17,113 research outputs found
The timescale for giant planet formation : constraints from the rotational evolution of exoplanet host stars
The timescale over which planets may form in the circumstellar disks of young
stars is one of the main issues of current planetary formation models. We
present here new constraints on planet formation timescales derived from the
rotational evolution of exoplanet host stars.Comment: SF2A 2008, Journees de l'Astrophysique Francaise, Paris : France
(2008
The magnetospheric accretion/ejection process in young stellar objects: open issues and perspectives
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.Comment: Summary talk, 4 pages, to appear in the Proceedings of "Physics at
the Magnetospheric Boundary", Geneva, 25-28 June 201
Results of the ROTOR-program. I. The long-term photometric variability of classical T Tauri stars
We present a unique, homogeneous database of photometric measurements for
Classical T Tauri stars extending up to 20 years. The database contains more
than 21,000 UBVR observations of 72 CTTs. All the data were collected within
the framework of the ROTOR-program at Mount Maidanak Observatory (Uzbekistan)
and together they constitute the longest homogeneous, accurate record of TTS
variability ever assembled. We characterize the long term photometric
variations of 49 CTTs with sufficient data to allow a robust statistical
analysis and propose an empirical classification scheme. Several patterns of
long term photometric variability are identified. The most common pattern,
exhibited by a group of 15 stars which includes T Tau itself, consists of low
level variability (Delta(V)<=0.4mag) with no significant changes occurring from
season to season over many years. A related subgroup of 22 stars exhibits a
similar stable long term variability pattern, though with larger amplitudes (up
to Delta(V)~1.6 mag). Besides these representative groups, we identify three
smaller groups of 3-5 stars each which have distinctive photometric properties.
The long term variability of most CTTs is fairly stable and merely reflects
shorter term variability due to cold and hot surface spots. Only a small
fraction of CTTs undergo significant brightness changes on the long term
(months, years), which probably arise from slowly varying circumstellar
extinction.Comment: 16 pages, 11 figures. Astron. Astrophys., in pres
Improved angular momentum evolution model for solar-like stars II. Exploring the mass dependence
We developed angular momentum evolution models for 0.5 and 0.8
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.Comment: 17 pages, 11 figures, accepted for publication in A&
Improved angular momentum evolution model for solar-like stars
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.Comment: 16 pages, 5 figures, 4 table, accepted for publication by A&A. New
version that include the linguistic correctio
Hall effect in the normal state of high Tc cuprates
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<ES they are electron-like, giving opposite contributions to RH. We are abble
to fit the experimental results for a wide range of hole doping, and to fit the
universal curve. For us kb*T0 is simply EF-ES, where EF is the Fermi level
varying with the doping.Comment: 7 pages, 11 figure
Investigating the rotational evolution of young, low mass stars using Monte Carlo simulations
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)Comment: 11 pages, 14 figures, accepted for publication in A&
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