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The Spreading Layer and Dwarf Novae Oscillations
If an accretion disk remains thin near the point where it reaches the white dwarf, accreted material will first reach the stellar surface at the equator. This material must slow its orbit as it comes into co-rotation with the white dwarf, dissipating kinetic energy into thermal energy and creating a hot band of freshly accreted material around the equator. We have calculated properties of this hot belt (based on the work by Inogamov & Sunyaev for accreting neutron stars). At low accretion rates the amount of spreading is found to be negligible and most of the dissipated energy is radiated back into the accretion disk. When the accretion rate is high, such as in a dwarf nova outburst or symbiotic binary, the material may spread high enough to be seen above the accretion disk. Related Links: What does a "spreading layer" look like? - What is a White Dwarf? - Animation of an accreting white dwarf "Spreading of Accreted Material on White Dwarfs," A. L. Piro & L. Bildsten, 2004, Ap. J., 610, 977. Dwarf nova outbursts often show coherent sinusoidal oscillations. Called dwarf nova oscillations (DNOs), they have periods of 3-40 sec and scale monotonically with luminosity. We propose that DNOs may be produced by nonradial oscillations in a thin hydrostatic layer of freshly accreted material, the "spreading layer" (SL), at the white dwarf equator. This would naturally explain a number of key properties of DNOs, including their frequency range, sinusoidal nature, sensitivity to accretion rate, and why they are only seen during outburst. In support of this hypothesis we construct a simple model that treats the SL as a cavity containing shallow surface waves. Our new idea for explaining DNOs will hopefully motivate more sophisticated models of nonradial oscillations in the SL so as to make a detailed test of our hypothesis. A Spreading Layer Origin for Dwarf Nova Oscillations," A. L. Piro & L. Bildsten, 2004, Ap. J. L., 616, L155. Below is a relatively concise summary of my ideas on the "spreading layer" picture of disk accretion and how it may be related to dwarf nova oscillations (both of which are described in more detail in two of my previous papers). This article will appear in a book of conference preceedings from the Astrophysics of Cataclysmic Variables and Related Objects conference I attended in Strasbourg, France over the summer of 2004. "The Spreading Layer and Dwarf Nova Oscillations," A. L. Piro & L. Bildsten, 2005, in "The Astrophysics of Cataclysmic Variables and Related Objects," eds. J. M. Hameury and J. P. Lasota (ASP Conf. Ser.) |
