Early evolution of the extraordinary Nova Delphini 2013 (V339 Del)^⋆

¹ Astronomical Institute, Slovak Academy of Sciences, 059 60 Tatranská Lomnica, Slovakia
e-mail: skopal@astro.sk
² Dr. Karl Remeis-Observatory & ECAP, Astronomical Institute, Friedrich-Alexander-Universität Erlangen-Nürnberg, Sternwartstraße 7, 96049 Bamberg, Germany
³ Crimean Astrophysical Observatory, 298409 Nauchny, Crimea, Russia
⁴ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, 606-8502 Kyoto, Japan
⁵ Fujii Kurosaki Observatory, 4500 Kurosaki, Tamashima, Kurashiki, 713-8126 Okayama, Japan
⁶ 67 rue Jacques Daviel, 76100 Rouen, France
⁷ Observatoire de la Tourbière, 38690 Chabons, France
⁸ Balmes 2, 08784 Piera, Barcelona, Spain
⁹ 1833 Bobwhite Dr. Ammon, Idaho, 83401, USA
¹⁰ Castanet Tolosan Observatory, 6 place Clemence Isaure, 31320 Castanet Tolosan, France
¹¹ Lieu-dit Durfort, 81150 Fayssac, France
¹² 6 rue Virgile, 42100 Saint-Étienne, France
¹³ Chelles Observatory, 23 avenue Hénin, 77500 Chelles, France
¹⁴ Durtal Observatory, 6 rue des Glycines, 49430 Durtal, France
¹⁵ Mirranook Observatory, Boorolong Rd Armidale, NSW, 2350, Australia
¹⁶ Société Astronomique de Bourgogne-Dijon, France
¹⁷ 23746 Schoolhouse Road, Manhattan, Illinois, 60442, USA

Received: 27 May 2014
Accepted: 25 July 2014

Abstract

Aims. We determine the temporal evolution of the luminosity (L_WD), radius (R_WD) and effective temperature (T_eff) of the white dwarf (WD) pseudophotosphere of V339 Del from its discovery to around day 40. Another main objective was studying the ionization structure of the ejecta.

Methods. These aims were achieved by modelling the optical/near-IR spectral energy distribution (SED) using low-resolution spectroscopy (3500–9200 Å), UBVR_CI_C and JHKLM photometry. Important insights in the physical conditions of the ejecta were gained from an analysis of the evolution of the Hα and Raman-scattered 6825 Å O vi line using medium-resolution spectroscopy (R ~ 10 000).

Results. During the fireball stage (Aug. 14.8–19.9, 2013), T_eff was in the range of 6000–12 000 K, R_WD was expanding non-uniformly in time from ~66 to ~300 (d/ 3 kpc) R_⊙, and L_WD was super-Eddington, but not constant. Its maximum of ~9 × 10³⁸ (d/ 3 kpc)² erg s^-1 occurred around Aug. 16.0, at the maximum of T_eff, half a day before the visual maximum. After the fireball stage, a large emission measure of 1.0−2.0 × 10⁶² (d/ 3 kpc)² cm^-3 constrained the lower limit of L_WD to be well above the super-Eddington value. The mass of the ionized region was a few × 10^-4 M_⊙, and the mass-loss rate was decreasing from ~5.7 (Aug. 22) to ~0.71 × 10^-4 M_⊙ yr^-1 (Sept. 20). The evolution of the Hα line and mainly the transient emergence of the Raman-scattered O vi 1032 Å line suggested a biconical ionization structure of the ejecta with a disk-like H i region persisting around the WD until its total ionization, around day 40. On Sept. 20 (day 35), the model SED indicated a dust emission component in the spectrum. The dust was located beyond the H i zone, where it was shielded from the hard, ≳10⁵ K, radiation of the burning WD at that time.

Conclusions. Our extensive spectroscopic observations of the classical nova V339 Del allowed us to map its evolution from the very early phase after its explosion. It is evident that the nova was not evolving according to the current theoretical prediction. The unusual non-spherically symmetric ejecta of nova V339 Del and its extreme physical conditions and evolution during and after the fireball stage represent interesting new challenges for the theoretical modelling of the nova phenomenon.