Is there partial mixing in the binary system components?

Данные об абсолютных элементах компонент двойных систем, находящихся на стадии горения водорода и относящихся к ранним спектральным подклассам В, проанализированы с привлечением моделей звезд, построенных с учетом частичного перемешивания вещества лучистой оболочки и конвективного ядра. Имеющиеся данные о массах, размерах и светимостях компонент двойных систем допускают возможность частичного перемешивания в их недрах аналогично перемешиванию в одиночных В-звездах главной последовательности таких же спектральных подклассов. Имеющихся данных недостаточно для того, чтобы наложить строгие ограничения на величину частичного перемешивания и выявить количественные различия в перемешивании у компонент двойных систем и одиночных звезд, если таковые имеются.The absolute elements of binary system components which are main sequence early type B-stars are analysed on the base of stellar models taking into account partial mixing of material in the radiative envelope and convective core. The data on masses, radii and luminosities of components, we have got by now, confirm the presence of partial mixing in the interiors of components. That data are not sufficient to put a restriction on the power of partial mixing and to find out a difference of partial mixing in the binary components and the single stars.Часть работ проведена при финансовой поддержке государства в лице Министерства образования и науки Российской Федерации (базовая часть госзадания, РК № AAAA-A17-117030310283-7), а также при финансовой поддержке Правительства Российской Федерации (постановление № 211, контракт № 02.A03.21.0006)

Formation of a rapidly rotating classical Be-star in a massive close binary system

This paper investigates the spin-up of a mass-accreting star in a close binary system passing through the first stage of mass exchange in the Hertzsprung gap. Inside an accreting star, angular momentum is carried by meridional circulation and shear turbulence. The circulation carries part of the angular momentum entering the accretor to its surface. The greater the rate of arrival of angular momentum in the accretor is, the greater this part. It is assumed that this part of the angular momentum can be removed by the disk further from the accretor. If the angular momentum in the matter entering the accretor is more than half the Keplerian value, then the angular momentum obtained by the accretor during mass exchange stage does not depend on the rate of arrival of angular momentum. The accretor may have the characteristics of a Be-star immediately after the end of mass exchange.Comment: 13 pages, 7 figure

Formation of a Rapidly Rotating Classical Be-star in a Massive Close Binary System

This paper investigates the spin-up of a mass-accreting star in a close binary system passing through the first stage of mass exchange in the Hertzsprung gap. Inside an accreting star, angular momentum is carried by meridional circulation and shear turbulence. The circulation carries part of the angular momentum of the accreted layers to the accretor’s surface. The greater the rate of arrival of angular momentum in the accretor is, the greater this part. It is assumed that this part of the angular momentum can be removed by the disk further from the accretor. If the angular momentum in the matter entering the accretor is more than half the Keplerian value, then the angular momentum obtained by the accretor during mass exchange stage does not depend on the rate of arrival of angular momentum. The accretor may have the characteristics of a Be-star immediately after the end of mass exchange. © 2023. National Astronomical Observatories, CAS and IOP Publishing Ltd.Ministry of Science and Higher Education of the Russian Federation, (FEUZ-2023-0019); Ministry of Science and Higher Education of the Russian FederationThis work was supported by the Ministry of Science and Education, FEUZ-2023-0019

The Spin-up of a Star Gaining Mass in a Close Binary System on the Thermal Time Scale

We investigate the exchange of mass in a binary system as a channel through which a Be star can receive a rapid rotation. The mass-transfer phase in a massive close binary system in the Hertzsprung-gap is accompanied by the spinning up of the accreting component. We consider a case when the mass of the accreting component increases by 1.5 times. The component acquires mass and angular momentum while in a state of critical rotation. The angular momentum of the component increases by 50 times. Meridional circulation effectively transports angular momentum inside the component during the mass-transfer phase and during the thermal timescale after the end of the mass-transfer phase. As a result of mass transfer, the component acquires the rotation typical of classical Be stars. © 2022. National Astronomical Observatories, CAS and IOP Publishing Ltd.Ministry of Science and Higher Education of the Russian Federation, (FEUZ-2020-0030)This work was supported by the Ministry of Science and Education, FEUZ-2020-0030

The Role of Meridional Circulation in the Formation of Classical Be Stars

Abstract: At the stage of mass exchange in a binary system, the meridional circulation brings to the surface of the star up to two-thirds of the angular momentum that entered the star along with the accreted matter. As a result, the mass and angular momentum of the star can increase due to accretion. After the end of accretion, the star has a rotation typical of rapidly rotating Be stars. It is assumed that the angular momentum carried by the meridional circulation to the star’s surface from the accreted matter is removed from the star by the accretion disk. The article is based on a talk presented at the astrophysical memorial seminar “Novelties in Understanding the Evolution of Binary Stars,” dedicated to the 90th anniversary of Professor M.A. Svechnikov. © 2023, The Author(s).Ministry of Education and Science of the Russian Federation, Minobrnauka: FEUZ-2023-0019This work was supported by the Ministry of Science and Higher Education of the Russian Federation, topic No. FEUZ-2023-0019

Increasing Spin of a B-Star During the Common Envelope Stage

Context. In its evolution, the MWC 656 binary system, consisting of a Be star and a black hole, has gone through the common envelope stage. The O and B stars of the early spectral subclasses can be characterised by lower rotational velocities and increased binary frequency. The B star in the MWC 656 system may have undergone rapid rotation during the common envelope stage. Aims. We study the change in the B star's state of rotation due to an increase in its angular momentum during the common envelope stage and immediately afterwards. Methods. We performed one-dimensional numerical calculations of angular momentum transport in the interior of a slightly evolved star on the main sequence with a mass of 16 M⊙. Meridional circulation and shear turbulence are taken into account. Results. Due to the arrival of angular momentum through the star's surface, the velocity of the meridional circulation increases by more than six orders of magnitude. Meridional circulation is the main mechanism for the transport of angular momentum into the star's deep interior. The acquired angular momentum enters the convective core during the common envelope stage if the angular velocity of the accreted matter exceeds half the Kepler value. Conclusions. The star acquires a rotation typical of classical Be stars if the angular velocity of accreted matter rotation is close to the maximum possible value during the common envelope stage (∼80% of the Kepler value). © ESO 2021.This work was supported by the Ministry of Science and Education, FEUZ-2020-0030

Turbulent entrainment at the boundaries of the convective cores of main-sequence stars

Extra mixing of matter in stellar interiors at the boundaries of the convective cores of mainsequence stars is considered for the first time using the physical model of turbulent entrainment developed by Arnett and collaborators based on three-dimensional hydrodynamical simulations. The model takes into account the energy that goes into mixing the matter of the convective core and layers stable against convection located above the core. It is shown that the extent of the region of extra mixing expressed in units of the pressure scale height is not constant, and decreases as the star evolves along the main sequence. Adequate allowance for extra mixing at the boundaries of convective cores is necessary to clarify the relative importance of different mixing mechanisms in stellar interiors, as well as to determine stellar parameters using asteroseismlogy. © 2013 Pleiades Publishing, Ltd

Is the additional increase of star luminosity due to partial mixing real?

The partial mixing of matter between the radiative envelope and convective core in an early B-type star produces an additional increase of star luminosity during main sequence evolution. High quality data on stellar mass and luminosity defined from studies of detached double-lined eclipsing binaries are used to check the existence of such additional increase. It is shown that the additional luminosity increase does not contradict observed high quality data, if the intensity of partial mixing is restricted by the observed increase in surface helium content. © 2019 National Astronomical Observatories, CAS and IOP Publishing Ltd.

Presupernova Structure of Massive Stars

Issues concerning the structure and evolution of core collapse progenitor stars are discussed with an emphasis on interior evolution. We describe a program designed to investigate the transport and mixing processes associated with stellar turbulence, arguably the greatest source of uncertainty in progenitor structure, besides mass loss, at the time of core collapse. An effort to use precision observations of stellar parameters to constrain theoretical modeling is also described.Comment: Proceedings for invited talk at High Energy Density Laboratory Astrophysics conference, Caltech, March 2010. Special issue of Astrophysics and Space Science, submitted for peer review: 7 pages, 3 figure

The Role of the Meridional Circulation in the Formation of Classical Be Stars

На стадии обмена веществом в двойной системе меридиональная циркуляция выносит к поверхности звезды до двух третей момента импульса, поступившего в звезду вместе с аккрецированным веществом. Отвод этой части момента импульса из звезды делает возможным увеличение ее массы и момента импульса вследствие аккреции и образование быстровращающейся Ве-звезды.At the stage of matter exchange in a binary system, the meridional circulation carries to the surface of the star up to two-thirds of the angular momentum received by the star along with the accreted matter. The removal of this part of the angular momentum from the star makes it possible to increase its mass and angular momentum due to accretion and to create of a rapidly rotating Be-star.Работа выполнена при финансовой поддержке Министерства науки и высшего образования Российской Федерации, тема FEUZ-2023-0019