Topics of Investigation (completely outdated!)

 White dwarf evolution
 Stellar Pulsations
 Asymptotic giant branch (AGB) stars and formation of white dwarfs
 Binary evolution

White dwarf evolution

White dwarf stars are the electron-degenerate stellar remnants of the majority of all stars. These stars constitute a natural laboratory to  confront our current theory of stellar evolution with the physical properties of dense matter.  In addition to their intrinsic interest to the  stellar evolution,  they have recently received particular attention as they may be linked to other astrophysical fields. They are strong candidates for the dark matter of the Galaxy,  (e.g. Alcock  et  al.,  1998,  ApJ, 491, L11;  1999  ApJ 518, 44). They  also provide estimations of ages  for different stellar systems such as the local Galactic disk  (Winget et  al.  1987 ApJ 315, L77; Wood 1992, ApJ, 386, 539)  and globular  and open clusters (cf. von Hippel & Gilmore 2000, AJ, 120, 1384).  In this sense, white dwarfs can be used  to trace the  evolutionary history of our Galaxy.

Our research group  has developed a numerical code (LPCODE) appropriate for the study of the evolution of degenerate configurations based on an updated and detailed constitutive physics such as a full network for thermonuclear  reactions, OPAL radiative opacities,  full-spectrum turbulence theories of convection, and detailed equations of state and neutrino emission rates.   We have also developed a set of routines that compute the evolution of the chemical abundance distribution caused by  gravitational  settling,  chemical and  thermal  diffusion of  nuclear species. In particular, we have studied the evolution of  low-mass, helium-core white dwarfs resulting from the evolution  of close binary systems (Althaus et al., 2001, MNRAS, 323, 471).  Our results shows that discrepancies between spin-down ages and  the  predictions  of standard white dwarf evolutionary models (van Kerkwijk  et al.,  2000, ApJ,  530, L37) appear to be the result of ignoring element  diffusion in evolutionary calculations. The recent detection of  low-mass white dwarfs in compact binaries belonging to globular clusters (see, i.e., Taylor, Grindlay, et al., 2001, ApJ, 553,  L169) has also sparked the attention of many researchers.  Indeed, the interest in studying low-mass  white dwarfs in globular clusters is motivated not only by their importance in the understanding of the formation and evolution of the compact binaries in which these stars are found but  also by the possibility they offer of constraining globular cluster dynamics and evolution.  It is worth mentioning that our evolutionary results for these stars have received tentative support from the optical detection of the helium white dwarf companion to the millisecond pulsar in 47 Tucanae (Edmonds et  al., 2001, ApJ,  557, L57).

On the  other hand, it is  well known that white dwarfs are excellent candidates to test the existence of several weakly interacting
massive particles such as  axions. In this connection, we started a collaborative effort with  Drs. Enrique Garcia-Berro  and Jordi Isern of the University of Barcelona. In  particular, we have used our evolutionary models to perform a comprehensive study of the pulsational  characteristics of  the variable hydrogen-rich white dwarf G117-B15A. This has allowed us to place tight constraints on the mass of the  axion (Corsico et  al.  2001, New Astronomy,  6, 197),  improving  previous efforts. In collaboration with the above mentioned authors we  are currently working on a project aimed at understanding the evolutionary and pulsational properties of massive white dwarf stars with  oxygen and neon cores.


 Stellar pulsations

White dwarf stars are  pulsationally unstable in three temperature regimes, with typical periods in the range 100-1000 seconds. Over the last past decade, the study of pulsational pattern of variable white dwarfs through asteroseismological techniques has become a very powerful tool for probing the internal structure and evolution of these stars. In particular, asteroseismology of massive white dwarfs has  recently drawn the attention of researchers in view of  the possibility it offers to place constraints on the crystallization process in the  interior of white dwarfs.  This has been motivated by the discovery of pulsation in the star BPM  37093, a  massive white dwarf which should be largely crystallized.

Our group has developed a pulsational code that compute the linear, adiabatic, non-radial stellar pulsations (Corsico, 2003, PhD., University of La Plata).  This code is fully coupled to the LPCODE evolutionary code, which has enabled us to study the pulsations of  variable white dwarfs. One of our main results concerns the mode trapping properties of white dwarfs.  We find that element  diffusion strongly  smoothes out  the chemical  profiles,  making the mode trapping caused by the outer chemical  interfaces notably less important (Corsico et  al., 2001, A&A, 380, L17).  In collaboration with Michael Montgomery of the University of Texas we started a joint project aimed at exploring the pulsational properties of massive white dwarfs on the basis of  new and improved evolutionary models for these  stars that take into account time-dependent element diffusion, nuclear  burning and  the history of the white dwarf progenitor.  Our first results suggest that the pulsational properties become very sensitive to the occurrence of core overshooting during the evolutionary stages prior to the white dwarf formation  (Althaus L.G., Serenelli A. M., Corsico  A. H. & Montgomery M. H., 2003, A&A, 404,593). In this connection, we are currently investigating the effect of a solid core on the pulsational pattern of crystallized white dwarfs. 

In  the context  of pulsating stars, variable white dwarfs with  helium-rich envelopes are  likewise  within  our current  research  interest. With  Alfred  Gautschy we  are  studying  the non-adiabatic  pulsational properties  of such  stars by employing detailed  stellar models  which explicitly account  for the evolution  of chemical  distribution  due to  diffusion processes  and  modern theories  of turbulent convection. Our first  results suggest a  weaker trapping effect in the periodicities than previously believed (Gautschy & Althaus, 2002, A&A, 382, 141). 

An analysis of the secular instability in intermediate mass stars with core helium burning is likewise within the scope of our interest. This
aspect is currently underwent in a joint project with Alfred Gautschy. 


Asymptotic giant branch (AGB) stars and formation of white dwarfs 

There are a number of relevant and open astrophysical problems about the  advanced evolutionary phases of  low-and intermediate-mass stars. Amongst  these are a quantification  according to first principles of the various mechanisms that lead  nuclearly processed  matter to the surface; the occurrence driving  mass loss and the relation between initial stellar mass and final white dwarf mass; the value of the minimum mass for intermediately degenerate carbon ignition; and the origin of neon- and  magnesium-rich  white dwarfs. Solution to these problems is required as a whole, and as an  input for Galactic chemical evolution, population synthesis, interpretation of colors of distant galaxies and so on.  All these problems deal with evolutionary phases following central helium  exhaustion, from the base of  the asymptotic giant branch (AGB) to the  final ejection of planetary nebula, after which the blueward excursion  leading to white dwarfs begins.

In this regard, we are studying some of the above-mentioned aspects on the basis of new and improved evolutionary models we are currently developing.  We mention the treatment of the abundance changes which consider nuclear burning,  time-dependent convective  mixing  and overshooting,  semiconvection,  salt finger  instability and element  diffusion. Our major aim is the computation of the whole evolution of  intermediate-mass stars from the main sequence stage through the thermally pulsing and mass loss phases on the AGB to the white dwarf regime.  Aspects such as the study of  diffusion-induced  hydrogen shell  flashes, the exploration of white dwarf formation 
the  study of pulsation of hot  white dwarfs, AGB and  post-AGB  stars, carbon  stars  and the  PG 1159--DB--DQ  evolutionary connection are  within the scope of our immediate objectives. 

Binary evolution 

White  dwarfs are often found  in binary systems containing usually another white  dwarf, a millisecond pulsar or a main sequence
star. Cataclysmic variables and X-ray binaries are commonly associated  with white dwarf stars. We are studying the  problem of close binary evolution and the formation of low-mass white dwarfs in globular clusters (Serenelli et al. 2002, MNRAS, 337, 1091).  These topics are currently of much interest for researchers. In particular,  the HST detection of a sequence of  low-mass white dwarf candidates in the cluster NGC 6397 (Taylor et al., 2001, ApJ, L169)  has prompted us to compute evolutionary models for such white dwarfs with the  aim of placing on theoretical grounds some expeculations about the formation and evolution of such white dwarf stars.

In the light of theoretical evidence suggesting that some of the presumed low-mass helium core white dwarfs could actually be white dwarfs with oxygen cores we have started a collabortive effort with Zhanwen Han at Oxford University with the aim of exploring the formation and evolution of carbon-oxygen  white  dwarfs  with stellar  masses as low as  0.3  solar masses. To this end we are computing the conservative close binary evolution of a 2.5 solar mass star in a close binary system from the main sequence to the white dwarf stage. The stellar mass of the secondary is 1.25 solar masses and the systems has an initial period of 3 days.  Our first results suggest that both the pulsational and evolutionary properties of oxygen core white dwarfs differ appreciably from those of their helium-core counterparts. In particular, our results indicate that future asteroseismology could be a promising way of distinguishing both
types of stars if low-mass white dwarfs were in fact found to pulsate.