done. In this proceeding we present the results of three dimensions numerical simulations of the non-linear evolution of the RWI in a non magnetized disc and its vertical structure

Toward a new paradigm for Type II migration

Context. Giant planets open gaps in their protoplanetary and subsequently suffer so-called type II migration. Schematically, planets are thought to be tightly locked within their surrounding disks, and forced to follow the viscous advection of gas onto the central star. This fundamental principle, however, has recently been questioned as migrating planets were shown to decouple from the gas’ radial drift. Aims. In this framework, we question whether the traditionally used linear scaling of migration rate of a giant planet with the disk’s viscosity still holds. Additionally, we assess the role of orbit-crossing material as part of the decoupling mechanism. Methods. We have performed 2D (r, θ) numerical simulations of point-mass planets embedded in locally isothermal α-disks in steady-state accretion, with various values of α. Arbitrary planetary accretion rates were used as a means to diminish or nullify orbit-crossing flows. Results. We confirm that the migration rate of a gap-opening planet is indeed proportional to the disk’s viscosity, but is not equal to the gas drift speed in the unperturbed disk. We show that the role of gap-crossing flows is in fact negligible. Conclusions. From these observations, we propose a new paradigm for type II migration: a giant planet feels a torque from the disk that promotes its migration, while the gap profile relative to the planet is restored on a viscous timescale, thus limiting the planet migration rate to be proportional to the disk’s viscosity. Hence, in disks with low viscosity in the planet region, type II migration should still be very slow

Gaps, rings, and non-axisymmetric structures in protoplanetary disks: Emission from large grains

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Determination of the equilibrium magnesium isotope fractionation factors between brucite and aqueous inorganic and organic species

International audienceMagnesium (Mg) is a major element in seawater, rock-forming minerals, and biological systems. Stable Mg isotopes fractionate during silicate weathering and carbonate mineralization, and hence are a promising tool to trace these processes. Magnesium can be present in natural aqueous solutions as a number of distinct inorganic and organic complexes including MgHCO3+, MgCO30, MgSO40, Mg(OH)+, Mg(citrate)- and Mg(EDTA)2-, in addition to Mg(H2O)62+ commonly referred to as Mg2+. The formation of these species can significantly alter the fractionation of Mg isotopes between minerals and natural fluids. To quantify these effects, isotope exchange experiments were performed at bulk chemical equilibrium between brucite and aqueous solutions containing different organic (citrate, EDTA) and inorganic (SO4-) ligands at 25 °C. The 'three isotope' method was used to determine the equilibrium Mg isotope fractionation factors Δeq26Mg between brucite and several major aqueous magnesium species. The experimentally measured equilibrium Mg isotope fractionation factor between brucite and aqueous Mg2+ was found to be Δeq26Mgbrucite-Mg2+ = -0.35 ± 0.39‰. First-principle calculations to retvieve the brucite β-factor were performed consistently with the calculations of Pinilla et al. (2015) for Mg2+(aq) β-factor. The combination of both studies yield values of Δeq26Mgbrucite-Mg2+ between +0.3 and +0.8 ± 1.0‰, which is the lowest theoretical estimate of this constant obtained to date. An average value Δeq26Mgbrucite-MgSO40 = 0.48 ± 0.16‰ was retrieved from the experiments for the isotope fractionation between brucite and aqueous MgSO40. Mg isotope equilibrium fractionation factors between brucite and aqueous Mg(citrate)- and between brucite and aqueous Mg(EDTA)2- retrieved from the experiments performed in the presence of these organic ligands are Δeq26Mgbrucite-Mg(citrate)- = 0.35 ± 0.21, and Δeq26Mgbrucite-Mg(EDTA)2- = 2.41 ± 0.20‰. The experimental values determined in this study for Δeq26Mgbrucite-Mg2+ agree with the experimental values reported by Li et al. (2014). There is also an excellent agreement between the experimental values of this study and Li et al. (2014) with the density functional theory (DFT) estimates from Schott et al. (2016) for Δeq26MgMg2+-Mg(EDTA)2-. In contrast, the Mg isotope fractionation factor measured in this study between aqueous Mg2+ and both aqueous Mg sulphate or citrate species is significantly smaller than predictions from the ab initio calculations reported by Schott et al. (2016). The results of the present study confirm that the mineral-fluid equilibrium fractionation of Mg isotopes is strongly dependent on the identity of the inorganic or organic ligands present in the aqueous fluid and the nature of the complexes, (e.g. inner-sphere versus outer-sphere complexes), formed by magnesium with these ligands. Therefore, Mg speciation in natural fluids and the structure of aqueous Mg complexes have to be known for an accurate interpretation of Mg isotopic signatures in natural environments

Vortex-like kinematic signal, spirals, and beam smearing effect in the HD 142527 disk

International audienceVortices are one of the most promising mechanisms to locally concentrate millimeter dust grains and allow the formation of planetesimals through gravitational collapse. The outer disk around the binary system HD 142527 is known for its large horseshoe structure with azimuthal contrasts of ~3–5 in the gas surface density and of ~50 in the dust. Using 13 CO and C 18 O J = 3–2 transition lines, we detect kinematic deviations to the Keplerian rotation, which are consistent with the presence of a large vortex around the dust crescent, as well as a few spirals in the outer regions of the disk. Comparisons with a vortex model suggest velocity deviations up to 350 m s −1 after deprojection compared to the background Keplerian rotation, as well as an extension of ±40 au radially and ~200° azimuthally, yielding an azimuthal-to-radial aspect ratio of ~5. Another alternative for explaining the vortex-like signal implies artificial velocity deviations generated by beam smearing in association with variations of the gas velocity due to gas pressure gradients at the inner and outer edges of the circumbinary disk. The two scenarios are currently difficult to differentiate and, for this purpose, would probably require the use of multiple lines at a higher spatial resolution. The beam smearing effect, due to the finite spatial resolution of the observations and gradients in the line emission, should be common in observations of protoplanetary disks and may lead to misinterpretations of the gas velocity, in particular around ring-like structures