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The expanding fireball of Nova Delphini 2013



A classical nova occurs when material accreting onto the surface of a white dwarf in a close binary system ignites in a thermonuclear runaway1,2. Complex structures observed in the ejecta at late stages3,4,5 could result from interactions with the companion during the common-envelope phase6,7. Alternatively, the explosion could be intrinsically bipolar, resulting from a localized ignition on the surface of the white dwarf8 or as a consequence of rotational distortion9,10. Studying the structure of novae during the earliest phases is challenging because of the high spatial resolution needed to measure their small sizes11. 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.

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Figure 1: Expansion curve of Nova Del 2013.
Figure 2: Infrared light curve of Nova Del 2013.
Figure 3: Changes in the flux ratio of the two-component model.
Figure 4: Model and reconstructed images of Nova Del 2013.


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We acknowledge the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. We thank O. Garde and other members of the Astronomical Ring for Access to Spectroscopy for use of their archive of Nova Del 2013 spectra. We thank G. J. Schwarz, S. N. Shore, and F. M. Walter for discussions that helped us to interpret the nova observations. 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. This publication made use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.

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Authors and Affiliations



Observations with the CHARA Array were originally proposed by B.K. and D.R.G. Modelling and interpreting the angular expansion curve and asymmetries were done by G.H.S., D.R.G., B.K., T.t.B., O.C., I.T.-B. and S.T.R. The CHARA data were reduced by T.t.B., J.D.M., O.C., I.T.-B., D.M., V.M., C.D.F., N.S. and G.H.S. The observations were planned and conducted by C.D.F., N.S., N.V., B.K., D.R.G., T.B., G.H.S., D.M., A.M., N.N., P.S., M.I., V.M., P.T., J.J., N.D.R., R.M.R., G.v.B., K.v.B. and R.T.Z. Observational setup and technical support were provided by J.S., L.S., N.H.T. and X.C. Administrative oversight and access to CHARA were provided by H.A.M. and T.t.B. Reconstructing and interpreting the nova images were done by G.H.S., F.B., J.D.M. and B.K. Infrared magnitudes derived from CHARA data were computed by N.S. and G.H.S. Infrared spectra were taken and reduced by D.P.K.B., N.M.A., V.J., P.S.M., J.B. and analysed by D.R.G. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to G. H. Schaefer.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Integrated emission line to continuum flux ratios measured from infrared spectroscopy.

The squares represent the H-band ratios while the diamonds represent the K′-band ratios. The rise in the emission line flux is consistent with an increasing contribution from optically thin emission. The down turn in the curve during the last measurement is probably caused by a rising contribution of the continuum owing to the formation of dust.

Source data

Extended Data Figure 2 Closure phases measured with MIRC on ut 2013 August 21.

The measurements are plotted against the length of the maximum baseline (B) for the group of three telescopes that were used to form the closure phase. The non-zero closure phases indicate an asymmetry in the light distribution that is not point symmetric. The colours of the symbols are used to differentiate the measurements from each grouping of three telescopes in the array. The closure phases are measured in eight wavelength channels. Error bars represent 1σ measurement uncertainties.

Extended Data Figure 3 Example of MIRC observations obtained on ut 2013 August 21.

The light is dispersed over eight wavelength channels in the H-band. a, Coverage of the interferometric baselines projected on the plane of the sky in right ascension (RA) and declination (Dec.) in units of spatial frequencies (u = Bx/λ, v = By). b, Squared, normalized visibility amplitude measurements, colour-coded to match the baselines on the left. The solid line shows the best-fitting uniform disk model. The small black dots show the best-fitting uniform ellipse model. Error bars represent 1σ measurement uncertainties.

Extended Data Figure 4 Interferometric visibilities of Nova Del 2013 measured with the CHARA Array.

The red line shows the best-fitting model for a uniformly bright, circular disk. The time since the explosion (in days) is indicated in the upper right corner of each panel. The measurements were obtained with the CLASSIC, CLIMB, and MIRC beam combiners (see Extended Table 1). Error bars represent 1σ measurement uncertainties.

Extended Data Figure 5 Time evolution of the two-component model of Nova Del 2013.

The model consists of a circular core surrounded by a halo ring. The expansion rate and size ratio were determined by minimizing χ2 across all of the nights while allowing the flux ratio to vary night by night. In plotting the images, we used the flux ratio measured directly for the first three nights, the linear fit plotted in Fig. 3 of the main paper for t = 4–27 days, and our measurement that the ring contributes an average of 68% of the light on the last five nights (dust emission in the outer layers). Each panel is 12 mas on a side. We scaled the model flux by the infrared magnitude measured on each night to show how the surface brightness changes. The time since the explosion (in days) is indicated in each panel. Intensity refers to the flux per unit area.

Extended Data Table 1 Journal of observations and angular diameters measurements of Nova Del 2013
Extended Data Table 2 Uniform ellipse models of Nova Del 2013
Extended Data Table 3 Infrared magnitudes of Nova Del 2013
Extended Data Table 4 Effective bandwidth of the CLASSIC/CLIMB observations

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Schaefer, G., Brummelaar, T., Gies, D. et al. The expanding fireball of Nova Delphini 2013. Nature 515, 234–236 (2014).

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