A. A. Boyarchuk

On the role of duplicity in the Be phenomenon I. General considerations and the first attempt at a 3-D gas-dynamical modelling of gas outflow from hot and rapidly rotating OB stars in binaries

This paper begins a new series of studies devoted to a critical re-examination of the role of duplicity for the Be phenomenon and for the variability patterns observed for many Be stars. Based on both dynamical and energy considerations and a numerical gas-dynamical modelling, a new hypothesis of the forma- tion of Be envelopes in binaries, via an outflow from a rapidly rotating B star in a detached binary, is outlined. It is shown that such an outflow is facilitated by the presence of a companion to the B star and leads to the formation of an envelope but not to any significant mass exchange between the binary components.

The effect of viscosity on the flow morphology in semidetached binary systems. Results of 3D simulations. II

Results of 3D gas dynamical simulation of mass transfer in binaries are presented for systems with various values of viscosity. Analysis of obtained solutions shows that in the systems with low value of viscosity the flow structure is qualitatively similar to one for systems with high viscosity. Presented calculations confirm that there is no shock interaction between the stream from L1 and the forming accretion disk (`hot spot) at any value of viscosity.We present the results of three-dimensional simulations of matter flows in semidetached binary systems with various viscosities. In low-viscosity systems, the flow structure displays the same qualitative features as in high-viscosity computations. A self-consistent solution shows the absence of a shock interaction between the stream flowing from the inner Lagrange point and the forming accretion disk (or hot spot) for any viscosity.Results of 3D gas dynamical simulation of mass transfer in binaries are presented for systems with various values of viscosity. Analysis of obtained solutions shows that in the systems with low value of viscosity the flow structure is qualitatively similar to one for systems with high viscosity. Presented calculations confirm that there is no shock interaction between the stream from L1 and the forming accretion disk (`hot spot) at any value of viscosity.

A comparative analysis of chemical abundances in the atmospheres of red giants of different age groups

We analyze previously published chemical abundances in the atmospheres of red giants. Excess abundances are observed not only for Na, but also for Al and Si, with the overabundances increasing with the stars’ luminosity. The observed anomalies provide evidence that, in addition to the CNO hydrogen-burning cycle, the Mg-Al and Ne-Na cycles operate in the interiors of main-sequence stars; their products are brought to the stellar atmospheres by convection after the transition to the red-giant phase. The abundance anomalies for s-process elements, also observed in the atmospheres of field stars, testify to the presence of a substantial number of neutrons. The s-process abundance anomalies are absent from giants of the young Hyades cluster.

Synthetic Doppler tomograms of gas flows in the binary system IP Peg

We have synthesized Doppler tomograms of gas flows in the binary system IP Peg using the results of three-dimensional gas-dynamical computations. Gas-dynamical modeling in combination with Doppler tomography enables identification of the key elements of flows in Doppler maps without solution of an ill-posed inverse problem. A comparison of the synthetic tomograms with observations shows that, in the quiescent state of the system, the most luminous components are (1) the shock wave induced by interaction between the circumbinary envelope and the stream from the Lagrange point L1 (the “hot line”) and (2) the gas condensation at the apogee of the quasi-elliptical disk. Both the single spiral shock wave arm in the gas-dynamical solution and the stream from L1 contribute little to the luminosity. In the active state of the system, when the stream from L1 does not play an appreciable role and the disk dominates, both areas of enhanced luminosity in the observational tomograms are associated with the two arms of the spiral shock wave in the disk.

The structure of matter flows in semi-detached binaries after the termination of mass transfer

We present the results of three-dimensional gas-dynamical simulations of matter flows in semi-detached binaries after termination of the mass transfer between the components of the system. The structure of the residual accretion disk is studied. When the mass transfer has ended, the quasi-elliptical disk becomes circular and its structure changes: tidal interactions result in the formation of a second arm in the spiral shock wave. In addition, a condensation (blob) moving through the disk with variable velocity is formed. The blob is maintained by interactions with the arms of the spiral shock and exists essentially over the entire lifetime of the disk. We also show that, for a viscosity corresponding to α∼0.01 (typical for observed accretion disks), the lifetime of the residual disk is about 50 orbital periods.

A Study of Red Giants in the Fields of Open Clusters. Cluster Members

We present the results of a comparative analysis of the atmospheric chemical abundances of red giants in several open clusters: the Hyades, Collinder 350, NGC 6475, and Ruprecht 147. We determined the atmospheric parameters of all the stars and the elemental abundances in their atmospheres, as well as their masses, Galactic velocities, and the elements of their orbits in the Galaxy. The observed excess [Na/Fe] and [Eu/Fe] abundances in the atmospheres of Hyades giants suggests that matter later used for star formation had been enriched in the ejecta from type II supernovae.

A possible manifestation of spiral shock waves in the accretion disks of cataclysmic variables

We present the results of three-dimensional numerical simulations of flow structures in binary systems with spiral shock waves. Variations of the mass-transfer rate perturb the equilibrium state of the accretion disk; consequently, a condensation (blob) behind the shock breaks away from the shock front and moves through the disk with variable speed. Our computations indicate that the blob is a long-lived formation, whose mean parameters do not vary substantially on timescales of several tens of orbital periods of the system. The presence of the spiral shocks maintains the compact blob in the disk: it prevents the blob from spreading due to the differential motion of matter in the disk, and dissipative spreading on this timescale is negligible. A number of cataclysmic variables display periodic or quasi-periodic photometric variations in their light curves with characteristic periods ∼0.1–0.2Porb, where Porb is the orbital period. The blobs formed in systems with spiral shock waves are examined as a possible origin for these variations. The qualitative (and, in part, quantitative) agreement between our computations and observations of IP Peg and EX Dra provides evidence for the efficacy of the proposed model.

Analysis of elemental abundances in the atmospheres of three Hyades red giants

Model atmospheres are fitted to spectroscopic data in order to analyze the elemental abundances in the atmospheres of three red giants in the Hyades cluster. The three stars have almost identical chemical compositions, with iron-group elements slightly overabundant compared to the solar values—a pattern that is typical of Hyades dwarfs. The overabundances of the light elements Na, Al, and Si are virtually equal to those observed for field giants. No enrichment in rare-earth elements relative to iron was found, in sharp contrast to field giants. It is concluded that these discrepancies are due to the age difference between the two groups of stars, which have resulted in different degrees of convective overshooting.

Interpretation of Light Curves of IP Peg in a Model with Shockless Interaction between the Gas Stream and Disk

We have analyzed light curves of the eclipsing cataclysmic variable IP Peg. A model with a shockless interaction between the gaseous stream and disk (i.e., an elliptical disk with a “hot line”) can describe the main characteristic features of the light curve of the interacting close binary better than a classical model with a “hot spot” at the outer boundary of a circular accretion disk. In particular, the hot-line model can reproduce the luminosity increase observed at phases ϕ∼0.1–0.2 and ϕ∼0.5–0.6, which is not possible in the standard hot-spot model. The advantages of the hot-line model are particularly striking for the IR light curves of IP Peg: the discrepancy χ2 between the theoretical and observed light curves is 49 for the model with a shockless interaction between the gaseous stream and disk, and 2681 for the standard hot-spot model.

The chemical composition of the red giant η Ser

The paper presents the results of an analysis of elemental abundances in the atmosphere of η Ser—a red giant of spectral type K2IIIabCN1, according to Hoffleit and Jaschek [4]. The resulting atmospheric parameters correspond to a K0 giant, and the peculiarities of the star’s chemical composition provide evidence that the assignment of η Ser to the class of CN stars was erroneous.