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Search Results: 1 - 10 of 297558 matches for " J. Bouvier "
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The magnetospheric accretion/ejection process in young stellar objects: open issues and perspectives
J. Bouvier
Physics , 2013, DOI: 10.1051/epjconf/20136409001
Abstract: This summary talk aims at highlighting some of the remaining open issues regarding the physics of the magnetospheric accretion/ejection process in young stellar objects (YSOs). It lists a number of questions related to YSOs magnetic fields and accretion regimes, the structure and properties of accretion shocks, the origin of multiple outflow components, and the impact of the star-disk magnetic interaction onto early angular momentum evolution.
Observational studies of stellar rotation
J. Bouvier
Physics , 2013, DOI: 10.1051/eas/1362005
Abstract: This course reviews the rotational properties of non-degenerate stars as observed from the protostellar stage to the end of the main sequence. It includes an introduction to the various observational techniques used to measure stellar rotation. Angular momentum evolution models developed over the mass range from the substellar domain to high-mass stars are briefly discussed
Transport properties of electrons and holes in a CuO2 layer doped by field effect
J. Bok,J. Bouvier
Physics , 2002,
Abstract: We propose a model for explaining the recent results obtained by J. Schon et al (1,2) on transport properties of electrons and holes in one plane of CuO2 in a layered cuprate (CaCuO2) where the carriers are created using a field effect transistor (FET). We use the known band structure of a CuO2 plane showing a van Hove singularity (vHs). When the energy of the hole lies near the vHs a variation of the resistivity linear with temperature (T) is calculated and when the FL lies far from the vHs, a quadratic law is obtained at low temperature and becomes linear at higher T. We find that the transition temperature T* is simply related to the distance between the Fermi level and the vHs. The behavior of the Hall coefficient is explained by the existence of hole type and electron type orbits in hole doped CuO2 planes. The fit with the experimental results is excellent.
Hall effect in the normal state of high Tc cuprates
J. Bok,J. Bouvier
Physics , 2003, DOI: 10.1016/j.physc.2003.12.019
Abstract: We propose a model for explaining the dependence in temperature of the Hall effect of high Tc cuprates in the normal state in various materials. They all show common features: a decrease of the Hall coefficient RH with temperature and a universal law, when plotting RH(T)/RH(T0) versus T/T0, where T0 is defined from experimental results. This behaviour is explained by using the well known electronic band structure of the CuO2 plane, showing saddle points at the energies ES in the directions (0,+/-pi) and (+/-pi,0). We remark that in a magnetic field, for energies E>ES the carrier orbits are hole-like and for E
Investigating the rotational evolution of young, low mass stars using Monte Carlo simulations
M. J. Vasconcelos,J. Bouvier
Physics , 2015, DOI: 10.1051/0004-6361/201525765
Abstract: We investigate the rotational evolution of young stars through Monte Carlo simulations. We simulate 280,000 stars, each of which is assigned a mass, a rotational period, and a mass accretion rate. The mass accretion rate depends on mass and time, following power-laws indices 1.4 and -1.5, respectively. A mass-dependent accretion threshold is defined below which a star is considered as diskless, which results in a distribution of disk lifetimes that matches observations. Stars are evolved at constant angular spin rate while accreting and at constant angular momentum when they become diskless. We recover the bimodal period distribution seen in several young clusters. The short period peak consists mostly of diskless stars and the long period one is mainly populated by accreting stars. Both distributions present a long tail towards long periods and a population of slowly rotating diskless stars is observed at all ages. We reproduce the observed correlations between disk fraction and spin rate, as well as between IR excess and rotational period. The period-mass relation we derive from the simulations exhibits the same global trend as observed in young clusters only if we release the disk locking assumption for the lowest mass stars. We find that the time evolution of median specific angular momentum follows a power law index of -0.65 for accreting stars and of -0.53 for diskless stars, a shallower slope that results from a wide distribution of disk lifetimes. Using observationally-documented distributions of disk lifetimes, mass accretion rates, and initial rotation periods, and evolving an initial population from 1 to 12 Myr, we reproduce the main characteristics of pre-main sequence angular momentum evolution, which supports the disk locking hypothesis. (abridged)
Protostellar spin-down: a planetary lift?
J. Bouvier,D. Cébron
Physics , 2015, DOI: 10.1093/mnras/stv1824
Abstract: When they first appear in the HR diagram, young stars rotate at a mere 10\% of their break-up velocity. They must have lost most of the angular momentum initially contained in the parental cloud, the so-called angular momentum problem. We investigate here a new mechanism by which large amounts of angular momentum might be shed from young stellar systems, thus yielding slowly rotating young stars. Assuming that planets promptly form in circumstellar disks and rapidly migrate close to the central star, we investigate how the tidal and magnetic interactions between the protostar, its close-in planet(s), and the inner circumstellar disk can efficiently remove angular momentum from the central object. We find that neither the tidal torque nor the variety of magnetic torques acting between the star and the embedded planet are able to counteract the spin up torques due to accretion and contraction. Indeed, the former are orders of magnitude weaker than the latter beyond the corotation radius and are thus unable to prevent the young star from spinning up. We conclude that star-planet interaction in the early phases of stellar evolution does not appear as a viable alternative to magnetic star-disk coupling to understand the origin of the low angular momentum content of young stars.
Proper motion of very low mass stars and brown dwarfs in the Pleiades cluster
E. Moraux,J. Bouvier,J. R. Stauffer
Physics , 2000, DOI: 10.1051/0004-6361:20000414
Abstract: We report proper motion measurements for 25 very-low mass (VLM) star and brown dwarf (BD) candidates of the Pleiades cluster previously identified by Bouvier et al. (1998). Proper motions are measured with an accuracy of 9 mas/yr, compared to an expected tangential motion of about 50 mas/yr for Pleiades members. Of the 25 candidates, 15 have a membership probability of 95% or more and 7 are rejected as being field dwarfs. The 3 remaining candidates exhibit independent evidence for membership (lithium absorption or long-term proper motion). From the firm identification of Pleiades VLM and BD members, the cluster's substellar mass function is revised to dN/dM \propto M^{-0.5} in the mass range from 0.04 to 0.3 M_solar.
Perspectives for the study of gas in protoplanetary disks and accretion/ejection phenomena in young stars with the near-IR spectrograph SPIROU at the CFHT
A. Carmona,J. Bouvier,X. Delfosse
Physics , 2013,
Abstract: Near-IR atomic and molecular transitions are powerful tools to trace the warm and hot gas in the circumstellar environment of young stars. Ro-vibrational transitions of H2 and H2O, and overtone transitions of CO at 2 micron centered at the stellar velocity trace hot (T~1500 K) gas in the inner few AU of protoplanetary disks. H2 near-IR lines displaying a blueshift of a few km/s probe molecular disk winds. H2 lines presenting blueshifts of hundreds of km/s reveal hot shocked gas in jets. Atomic lines such as the HeI line at 10830 A and the Hydrogen Paschen beta and Brakett gamma lines trace emission from accretion funnel flows and atomic disk winds. Bright forbidden atomic lines in the near-IR of species such as [Fe II], [N I], [S I], [S II], and [C I] trace atomic and ionized material in jets. The new near-IR high resolution spectrograph SPIROU planned for the Canada France Hawaii Telescope will offer the unique capability of combining high-spectral resolution (R~75000) with a large wavelength coverage (0.98 to 2.35 micron) in one single exposure. This will provide us with the means of probing accretion funnel flows, winds, jets, and hot gas in the inner disk simultaneously. This opens the exiting possibility of investigating their combined behavior in time by the means of monitoring observations and systematic surveys. SPIROU will be a powerful tool to progress our understanding of the connexion between the accretion/ejection process, disk evolution, and planet formation.
Improved angular momentum evolution model for solar-like stars II. Exploring the mass dependence
Florian Gallet,Jér?me Bouvier
Physics , 2015, DOI: 10.1051/0004-6361/201525660
Abstract: We developed angular momentum evolution models for 0.5 and 0.8 $M_{\odot}$ stars. The parametric models include a new wind braking law based on recent numerical simulations of magnetised stellar winds, specific dynamo and mass-loss rate prescriptions, as well as core/envelope decoupling. We compare model predictions to the distributions of rotational periods measured for low mass stars belonging to star forming regions and young open clusters. Furthermore, we explore the mass dependence of model parameters by comparing these new models to the solar-mass models we developed earlier. Rotational evolution models are computed for slow, median, and fast rotators at each stellar mass. The models reproduce reasonably well the rotational behaviour of low-mass stars between 1 Myr and 8-10 Gyr, including pre-main sequence to zero-age main sequence spin up, prompt zero-age main sequence spin down, and early-main sequence convergence of the surface rotation rates. Fast rotators are found to have systematically shorter disk lifetimes than moderate and slow rotators, thus enabling dramatic pre-main sequence spin up. They also have shorter core-envelope coupling timescales, i.e., more uniform internal rotation. As to the mass dependence, lower mass stars require significantly longer core-envelope coupling timescale than solar-type ones, which results in strong differential rotation developing in the stellar interior on the early main sequence. Lower mass stars also require a weaker braking torque to account for their longer spin down timescale on the early main sequence, while they ultimately converge towards lower rotational velocities than solar-type stars on the longer term due to their reduced moment of inertia. We also find evidence that the mass-dependence of the wind braking efficiency may be related to a change of the magnetic topology in lower mass stars.
Improved angular momentum evolution model for solar-like stars
Florian Gallet,Jér?me Bouvier
Physics , 2013, DOI: 10.1051/0004-6361/201321302
Abstract: We present new models for the rotational evolution of solar-like stars between 1 Myr and 10 Gyr with the aim to reproduce the distributions of rotational periods observed for star forming regions and young open clusters within this age range. The models include a new wind braking law based on recent numerical simulations of magnetized stellar winds and specific dynamo and mass-loss prescriptions are adopted to tie angular momentum loss to angular velocity. The model additionally assume constant angular velocity during the disk accretion phase and allow for decoupling between the radiative core and the convective envelope as soon as the former develops. We have developed rotational evolution models for slow, median and fast rotators with initial periods of 10, 7, and 1.4d, respectively. The models reproduce reasonably well the rotational behaviour of solar-type stars between 1 Myr and 4.5 Gyr, including PMS to ZAMS spin up, prompt ZAMS spin down, and the early-MS convergence of surface rotation rates. We find the model parameters accounting for the slow and median rotators are very similar to each other, with a disk lifetime of 5 Myr and a core-envelope coupling timescale of 28-30 Myr. In contrast, fast rotators have both shorter disk lifetime (2.5 Myr) and core-envelope coupling timescale (12 Myr). We emphasize that these results are highly dependent on the adopted braking law. We also report a tentative correlation between initial rotational period and disk lifetime, which suggests that protostellar spin-down by massive disks in the embedded phase is at the origin of the initial dispersion of rotation rates in young stars. We conclude that this class of semi-empirical models successfully grasp the main trends of the rotational behaviour of solar-type stars as they evolve and make specific predictions that may serve as a guide for further development.
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