A multiwavelength light curve model of the classical nova V339 Del: A mechanism for the coexistence of dust dip and supersoft X-rays

The classical nova V339 Del 2013 is characterized by a 1.5 mag dip of the $V$ light curve owing to a dust shell formation, during which soft X-ray emissions coexist. We present Str\"omgren $y$ band light curve, which represents continuum emission, not influenced by strong [O III] emission lines. The $y$ light curve monotonically decreases in marked contrast to the $V$ light curve that shows a 1.5 mag dip. We propose a multiwavelength light curve model that reproduces the $y$ and $V$ light curves as well as the gamma-ray and X-ray light curves. In our model, a strong shock arises far outside the photosphere after optical maximum, because later ejected matter collides with earlier ejected gas. Our shocked shell model explains optical emission lines, H$\alpha$, hard X-ray, and gamma-ray fluxes. A dust shell forms behind the shock that suppresses [O III]. This low flux of [O III] shapes a 1.5 mag drop in the $V$ light curve. Then, the $V$ flux recovers by increasing contribution from [O III] lines, while the $y$ flux does not. However, the optical depth of the dust shell is too small to absorb the photospheric (X-ray) emission of the white dwarf. This is the reason that a dust shell and a soft X-ray radiation coexist. We determined the white dwarf mass to be $M_{\rm WD}=1.25\pm 0.05~M_\odot$ and the distance modulus in the $V$ band to be $(m-M)_V=12.2 \pm 0.2$; the distance is $d= 2.1\pm 0.2$ kpc for the reddening of $E(B-V)=0.18$.