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A classical nova occurs when material accreting onto the surface of a whitedwarf in a close binary system ignites in a thermonuclear runaway. Complexstructures observed in the ejecta at late stages could result from interactionswith the companion during the common envelope phase. Alternatively, theexplosion could be intrinsically bipolar, resulting from a localized ignitionon the surface of the white dwarf or as a consequence of rotational distortion.Studying the structure of novae during the earliest phases is challengingbecause of the high spatial resolution needed to measure their small sizes.Here we report near-infrared interferometric measurements of the angular sizeof Nova Delphini 2013, starting from one day after the explosion and continuingwith extensive time coverage during the first 43 days. Changes in the apparentexpansion rate can be explained by an explosion model consisting of anoptically thick core surrounded by a diffuse envelope. The optical depth of theejected material changes as it expands. We detect an ellipticity in the lightdistribution, suggesting a prolate or bipolar structure that develops as earlyas the second day. Combining the angular expansion rate with radial velocitymeasurements, we derive a geometric distance to the nova of 4.54 +/- 0.59 kpcfrom the Sun.

The Expanding Fireball of Nova Delphini 2013