HST images of Nova Cyngi 1992, taken in 1993 and 1994.

Classical Novae, like Type Ia supernovae, are thermonuclear flashes powered by the accretion of material from a companion onto a white dwarf. In the case of a nova, however, only the surface layers — the accreted envelope — are burned, and ejected off into the cosmos; the white dwarf can then return to accrete more material. The detailed picture of novae however remain unclear; it is a difficult problem, because it depends sensitively on how matter is accreted, and how burning and convection occur on the surface of the white dwarf. Accretion and convection are notoriously difficult problems to model correctly in any great detail. However, with more computer power, more sophisticated simulations, and new mixing mechanisms, progress is possible.

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Wind-driven mixing through the resonant driving of a gravity wave, from Alexakis et al (2004).

Classical novae result from the accretion of material from a giant companion onto the surface of a white dwarf. The accretion continues until the base of the new atmosphere becomes hot enough that burning occurs, blowing off a great deal of the accreted material and its ash.

However, the abundances of the ejecta, and the energetics, imply that a significant amount of material is dredged up from the white dwarf itself and mixed into the atmosphere. Although much work has gone into understanding how this might occur, it remains unclear; it is hard to get light material to dig up heavier material.

I was involved with a project that used simulations to test whether a new mechanism could be responsible for this dredge-up — the driving of winds on the surface of the white dwarf, which then break and mix material into the surface. This mechanism is partly responsible for the mixing of water into air above bodies of water, where the density difference is much greater (about a factor of 1000) than in this case (about a factor of 10). This simulation effort garnered some press attention, including an article in USA Today.

Meaningful future work on this project will require use of a hydrodynamic solver that can take very long timesteps during the lead up to convection and the onset of convection; such solvers are just now being developed.

A. Alexakis, A. C. Calder, L. J. Dursi, R. Rosner, J. W. Truran, B. Fryxell, M. Zingale, F. X. Timmes, P. Ricker, and K. Olson. On the Nonlinear Evolution of Wind-Driven Gravity Waves, Physics of Fluids, 16:3256-3268, Sept 2004.

A. Alexakis, A. C. Calder, A. Heger, E. F. Brown, L. J. Dursi, J. W. Truran, R. Rosner, D. Q. Lamb, F. X. Timmes, B. Fryxell, M. Zingale, P. M. Ricker, and K. Olson. On Heavy Element Enrichment in Classical Novae. ApJ, 602:931-937, February 2004.

M. Zingale, L. J. Dursi, J. ZuHone, A. C. Calder, B. Fryxell, T. Plewa, J. W. Truran, A. Caceres, K. Olson, P. Ricker, K. Riley, R. Rosner, A. Siegel, F. X. Timmes, and N. Vladimirova. Mapping Initial Hydrostatic Models in Godunov Codes. ApJSS , 143(2):539-566, December 2002.

L. J. Dursi, A. C. Calder, A. Alexakis, J. W. Truran, M. Zingale, B. Fryxell, P. Ricker, F. X. Timmes, and K. Olson. Onset of Convection on a Pre-Runaway White Dwarf . In Classical Nova Explosions: International Conference on Classical Nova Explosions. AIP Conference Proceedings, Vol. 637. Sitges, Spain, 20-24 May, 2002. Edited by Margarita Hernanz and Jordi José. American Institute of Physics, 2002, p.139-143 , pages 139-143, 2002.

A. C. Calder, A. Alexakis, L. J. Dursi, R. Rosner, J. W. Truran, B. Fryxell, P. Ricker, M. Zingale, K. Olson, F. X. Timmes, and P. MacNeice. Mixing by Non-linear Gravity Wave Breaking on a White Dwarf Surface. In Classical Nova Explosions: International Conference on Classical Nova Explosions. AIP Conference Proceedings, Vol. 637. Sitges, Spain, 20-24 May, 2002. Edited by Margarita Hernanz and Jordi José. American Institute of Physics, 2002, p.134-138, pages 134-138, 2002.


Talks

• Using Numerical Simulations to explore a Mixing Mechanisms for Nova Enrichment
  Krell/DOE CSGF Conference, July 2002
  [PDF] [PPT]
  A presentation to a general computational science audience about our work on classical novae.