Rise and fall of the dust shell of the classical nova V339 Delphini

We present infrared spectroscopy of the classical nova V339 Del, obtained over an ∼2-yr period. The infrared emission lines were initially symmetrical, with half width half-maximum velocities of 525 km s⁻¹. In later (t ≳ 77 d, where t is the time from outburst) spectra, however, the lines displayed a distinct asymmetry, with a much stronger blue wing, possibly due to obscuration of the receding component by dust. Dust formation commenced at approximately day 34.75 at a condensation temperature of 1480 ± 20 K, consistent with graphitic carbon. Thereafter, the dust temperature declined with time as T_d ∝ t^−0.346, also consistent with graphitic carbon. The mass of dust initially rose, as a result of an increase in grain size and/or number, peaked at approximately day 100, and then declined precipitously. This decline was most likely caused by grain shattering due to electrostatic stress after the dust was exposed to X-radiation. The appendix summarizes Planck means for carbon and the determination of grain mass and radius for a carbon dust shell.