White Dwarfs and the Age of the Galaxy Kurtis A. Williams & Prof. James Liebert

The Milky Way’s halo and globular clusters all seem to be old. What about the rest of the galaxy? Globular clusters Bulge Disk Halo Credit: 2MASS Project

Our tools of choice for measuring ages are white dwarfs.

Remember, white dwarfs are composed of the carbon and oxygen ashes of stars less massive than about 8M .

Photo: Psycho Therapy Clothing The physics of white dwarfs is dominated by “electron degeneracy.”

As white dwarfs radiate energy, they cool off and fade away.

Determining ages from white dwarfs requires looking at the coolest and faintest white dwarfs.

Our team is trying to model and test the cooling rates of white dwarfs, and then use those rates to get the age of the oldest open star clusters and the disk of the Milky Way. Prof. James Liebert Prof. Don WingetDr. Ted von Hippel Steven Degennaro, Elizabeth Jeffery, Hugh Harris, Jeff Munn, Mukremin Kilic, John Thorstenson, Harry Shipman, the Sloan Digital Sky Survey Consortium, and many more…

Models of white dwarfs are exploring some exotic physics that is impossible to test on the Earth. Electron degeneracy Gravitational settling Neutrino production Crystallization Debye cooling BPM = carats

For young white dwarfs, models agree. But for the oldest white dwarfs, there are big differences. Brad Hansen (1999)

We can use open star clusters to help us choose the right white dwarf models. Messier 35

Open star clusters have ages from <1 Myr to 7 Gyr, and so we can check all parts of white dwarf models. von Hippel (2005)

Messier 35 is an adolescent star cluster (175 Myr old); its white dwarf age agrees with the star cluster age. 150 Myr 175 Myr 200 Myr Williams et al. (2008)

NGC 6791 is one of the oldest open clusters. Its white dwarf age is a little mysterious. Bedin et al. (2008) Sloan Digital Sky Survey

I have Hubble imaging of another old star cluster, NGC 188, in order to study its white dwarfs. Palomar Sky Survey Age = 7 Gyr

Concurrently, we are looking for the oldest and coolest white dwarfs in our Milky Way galaxy. NASA/JPL-Caltech/R. Hurt

Cool white dwarfs are hard to find; They are extremely faint, and their colors look like normal stars. Richer et al. (2000)

But white dwarfs tend to be nearby, and have high “proper motions.”

To find the oldest white dwarfs, we compare pictures of the same field of stars several years apart and look for moving stars. POSS 1 (early1950s) POSS 2 (80s and 90s) van Maanen 2 12th Magnitude DZ, Sun’s 25th nearest neighbor 13.9 light years -- proper motion is 2.95 arc sec per year (39th largest proper motion) It’s about the T eff of the Sun.

In the 1980s, Prof. Liebert and Prof. Winget analyzed the coolest white dwarfs known. Liebert, Dahn and Monet (1988) Prediction for a galaxy constantly making stars for 9 Gyr Only 3 stars!

In the 1990s, our team combined the Sloan Digital Sky Survey with the older Palomar Sky Survey to find fainter white dwarfs. Harris et al. (2006)

We are now re-imaging the entire Sloan Digital Sky Survey (1/4 of the visible sky, or 5000 sq. degrees) to find even fainter white dwarfs.