Pulsating White Dwarf Stars and Precision Asteroseismology

Abstract

■ AbstractGalactic history is written in the white dwarf stars. Their surface properties hint at interiors composed of matter under extreme conditions. In the forty years since their discovery, pulsating white dwarf stars have moved from side-show curiosities to center stage as important tools for unraveling the deep mysteries of the Universe. Innovative observational techniques and theoretical modeling tools have breathed life into precision asteroseismology. We are just learning to use this powerful tool, confronting theoretical models with observed frequencies and their time rate-of-change. With this tool, we calibrate white dwarf cosmochronology; we explore equations of state; we measure stellar masses, rotation rates, and nuclear reaction rates; we explore the physics of interior crystallization; we study the structure of the progenitors of Type Ia supernovae, and we test models of dark matter. The white dwarf pulsations are at once the heartbeat of galactic history and a window into unexplored and exotic physics. A TASTE OF HISTORYThe history of science contains blueprints for the future. We see common threads running through much of the stories of past progress and discovery. Looking back at the history of scientific achievement, we see this theme repeated many times: meaningful progress requires that theory and experiment advance together. They must coordinate like two legs of a child learning to walk. If one leg gets too far ahead, the child falls. To move forward again, the child is forced to gather both legs together to stand. Examples when one leg gets farther in front are numerous in all fields of science. An early astronomical example is the Greek idea that the Earth might orbit the Sun. Searching for and not finding parallax, they abandoned the idea---the observational techniques of the time were not adequate. More recently, the cosmic background radiation might have been discovered in the 1940s through the observations of interstellar lines, were it not for lack of a theoretical context. Our analogy works in another way: It doesn't matter too much which leg takes the first step as long as they stay close enough together. Kepler built his laws on Tycho's careful observations; Newton "stood on the shoulders of giants;" Einstein's general relativistic description of gravity required Eddington and a total solar eclipse to be taken seriously.The attentive reader will see many parallels throughout this review between the quantum theory of the atom and white dwarf stars. The structure of both is determined through the delicate balance of a strong central force against the uncertainty and exclusion principles. Not surprisingly, their stories are intertwined; the white dwarf stars have a long history as a cosmic laboratory for quantum physics and general relativity.We are not historians; the science we review is of a more humble nature than these lofty examples. We offer the history of the field---from the inevitably biased viewpoint of your guides---as an introduction and a starting point for a blueprint...

Kepler and the seven dwarfs: detection of low-level day-time-scale periodic photometric variations in white dwarfs

“…The pulsations typically have periods of ∼ 100-1000 s, and up to ∼ 1 hr in very low-mass WDs (Winget & Kepler 2008). WD rotation can induce splitting of pulsation frequencies in the power spectrum.…”

New evolutionary sequences for extremely low-mass white dwarfs

“…In addition, they are used to place constraints on properties of elementary particles, such as axions (Isern et al 1992(Isern et al , 2008Córsico et al 2012a,b) and neutrinos (Winget et al 2004), or on alternative theories of gravitation (García-Berro et al 1995. These and other potential applications of white dwarfs require a detailed and precise knowledge of the main physical processes that control their evolution (see Fontaine & Brassard 2008;Kepler 2008, andAlthaus et al 2010a, for a review).…”

Triennial Report (2012-2015): The Legacy Issue

“…Reviews of pulsating white dwarfs have been published by Winget & Kepler (2008), Fontaine & Brassard (2008), and Althaus et al (2010). Kepler et al (2005b) and Mukadam et al (2013) measured the rate of evolution of T eff 12 000 K white dwarfs using the rate of change of their pulsation periods, while Costa et al (2008a) measuredṖ for a T eff 140 000 K, PG 1159-035 pulsator.…”