Carbon monoxide emission and the Eta Carinae stage of Nova NQ Vulpeculae

Abstract

The detection of first-overtone vibration-rotation emission bands of CO at 2.3 microns from Nova NQ Vul is reported. These bands were detected in a 1.6-2.4-micron spectrum obtained shortly after the first sharp drop in visual magnitude. It is suggested that the previously discovered 5-micron excess can be identified with emission in the stronger fundamental bands of CO and that the CO emission originates within or near the nova ejecta. A temperature of 3500 + or - 750 K is derived, a lower limit of 3 is placed on the C-12/C-13 isotopic abundance ratio, and the fundamental-band CO emission at 4.8 microns is predicted to be approximately 40 times stronger than the first-overtone emission at 2.3 microns. Other emission lines in the spectrum are identified as recombination lines of H, C, and Na. A model of the ionization structure of the ejecta is presented which can explain in a semiquantitative way several important features of the spectrum.