Transcript Current White Dwarf Research

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
Disk
Halo
Bulge
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.
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.
Dr. Ted von Hippel
Prof. James Liebert
Prof. Don Winget
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 37093 = 1034 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.
Sloan Digital Sky Survey
Bedin et al. (2008)
I have Hubble imaging of another old
star cluster, NGC 188, in order to
study its white dwarfs.
Age = 7 Gyr
Palomar Sky Survey
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 Teff of the Sun.
In the 1980s, Prof. Liebert and Prof.
Winget analyzed the coolest white dwarfs
known.
Prediction
for a galaxy
constantly
making stars
for 9 Gyr
Only 3
stars!
Liebert, Dahn and
Monet (1988)
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.