Bimodal brightness oscillations in models of young binary systems

Abstract

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.

Modulation of circumstellar extinction in a young binary system with a low-mass companion in a noncoplanar orbit

“…4 and 5). Situations similar to that described above are also possible in models with coplanar orbits (see Demidova et al 2010aDemidova et al , 2010b. However, as our comparative analysis showed, other things being equal, the brightness modulation depth in models with noncoplanar CB disk and orbit is appreciably larger than that in coplanar binary systems.…”

“…The technique for calculating the phase dependence of the dust column density was described in detail by Demidova et al (2010a). Let us briefly recall its main points.…”

“…Our computations showed that this provided a satisfactory removal of the trend for all of the models under consideration. Subsequently, we passed from the test particle column density n(t) to the column density of real dust grains n d (t) (for details, see Demidova et al 2010a). To reduce the influence of random fluctuations when the phase dependences of n are computed, the current values of n(t) are folded with the orbital period for a time interval of 50 binary revolutions.…”

“…This problem was initiated by the observations of large-scale activity cycles in UX Ori stars (Grinin et al 1998) and by the observations of anomalously long eclipses in some young stars (Cohen et al 2003; Barsunova et al 2005; Nordhagen et al 2006; Grinin et al 2008), which cannot be explained in terms of classical models of eclipsing binary systems. Sotnikova and and Demidova et al (2010aDemidova et al ( , 2010b showed that the streams of matter and density waves produced in young binary systems by the orbital motion of their companions can cause (at low inclinations of the orbital plane to the line of sight) various (in shape and duration) periodic fadings of the primary component. One of the main conditions in the computations was coplanarity of the orbital plane and the circumbinary (CB) disk.…”

“…In the previous papers of this series (Sotnikova and Grinin 2007; Demidova et al 2010aDemidova et al , 2010b, we studied the behavior of the circumstellar extinction and brightness in young binary systems accreting matter from the remnants of a protostellar cloud. This problem was initiated by the observations of large-scale activity cycles in UX Ori stars (Grinin et al 1998) and by the observations of anomalously long eclipses in some young stars (Cohen et al 2003; Barsunova et al 2005; Nordhagen et al 2006; Grinin et al 2008), which cannot be explained in terms of classical models of eclipsing binary systems.…”