The Early Spectrophotometric Evolution of V1186 Scorpii (Nova Scorpii 2004 No. 1)

Abstract

We report optical photometry and optical through mid-infrared spectroscopy of the classical nova V1186 Sco. This slowly developing nova had a complex light curve with multiple secondary peaks similar to those seen in PW Vul. The time to decline 2 mag, t2, was 20 days, but the erratic nature of the light curve makes determination of intrinsic properties based on the decline time (e.g., luminosity) problematic, and the often-cited relationship of maximum magnitude versus the rate of decay of the light curve of Della Valle & Livio fails to yield a plausible distance. Spectra covering 0.35-35 μm were obtained in two separate epochs during the first year of outburst. The first set of spectra, taken about 2 months after visible maximum, are typical of a CO-type nova with narrow-line emission from H I, Fe II, O I, and He I. Later data, obtained between 260 and 380 days after maximum, reveal an emerging nebular spectrum. Spitzer spectra show weakening hydrogen recombination emission with the emergence of [Ne II] (12.81 μm) as the strongest line. Strong emission from [Ne III] (15.56 μm) is also detected. Photoionization models with low effective temperature sources and only marginal neon enhancement (Ne ~ 1.3 Ne⊙) are consistent with these IR fine-structure neon lines indicating that V1186 Sco did not occur on a ONeMg white dwarf. In contrast, the slow and erratic light-curve evolution, spectral development, and photoionization analysis of the ejecta imply that the outburst occurred on a low-mass CO white dwarf. We note that this is the first time strong [Ne II] lines have been detected so early in the outburst of a CO nova and suggest that the presence of mid-infrared neon lines is not directly indicative of a ONeMg nova event.