Schaefer, G. Hten Brummelaar, TheoGies, D. RFarrington, C. DKloppenborg, BChesneau, OMonnier, J. DRidgway, S. TScott, NTallon-Bosc, IMcAlister, H. ABoyajian, TMaestro, VMourard, DMeilland, ANardetto, NStee, PSturmann, JVargas, NBaron, FIreland, MichaelBaines, EllynChe, X.Jones, J.Richardson, N. D.Roettenbacher, R. M.Sturmann, L.Turner, N. H.Tuthill, P.van Belle, Gerard T.von Braun, K.Zavala, R. T.Banerjee, D. P. K.Ashok, N. M.Joshi, V.Becker, J.Muirhead, P. S.2015-03-242015-03-240028-0836http://hdl.handle.net/1885/13031A classical nova occurs when material accreting onto the surface of a white dwarf in a close binary system ignites in a thermonuclear runaway. Complex structures observed in the ejecta at late stages could result from interactions with the companion during the common-envelope phase. Alternatively, the explosion could be intrinsically bipolar, resulting from a localized ignition on the surface of the white dwarf or as a consequence of rotational distortion. Studying the structure of novae during the earliest phases is challenging because of the high spatial resolution needed to measure their small sizes. Here we report near-infrared interferometric measurements of the angular size of Nova Delphini 2013, starting one day after the explosion and continuing with extensive time coverage during the first 43 days. Changes in the apparent expansion rate can be explained by an explosion model consisting of an optically thick core surrounded by a diffuse envelope. The optical depth of the ejected material changes as it expands. We detect an ellipticity in the light distribution, suggesting a prolate or bipolar structure that develops as early as the second day. Combining the angular expansion rate with radial velocity measurements, we derive a geometric distance to the nova of 4.54 ± 0.59 kiloparsecs from the Sun.This material is based upon work supported by the National Science Foundation under grant number AST-1009080. The CHARA Array is funded by the National Science Foundation through NSF grants AST 0908253 and AST 1211129, and by Georgia State University through the College of Arts and Sciences.3 pages© 2014 Macmillan Publishers Limited http://www.sherpa.ac.uk/romeo/issn/0028-0836/ author can archive pre-print (ie pre-refereeing), subject to Restrictions, author can archive post-print (ie final draft post-refereeing), 6 months embargo (Sherpa/Romeo as of 18/5/2015).cometfireinterferometrynear infraredoptical depthplanetary rotationspatial resolutionthermochemistryastronomygeometryinfrared spectroscopyinfrared radiationvelocitybrightness2014-11-1310.1038/nature138342016-06-14