Masayuki Y. Fujimoto

Hydrogen burning and dredge-up during the major core helium flash in a Z = 0 model star

The basic evolution of the core helium flash in a Z = 0 model star is studied, analyzing the manner in which both a hydrogen-burning and a helium-burning convective shell can form. The manner in which CN-enhanced material is dredged up to the stellar surface is addressed. A numerical experiment is presented to determine the importance of opacity during the dredge-up phase. The implications that the Z = 0 model has for understanding of Population III stars is discussed. 23 refs.

Diffusion and mixing in accreting white dwarfs

Numerical experiments have been conducted to determine the degree of enhancement of CNO elements in the envelope of a 1 M ⊙ carbon-oxygen white dwarf accreting hydrogen-rich material at rates of 10 −10 , 10 −9 , and 10 −8 M ⊙ yr −1 . Three initial configurations have been adopted : (1) no initial surface helium layer, 10 9 yr of cooling prior to start of accretion; (2) no initial surface helium layer, a steady state interior thermal structure that is expected after many thermonuclear outbursts; and (3) an initial layer of 10 −3 M ⊙ of helium, 10 9 yr of cooling, with diffusion, before the start of accretion at the rate 10 −10 M ⊙ yr −1

The decline and fall of classical novae

We address questions concerning the evolution of classical nova systems through outburst and the time scale for the return to their preoutburst state. Observations indicate that evolution on a purely nuclear burning time scale is not sufficient. We call attention to the sequence of events involving common envelope evolution which naturally occurs following thermonuclear runaways in the accreted hydrogen shells on the white dwarf components of these close binary systems. We estimate that a significant fraction of the envelope may be lost as a result of dynamical friction during a relatively short-lived common envelope phase which is experienced by most classical nova systems. The residual hydrogen-rich envelope matter may be consumed by nuclear burning or wind-driven mass loss or both on time scales compatible with observations of nova systems. As mass is consumed, the photospheric radius decreases and consequently the photospheric temperature rises. The associated hardening of the radiation from novae causes them progressively to become UV, EUV, and ultimately soft X-ray sources. Estimates of the fluxes during these late stages are provided and shown to be compatible with their being detected with EXOSAT or equivalent X-ray facilities. Such detection would provide strong confirmation of theoretical models for the classicalmorexa0» novae. We also point out that infrared emission from X-ray heated grains may be detectable during the soft X-ray phase.«xa0less

On mass-transfer rates in classical nova precursors

A simple model is presented to describe the evolution of cataclysmic variables (CVs). Mass transfer in long-period CVs (P(orb) > 3 hr) is assumed to be due to a magnetic stellar wind and in short-period CVs (P(orb) < 2 hr) to be due to gravitational wave radiation. The critical accreted mass for a classical nova event is adopted to be that for which the thermal structure of the white dwarf is in steady state after many nova events

Diffusion and mixing in classical nova precursors

Numerical experiments have been conducted to determine the degree of enhancement of CNO elements in the envelope of an initially cold 1 M ○. carbon-oxygen white dwarf accreting hydrogen-rich material at rates expected in cataclysmic variables. Only ordinary particle diffusion and convective mixing have been considered, and the enhancements found are a consequence of the diffusive intermixing of hydrogen with carbon and oxygen; this mixing causes a hydrogen-burning thermonuclear runaway to begin below the surface of the original white dwarf

Behavior of low-mass main-sequence stars in response to accretion

The structural response of a low-mass (0.75 solar mass) main-sequence star to mass accretion is examined, assuming that matter is added onto the star with the same entropy as at the photosphere. Calculations show that the accreting star passes through a brief phase of underluminosity and subradius after the onset of accretion. This phase is followed by a sudden increase in luminosity rates and radius. These phases are characterized by a surface convective zone which first absorbs most of the heat liberated during accretion and then retreats and dies away. During the underluminosity and subradius phase, the star grows in mass without expanding very much and returns to the main-sequence state. If accretion is terminated while surface convection persists, the star flares, convection recedes and the star cools down. 14 refs.

Two-component gravitating systems and the red giant-like structure

The present study investigates the equilibria and evolution of gravitating systems composed of two components by approximating their equations of states to polytropes. The structures are explored in hydrostatic equilibrium systematically under the condition that two components interact with each other only through gravity. The systems are found to be characterized by four parameters, the ratio of central densities and the ratio of central thermal energies per unit mass, and the polytropic indices of two components. If the central density is much higher, the structure is little affected by the presence of the other component. If the difference in the central thermal energies is smaller than specified by beta-cri, the system adopts an equilibrium configuration for any mass ratio. Two-component systems necessarily evolve to have the red giantlike structure if one component suffers cooling faster than the other. It is concluded that the red giant structure is a general characteristic of gravitating systems for which there is an appropriate mechanism to decouple the constituent into the hotter and cooler components.

Shell flashes interacting with the core of neutron stars

Taking the whole internal structure into account, and using a fully general relativistic stellar evolutionary code, shell flashes on accreting neutron stars are computed. For high accretion rates, their recurrence is shown to be little influenced by the thermal state of the core, no matter how cool it is. For low accretion rates, however, their progress depends strongly on the interior temperature. When the interior is cool, as expected in the case of X‐ray transients, shell flashes are of long duration. Observational tests are proposed for probing the interior of neutron stars.

Response of Low-Mass Main Sequence Stars to Accretion

When the massive component in a close binary system evolves to fill its Roche lobe, mass transfer occurs and gas is accreted onto the companion star. Recently, the response of the unevolved secondary to accretion has been studied by a number of authors, but the emphasis has been on relatively massive stars which have a radiative envelope (Ulrich and Burger 1976; Flannery and Ulrich 1977; Kippenhalm and Meyer-Hofmeister 1977; Neo et al. 1977). The results show that the mass accepting star becomes overluminous and grows in radius until rapid mass transfer ultimately brings the two stars into contact. Such changes in the structure are caused by the steep increase in the specific entropy in the outermost layers and only a small amount of mass (about a tenth of the initial mass of the star) is accreted before contact is made. Thereafter, the expansion of the common envelope will lead to mass loss from the system. It is also found that, for a given accretion rate, the radial increase is much more conspicuous for a smaller mass star. Thus, a characteristic transfer rate which will lead to an increase in radius by, say, a factor of ten is much smaller for a less massive star and becomes as small as 10-6M⊙yr-1 for a model with an initial mass Mi=0.75M⊙, as computed by Neo et al. (1977) assuming a radiative envelope. In such a low mass main sequence star, however, surface convection develops and therefore. response to the accretion is expected to be quite different from that of massive main sequence stars. The evolution of binary systems containing a low mass star is important since such systems may be progenitors of cataclysmic binaries and/or progenitors of Type I supernovae. In this paper, we will focus on the evolution of a low mass main sequence star during accretion.