Dina Prialnik

An Extended Grid of Nova Models. II. The Parameter Space of Nova Outbursts

This paper is a sequel to an earlier paper devoted to multiple, multicycle nova evolution models (Prialnik & Kovetz, Paper I), which showed that the different characteristics of nova outbursts can be reproduced by varying the values of three basic and independent parameters: the white dwarf mass, MWD, the temperature of its isothermal core, TWD, and the mass transfer rate, . Here we show that the parameter space is constrained by several analytical considerations and find its limiting surfaces. Consequently, we extend the grid of multicycle nova evolution models presented in Paper I to its limits, adding multicycle nova outburst calculations for a considerable number of new parameter combinations. In particular, the extended parameter space that produces nova eruptions includes low mass transfer rates down to 5 × 10-13 M☉ yr-1 and more models for low TWD. Resulting characteristics of these runs are added to the former parameter combination results to provide a full grid spanning the entire parameter space for carbon-oxygen white dwarfs. The full grid covers the entire range of observed nova characteristics, even those of peculiar objects, which have not been numerically reproduced until now. Most remarkably, runs for very low lead to very high values for some characteristics, such as outburst amplitude A 20, high super-Eddington luminosities at maximum, heavy element abundance of the ejecta Zej ≈ 0.63, and high ejected masses mej ≈ 7 × 10-4 M☉.

Evolution of a classical nova model through a complete cycle

A classical nova model is evolved through a complete cycle, including accretion leading to outburst, mass loss, accretion again, ending in another outburst. Mass loss is found to occur in three different stages: shock ejection, continuous mass loss through an optically thick wind, and concentration of the central star, leaving behind a thin, slowly expanding nebula. There is a striking correspondence between the evolution of the model and the observed development of the nova spectrum. The entire accreted mass is ejected, as well as a small amount of the underlying WD material. The composition of the ejecta is very similar to that obverved in nova shells, with a similar estimated Z. A remnant helium-rich envelope of small mass is left on the WD. 52 references.

Crystallization, sublimation, and gas release in the interior of a porous comet nucleus

A numerical code is developed for evolutionary calculations of the thermal structure and composition of a porous comet nucleus made of water ice, in amorphous or crystalline form, other volatiles, dust, and gases trapped in amorphous ice. Bulk evaporation, crystallization, gas release, and free (Knudsen) flow of gases through the pores are taken into account. The numerical scheme yields exact conservation laws for mass and energy. The code is used to study the effect of bulk evaporation of ice in the interior of a comet nucleus during crystallization. It is found that evaporation controls the temperature distribution; the vapor prevents cooling of the crystallized layer of ice, by recondensation and release of latent heat. Thus high temperatures are maintained below the surface of the nucleus and down to depths of tens or hundreds of meters, even at large heliocentric distances, as long as crystallization goes on. Gas trapped in the ice and released during the phase transition flows both toward the interior and toward the surface and out of the nucleus. The progress of crystallization is largely determined by the contribution of gas fluxes to heat transfer.

Evaporation from a porous cometary nucleus

In a porous cometary nucleus, ice sublimates from the volume of a surface layer rather than just from the upper boundary. Given a model for the porous medium, the equations of mass and heat transfer can be solved for any desired orbit. The temperature profile and the vapor flux as a function of depth in the upper layer of the nucleus may thus be obtained. Calculations are performed for a spherically symmetric icy nucleus in the orbit of Comet P/Halley, assuming different values of porosity and different models for the ice structure. The upper layer may be divided in two zones: in the uppermost zone, whose thickness ranges from 100 microns to about 1 mm, the vapor flux is directed outward, whereas in the lower zone, which is 1000 times thicker, the vapor flows in the opposite direction. The sublimation rate as a function of heliocentric distance depends strongly on the porosity of the nucleus and is little affected by other parameters related to the structure of the ice. This allows the determination of the porosity coefficient of a comet from observation of its water production rates at large heliocentric distances. 18 refs.

Radiogenic heating of comets by 26Al and implications for their time of formation.

The effect of radiogenic heating on the thermal evolution of spherical icy bodies with radii 1 km < R < 100 km was investigated. The radioisotopes considered were 26Al, 40K, 232Th, 235U, and 238U. Except for the 26Al abundance, which was varied, the other initial abundances were kept fixed, at values derived from those of chondritic meteorites and corresponding to a gas-to-dust ratio of 1. The initial models were homogeneous and isothermal (To = 10 K) amorphous ice spheres, in a circular orbit at 10(4) AU from the Sun. The main object of this study was to examine the conditions under which the transition temperature from amorphous into cubic ice (Ta = 137 K) would be reached. It was shown that the influence of the short-lived radionuclide 26Al dominates the effect of other radioactive species for bodies of radii up to approximately 50 km. Consequently, if we require comets to retain their ice in amorphous form, as suggested by observations, an upper limit of approximately 4 x 10(-9) is obtained for the initial 26Al abundance in comets, a factor of 100 lower than that of the inclusions in the Allende meteorite. A lower limit for the formation time of comets may thus be derived. The possibility of a coexistence of molten cometary cores and extended amorphous ice mantles is ruled out. Larger icy spheres (R > 100 km) reached Ta even in the absence of 26Al, due to the decay of the other radionuclides. As a result, a crystalline core formed whose relative size depended on the composition assumed. Thus the outermost icy satellites in the solar system, which might have been formed of ice in the amorphous state, have probably undergone crystallization and may have exhibited eruptive activity when the gas trapped in the amorphous ice was released (e.g., Miranda).

The Composition of Nova Ejecta from Multicycle Evolution Models

Following a previous systematic study involving calculations of evolutionary sequences of nova outbursts through several cycles, for combinations of parameters—the accreting white dwarf (WDs) mass, its core temperature, and the mass transfer rate—spanning the entire parameter space, assuming CO WDs (with C and O in equal mass fractions), we now consider the detailed composition of the ejecta for the subset of models which simulated classical nova outburst. We also investigate the effect of the additional input parameter—the WD composition—on nova characteristics by calculating evolutionary sequences with pure-carbon and pure-oxygen WD progenitors. The stellar evolution code used includes an extended nuclear reactions network, OPAL opacities, and diffusion of all elements. Our main conclusions are that CO progenitors reproduce most of the observed abundances and abundance ratios, and that correlations between them, if any, are in very good agreement with observations. The WD composition is generally not reflected in the abundances of the ejecta: whereas a large fraction of the carbon is always turned into nitrogen, oxygen is in some cases unaffected and in others almost completely destroyed. Hence ejecta abundances cannot be used to deduce the WD composition. Ejected masses of pure O WD progenitors exceed those of CO progenitors, sometimes by a factor of 4. The isotopes 13C and 17O are in all cases significantly overabundant, compared to the solar composition:12C/13C varies between 0.97 and 3.8 (by number) and 16O/17O varies between 1.8 and 55;15N, however, is sometimes greatly enhanced and sometimes underabundant,14N/15N varying over a very wide range, from 2.4 to 33,000.

The formation of a permanent dust mantle and its effect on cometary activity

The growth of a permanent, permeable, dust mantle on the surface of a comet nucleus, composed initially of dusty amorphous water ice, is investigated. Numerical simulations of the evolution of one-dimensional comet nucleus models, in Comet Halleys orbit, are carried out for various parameters, allowing for the crystallization of the amorphous ice. It is assumed that the mantle forms gradually, by the accumulation of a constant fraction (0.001-0.01) of the dust, which is not carried away with the sublimating ice. It is found that an approximately 1-cm-thick dust mantle diminishes the average sublimation rate by a factor of approximately 5, and a further growth of the dust mantle may decrease the surface activity of the nucleus by another factor of 10. Therefore, the activity of a dust-covered nucleus is expected to result mainly from exposed patches of ice and from craters, such as were observed on Comet Halley by Giotto. These are formed by explosions of gas-filled pockets in the crystalline outer layer of the nucleus. The insulating effect of the dust mantle causes the crystallization of the amorphous ice to proceed at a slower rate than in the case of a bare icy nucleus. Thus, the thickness of the outer crystalline shell, overlying the amorphous ice core, is always greater than 15 m, but does not exceed a few tens of meters. This size range is compatible with the amount of gas released in the numerous small explosions which were observed on Comet Halley.

What does an erupting nova do to its red dwarf companion

During nova eruptions and for decades afterward, the red dwards in cataclysmic binaries are irradiated with hundreds of times more luminosity than they themselves produce. Simulations of the time-dependent irradiation of three red dwarf models (0.25, 0.50, and 0.75 solar mass) are presented. The mass transfer rates forced by irradiation after nova eruption are found to be enhanced by two orders of magnitude because of the irradiation. The time scale for irradiation to become unimportant is that of the white dwarf cooling time scale, a few centuries. These two results support the hibernation scenario of novae, which suggests that novae remain bright for a few centuries after eruption because of irradiation-induced mass transfer. After irradiation decreases mass transfer slows, and some very old novae may then become extremely faint. 26 references.

Gas release in comet nuclei

The evolution of a comet nucleus is investigated, taking into account the crystallization process by which the gas trapped in the ice is released to flow through the porous ice matrix. The equations of conservation of the energy and of the masses of ice and gas are solved throughout the nucleus, to obtain the evolution of the temperature, gas pressure and density profiles. A spherical nucleus composed of cold, porous amorphous ice, with 10% of CO trapped in it, serves as initial model. Several values of density (porosity) and pore size are considered. For each combination of parameters the model is evolved for 20-30 revolutions in comet P/Halleys orbit. Two aspects of the release of gas upon crystallization are analyzed and discussed: (a) the resulting continuous outward flux with high peaks at the time of crystallization, which is a cyclic process in the low-density models and sporadic in the high-density ones; (b) the internal pressures obtained down to depths of a few tens to approximately 200 m (depending on parameters), that are found to exceed the compressional strength of cometary ice. As a result, both cracking and explosions of the overlying ice layer and ejection of gas and ice/dust grains are expected to follow crystallization. They should appear as outbursts or sudden brightening of the comet. The model of 0.2 g cm-3 density is found to reproduce quite well many of the light-curve and activity characteristics of comet P/Halley.

The formation of an ice crust below the dust mantle of a cometary nucleus

Using a cometary model which assumes a porous cometary nucleus covered by an equally porous thin permanent dust mantle, the possible formation of an ice crust below the surface of the comet nucleus by the condensation of the inward-flowing vapor is investigated. It is found that, for any given porosity value, there is a range of dust-mantle thicknesses at which the formation of a crust is favored. Below this range, sublimation is too strong, making it impossible for any change in the ice structure within a surface layer to occur before the layer evaporates; above this range, the thick dust mantle quenches the vapor production rate. The consequences of the ice crust formation are examined for two models, with the dust mantles 5-mm and 1-mm thick, respectively. 25 refs.