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The Faintest Known White Dwarf (1Mar96)
(from NOAO HIGHLIGHTS!, NOAO Newsletter No. 45, March 1996)

The white dwarf luminosity function incorporates information on the age
and star formation history of the Galactic disk that is independent of
other techniques. In particular, the least luminous white dwarfs set
lower limits on the disk age, and thus the age of the Universe. The
faintest known white dwarfs have Mv ~ 16.2. Age estimates for these
stars are in the range 6.5-11 Gyr, with the variation largely due to
uncertainty in the core composition. The white dwarf luminosity
function declines abruptly at this absolute magnitude, which has been
interpreted as being due to the age of the Galactic disk. The question
remains, do even less luminous and older white dwarfs--however
rare--exist? The search for very low luminosity white dwarfs is still
very active. A concerted effort has been carried out for a number of
years by Maria Teresa Ruiz and her colleagues at Cerro Calan
Observatory (Chile). Selection of promising candidates is done with a
classic technique, blinking plate pairs from the ESO Schmidt telescope
to identify faint stars with substantial proper motion. The magnitude
limit is about m sub R ~ 21 and motions as small as 0.1" yr sup -1 are
detectable with a few years' time base. Followup observations at CTIO and
other facilities in Chile help winnow the most interesting objects out of
the hundreds found on a plate pair.

One such object is the cold DC type white dwarf ESO 439-26. Following
its initial discovery in 1988, preliminary spectroscopic and
astrometric data suggested it to be of very low luminosity. Several
years' subsequent investigation at CTIO have now established this
definitively. This result is based on another classic technique, the
determination of trigonometric parallax, applied with modern CCD
detectors in a program led by Claudio Anguita (Chile). Observations
were carried out on the CTIO 1.5-m telescope over a five-year period,
using initially a 312 x 508 RCA CCD and more recently a Tek 1K device.
With 0.3" pixels and seeing at 1.2" or better, 40 frames yield a
parallax of 0.024" +/- 0.003", or M sub V = 17.4--more than one magnitude
fainter than the faintest previously known white dwarfs. The existence
of such an extreme low-luminosity white dwarf may imply a very large
value for the age of the local Galactic disk.

Alternatively, the low luminosity of ESO 439-26 could be accounted for
if it is a massive white dwarf with a correspondingly small
radius--also a rare beast, but not one implying great age. Only a
detailed comparison of the observed absolute flux distribution with
theoretical models can resolve the ambiguity. Ruiz and Anguita,
together with S.K. Leggett (IRTF) and P. Bergeron (Montreal), have used
BVRI photometry obtained on the CTIO 1.5-m and 0.9-m telescopes and
model atmosphere calculations by Bergeron and colleagues to make this
comparison. They find the high mass interpretation to be the correct
one, independent of details of atmospheric composition. The best fit to
the photometry gives T sub eff = 4560 K, log g = 9.0, and M = 1.2 M sub O--
twice that of a typical white dwarf. Comparison with carbon core
evolutionary models yield an upper age limit of 6.4 Gyr for the best
fit solution. More complex interiors models would tend to reduce the
age significantly, but are not yet available at appropriately cool
temperatures.

Infrared JHK photometry, in progress, will help constrain the
atmospheric composition. Interestingly, spectroscopy does not. Even
though hydrogen can be detected in low mass white dwarfs of comparable
effective temperature, the large surface gravity of this small, massive
object collisionally broadens hydrogen lines to the level of
undetectability. Although not of great age, the high mass of ESO 439-26
makes it unique in one respect. Comparison with model isochrones
indicate it is in an advanced state of crystallization.

This work appeared in the 20 December 1995 Astrophysical Journal
Letters.

         		[Figure not included]

	ESO 439-26 in the V-I M sub V plane compared with
	previously known low luminosity white dwarfs (open
	circles) and photometric sequences by Bergeron et al.
	for several masses. T sub eff is marked by squares
	on each sequence at 5500, 5000, 4500, and 4000 K.

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