Progenitor and Remnant of the Luminous Red Nova V838 Monocerotis

—The article presents the results of multicolor photometry, medium and low resolution spectroscopy of the red nova V838 Mon remnant for 16 years after the 2002 outburst. We also used the archival photometry with the photographic plates of the Sonneberg and Moscow collections from 1928 to 1994. Analysis of these observational data confirmed that the progenitor of the V838 Mon explosion was a wide pair of B3V type stars of reduced luminosity. A brighter component exploded; it was 36 per cent brighter than its companion, and located on the zero-age main sequence of the Spectrum–Luminosity Diagram. Immediately after the outburst, in the fall of 2002, the remnant was a brown L-type supergiant (sgL), but in the fall of 2003 its spectrum changed to M type with a blue radiation excess appeared in the spectral energy distribution, which we interpreted as the reflection effect of the B type companion on the dust formed on the M star. In 2008, the companion was engulfed by the expanding explosion remnant, a type M supergiant (sgM). When the companion was immersing in the expanding M-star, a void was discovered under the M-star upper layer, in which the companion moved for about 200 days. Over the past 10 years, the luminosity of the M star has increased in the V filter by a factor of 10, and the spectral type has changed from M7.5 to M5.5. Based on radial velocities in the BaII 6497 A and CaI 6572 A lines, a deceleration of the expanding envelope of the M supergiant was detected, and in 2018, the envelope velocity approached to the heliocentric velocity of the star +71 km s−1. Quasi-periodic changes with a period of 320 days appeared then in the light curves, especially clearly expressed in I filter. We assume that the remnant has an elongated structure, and its rotation period is about 640 days. This is probably a gigantic contact system that will become a detached binary system in future development. The observations do not confirm the assumption that the explosion of one of the V838 Mon components was due to the merger of a compact binary system components located in a hierarchical triple one. Two hypotheses were proposed on the nature of the explosion of one of the V838 Mon components, directly based on the early age of this system: (1) the ignition of thermonuclear burning of hydrogen in the core after the gravitational compression of a protostar; (2) the fragmentation of the core inside a rapidly rotating star in the stage of gravitational compression of a protostar, and later, the subsequent defragmentation (merger of the core components) due to the loss of torque.