CI Cam is a B[e] phenomenon star, characterized by the presence of forbidden emission lines and an infrared excess originating from circumstellar dust. In 1998, CI Cam underwent a powerful outburst spanning the entire electromagnetic spectrum, from gamma rays to radio wavelengths. The physical origin of this event has remained debated for more than two decades. Using photometric and spectroscopic monitoring, researchers from the Special Astrophysical Observatory of the Russian Academy of Sciences and the Sternberg Astronomical Institute of Lomonosov Moscow State University have demonstrated that the primary component of CI Cam—a B0–B2 star of luminosity class I–III—exhibits radial pulsations with periods in the range of 0.402–0.416 day and hosts a companion moving on an eccentric orbit with a period of 19.407 day and an eccentricity of 0.49. The orbital motion of the companion is traced through systematic shifts in the He II λ4686 emission line (Fig. 1).

    During the night of 31 October 2023, high-resolution spectra of CI Cam were obtained with the BTA telescope equipped with the high-resolution optical spectrograph (MSS). The observations were carried out at orbital phases corresponding to the passage of the companion through the descending node near periastron and covered a full pulsation cycle of the B star (Fig. 2). Threefold variations of the He II-line intensity with a period equal to half the pulsation period were detected (Fig. 3). This behavior provides direct evidence that the companion is repeatedly impacted by successive pulsation-driven shock waves. As these waves decelerate in the gravitational field of the massive B star (12–20 M⊙), they reach the orbital level with high frequency.

    The observations confirm the high ellipticity of the orbit and the large orbital velocity of the companion at periastron exceeding 200 km s⁻¹. At this phase, a helium Of-subdwarf plunges into the dense regions of the stellar wind and accretes hydrogen-rich material from its companion. The luminosity of the system near periastron increases by approximately 1800 L⊙, corresponding to about 4% of the total system luminosity.

    In this configuration, accretion proceeds in a dynamical, wind-sweeping regime, in which the rapidly moving companion efficiently captures the incoming flow of matter. The proposed scenario naturally explains the 1998 outburst as a thermonuclear explosion in the layer of hydrogen accumulated on the surface of the helium subdwarf hosting a degenerate core.

Fig. 1. Radial velocity curve of the He II 4686 Å emission. Orbital phases are indicated at the top. Observations on October 31, 2023 with the BTA/MSS are marked in orange.

Fig. 2. A threefold intensity modulation of the He II line intensity over the observation period equal to the pulsation period (~9.5 hours).

Fig. 3. Variations in the equivalent width of He II emission and control lines in the CI Cam spectra.

Published:
Barsukova E.A., Goranskij V.P., Burenkov A.N., Yakunin I.A., Investigation of the Nature of the B[e] Star CI Cam in the Optical Range. Galaxies, 2025, V.13(3), 61, p.1-11. https://doi.org/10.3390/galaxies13030061