56 research outputs found
Time-dependent spectral-feature variations of stars displaying the B[e] phenomenon; I. V2028 Cyg
We present results of nearly six years of spectroscopic observations of the
B[e] star V2028 Cyg. The presence of the cold-type absorption lines combined
with a hot-type spectrum indicate the binarity of this object. Since B[e] stars
are embedded in an extended envelope, the usage of common stellar atmosphere
models for the analysis is quite inappropriate. Therefore, we focus on the
analysis of the long-term spectral line variations in order to determine the
nature of this object. We present the time dependences of the equivalent width
and radial velocities of the H alpha line, [O I] 6300 A, Fe II 6427, 6433, and
6456 A lines. The bisector variations and line intensities are shown for the H
alpha line. The radial velocities are also measured for the absorption lines of
the K component. No periodic variation is found. The observed data show
correlations between the measured quantities, which can be used in future
modelling
2.5-MHD models of circumstellar discs around FS~CMa post-mergers : II. Stationary accretion stage
We study the star-disc interaction in the presence of the strong magnetic
field () of a slowly rotating star. This situation describes a
post-merger of the spectral type B and has not been previously investigated. We
perform a set of resistive and viscosity -magnetohydrodynamical
simulations using the PLUTO code. Based on our previous work, we consider the
initial gas disc density since it
describes the conditions around IRAS 17449+2320 well. We find that the fall of
gas towards the star occurs in the mid-plane, and remarkably, intermittent
backflow takes place in the mid-plane in all of our models for
. However, we do not rule out that the funnel effect may occur
and cause the accretion closer to the poles. Also, when larger values of
viscosity () and stellar rotation rate () are
considered, we find that the disc exhibits a thickening which is characteristic
of FS~CMa-type stellar objects. Additionally, we find that the poloidal
magnetic field lines twist over short periods of time, leading to magnetic
reconnection causing coronal heating that could explain the presence of the
Raman lines found observationally in several FS~CMa stars. Lastly, we find the
formation of several knots in the magnetic field lines near and in the
mid-plane of the disc which produce perturbations in the density and velocity
components, as well as the formation of shallow gaps whose position depends on
the inflation of the magnetic field lines.Comment: 10 pages, 9 figure
2.5-MHD models of circumstellar discs around FS~CMa post-mergers : I. Non-stationary accretion stage
We investigate the dynamic evolution of gaseous region around FS~CMa
post-mergers. Due to the slow rotation of a central B-type star, the dynamics
is driven mainly by the magnetic field of the central star. Recent observations
have allowed us to set a realistic initial conditions such as, the magnetic
field value (), the mass of the central star
(), and the initial disc density
. We
use the PLUTO code to perform 2.5D-MHD simulations of thin and thick discs
models. Especially relevant for the interpretation of the observed properties
of FS~CMa post-mergers are the results for low-density discs, in which we find
formation of a jet emerging from inner edge of the disc, as well as the
formation of the so called "hot plasmoid" in the corona region. Jets are
probably detected as discrete absorption components in the resonance lines of
FS~CMa stars. Moreover, the magnetic field configuration in the low-density
plasma region, favors the appearance of magnetocentrifugal winds from the disc.
The currents toward the star created by the magnetic field may explain
accidentally observed material infall. The disc structure is significantly
changed due to the presence of the magnetic field. The magnetic field is also
responsible for the formation of a hot corona as observed in several FS~CMa
stars through the Raman lines. Our results are valid for all magnetic stars
surrounded by a low density plasma, i.e., some of stars showing the B[e]
phenomenon.Comment: 12 pages, 6 figure
Principal component analysis - an efficient tool for variable stars diagnostics
We present two diagnostic methods based on ideas of Principal Component
Analysis and demonstrate their efficiency for sophisticated processing of
multicolour photometric observations of variable objects.Comment: 8 pages, 4 figures. Published alread
Multitechnique testing of the viscous decretion disk model I. The stable and tenuous disk of the late-type Be star CMi
The viscous decretion disk (VDD) model is able to explain most of the
currently observable properties of the circumstellar disks of Be stars.
However, more stringent tests, focusing on reproducing multitechnique
observations of individual targets via physical modeling, are needed to study
the predictions of the VDD model under specific circumstances. In the case of
nearby, bright Be star CMi, these circumstances are a very stable
low-density disk and a late-type (B8Ve) central star. The aim is to test the
VDD model thoroughly, exploiting the full diagnostic potential of individual
types of observations, in particular, to constrain the poorly known structure
of the outer disk if possible, and to test truncation effects caused by a
possible binary companion using radio observations. We use the Monte Carlo
radiative transfer code HDUST to produce model observables, which we compare
with a very large set of multitechnique and multiwavelength observations that
include ultraviolet and optical spectra, photometry covering the interval
between optical and radio wavelengths, optical polarimetry, and optical and
near-IR (spectro)interferometry. Due to the absence of large scale variability,
data from different epochs can be combined into a single dataset. A parametric
VDD model with radial density exponent of = 3.5, which is the canonical
value for isothermal flaring disks, is found to explain observables typically
formed in the inner disk, while observables originating in the more extended
parts favor a shallower, = 3.0, density falloff. Modeling of radio
observations allowed for the first determination of the physical extent of a Be
disk (35 stellar radii), which might be caused by a binary
companion. Finally, polarization data allowed for an indirect measurement of
the rotation rate of the star, which was found to be , i.e.,
very close to critical.Comment: 19 pages (35 including online material), 17 figures, 2 online
figures, 2 online tables with dat
Time-dependent spectral-feature variations of stars displaying the B[e] phenomenon III. HD 50138
We analyse spectroscopic observations of the B[e] star HD 50138 (MWC 158,
V743 Mon, or IRAS 06491-0654), a member of the FS CMa group, obtained over the
last twenty years. Four different epochs are identified in the observational
data, where the variability of the spectral features is substantially
different. Additionally, two long periods of (3 000 +/- 500) and (5 000 +/-
1000) days are found in the variations of the equivalent widths of the H alpha
and [OI] 6300 A lines and radial velocities of the H alpha line violet peak.
Modest signatures of a regular period of ~34 days in the radial velocities of
the H alpha red peak and H beta central depression are found in the season
2013/2014. The H alpha V/R changes indicate a periodicity of ~50 days. The
correlations between individual spectral features significantly restricts the
model of the object and suggest that it is most likely a binary system with a
highly distorted disc with spiral arms around the primary component. At the
same time, no obvious signs of the secondary component has been found in the
object's spectrum
Radiative transfer in moving media II. Solution of the radiative transfer equation in axial symmetry
A new method for the formal solution of the 2D radiative transfer equation in
axial symmetry in the presence of arbitrary velocity fields is presented. The
combination of long and short characteristics methods is used to solve the
radiative transfer equation. We include the velocity field in detail using the
Local Lorentz Transformation. This allows us to obtain a significantly better
description of the photospheric region, where the gradient of the global
velocity is too small for the Sobolev approximation to be valid. Sample test
calculations for the case of a stellar wind and a rotating atmosphere are
presented.Comment: 11 pages, 19 figures. accepted by Astronomy and Astrophysic
Properties and nature of Be stars 30. Reliable physical properties of a semi-detached B9.5e+G8III binary BR CMi = HD 61273 compared to those of other well studied semi-detached emission-line binaries
Reliable determination of the basic physical properties of hot emission-line
binaries with Roche-lobe filling secondaries is important for developing the
theory of mass exchange in binaries. It is a very hard task, however, which is
complicated by the presence of circumstellar matter in these systems. So far,
only a small number of systems with accurate values of component masses, radii,
and other properties are known. Here, we report the first detailed study of a
new representative of this class of binaries, BR CMi, based on the analysis of
radial velocities and multichannel photometry from several observatories, and
compare its physical properties with those for other well-studied systems. BR
CMi is an ellipsoidal variable seen under an intermediate orbital inclination
of ~51 degrees, and it has an orbital period of 12.919059(15) d and a circular
orbit. We used the disentangled component spectra to estimate the effective
temperatures 9500(200) K and 4655(50) K by comparing them with model spectra.
They correspond to spectral types B9.5e and G8III. We also used the
disentangled spectra of both binary components as templates for the 2-D
cross-correlation to obtain accurate RVs and a reliable orbital solution. Some
evidence of a secular period increase at a rate of 1.1+/-0.5 s per year was
found. This, together with a very low mass ratio of 0.06 and a normal mass and
radius of the mass gaining component, indicates that BR CMi is in a slow phase
of the mass exchange after the mass-ratio reversal. It thus belongs to a still
poorly populated subgroup of Be stars for which the origin of Balmer emission
lines is safely explained as a consequence of mass transfer between the binary
components.Comment: 17 pages, 5 figures, accepted for publication in Astronomy and
Astrophysics. appears in Astronomy and Astrophysics 201
Improved model of the triple system V746 Cas that has a bipolar magnetic field associated with the tertiary
V746 Cas is known to be a triple system composed of a close binary with an
alternatively reported period of either 25.4d or 27.8d and a third component in
a 62000d orbit. The object was also reported to exhibit multiperiodic light
variations with periods from 0.83d to 2.50d, on the basis of which it was
classified as a slowly pulsating B star. Interest in further investigation of
this system was raised by the detection of a variable magnetic field. Analysing
spectra from four instruments, earlier published radial velocities, and several
sets of photometric observations, we arrived at the following conclusions: (1)
The optical spectrum is dominated by the lines of the B-type primary
(Teff1~16500(100) K), contributing 70% of the light in the optical region, and
a slightly cooler B tertiary (Teff3~13620(150) K). The lines of the low-mass
secondary are below our detection threshold; we estimate that it could be a
normal A or F star. (2) We resolved the ambiguity in the value of the inner
binary period and arrived at a linear ephemeris of T_super.conj.=HJD
2443838.78(81)+25.41569(42)xE. (3) The intensity of the magnetic field
undergoes a~sinusoidal variation in phase with one of the known photometric
periods, namely 2.503867(19)d, which we identify with the rotational period of
the tertiary. (4) The second photometric 1.0649524(40)d period is identified
with the rotational period of the B-type primary, but this interpretation is
much less certain and needs further verification. (5) If our interpretation of
photometric periods is confirmed, the classification of the object as a slowly
pulsating B star should be revised. (6) Applying an N-body model to different
types of available observational data, we constrain the orbital inclination of
the inner orbit to ~60 deg to 85 deg even in the absence of eclipses, and
estimate the probable properties of the triple system and its components.Comment: Accepted for publication in Astronomy and Astrophysic
Influence of rotation velocity gradient on line profiles of accretion discs of CVs
We show the influence of the Keplerian velocity shear on the line profiles of cataclysmic variable discs. The complete disc structure is taken into account. The radial disc structure follows the alpha disc approximation. Based on this assumption, the vertical structure is computed using the detailed non-LTE code AcDc. The obtained opacities and source functions are interpolated in the 2D grid, where the radiative transfer is calculated with the inclusion of the velocity field gradient. © International Astronomical Union 2012
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