Letters of Intent received in 2016

LoI 2018-1886
White Dwarfs: probes of exo-planets, stellar and galactic evolution and cosmology

Topics

Fundamental physics from observations of white dwarfs

White dwarf mass radius relation and degenerate matter

White dwarfs in binaries and Type 1a supernovae

Composition of extrasolar planetary debris

Circumstellar material and structure of the local ISM

End points of stellar evolution

Atomic physics in the laboratory and in white dwarf atmospheres

Rationale

White dwarfs are ubiquitous in astronomy. Over 90 percent of all stars currently on the main sequence will end their lives as white dwarfs. As such, they are important laboratories for the study of stellar evolutionary processes, the evolution and history of the Milky Way Galaxy, the evolution of planetary systems and the behaviour of matter at extremes of temperature and density. Surveys such as SDSS and SPY have given us access to unprecedented numbers of members of the white dwarf population. Recent studies incorporating these databases have initiated a revolution in our understanding of the global properties of this white dwarf population that will continue to grow with the upcoming Gaia and LSST results. For example, consider white dwarf evolution. Once a white dwarf is formed, its evolution is dominated by cooling. As they cool over timescales of billions of years, determinations of the age of the oldest (coolest) white dwarfs place limits on the ages of the components of the Galaxy, such as the thin and thick disks, and the the system of open and globular clusters. The detailed temperature distribution of the white dwarf population contains information about the star formation history of the Galaxy.

White dwarfs are also extremely important indicators for cosmology. Type Ia supernovae are the standard candles indicating the acceleration of cosmic expansion. It is extremely important to have a clear understanding of the progenitor systems, yet the evolutionary path leading to these explosions is poorly understood. Recent surveys have begun to reveal the properties of single and double degenerate progenitors, but more work is needed.

In the past few years, white dwarfs have also begun to influence our understanding of the evolution of planetary systems. We have strong evidence that white dwarfs harbor planetary systems. In a very exciting development, we now know that white dwarfs can disrupt minor planetary bodies and the resulting debris is accreted onto the white dwarf. White dwarfs have a unique photospheric characteristic. The high surface gravity (log g ~ 8) naturally leads to chemically pure hydrogen or helium photopheres. This means that the spectral features produced by the accreted material are not contaminated by original photospheric abundances. The observed features provide a unique opportunity to study the bulk composition of extraolar planetary material. In a subset of accreting white dwarfs we find spectral features of highly ionized heavy elements. Accurate measurements of observed wavelengths can be compared with laboratory measurements to probe the possible variation of the fine structure constant in a strong gravitational field.

The 2018 IAU General Assembly presents an opportunity to formulate the direction of white dwarf studies in the next decade. The relevance of white dwarfs to so many astrophysical fields makes an IAU Symposium devoted to them and their important scientific results particularly timely. There has not been an IAU Symposium devoted to white dwarfs for many years. Indeed, the last one we can find references for was held in 1970, and that meeting preceeded the 14th General Assembly. There was an IAU colloquium in 1988 and a specialist discussion meeting at the IAU General Assembly in 2003. The proposed 2018 Symposium will bring together scientists from a range of fields including laboratory astrophysics, observational astronomy, theoretical modeling and fundamental physics, necessary to deliver new advances in the diverse scientific endeavors outlined in this proposal. Involvement of experts from such a wide range of astrophysical disciplines is best achieved by holding the Symposium as part of the General Assembly, rather than as an isolated meeting. The SOC will structure this Symposium to highlight topics that are both timely and of interest to the general astronomical community.