Absolute Proper Motions to B~22.5: IV. Faint, Low Velocity White Dwarfs and the White Dwarf Population Density Law

The reduced proper motion diagram (RPMD) for a complete sample of faint stars with high accuracy proper motions in the North Galactic Pole field SA57 is investigated. Eight stars with very large reduced proper motions are identified as faint white dwarf candidates. We discriminate these white dwarf candidates from the several times more numerous QSOs based on proper motion and variability. We discuss the implausibility that these stars could be any kind of survey contaminant. If {\it bona fide} white dwarfs, the eight candidates found here represent a portion of the white dwarf population hitherto uninvestigated by previous surveys by virtue of the faint magnitudes and low proper motions. The newly discovered stars suggest a disk white dwarf scaleheight larger than the values of 250-350 pc typically assumed in assessments of the local white dwarf density. Both a <V/V_{max}> and a more complex maximum likelihood analysis of the spatial distribution of our likely thin disk white dwarfs yield scaleheights of 400-600 pc while at the same time give a reasonable match to the local white dwarf volume density found in other surveys. Our results could have interesting implications for white dwarfs as potential MACHO objects. We can place some direct constraints (albeit weak ones) on the contribution of halo white dwarfs to the dark matter of the Galaxy. Moreover, the elevated scale height that we measure for the thin disk could alter the interpretation of microlensing results to the extent of making white dwarfs untenable as the dominant MACHO contributor. (Abridged)Comment: 38 pages, 5 figures, to appear in April Ap

Regional validation of General Circulation Models

SIGLEAvailable from British Library Document Supply Centre- DSC:D84379 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Estimates of low frequency natural variabilit in near-surface air temperature

Estimates of the spectrum of natural variability are critical to the problem of detecting an anthropogenic signal in global climate observations. Without such information it is impossible to say that current climate change is different or unique from changes that have happened in the past and, therefore, potentially due to man induced causes. We have estimated the spectrum of natural variability from a globally distributed set of palaco temperature proxies and compared it with comparable estimates from two long control integrations of coupled general circulation models - the type used to predict anthropogenic change due to greenhouse gases. None of the three estimates of the natural variability spectrum agree with each other on the low-frequency, near-global time/space scales. Until this dichotomy is resolved, it will be hard to say, with confidence, that an anthropogenic climate signal has or has not been detected

Regional validation of general circulation models

Reprint of Ph.D thesis by Santer, B.DAvailable from British Library Document Supply Centre- DSC:7759.149(CRURP--9) / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo