The innermost portions of the Milky Way's stellar halo have avoided scrutiny until recently. The lack of wide-area survey data, made it difficult to reconstruct an uninterrupted view of the density distribution of the metal-poor stars inside the Solar radius. In this study, we utilize red giant branch (RGB) stars from Gaia, with metallicities estimated using spectro-photometry from Gaia Data Release 3. Accounting for Gaia's selection function, we examine the spatial distribution of metal-poor ([M/H]<-1.3) RGB stars, from the Galactic centre (r 1 kpc) out to beyond the Solar radius (r 18 kpc). Our best-fitting single-component cored power-law model shows a vertical flattening of 0.5 and a slope -3.4, consistent with previous studies. Motivated by the mounting evidence for two distinct stellar populations in the inner halo, we additionally test a range of two-component models. One of the components models the tidal debris from the Gaia Sausage/Enceladus merger, while the other captures the Aurora population – stars that predate the Galactic disk formation. Our best-fit two-component model suggests that both populations contribute equally around the Solar radius, but Aurora dominates the inner halo with a steeper power-law index of -4.5, in agreement with the nitrogen-rich star distribution measured by Horta et al. (2021).

We present a model for the transport of anisotropic turbulence in an accretion disc. The model uses the Reynolds stress tensor approach in the mean field approximation. To study the role of convection in a protoplanetary disc, we combine the turbulence model with a radiative transfer calculation, and also include convection using the mixing length approximation. We find that the turbulence generated by convection causes the angular momentum of the accretion disc to be directed outwards. We also confirm the conclusions of other authors that turbulent convection is unable to provide the observed disc accretion rates as well as a heat source sufficient for the convection to be self-sustaining. The reasons for the latter are the strong anisotropy of the turbulence together with the low efficiency of the energy transfer from the background velocity shear to the turbulent stress tensor.

Gaia can not individually resolve very close binary systems, however, the collected data can still be used to identify them. A powerful indicator of stellar multiplicity is the sources reported Renormalized Unit Weight Error (ruwe), which effectively captures the astrometric deviations from single-source solutions. We aim to characterise the imprints left on ruwe caused by binarity. By flagging potential binary systems based on ruwe, we aim to characterise which of their properties will contribute the most to their detectability. We develop a model to estimate ruwe values for observations of Gaia sources, based on the biases to the single-source astrometric track arising from the presence of an unseen companion. Then, using the recipes from previous GaiaUnlimited selection functions, we estimate the selection probability of sources with high ruwe, and discuss what binary properties contribute to increasing the sources ruwe. We compute the maximum ruwe value which is compatible with single-source solutions as a function of their location on-sky. We see that binary systems selected as sources with a ruwe higher than this sky-varying threshold have a strong detectability window in their orbital period distribution, which peaks at periods equal to the Gaia observation time baseline. We demonstrate how our sky-varying ruwe threshold provides a more complete sample of binary systems when compared to single sky-averaged values by studying the unresolved binary population in the Gaia Catalogue of Nearby Stars. We provide the code and tools used in this study, as well as the sky-varying ruwe threshold through the GaiaUnlimited Python package

Many of observed hot Jupiters are subject to atmospheric outflows. Numerical simulations have shown that the matter escaping from the atmosphere can accumulate outside the orbit of the planet, forming a torus. In a few 10^8 yr, the mass of the torus can become large enough to exert a significant gravitational effect on the planet. Accumulation of mass, in its own turn, is hindered by the activity of the star, which leads to the photoevaporation of the torus matter. We explore the role of these and other factors in the planet's migration in the epoch when the protoplanetary disk has already disappeared. Using HD209458 system as an example, we show that the gravitational interaction with the torus leads to the possibility of migration of the planet to its observable position, starting from an orbit >= 0.3 AU.

A hierarchical numerical model for investigation of the accretion processes in the magnetic cataclysmic variable EX Hya has been described. The coarsest level of the hierarchy corresponds to the accretion disk formation due to the mass transfer. The intermediate level describes the structure of the white dwarf magnetosphere. The finest level corresponds to the formation of the accretion curtain at the inner edge of the accretion disk. The model is based on the equations of the modified MHD, where a strong external magnetic field acts as an efficient fluid with which the plasma interacts. Results of simulations show that the accretion in EX Hya proceeds via accretion columns that have curtain-like shapes. These columns are very thin and the accretion regions cover a small fraction of the accretor surface. Numerical results agree with the parameters of the EX Hya system as determined by Semena et al. (2014).

— As is known, the influence of the ionizing radiation and the gravity of a star on a hot Jupiter results in the outflow of its atmosphere. Due to the gravity of the planet, the outflowing matter acquires angular momentum, after which it is accumulated on a higher circular orbit around the star and forms a disk or a torus. The angular momentum interexchange between the torus and the planet induces the migration of the planet to the star. In this paper, we consider the effectiveness of this migration mechanism by the example of the HD 209458 system. It turns out that, after 4.5 × 10 9 years since the evaporation of the protoplanetary disk, the planet might migrate from an orbit at ≳0.67 AU to its current orbit at 0.045 AU.

The accretion disk response to a loss of 5% of the mass of the accretor formed by a stellar-mass binary black hole coalescence has been simulated. The parameters of the initial binary system correspond to the gravitational-wave source GW170814. We performed a series of simulations of models with radiative and convective heat transfer mechanisms and various accretion rates. The models took into account radiation pressure and adiabatic vertical gas expansion. Reducing the mass of the accretor due to emission of gravitational waves led to a strong hydrodynamic disturbance of the disk that developed in less than one second into a shock wave. As a result of shock heating, the luminosity of convective disks increased by 3–5 orders of magnitude and reached 10 43 erg s −1 . The luminosity of radiative disks increased by 1–2 orders of magnitude, to 10 40 erg s −1 . If the source is as far as 540 Mpc (as in the case of the GW170814 event), disk brightening could be detected by the XMM-Newton X-ray observatory.

A means of generating the radio emission of polars is proposed, based on cyclotron radiation of thermal electrons in a fluctuating magnetic field. The source of these fluctuations is Alfvén turbulence. Expressions for the radiation spectrum and degree of polarization are obtained. The radio flux from the accretion flow is computed, using the polar AM Her as an example. The proposed model for the emission can reproduce the observed fluxes in the VLA frequency range with realistic plasma characteristics.

Estimates of typical parameters of accretion flows in the representative intermediate polar EX Hydrae, the polar AM Herculis, and the “hot Jupiter” WASP-12b are presented. Dimensionless parameters of astrophysical systems are compared with those of laboratory experiments on laser ablation in magnetic fields. It is shown that laboratory simulations of astrophysical flows is possible in principle, provided that some adjustment to the magnetic field, plasma density, and plasma velocity are made.

Results of three-dimensional gas-dynamical numerical simulations of the structure of matter flows in semi-detached binary systems with various component-mass ratios are considered. The main elements of the flows in the models studied are described. The characteristics of density waves inside the accretion disk for various component-mass ratios are considered. The influence of the precessional density wave on the development of instability in the accretion disks and the characteristics of developing turbulence are analyzed. Values of the Shakura–Syunyaev coefficient α for the simulated systems are calculated.