**AAS 195th Meeting, January 2000**

*Session 36. White Dwarfs New and Old*

Display, Thursday, January 13, 2000, 9:20am-6:30pm, Grand Hall
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## [36.03] Implications of the Assumption of Spherical Symmetry on Nova Expansion Parallaxes

*J.J.B. Harlow (U. of Pacific), R.A. Wade, R.B. Ciardullo (Penn State)*

Nova expansion parallax measurements are used in studies of
individual novae and in statistical studies such as the
Magnitude at Maximum -- Rate of Decline (MMRD) relation.
Such distance estimates compare the observed angular size of
a nova shell with its linear size as inferred from the
Doppler measured expansion rate and the known age. An
implicit simplifying assumption in this method is that the
expanding nova shells are spherically symmetric. However,
most nova shells are actually prolate in shape. Given
specific information on the axis ratio and orientation of an
individual nova shell, the non-sphericity of the expansion
can be taken into account, but this information is often
lacking.

We employ a simple geometric analysis to compute the error
introduced by the assumption of spherical symmetry when the
actual shape of the nova shell is prolate. If the true shape
of an individual nova shell is an ellipsoid with axis ratio
a:b:b, we show that its distance will be mis-estimated by
as much as 25% for b/a=0.8 or as much as 200% for
b/a=0.5. We explore the systematic bias on an ensemble of
distances measured with nova expansion parallaxes, assuming
a random distribution of orientation angles with respect to
the line of sight, and various distributions of axis ratios,
b/a. Depending on the method used to measure the angular
size of individual nova shells, we show that the mean
distance to a sample of novae will be mis-estimated by as
much as 12%, assuming a flat distribution of axis ratios in
the range 0.6 to 1.0. We show that this systematic bias can
be substantially reduced if the angular sizes of the
individual nova shells are quantified as carefully measured
mean diameters.

If you would like more information about this abstract, please
follow the link to http://astro.sci.uop.edu/~harlow/novashells.html.
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The author(s) of this abstract have provided an email address
for comments about the abstract:
jharlow@uop.edu

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