Thick to Thin: The Evolutionary Connection Between PG 1159 Stars and the Thin Helium-Enveloped Pulsating White Dwarf GD 358

Seismological observations with the Whole Earth Telescope (WET) allow the determination of the subsurface compositional structure of white dwarf stars. The hot DO PG 1159 has a helium surface layer with a mass of 0.001 Msun, while the cooler DB white dwarf GD 358 has a much thinner surface helium layer of 10^-6 Msun. These results imply that either there is no evolutionary relation between these two stars, or that there is an unknown mass loss mechanism. To investigate possible evolutionary links between these objects, we computed evolutionary sequences of white dwarf models including time-dependent diffusion. Our initial model is based on the PG~1159 pulsational data, and has a surface composition of 30% helium, 35% carbon, and 35% oxygen. Below this is a thin transition zone where the helium fraction falls to zero. As expected, diffusion caused a separation of the elements; a thickening surface layer of nearly pure helium overlays a deepening transition zone where the composition returns to the original surface composition. When the model reached the temperature range of GD~358 and the pulsating DB white dwarfs, this pure helium surface layer was 3x10^-6 stellar masses deep. The resulting evolved model is very similar to the model used by Bradley and Winget (1994) to match the pulsation observations of GD 358. The pulsation periods of this model also show a good fit to the WET observations. These results demonstrate the plausibility of a direct evolutionary path from PG 1159 stars to the much cooler DB white dwarfs by inclusion of time-dependent diffusion. A problem still remains in that our models have no hydrogen, and thus must retain their DB nature while their surface tempeture drops from 45,000K to 30,000K. Since there are no known DB stars in this range, we plan to address this problem in future calculations.Comment: LaTeX, 10 pages, using AAS Macros. 2 PostScript figures. Accepted for publication in The Astrophysical Journal Letters

Chameleon stars: diffusion and spectroscopic transformations in white dwarfs

White dwarfs stars are the end product of stellar evolution, generating no energy by nuclear fusion, slowly cooling over time. As they cool, their intense gravity causes the lightest element--either hydrogen or helium--to float to the surface. This gravitational settling is responsible for the observed nearly pure surface composition of white dwarfs. In this investigation, I model this settling process in these stars by constructing a sequence of models which represent the star at different stages as it cools;As white dwarfs cool, at certain temperatures they undergo nonradial pulsations. Most importantly, a change in the composition as a function of the depth within the star causes differences in the pulsations of the star. Pulsations are then a natural way of measuring the depth of surface layers of pure composition formed by gravitational settling;I calculated a sequence of models representing the evolution of white dwarfs. The initial model was of a young, hot white dwarf at a surface temperature of Teff~ 130,000K, representing the pulsating white dwarf PG 1159. The model has a surface layer of mixed He, C, and O containing about 10-3 of the stellar mass. This model was evolved until it cooled to 25,000K, where it again undergoes pulsations. By this temperature, settling causes the formation of a surface layer of helium containing about 10-6 of the stellar mass. Observations of GD 358, a pulsating helium-rich white dwarf at Teff~25,000K, show that it has a surface helium layer of roughly this thickness. This indicates that stars like PG 1159 may evolve to stars like GD 358, despite the incongruity in surface layer mass;Finally, there exists a range of temperature, from 45,000K to 30,000K, where all observed white dwarfs have hydrogen dominated surfaces. I added hydrogen to the PG 1159 models to investigate its effects. The calculations show that hydrogen diffuses very quickly to the surface, and would be present at the surface in detectable quantities. This contradiction with observations of PG 1159, which shows no detectable hydrogen, implies some other mechanism, such as mass loss, which alters the surface abundance of white dwarfs

Wnt Signaling in Vertebrate Neural Development and Function

Members of the Wnt family of secreted signaling proteins influence many aspects of neural development and function. Wnts are required from neural induction and axis formation to axon guidance and synapse development, and even help modulate synapse activity. Wnt proteins activate a variety of downstream signaling pathways and can induce a similar variety of cellular responses, including gene transcription changes and cytoskeletal rearrangements. This review provides an introduction to Wnt signaling pathways and discusses current research on their roles in vertebrate neural development and function