Mario Livio

The white dwarf mass distribution in classical nova systems

A detailed model of white dwarf mass distribution in newly formed cataclysmic binaries produced by close binary evolution is presently used to derive the distributions for such systems subsequent nova outbursts, in view of selection effects due to the dependences of envelope ignition mass, companion mass, and outburst luminosity on the masses of white dwarfs in nova systems. As much as 25-27 percent of observed nova outbursts should occur on O-Ne-Mg white dwarfs, with the rest occurring almost entirely on C-O white dwarfs. Some evidence of a bimodal character is noted in the empirical mass distribution for white dwarfs, as anticipated by the models. 45 refs.

On the stream-disk interaction in accreting compact objects

The interaction between the stream of material originating from the inner Lagrangian point L(1) and the accretion disk is studied in the framework of an idealized picture of hypersonic flow. When strong simplifying assumptions are made, fully analytic solutions are found for the physical quantities of the shock matter flowing above and below the disk surface. The three-dimensional problem is solved numerically, using a pseudoparticle method. The results are found to depend critically on the strength of the viscous interaction which determines the rate of stream stripping. Various degrees of stream penetration and the formation of dense bulges can be obtained. The results are discussed in relation to observations of compact X-ray sources and cataclysmic variables. 30 references.

Accretion from Stellar Winds

Recent calculations have demonstrated that accretion from a stellar wind is very probably unsteady. The average rate of accretion of angular momentum is lower by about a factor 5 than the rate at which angular momentum is deposited into the Bondi-Hoyle accretion cylinder. This makes disk formation from wind accretion very difficult, in particular in the case of massive x-ray binaries. A combination of x-ray, uv and optical observations of symbiotic and related systems, as well as spin-up information on x-ray binaries, can be used to determine whether an accretion disk does form. Such observations can provide us with valuable information on the process of accretion from an inhomogeneous medium.

The nature of recurrent novae

The observational properties and outburst histories of each member of the small class of recurrent novae are reviewed, with the objective of identifying the outburst mechanisms for these systems. Examples are found of both accretion-powered outbursts (T CrB and RS Oph) and thermonuclear-powered outbursts (T Pyx and U Sco). Outburst properties which might be used in future to distinguish between these mechanisms are identified. Many erstwhile recurrent novae (VY Aqr, RZ Leo, V616 Mon, V1195 Oph, V529 Ori, WZ Sge, and V1017 Sgr) appear to be misclassified examples of other types of cataclysmic variables.

Elemental mixing in classical nova systems

Recent theoretical models of mixing mechanisms in classical novae are reviewed. Observational data on heavy-element and He abundances in nova envelopes are presented in tables, and consideration is given to models based on diffusion-induced convection, shear mixing, convective-overshoot-induced flame propagation, and convection-induced shear mixing. It is found that all of the models have difficulties in accounting for the observed abundances; a number of observational tests are proposed to aid in the unambiguous identification of the mixing mechanism. 43 refs.

Local detonations with application to type I supernovae

A two-dimensional hydrodynamical calculation is performed of a detonation starting from a point, at the bottom of the helium layer in an accreting white dwarf. It is found that, in those cases in which strong double detonations occurred in spherically symmetric calculations, off-center point ignitions also result in double detonations. Spherical damping does not occur. 21 refs.

The theoretical frequency of classical nova outbursts as a function of white dwarf mass

Using the distribution of white dwarf masses in zero-age cataclysmic variables (CVs) calculated by Politano and Webbink (these proceedings), the relative frequency of classical nova outbursts as a function of white dwarf mass is calculated. These results are compared with the results obtained by Truran and Livio (1986), who calculated the same function, but used a distribution of white dwarf masses in CVs calculated from a Salpeter initial mass function and a particular progenitor mass-white dwarf mass relationship for single stars.

SN Ia Models by Coalescence of Two Carbon-Oxygen White Dwarfs

White dwarf coalescence models for type Ia supernovae (SNe Ia) have been recently proposed [1,2] as an alternative to the standard scenario in which a white dwarf accretes hydrogen rich material from a companion star. The need for new models appeared after it was realized that it might be difficult to increase the white dwarf mass up to the Chandrasekhar limit by accretion, at least in a sufficiently large number of systems to account for the SN Ia rate in the Galaxy. Indeed, mass loss during hydrogen and helium flashes and off-center helium detonation considerably restrict the range of values of the accretion rate, white dwarf and companion masses for which central carbon ignition is possible. Another difficulty is the risk of contamination of the spectrum by stripped hydrogen from the companion. Even if a small window of parameters giving a SN Ia explosion may remain (the theoretical modelling of all the processes involved in the presupernova evolution is too uncertain to draw definite conclusions) the search for a new class of progenitors may turn to be more promising.

The influence of our Galaxy on the evolution of the Magellanic Clouds

Building upon the assumption of the existence of a hot gaseous corona around the Milky Way Galaxy and a calculation of possible past orbits of the Magellanic Clouds in the gravitational potential of the Galaxy, the question as to whether significant interchange of material could have occurred with the Large Magellanic Cloud is examined. It is estimated that, indeed, the LMC could have exchanged a nontrivial fraction, of the order of 30 percent, of its mass with the corona, over an assumed 10 to the 10th yr of interaction. It therefore follows that, if the Clouds were bound to the Galaxy during the past 10 to the 10th yr, their chemical and dynamical histories would have been strongly influenced by those of the Galaxy itself. It is concluded that either (1) the Clouds have not been bound to the Galaxy over most of its 10 to the 10th yr lifetime, or (2) it is not possible to treat their evolutions as those of isolated systems. 32 refs.

A decade of extrasolar planets around normal stars: Preface

Humans have long thought that planetary systems similar to our own should exist around stars other than the Sun, yet the search for planets outside our Solar System has had a dismal history of discoveries that could not be confirmed. However, this all changed in 1995, with the past decade witnessing astonishing progress in this field; we now know of more than 200 extrasolar planets. These findings mark crucial milestones in the search for extraterrestrial life – arguably one of the most intriguing endeavors of modern science. These proceedings from the Space Telescope Science Institute Symposium on extrasolar planets explore one of the hottest topics in astronomy today. Discussions include the Kepler mission, observational constraints on dust disk lifetimes and the implications for planet formation, and gravitational instabilities in protoplanetary disks. With review papers written by world experts in their fields, this is an important resource on extrasolar planets.