1,970 research outputs found
Three-dimensional simulations of rotationally-induced line variability from a Classical T Tauri star with a misaligned magnetic dipole
We present three-dimensional (3-D) simulations of rotationally induced line
variability arising from complex circumstellar environment of classical T Tauri
stars (CTTS) using the results of the 3-D magnetohydrodynamic (MHD) simulations
of Romanova et al., who considered accretion onto a CTTS with a misaligned
dipole magnetic axis with respect to the rotational axis. The density, velocity
and temperature structures of the MHD simulations are mapped on to the
radiative transfer grid, and corresponding line source function and the
observed profiles of neutral hydrogen lines (H-beta, Pa-beta and Br-gamma) are
computed using the Sobolev escape probability method. We study the dependency
of line variability on inclination angles (i) and magnetic axis misalignment
angles (Theta). By comparing our models with the Pa-beta profiles of 42 CTTS
observed by Folha & Emerson, we find that models with a smaller misaligngment
angle (Theta<~15 deg.) are more consistent with the observations which show
that majority of Pa-beta are rather symmetric around the line centre. For a
high inclination system with a small dipole misalignment angle (Theta ~ 15
deg.), only one accretion funnel (on the upper hemisphere) is visible to an
observer at any given rotational phase. This can cause an anti-correlation of
the line equivalent width in the blue wing (v0)
over a half of a rotational period, and a positive correlation over other half.
We find a good overall agreement of the line variability behaviour predicted by
our model and those from observations. (Abridged)Comment: 15 pages, 13 figures. Accepted for publication in MNRAS. A version
with full resolution figures can be downloaded from
http://www.physics.unlv.edu/~rk/preprint/inclined_dipole.pd
Testing the Disk-Locking Paradigm: An Association Between U-V Excess and Rotation in NGC 2264
We present some results from a UVI photometric study of a field in the young
open cluster NGC 2264 aimed, in part, at testing whether accretion in pre-main
sequence stars is linked to rotation. We confirm that U-V excess is well
correlated with H-alpha equivalent width for the stars in our sample. We show
that for the more massive stars in the cluster sample (roughly 0.4-1.2 M_sun)
there is also a significant association between U-V excess and rotation, in the
sense that slow rotators are more likely to show excess U-band emission and
variability. This constitutes significant new evidence in support of the
disk-locking paradigm.Comment: Accepted by ApJ Letter
Modeling T Tauri Winds from He I 10830 Profiles
The high opacity of He I 10830 makes it an exceptionally sensitive probe of
the inner wind geometry of accreting T Tauri stars. In this line blueshifted
absorption below the continuum results from simple scattering of stellar
photons, a situation which is readily modeled without definite knowledge of the
physical conditions and recourse to multi-level radiative transfer. We present
theoretical line profiles for scattering in two possible wind geometries, a
disk wind and a wind emerging radially from the star, and compare them to
observed He I 10830 profiles from a survey of classical T Tauri stars. The
comparison indicates that subcontinuum blueshifted absorption is characteristic
of disk winds in ~30% of the stars and of stellar winds in ~40%. We further
conclude that for many stars the emission profile of helium likely arises in
stellar winds, increasing the fraction of accreting stars inferred to have
accretion-powered stellar winds to ~60%. Stars with the highest disk accretion
rates are more likely to have stellar wind than disk wind signatures and less
likely to have redshifted absorption from magnetospheric funnel flows. This
suggests the possibility that when accretion rates are high, disks can extend
closer to the star, magnetospheric accretion zones can be reduced in size and
conditions arise that favor radially outflowing stellar winds.Comment: 41 pages, 11 figures. Accepted by Astrophysical Journa
Accretion dynamics in the classical T Tauri star V2129 Oph
We analyze the photometric and spectroscopic variability of the classical T
Tauri star V2129 Oph over several rotational cycles to test the dynamical
predictions of magnetospheric accretion models. The photometric variability and
the radial velocity variations in the photospheric lines can be explained by
rotational modulation due to cold spots, while the radial velocity variations
of the He I (5876 \AA) line and the veiling variability are due to hot spot
rotational modulation. The hot and cold spots are located at high latitudes and
about the same phase, but the hot spot is expected to sit at the chromospheric
level, while the cold spot is at the photospheric level. Using the
dipole+octupole magnetic-field configuration previously proposed in the
literature for the system, we compute 3D MHD magnetospheric simulations of the
star-disk system. We use the simulation's density, velocity and scaled
temperature structures as input to a radiative transfer code, from which we
calculate theoretical line profiles at all rotational phases. The theoretical
profiles tend to be narrower than the observed ones, but the qualitative
behavior and the observed rotational modulation of the H\alpha and H\beta
emission lines are well reproduced by the theoretical profiles. The
spectroscopic and photometric variability observed in V2129 Oph support the
general predictions of complex magnetospheric accretion models with
non-axisymmetric, multipolar fields.Comment: Accepted by Astronomy and Astrophysic
Strain measurement at the knee ligament insertion sites
We describe the modification of an existing method of ligament strain measurement at the knee joint in detail. At ten fresh joint specimens we used that technique where strain gauges are attached to the ligamentous insertions and origins. We both improved the preparation of the attachment site and the application of the strain gauges. In a special apparatus the specimens were moved from 0degrees extension to 100degrees flexion while simulating muscle strength and axial force. Testing was performed at the posterior cruciate ligament with both intact and transsected anterior cruciate ligament. In contrast to other existing techniques it does not affect the motion of the joint or the integrity and the function of the ligaments. Unlike the original description of that method we could register a loading behaviour of the posterior cruciate ligament that is similar to those reported in the literature
MHD Simulations of Magnetospheric Accretion, Ejection and Plasma-field Interaction
We review recent axisymmetric and three-dimensional (3D) magnetohydrodynamic
(MHD) numerical simulations of magnetospheric accretion, plasma-field
interaction and outflows from the disk-magnetosphere boundary.Comment: 11 pages, 8 figures, conference proceedings: "Physics at the
Magnetospheric Boundary", Geneva, Switzerland, 25-28 June, 201
Facing the wind of the pre-FUor V1331 Cyg
The mass outflows in T Tauri stars (TTS) are thought to be an effective
mechanism to remove angular momentum during the pre-main-sequence contraction
of a low-mass star. The most powerful winds are observed at the FUor stage of
stellar evolution. V1331 Cyg has been considered as a TTS at the pre-FUor
stage. We analyse high-resolution spectra of V1331 Cyg collected in 1998-2007
and 20-d series of spectra taken in 2012. For the first time the photospheric
spectrum of the star is detected and stellar parameters are derived: spectral
type G7-K0 IV, mass 2.8 Msun, radius 5 Rsun, vsini < 6 km/s. The photospheric
spectrum is highly veiled, but the amount of veiling is not the same in
different spectral lines, being lower in weak transitions and much higher in
strong transitions. The Fe II 5018, Mg I 5183, K I 7699 and some other lines of
metals are accompanied by a `shell' absorption at radial velocity of about -240
km/s. We show that these absorptions form in the post-shock gas in the jet,
i.e. the star is seen though its jet. The P Cyg profiles of H-alpha and H-beta
indicate the terminal wind velocity of about 500 km/s, which vary on
time-scales from several days to years. A model of the stellar wind is
developed to interpret the observations. The model is based on calculation of
hydrogen spectral lines using the radiative transfer code TORUS. The observed
H-alpha and H-beta line profiles and their variability can be well reproduced
with a stellar wind model, where the mass-loss rate and collimation (opening
angle) of the wind are variable. The changes of the opening angle may be
induced by small variability in magetization of the inner disc wind. The
mass-loss rate is found to vary within (6-11)x10^{-8} Msun/yr, with the
accretion rate of 2.0x10^{-6} Msun/yr.Comment: 11 pages, 12 figures; accepted for publication in MNRAS.
Typographical errors have been corrected after the proof stag
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