Cool white dwarfs and the age of the galaxy

Abstract

White dwarf stars represent the most common endpoint of stellar evolution and therefore provide a reliable estimate of the star formation history and age of different Galactic populations. The major observational requirement for accurate age measurements is to have a large sample of cool, old white dwarfs. The intrinsic faintness of the coolest white dwarfs has made them difficult to observe, and the previous studies of the Galactic disk and halo suffered from small samples of cool white dwarfs. The most commonly used sample of cool white dwarfs in the Galactic disk included only 43 stars. The current best estimate for the age of the disk is about 8 billion years. Due to small number statistics, the error in this age estimate is ∼1.5 billion years. In order to reduce the age uncertainty to less than 10%, we have created a large sample of cool white dwarfs from the Sloan Digital Sky Survey. We have used a reduced proper motion diagram to effectively identify white dwarfs among many other field stars and assembled a new white dwarf luminosity function including 6000 white dwarfs. This new luminosity function is consistent with an 8 billion years old population and -when supplied with our ongoing near infrared photometric observations- will provide an accurate age estimate for the Galactic disk. In addition to being accurate cosmochronometers, white dwarfs have become important in the search for dark matter and micro-lensing objects. For many years, astronomers have been trying to understand dark matter. Claims by several investigators that they had found a large number of faint halo white dwarfs in deep field images suggested that halo white dwarfs may exist in large numbers and explain part of the missing matter. In order to test this claim, we have looked at the deepest images of the Universe taken with the Hubble Space Telescope. We have obtained proper motion measurements of the point sources in the Hubble Deep Field North and South, and showed that the observed number of white dwarfs is consistent with the standard Galactic models. We have demonstrated that the white dwarfs in these fields do not contribute to dark matter in the Galaxy. By studying possible planetary systems around white dwarfs, we can predict the future of our solar system and see if the Earth is going to survive the demise of a giant Sun. Even though planets around white dwarfs are waiting to be discovered, the existence of debris disks around several white dwarfs suggest that planets may exist around white dwarfs. Planets in previously stable orbits around a star undergoing mass loss may become unstable, and some of these systems may result in close encounters which could result in tidal stripping of a parent body that would end up in a circumstellar debris disk around a white dwarf. Until recently, there was only a single white dwarf known to have a circumstellar debris disk. We have found four more debris disks around white dwarfs, all of which turned out to be DAZs - white dwarfs with hydrogen rich atmospheres that have trace amounts of metals. Our observations strengthened the connection between the debris disk phenomenon and the observed metal abundances in cool DAZ white dwarfs. We have demonstrated that accretion from circumstellar debris disks can explain the metal abundances in at least 15% of the DAZ white dwarfs, a problem that has been puzzling astronomers for decades.