V694 Mon is a well-known symbiotic stellar system exhibiting highly collimated outflows with terminal velocities reaching up to 6500 km s⁻¹. Prior to the recent evolutionary transition, the system consisted of a hot component surrounded by an accretion disk and a cool M5-type giant, embedded in an extended nebular envelope. The jets were observed as absorption features superimposed on the spectrum of the hot component. The cool donor revealed itself through prominent TiO molecular bands in optical spectra and through pulsations detected in the near-infrared. The earliest photographic observations of the object date back to 1899–1900 and were obtained at the Moscow Observatory; these plates were subsequently recovered from the archives of the Sternberg Astronomical Institute of Lomonosov Moscow State University. Since that epoch, the optical brightness of the system has increased by 100 times (Fig. 1), and recurrent outbursts with a period of 5.2 yr have been recorded.

    In October 2018, researchers from the Special Astrophysical Observatory of the Russian Academy of Sciences and the Sternberg Astronomical Institute reported abrupt and profound changes in the system’s behavior. The jet absorption lines and rapid photometric flickering disappeared from the spectra, a sustained increase in brightness was detected about 2.5 yr prior to the predicted outburst, and the pulsations of the cool component ceased. These phenomena are interpreted as signatures of the termination of disk accretion and the subsequent destruction of the accretion disk. Over the following six years, the Roche lobe of the hot component—defining the region of its dominant gravitational influence—was progressively filled by the matter transferred from the cool giant. As a result, a star with a normal stellar photosphere and a spectral type of A4 I formed within this volume. Although a dynamical regime of mass transfer in binaries with massive donors had been anticipated theoretically, the extremely rapid timescale of the process proved to be unexpected. Spectroscopic observations obtained with the Zeiss-1000 telescope equipped with the UAGS spectrograph trace this transformation in detail (Fig. 2). Between November 2018 and December 2020, the density of the matter within the Roche lobe increased steadily, and the spectrum evolved from being emission-dominated to absorption-dominated. By November 2021, the system displayed an A-type stellar spectrum with unusually strong absorption lines formed in an extended atmosphere, followed by a hydrodynamic phase leading to the establishment of internal stellar equilibrium. By 2023, the object was spectroscopically indistinguishable from a standard A4 I supergiant and radiated as a star with a mass of approximately 6.5 M⊙.

    In February 2022, observations with the BTA telescope and the high-resolution optical spectrograph MSS revealed a pronounced outburst of emission lines from s-process elements superimposed on the A4 I spectrum (Fig. 3). These elements are synthesized deep within the envelope of the cool donor via slow neutron-capture reactions operating in the hydrogen-burning shell. The s-process–enriched matter was subsequently transported to the outer regions, where it became directly observable. Emission features of cerium, zirconium, lanthanum, niobium, neodymium, yttrium, gallium, dysprosium, erbium, hafnium, samarium, and several isotopes of other heavy elements were identified in the spectra.

    In the study, unique information is obtained about the critical phase of evolution in a mass-transfer binary system, as well as about the evolution with a common envelope, and the conclusions on the impact of a natural thermonuclear reactor on the deep internal layers of the star’s envelope.

Fig. 1. Images of V694 Mon: the first one taken in the late 19th century (on the left) and a CCD image in the 21st century (on the right). The star is shown in circles.

Fig. 2. Evolution of the spectrum of V694 Mon during the process of mass transfer from the red giant to the hot component and the formation of an A4 I-type star in the Roche lobe of the hot component (Zeiss-1000/UAGS). The spectrum of the A4 I standard is on the bottom right.

Fig. 3. Fragment of the spectrum of the s-process (BTA/MSS). The line profiles of zirconium, cerium, and neodymium coincide with the catalog values (the vertical lines).

Published:
Goranskij V.P., Barsukova E.A., Burenkov A.N., Metlova N.V., Zharova A.V., Yakunin I.A. V694 Mon: A Recent Event of Mass Transfer in the Dynamical Mode, Galaxies, 2025, 13(3), 59. https://doi.org/10.3390/galaxies13030059.