T. V. Demidova

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 calculated the phase light curves for the different orientations of the orbit. 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.

Structures in circumbinary disks: Prospects for observability

During the past decade circumbinary disks have been discovered around various young binary stars. Hydrodynamical calculations indicate that the gravitational interaction between the central binary star and the surrounding disk results in global perturbations of the disk density profile. We study the observability of characteristic large-scale disk structures resulting from the binary-disk interaction in the case of close binary systems. We derived the structure of circumbinary disks from smoothed-particle hydrodynamic simulations. Subsequently, we performed radiative transfer simulations to obtain scattered light and thermal reemission maps. We investigated the influence of the binary mass ratio, the inclination of the binary orbit relative to the disk midplane, and the eccentricity of the binary orbit on observational quantities. We find that ALMA will allow tracing asymmetries of the inner edge of the disk and potentially resolving spiral arms if the disk is seen face-on. For an edge-on orientation, ALMA will allow detecting perturbations in the disk density distribution through asymmetries in the radial brightness profile. Through the asymmetric structure of the disks, areas are formed with a temperature

Anisotropic illumination of a circumbinary disk in the presence of a low-mass companion

2.6

On the origin of the azimuthal asymmetry in pole-on protoplanetary disks: The case of LkHα 101

times higher than at the same location in equivalent unperturbed disks. The time-dependent appearance of the density waves and spiral arms in the disk affects the total re-emission flux of the object by a few percent.

On the Nature of the Unique Eclipsing System H 187 (HMW 15)

The model of an young star with a protoplanetary disk and a low-mass companion (q ≤ 0.1), which is moving on a circular orbit, inclined to the disk plane, is considered. The hydrodynamic models of such a system were calculated by SPH method. It was shown the distortions in the disk, caused by the orbital motion of the companion, lead to the strong dependence of illumination conditions of the disk on the azimuth (because of extinction between the star and the disk surface) and, therefore, it leads to the appearance of large-scale asymmetry in images of disks. The calculations showed the dependence of the illumination on the azimuth was stronger in the central part of the disk than on the periphery. Bright and dark domains are located not symmetry with respect to the line of nodes. The sizes and locations of the domains are depended on the model parameters as well as on the phase of the orbital period. The bright and dark domains do not follow the companion, but they make small amplitude oscillations with respect to some direction. The properties of the model, which were written above, open new opportunities of searching low-mass companions in the vicinity of young stars. The stars with protoplanetary disks, which are observed face-on or under the small inclination angle i, are the best ones for this purpose.The model of a young star with a protoplanetary disk and a low-mass companion (q ≤ 0.1) moving in a circular orbit inclined to the disk plane is considered. Hydrodynamic models of such a system have been calculated by the SPH method. The perturbations in the disk caused by the orbital motion of the companion are shown to lead to a strong dependence of the disk illumination conditions on azimuth (because of extinction variations between the star and the disk surface) and, as a result, to the appearance of a large-scale asymmetry in the disk images. Calculations show that the dependence of the disk illumination on azimuth is stronger in the central part of the disk than on the periphery. The bright and dark (shadow) regions are located asymmetrically relative to the line of nodes. The sizes of these regions and their positions on the disk depend on model parameters and orbital phase. During the orbital motion, the bright and dark regions do not follow the companion but execute small-amplitude oscillations relative to some direction. The model properties described above open up new possibilities for detecting low-mass companions in the vicinity of young stars. Stars with protoplanetary disks seen face-on or at low inclinations i are best suited for this purpose.

SPH simulations of structures in protoplanetary disks

The model of a protoplanetary disk of a star with a low-mass companion (M2 : M1 ≤ 0.1), moving on a circular orbit, which slightly inclined to the disk plane (≤ 10◦), is considered. The SPH-method is used to calculate gas-dynamic flows. The motion of the companion on the orbit leads to the inhomogeneous distribution of the matter in the disk: there are a cleared gap, density waves and streams of matter. Because of the disturbances the inner part of the disk is tilted to its periphery and also do not coincides with the orbit plane of the companion. It leads to anisotropic illumination of the disk by the star and, as a consequence, to the appearance of large-scale inhomogeneity on the disk image: it has a bright area in the form of a “horseshoe” and a small shadow zone located asymmetric with respect to the line of nodes. The asymmetry of the disk image is clearly visible even if it is observed pole-on. The motion of the companion on the orbit does not lead to the synchronous movement of the shadow and bright areas: they only make small oscillations relative to the some direction. By using the proposed model we an fairly accurately reproduce the asymmetric image of the disk of star LkHα 101, observed almost pole-on. The study of such asymmetric disks opens up new opportunity for searching massive bodies around young stars.The model of a protoplanetary disk around a star with a low-mass companion (M2: M1 ≤ 0.1) moving in a circular orbit inclined at a small angle to the disk plane (≤10°) is considered. The SPH method is used to calculate the hydrodynamic flows. The orbital motion of the companion leads to a nonuniform distribution of matter in the disk: a matter-free gap, density waves, and gas flows are formed in it. As a result of perturbations, the inner part of the disk is inclined relative to its periphery and does not coincide with the orbital plane of the companion either. This leads to an anisotropic illumination of the disk by the star and, as a consequence, to the appearance of a large-scale inhomogeneity in the disk image: it has a bright horseshoe-shaped region and a small shadow zone located asymmetrically relative to the line of nodes. An asymmetry of the disk image is clearly seen even when it is viewed pole-on. The orbital motion of the companion does not lead to any synchronous motion of the dark (shadow) and bright regions: they only execute small oscillations relative to some preferential direction. The asymmetric image of the disk around the star LkHα 101 seen nearly pole-on can be reproduced rather accurately within the proposed model. A study of such asymmetric disks opens up new opportunities for the search of massive bodies in the neighborhoods of young stars.

Simulations of the Dynamics of the Debris Disks in the Systems Kepler-16, Kepler-34, and Kepler-35

We present our photometric observations of the T Tauri star H 187. They confirm our conclusion that a new extended eclipse has begun in this young object. By the end of 2005, H 187 reached its minimum light following which its brightness began to slowly increase. Comparison with the previous ∼3.5-yr-long eclipse observed by Cohen et al. shows that the new eclipse follows the previous eclipse fairly closely and, hence, it was caused by a second passage of the same extended dust or gas-dust cloud around the object. We have estimated the period between these events to be 4.7 yr. The object reddened during the eclipse, suggesting that the extinction was produced by small grains ∼0.1μm in size. Possible mechanisms of such unusual eclipses are discussed. We draw an analogy between these eclipses and the cycles of photometric activity observed in UX Ori stars. Light curves similar to those observed for H 187 are shown to be obtained in the model of a young binary system with a low-mass companion accreting matter from the remnants of a protostellar cloud at a rate of ∼10−9M⊙ yr−1.

Habitat-dependent differences in architecture and microclimate of the burrows of Sundevall's jird (Meriones crassus) (Rodentia: Gerbillinae) in the Negev Desert, Israel

Using the GADGET-2 code modified by us, we have computed hydrodynamic models of a protoplanetary disk perturbed by a low-mass companion. We have considered the cases of circular and eccentric orbits coplanar with the disk and inclined relative to its midplane. During our simulations we computed the column density of test particles on the line of sight between the central star and observer. On this basis we computed the column density of circumstellar dust by assuming the dust and gas to be well mixed with a mass ratio of 1: 100. To study the influence of the disk orientation relative to the observer on the interstellar extinction, we performed our computations for four inclinations of the line of sight to the disk plane and eight azimuthal directions. The column densities in the circumstellar disk of the central star and the circumbinary disk were computed separately. Our computations have shown that periodic column density oscillations can arise in both inner and circumbinary disks. The amplitude and shape of these oscillations depend on the system’s parameters (the orbital eccentricity and inclination, the component mass ratio) and its orientation in space. The results of our simulations can be used to explain the cyclic brightness variations of young UX Ori stars.

SPIRAL PATTERNS IN PLANETESIMAL CIRCUMBINARY DISKS

We investigate the long-term dynamics of planetesimals in debris disks in models with the parameters of the binary star systems Kepler-16, Kepler-34, and Kepler-35 with planets. Our calculations show that the formation of a stable ring coorbital with the planet is possible for Kepler-16 and Kepler-35. In Kepler-34 significant orbital eccentricities of the binary system and the planet can prevent the formation of such a structure. The detection of circumbinary ring-like structures in observations of binary star systems can be evidence for the existence of planets retaining coorbital rings of dust and planetesimals.