6,910 research outputs found
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
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&
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
Line and continuum radiative transfer modelling of AA Tau
We present photometric and spectroscopic models of the Classical T Tauri star
AA Tau. Photometric and spectroscopic variability present in observations of AA
Tau is attributed to a magnetically induced warp in the accretion disc,
periodically occulting the photosphere on an 8.2--day timescale. Emission line
profiles show signatures of both infall, attributed to magnetospherically
accreting material, and outflow. Using the radiative transfer code TORUS, we
have investigated the geometry and kinematics of AA Tau's circumstellar disc
and outflow, which is modelled here as a disc wind. Photometric models have
been used to constrain the aspect ratio of the disc, the offset angle of the
magnetosphere dipole with respect to the stellar rotation axis, and the inner
radius of the circumstellar disc. Spectroscopic models have been used to
constrain the wind and magnetosphere temperatures, wind acceleration parameter,
and mass loss rate. We find observations are best fitted by models with a mass
accretion rate of M yr, a dipole offset of
between and , a magnetosphere that truncates the disc from
5.2 to 8.8 R, a mass-loss-rate to accretion-rate ratio of ~ 0.1, a
magnetosphere temperature of 8500 -- 9000 K, and a disc wind temperature of
8000 K.Comment: 22 pages, 32 figures, 4 tables. Accepted by MNRAS. V3: Corrected typ
Diamagnetic Blob Interaction Model of T Tauri Variability
Assuming a diamagnetic interaction between a stellar-spot originated
localized magnetic field and gas blobs in the accretion disk around a T- Tauri
star, we show the possibility of ejection of such blobs out of the disk plane.
Choosing the interaction radius and the magnetic field parameters in a suitable
way gives rise to closed orbits for the ejected blobs. A stream of matter
composed of such blobs, ejected on one side of the disk and impacting on the
other, can form a hot spot at a fixed position on the disk (in the frame
rotating with the star). Such a hot spot, spread somewhat by disk shear before
cooling, may be responsible in some cases for the lightcurve variations
observed in various T-Tauri stars over the years. An eclipse-based mechanism
due to stellar obscuration of the spot is proposed. Assuming high disk
inclination angles it is able to explain many of the puzzling properties of
these variations. By varying the field parameters and blob initial conditions
we obtain variations in the apparent angular velocity of the hot spot,
producing a constantly changing period or intermittent periodicity
disappearance in the models.Comment: 6 pages, 4 figures, aas2pp4 styl
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
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