The True Incidence of Magnetism among Field White Dwarfs

We study the incidence of magnetism in white dwarfs from three large and well-observed samples of hot, cool, and nearby white dwarfs in order to test whether the fraction of magnetic degenerates is biased, and whether it varies with effective temperature, cooling age, or distance. The magnetic fraction is considerably higher for the cool sample of Bergeron, Ruiz, and Leggett, and the Holberg, Oswalt, and Sion sample of local white dwarfs that it is for the generally-hotter white dwarfs of the Palomar Green Survey. We show that the mean mass of magnetic white dwarfs in this survey is 0.93 solar masses or more, so there may be a strong bias against their selection in the magnitude-limited Palomar Green Survey. We argue that this bias is not as important in the samples of cool and nearby white dwarfs. However, this bias may not account for all of the difference in the magnetic fractions of these samples. It is not clear that the magnetic white dwarfs in the cool and local samples are drawn from the same population as the hotter PG stars. In particular, two or threee of the cool sample are low-mass white dwarfs in unresolved binary systems. Moreover, there is a suggestion from the local sample that the fractional incidence may increase with decreasing temperature, luminosity, and/or cooling age. Overall, the true incidence of magnetism at the level of 2 megagauss or greater is at least 10%, and could be higher. Limited studies capable of detecting lower field strengths down to 10 kilogauss suggest by implication that the total fraction may be substantially higher than 10%.Comment: 16 pages, 2 figures, Astronomical Journal in press -- Jan 2003 issu

Effects of a ceramic coating on metal temperatures of an air-cooled turbine vane

The metal temperatures of air cooled turbine vanes both uncoated and coated with the NASA thermal barrier system were studied experimentally. Current and advanced gas turbine engine conditions were simulated at reduced temperatures and pressures. Airfoil metal temperatures were significantly reduced, both locally and on the average, by use of the the coating. However, at low gas Reynolds number, the ceramic coating tripped a laminar boundary layer on the suction surface, and the resulting higher heat flux increased the metal temperatures. Simulated coating loss was also investigated and shown to increase local metal temperatures. However, the metal temperatures in the leading edge region remained below those of the uncoated vane tested at similar conditions. Metal temperatures in the trailing edge region exceeded those of the uncoated vane

Erratum: "Post-T Tauri Stars in the Nearest OB Association" (AJ, 124, 1670 [2002])

A few typos in Mamajek, Meyer, & Liebert (2002, AJ, 124, 1670) have been corrected in this erratum (including two stellar misidentifications and an incorrect power in the units of a slope). The most significant is the correction of a sign error in the published polynomial conversion between Tycho and Johnson-Cousins (B-V) colors.Comment: 1 page, to appear in April 2006 Astronomical Journa

The open cluster initial-final mass relationship and the high-mass tail of the white dwarf distribution

Recent studies of white dwarfs in open clusters have provided new constraints on the initial - final mass relationship (IFMR) for main sequence stars with masses in the range 2.5 - 6.5 Mo. We re-evaluate the ensemble of data that determines the IFMR and argue that the IFMR can be characterised by a mean initial-final mass relationship about which there is an intrinsic scatter. We investigate the consequences of the IFMR for the observed mass distribution of field white dwarfs using population synthesis calculations. We show that while a linear IFMR predicts a mass distribution that is in reasonable agreement with the recent results from the PG survey, the data are better fitted by an IFMR with some curvature. Our calculations indicate that a significant (~28%) percentage of white dwarfs originating from single star evolution have masses in excess of ~0.8 Mo, obviating the necessity for postulating the existence of a dominant population of high-mass white dwarfs that arise from binary star mergers.Comment: 5 pages, 2 color Postscript figures. Accepted for publication in MNRA