Implications of the Assumption of Spherical Symmetry on Nova Expansion Parallaxes

Details Implications of the Assumption of Spherical Symmetry on Nova Expansion Parallaxes Implications of the Assumption of Spherical Symmetry on Nova Expansion Parallaxes

Dec 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aas...195.3603h&link_type=abstract

American Astronomical Society, 195th AAS Meeting, #36.03; Bulletin of the American Astronomical Society, Vol. 31, p.1419

Astronomy and Astrophysics

Astronomy

Scientific paper

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.

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