Xcov 18 Scientific Justification
PG 0122+200
Principle Investigators : Vauclair, O'Brien
ABSTRACT
As the coolest of the known DOVs, PG 0122 not only defines the red edge
of the DOV instability strip, but its cooling is calculated to be dominated
by neutrino, rather than photon, emission (O'Brien, 1998). A campaign has
therefore begun (with a multi-site component in October 1999) to establish
the timebase of observations needed for a significant determination of a
Pdot. The rate of period change we
expect to measure will be telling us directly about the physics of the
core of the star, in general, and of neutrinos, specifically.
JUSTIFICATION
Among the pulsating white dwarfs, the DOV variables offer the opportunity
to describe the interesting transition phase between the planetary
nebula and the white dwarf cooling sequence. Whole Earth Telescope
observations led to the determination of internal structure and rate of
evolution for the class prototype, PG 1159, through measurement of
the period structure of its light curve and the rate of period change,
respectively (Winget et al. 1991). Another DOV pulsator, PG 0122+200,
is particularly interesting for two reasons. First: it is the coolest
DOV pulsator and therefore defines the red edge of the instability strip
(Dreizler et al. 1995). Understanding its internal structure is necessary
to understand why the driving mechanism stops being efficient at effective
temperatures cooler than that of PG 0122 (75,000 K). Second: theoretical
models show that, at the temperature of PG 0122, and for its approximate
mass, the neutrino emission exceeds the photon emission by a factor of
about 2.5 (O'Brien et al. 1998). As a result, the evolutionary time scale
at this phase of evolution is entirely dominated by neutrino losses.
Determining the evolution of PG 0122 will test our theories neutrino
production and emission in dense plasma.
Previous observations of PG 0122 were obtained in 1987, 1990 and 1996.
Obtaining a new set of data in 1999 will allow us to measure the
accumulated phase drift of the pulsations from year to year caused by a
changing period, dP/dt. To do this, we must determine the period with
sufficient precision to compare the phase this year with that in previous
years. Observations spanning two consecutive months will achieve this
necessary precision. The primary goal of this proposal with respect
to PG 0122 is to provide the final vital link in a decade-long chain
of observations and achieve the first measurement of the evolutionary
time scale of a star whose evolution is dominated by neutrino cooling.
In addition, the discovery of new pulsation modes (which occurred with
every previous observation) could finally allow us to decode the internal
structure of PG 0122 as was done for PG 1159.
References
Dreizler, S., Werner, K., & Heber, U. 1995, in 9th European
Workshop on White Dwarf Stars, ed. D. Koester & K. Werner
(Dordrecht: Kluwer), p. 160
O'Brien, M.S. et al. 1998, ApJ 495, 458
Winget, D.E. et al. 1991, ApJ 378, 326
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Last updated on 30 Jul 1999.