2023ApJ...943...31M

We present the high-resolution soft X-ray spectrum of Nova Delphini 2013. Two spectra were taken with the Low Energy Transmission Grating Spectrometer on the Chandra X-ray Observatory, on 2013 November 9 and 2013 December 6, 87 and 114 days after the nova eruption, respectively. The spectra are of very high statistical quality, and reveal clear spectral evolution between the two observations. The source is bright enough on the two occasions that the third spectral order, with resolving power up to ∼3000, can easily be seen. We observe the photospheric emission spectrum of the hot white dwarf, which exhibits a rich absorption line spectrum from an atmosphere of effective temperature likely near T_eff ∼ 640,000 K, and complex chemical abundances. Superimposed on this photospheric spectrum, we detect the absorption spectrum of a shell of highly ionized gas, comprising absorption by the K-shell ions of C and N, blueshifted (outflowing) by ∼1400 km s^–1, and with a velocity width of ∼1000 km s^–1. The abundance ratio C/N is clearly very nonsolar, and indicative of thermonuclear fusion by the CNO cycle. We discuss the physical properties (kinematics, ionization balance, radiative transfer, mechanical energy balance, chemical abundances) of this hot shell, in the context of the physics of nova eruptions.