24 research outputs found
Bimodal Brightness Oscillations in Models of Young Binary Systems
We consider a model for the cyclic activity of young binary stars that
accrete matter from the remnants of a protostellar cloud. If the orbit of such
a binary system is inclined at a small angle to the line of sight, then the
streams of matter and the density waves excited in the circumbinary disk can
screen the primary component of the binary from the observer. To study these
phenomena by the SPH (smoothed particle hydrodynamics) method, we have computed
grids of hydrodynamic models for binary systems based on which we have
constructed the light curves as a function of the orbital phase.The main
emphasis is on investigating the properties of the brightness oscillations.
Therefore, the model parameters were varied within the following ranges: the
component mass ratio q = M2 : M1 = 0.2 - 0.5 and the eccentricity e = 0 - 0.7.
The parameter that defined the binary viscosity was also varied. We adopted
optical grain characteristics typical of circumstellar dust. Our computations
have shown that bimodal oscillations are excited in binaries with eccentric
orbits, provided that the binary components do not differ too much in mass. In
this case, the ratios of the periods and amplitudes of the bimodal oscillations
and their shape depend strongly on the inclination of the binary plane and its
orientation relative to the observer. Our analysis shows that the computed
light curves can be used in interpreting the cyclic activity of UX Ori stars.Comment: 14 pages, 5 figures, 1 tabl
Modulation of Circumstellar Extinction in a Young Binary System with a Low-Mass Companion in a Noncoplanar Orbit
The cyclic activity model of a young star with the low-mass secondary
component (q = M2/M1 <= 0.1) accreting a matter from circumbinary disk is
considered. It is assumed that the orbit is circular and the disk and orbital
planes are non-coplanar. Sets of hydrodynamics models of such a system have
been calculated by the SPH method and then the variations of the circumstellar
extinction and phase light curves were determined. The calculations showed that
depending on the model parameters and orientation of the system in regards to
an observer the different in shape and amplitude light curves can be observed.
An important property of the considered models is also the dependence of the
mass accretion rate onto the components on the phase of the orbital period. The
results of the calculation can be used for analysis of the cyclic activity of
UX Ori stars and young stars with the long-lasting eclipses.Comment: 14 pages, 7 figure
Brightness Oscillations in Models of Young Binary Systems with Low-Mass Secondary Components
We consider a model for the cyclic brightness variations of a young star with
a low-mass companion that accretes matter from the remnants of a protostellar
cloud. At small inclinations of the binary orbit to the line of sight, the
streams of matter and the density waves excited in the circumbinary disk can
screen the primary component of the binary from the observer. To study these
phenomena, we have computed grids of hydrodynamic models for binary systems by
the SPH method based on which we have constructed the phase light curves as a
function of the rotation angle of the apsidal line relative to the observer.
The model parameters were varied within the following ranges: the component
mass ratio q = 0.01-0.1 and the eccentricity e = 0-0.5. We adopted optical
grain characteristics typical of circumstellar dust. Our computations have
shown that the brightness oscillations with orbital phase can have a complex
structure. The amplitudes and shapes of the light curves depend strongly on the
inclination of the binary orbit and its orientation relative to the observer
and on the accretion rate. The results of our computations are used to analyze
the cyclic activity of UX Ori stars.Comment: 13 pages,8 figures, 1 tabl
Hydrodynamic Processes in Young Binary Systems as a Source of Cyclic Variations of Circumstellar Extinction
Hydrodynamic models of a young binary system accreting matter from the
remnants of a protostellar cloud have been calculated by the SPH method. It is
shown that periodic variations in column density in projection onto the primary
component take place at low inclinations of the binary plane to the line of
sight. They can result in periodic extinction variations. Three periodic
components can exist in general case. The first component has a period equal to
the orbital one and is attributable to the streams of matter penetrating into
the inner regions of the binary. The second component has a period that is a
factor of 5-8 longer than the orbital one and is related to the density waves
generated in a circumbinary (CB) disk. The third, longest period is
attributable to the precession of the inner asymmetric region of CB disk. The
relationship between the amplitudes of these cycles depends on the model
parameters as well as on the inclination and orientation of the binary in
space. We show that at a dust-to-gas ratio of 1:100 and and a mass extinction
coefficient of 250 cm g, the amplitude of the brightness variations
of the primary component in the V-band can reach at a mass accretion rate
onto the binary components of yr and a
inclination of the binary plane to the line of sight. We discuss possible
applications of the model to pre-main-sequence stars.Comment: 13 pages, 6 figures, published in Astronomy Letters (v.33, 2007
Flickering in FU Orionis
We analyze new and published optical photometric data of FU Orionis, an
eruptive pre-main sequence star. The outburst consists of a 5.5 mag rise at B
with an e-folding timescale of roughly 50 days. The rates of decline at B and V
are identical, 0.015 +- 0.001 mag per yr. Random fluctuations superimposed on
this decline have an amplitude of 0.035 +- 0.005 mag at V and occur on
timescales of 1 day or less. Correlations between V and the color indices U-B,
B-V, and V-R indicate that the variable source has the optical colors of a G0
supergiant. We associate this behavior with small amplitude flickering of the
inner accretion disk.Comment: 19 pages of text, 3 tables, and 6 figures to be published in the
Astrophysical Journal, 10 March 200
The Disk Wind in the Young Binaries and the Origin of the Cyclic Activity of Young Stars
We present results of numerical modeling of the cyclic brightness modulation
in the young binary systems with the eccentric orbits and low-mass secondary
components. Brightness variations of the primary is due to the periodical
extinction variations on the line-of-sight caused by the disk wind of the
secondary and a common envelope it produces. A matter distribution in the
envelope has been calculated in the ballistic approach. Calculations showed
that for the young binaries with the elliptic orbits parameters of the
photometric minima (their depth, duration and the shape of light curves) depend
not only on the disk wind parameters and an inclination of the binary orbit to
the line-of-sight but also on the longitude of the periastron. A modulation of
the scattered radiation of the common envelope with a phase of the orbital
period has been investigated in the single scattering approach. It is shown
that an amplitude of the modulation is maximal when the system is seen edge-on
and has also a non-zero value in the binaries observed pole-on. Possible
applications of the theory to the young stellar objects are discussed. In
particular, an attention is payed to a resemblance of the light curves in some
models with light curves of the objects suspected as candidates to FUORs.Comment: 18 pages, 9 figures, accepted by Astronomy Letter
Optical spectra of five UX Orionis-type stars
We present in this paper several high resolution ()
spectra of five UXORs (UX Ori, CQ Tau,
BF Ori, RR Tau, WW Vul), which cover
the entire visual range, from 3900 to about 8700 Å. There
are between 4 and 7 spectra per star,
obtained over a time interval of two years.
Simultaneous or quasi-simultaneous
photometric observations were also obtained
at the Crimean Astrophysical Observatory.
The complete, reduced and normalized spectra are available
in electronic form. We show for each
star a selection of the most interesting lines, and the full spectrum
of UX Ori computed by averaging the spectra
obtained when the star was at maximum light. For UX Ori
we show also the synthetic spectrum and provide an
identification of most of the lines.
The spectra are too sparse to form true time sequences; however, they provide
an extremely useful database for studies of UX Ori-type stars.
We discuss briefly the main features of the spectra.
We show that they contain many time-stable photospheric lines
that can be described
to a good approximation by the synthetic spectra of normal
A stars with
and we derive for each
star effective temperature, gravity and rotational velocity.
We examine the time variability of selected lines
and study their connection with the photometric activity of the stars.
Two different types of spectral variability are identified.
One is common to all stars with circumstellar (CS) gas
and is caused by perturbations of the physical and kinematic conditions
of the emitting region.
There is no correlation between this type of activity
and the brightness variations of the star.
On the contrary, a second type of spectral variability correlates
well with the brightness variations and is very likely
connected with the
screening effect of an opaque dust cloud which sporadically
intersects the line of sight.
This type
of variability has been observed in its simplest form in one of
the RR Tau spectra,
where
the equivalent width of the forbidden line [O