Charles F. Claver
Tucson Nighttime Scientific Staff
Areas of Interest
Stellar Ages, White Dwarf Structure and Evolution, Stellar Photometry,
Optical Instrumentation
Recent Research Results
Claver's research focuses on obtaining an independent estimate of the
Universe's age and, as a by product of doing so, resolve the apparent
discrepancy between the expansion age and the main-sequence age of the
oldest stars we see - the globular clusters. The age dating technique
Claver uses exploits the relatively simple physics found in the cooling
remnants of stellar evolution - white dwarf stars. The age of a white
dwarf is directly related to its luminosity, and any reasonable
estimates for the age of the Universe still allow for the oldest white
dwarfs to be visible. Therefore, a census of white dwarfs according to
their brighteness - called a luminosity function - in any stellar
population will show an abrupt cutoff at low luminosities that is
dependent on the population's age. Claver has used this fact and
observations of white dwarfs in the open clusters Praesepe and NGC 752
to show the white dwarf and main-sequence ages are in good agreement up
to 3 billion years old. If the agreement persists at older ages so that
we can be confident in the ages of the globular cluster, then the
current estimate of Hubble's constant does not allow for a simple
inflationary cosmology and a more complex one is demanded. Otherwise,
if the agreement breaks down beyond ages of 3 billion years, we must be
suspect of the estimates of stellar ages and re-evaluate the state of
stellar evolution calculations. To date all estimates of the white
dwarf luminosity function for the Galaxy's disk suffer from poor
statistics at their faint ends and prevent us from fully utilizing the
excellent clocks offered to us by white dwarf stars. Thus, age
estimates for the Galactic disk from these luminosity functions range
from 8-13 billion years, which is not precise enough to help resolve
the age dilemma. As part of Claver's thesis work he has developed a
method for identifying cool white dwarf candidates from photometry
alone. With his technique Claver has initiated a deep photometric
survey to search for cool white dwarfs in the field. To date Claver's
survey has covered enough area to improve the number statistics in the
cool part of the disk white dwarf luminosity function by a factor of
3-5 over previous estimates.
Future Research Plans
Over the next several observing seasons Clavers plans are to continue
spectroscopic follow-up observations with WIYN Hydra of his cool white
dwarf photometric survey. The cool white dwarfs identified will be used
to redefine the cool part of the white dwarf luminosity function
(WDLF). These data are important not only for estimating the Galaxy's
age, but also for placing an observational constraint on the importance
of phase separation of a carbon-oxygen mixture in crystallization of
white dwarf cores. Phase separation, if it happens, releases
additional energy into the white dwarf core further delaying the
cooling process beyond the delay caused by the release of latent heat.
The exact nature of white dwarf crystallization causes observable
features in the WDLF and has a large effect on the inferred white dwarf
cooling ages. He will also work toward increasing the area of his
survey in order to increase the detection sensitivity of older, cooler
white dwarfs belonging to the Galactic Halo. Also, Claver plans to
extend his work on calibrating the stellar chronology in star clusters
to ages older than 3 billion years. Specifically, he plans to search
for and identify the oldest white dwarfs in the clusters M67 and NGC
188 in the North and IC 4651 and NGC 3680 in the south using both
ground- and space-based telescopes. These clusters will extend the
calibration to roughly 8 billion years, which is sufficient to
constrain the source of the present differences in the Universe's
expansion age and its oldest stars.
Service
Within the Kitt Peak Scientific Staff Claver holds the title of Imaging
Scientist. As part of his service activities he has begun a coherent
comprehensive look at the imaging quality produced by Kitt Peak
telescopes with the aim of having all Kitt Peak telescopes deliver the
excellent seeing of which the site is capable. To this end, Claver has
taken on the responsibility of overseeing and maintaining optical
alignment of Kitt Peak telescopes, as well as debugging problems when
they occur. Claver also contributes to Kitt Peak service by being a
staff contact and providing some of the start-ups for visiting direct
imaging observers. As a member of the 4-m imaging improvement group,
Claver is investigating the performance of the 4-m primary support
system to determine if and where significant improvements can be made
in the delivered image quality of this valuable telescope. He is also
participating in critical design reviews of the new wide field 4-m
Prime-Focus Corrector and Mosaic project. In addition, Claver has
initiated a study of high frequency image motion at WIYN. Analysis of
these data by Claver and others is providing essential information for
the development of an Adaptive Optics program for WIYN.
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Posted: 06Dec1996