Dean Townsley's Current Research

My current research has two main foci. The first is Type Ia supernovae theory, in which I am mainly interested in determining how features of the progenitor star will manifest in the outcome of the explosion. Since our understanding of the mechanism which produces Type Ia supernovae remains incomplete, I am also interested in improving our ability to realistically simulate the explosion process. The second major focus of my research is on properties of the white dwarf (WD) primary in mass transferring white dwarf-main sequence star (WD-MS) and WD-WD binaries, cataclysmic variables. A major goal of this work is to better understand the population of these objects, which is also related to the population of progenitors for Type Ia Supernovae.

Progenitor Structure and Type Ia Supernovae

Products of fusion just after transition from deflagration to detonation. The Blue line represents the star's surface and the colors indicate various products of the fusion of Carbon and Oxygen: Red is mostly Silicon and Oxygen, Green mostly Silicon, and Black mostly Iron-group material.

While Type Ia supernovae as a class show remarkble regularity compared to other supernova types, there is still important inhomogeneity within the population. The ability to use SN Ia to measure luminosity distances, and therefore the expansion history of the universe, to high precision depends upon an understanding and characterization of this irregularity among explosions. The contribution of SN Ia to the composition of insterstellar gas, and how it changes with time might depend in important ways on the diversity of products of the explosion.

At the simplest level diversity in observed SN Ia is related to the properties of the progenitor just prior to explosion: its compositional structure, mass, temperature, and spin or rotational structure. Additional diversity will arise from chaotic processes occuring during the explosion. With these sources in mind, diversity can be thought of as having two forms: random and systematic. Random diversity, whether it arises from chaotic processes in the explosion or earlier in the evolution of the system, such as during binary formation, is innocuous for luminosity distance measures. In contrast systematic diversity must be accounted for because stellar, and therefore progenitor, populations differ in specific ways in the acient universe. The mix of active and passive star-forming regions, the metallicity distribution, and the average progenitor age will all depend on look-back time.

While current understanding of the supernova explosion mechanism is incomplete, the state of the art is advanced enough to produce simulation outcomes that are physically motivated, if not fully consistent, and similar to observed explosions. In general simulations can give a much wider variety of outcomes than are observed, due to uncertainties in some physical aspects which must be parameterized. Thus by making some simplifying assumptions, which can hopefully be updated in the near future, it is possible to study the dependence of the explosion on variations in progenitor structure.

White Dwarf Primaries in Cataclysmic Variables

Comparing theoretical and observed effective temperatures of CV primaries. From Townsley & Gänsicke 2009.

Binary systems which tranfer matter onto their WD primary display a wide variety of dynamic stellar phenomena. These include Dwarf Novae, Classical Novae, and Supernovae in addition to a variety of variability related to the orbital motion, the accretion disk and pulsations of the primary WD itself. Building on my extensive work on the surface and interior thermal structure of the WD in these systems, it is possible to use observations to constrain long-term mass transfer rates and thereby evolution of these binary systems. Many general features governing the evolution of these short-period binaries remain poorly understood. The mass transfer is driven by angular momentum loss from the system, but the nature of the mechanism is only very grossly characterized, with many details not understood.

The existence of WD primaries which exhibit pulsations offers the exciting opportunity to gain information about the interior of these stars. Many features of the star, such as mass, spin and internal compositional structure will be reflected in the frequencies at which variations occur. The variation of the surface properties in response to changes in the accretion rate on observable timescales (months-years) also offer a unique opportunity for the study of the driving of these oscillations. This is important as a counterpoint to the study of isolated WDs which exhibit similar oscillations but have surface properties which do not vary in time.