288 research outputs found
On the nature of veiling of classical T Tauri stars spectra in the near-IR spectral band
It is shown that the existence of a hot accretion spot on the surface of
classical T Tauri stars allows to explain observed veiling of their
photospheric spectrum not only in the visible but also in the near infrared
spectral band.Comment: 6 pages, 2 figures, accepted for publication in Astronomy Letter
Quasioptical modeling of wave beams with and without mode conversion: III. Numerical simulations of mode-converting beams
This work continues a series of papers where we propose an algorithm for
quasioptical modeling of electromagnetic beams with and without mode
conversion. The general theory was reported in the first paper of this series,
where a parabolic partial differential equation was derived for the field
envelope that may contain one or multiple modes with close group velocities. In
the second paper, we presented a corresponding code PARADE (PAraxial RAy
DEscription) and its test applications to single-mode beams. Here, we report
quasioptical simulations of mode-converting beams for the first time. We also
demonstrate that PARADE can model splitting of two-mode beams. The numerical
results produced by PARADE show good agreement with those of one-dimensional
full-wave simulations and also with conventional ray tracing (to the extent
that one-dimensional and ray-tracing simulations are applicable)
Mode conversion in cold low-density plasma with a sheared magnetic field
A theory is proposed that describes mutual conversion of two electromagnetic
modes in cold low-density plasma, specifically, in the high-frequency limit
where the ion response is negligible. In contrast to the classic
(Landau--Zener-type) theory of mode conversion, the region of resonant coupling
in low-density plasma is not necessarily narrow, so the coupling matrix cannot
be approximated with its first-order Taylor expansion; also, the initial
conditions are set up differently. For the case of strong magnetic shear, a
simple method is identified for preparing a two-mode wave such that it
transforms into a single-mode wave upon entering high-density plasma. The
theory can be used for reduced modeling of wave-power input in fusion plasmas.
In particular, applications are envisioned in stellarator research, where the
mutual conversion of two electromagnetic modes near the plasma edge is a known
issue
Results of magnetic field measurements in young stars DO Tau, DR Tau, DS Tau
Results of measurements of the longitudinal magnetic field in a hot accretion
spot in three classical T Tauri stars (CTTS) are presented. The magnetic field
in the formation region of the narrow component of the emission line HeI 5876
{\AA} was found for each star in our sample at a level of more than 2\sigma. In
case of DS Tau we have found the field in the narrow components of NaI D lines,
which was equal to +0.8 \pm 0.3 kG, i.e. it was equal to the field measured on
the narrow component of HeI 5876 \AA. Our results indicate that the magnetic
field in the hot spots can be studied for CTTS down to 13^m that allow in the
future to double a number of CTTS with measured field in the accretion zone.Comment: 12 pages, 2 figures, accepted for publication in Astrophysical
Bulleti
A non-LTE modeling of narrow emission components of He and Ca lines in optical spectra of CTTS
A spectrum of a hot spot, produced by radiation of accretion shock at T Tauri
star's surface, has been calculated taking into account non-LTE effects for
HeI, HeII, CaI and CaII, using LTE-calculations of spot's atmospheric
structure, calculated by Dodin & Lamzin (2012). Assuming that pre-shock gas
number density N_0 and its velocity V_0 are the same across the accretion
column, we calculated spectra of a system "star + round spot" for a set of N_0,
V_0 values and parameters, which characterized the star and the spot.
It has been shown that theoretical spectra with an appropriate choice of the
parameters reproduce well observed veiling of photospheric absorption lines in
optical band as well as profiles and intensities of so-called narrow components
of HeII and CaI emission lines in spectra of 9 stars. We found that the
accreted gas density N_0>10^{12} cm^{-3} for all considered stars except DK
Tau. Observed spectra of 8 stars were succesfully fitted, asuming solar
abundance of calcium, but it appeared possible to fit TW Hya spectrum only
under assumption that calcium abundance in accreted gas was three times less
than solar. We derive spot's parameters by comparison of theoretical and
observed spectra, normalised to continuum level, so our results are independent
on unknown value of interstellar extinction.
We have found that the predicted flux in CaII lines is less than observed
one, but this discrepancy can be resolved if not only high-density but also
lower density gas falls onto the star. Theoretical equivalent widths as well as
relative intensities of He\,I subordinate lines disagree significantly with
observations, presumbly due to a number of reasons: necessity to take into
account non-LTE thermal structure of upper layers of a hot spot, poorly known
collisional atomic data for HeI upper levels and inhomogeneity of the hot spot.Comment: accepted by Astronomy Letter
Quasioptical modeling of wave beams with and without mode conversion: I. Basic theory
This work opens a series of papers where we develop a general quasioptical
theory for mode-converting electromagnetic beams in plasma and implement it in
a numerical algorithm. Here, the basic theory is introduced. We consider a
general quasimonochromatic multi-component wave in a weakly inhomogeneous
linear medium with no sources. For any given dispersion operator that governs
the wave field, we explicitly calculate the approximate operator that governs
the wave envelope to the second order in the geometrical-optics
parameter. Then, we further simplify this envelope operator by assuming that
the gradient of transverse to the local group velocity is much larger
than the corresponding parallel gradient. This leads to a parabolic
differential equation for ("quasioptical equation") in the basis of the
geometrical-optics polarization vectors. Scalar and mode-converting vector
beams are described on the same footing. We also explain how to apply this
model to electromagnetic waves in general. In the next papers of this series,
we report successful quasioptical modeling of radiofrequency wave beams in
magnetized plasma based on this theory.Comment: This is the first part of the three-paper series that also includes
arXiv:1903.01357 and arXiv:1903.0136
Quasioptical modeling of wave beams with and without mode conversion: II. Numerical simulations of single-mode beams
This work continues a series of papers where we propose an algorithm for
quasioptical modeling of electromagnetic beams with and without mode
conversion. The general theory was reported in the first paper of this series,
where a parabolic partial differential equation was derived for the field
envelope that may contain one or multiple modes with close group velocities.
Here, we present a corresponding code PARADE (PAraxial RAy DEscription) and its
test applications to single-mode beams in vacuum and also in inhomogeneous
magnetized plasma. The numerical results are compared, respectively, with
analytic formulas from Gaussian-beam optics and also with cold-plasma ray
tracing. Quasioptical simulations of mode-converting beams are reported in the
next, third paper of this series
Orbital motions and light curves of young binaries XZ Tau and VY Tau
The results of our speckle interferometric observations of young binaries VY
Tau and XZ Tau are presented. For the first time, we found a relative
displacement of VY Tau components as well as a preliminary orbit for XZ Tau. It
appeared that the orbit is appreciably non-circular and is inclined by from the plane of the sky. It means that the rotation axis of XZ
Tau A and the axis of its jet are significantly non-perpendicular to the
orbital plane. We found that the average brightness of XZ Tau had been
increasing from the beginning of the last century up to the mid-thirties and
then it decreased by mag. The maximal brightness has been
reached significantly later on the time of periastron passage. The total
brightness of XZ Tau's components varied in a non-regular way from 1970 to 1985
when eruptions of hot gas from XZ Tau A presumably had occurred. In the early
nineties the variations became regular following which a chaotic variability
had renewed. We also report that a flare activity of VY Tau has resumed after
40 yr pause, parameters of the previous and new flares are similar, and the
flares are related with the A component.Comment: 18 pages, 4 figures, accepted for publication in the Astronomy
Letter
The jet of the young star RW Aur A and related problems
Comparing the images of the jet of the young star RW Aur A, separated by a
period of 21.3 years, we found that the outermost jet's knots have emerged
yr ago. We argue that at that moment the jet itself has appeared
and intensive accretion onto the star has began due to the rearrangement of its
protoplanetary disk structure caused by the tidal effect of the companion RW
Aur B. More precisely, we assume that the increase of accretion is a response
to changing conditions in the outer disk regions, which followed after the
sound wave, generated by these changes, crossed the disk in a radial direction.
The difference in the parameters of the blue and red lobes of the RW Aur A jet,
according to our opinion, is a result of the asymmetric distribution of the
circumstellar matter above and below the disk, due to a fly-by of the
companion. It was found from the analysis of RW Aur historical light curve that
deep and long days) dimmings of RW Aur A observed after 2010
yr, had no analogues in the previous 110 years. We also associate the change in
the character of the photometric variability of the star with the rearrangement
of the structure of inner a.u.) regions of its protoplanetary disk and
discuss why these changes began only 350 years after the beginning of the
active accretion phase.Comment: accepted by The Astrophysical Bulleti
Analysis of colour and polarimetric variability of RW Aur A in 2010-2018
Results of photometry and polarimetry of a young star RW
Aur A observed during unprecedented long and deep (up to
mag) dimming events in 2010-11 and 2014-18 are presented. The polarization
degree of RW Aur A at this period has reached 30 per cent in the band.
As in the case of UX Ori type stars (UXORs), the so-called 'bluing effect' in
the colour-magnitude versus diagrams of the
star and a strong anticorrelation between and brightness were observed. But
the duration and the amplitude of the eclipses as well as the value and
orientation of polarization vector in our case differ significantly from that
of UXORs. We concluded that the dimmings of RW Aur A occurred due to eclipses
of the star and inner regions of its disc by the axisymmetric dust structure
located above the disc and created by the disc wind. Taking into account both
scattering and absorption of stellar light by the circumstellar dust, we
explain some features of the light curve and the polarization degree -
magnitude dependence. We found that near the period of minimal brightness
mass-loss rate of the dusty wind was M yrComment: 19 pages, 17 figures, accepted for publication in MNRA
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