Investigation of the Nature of the B[e] Star CI Cam in the Optical Range

TL;DR

This 24-year study of CI Cam combines photometric and spectroscopic data to analyze its orbital dynamics, pulsations, and spectral characteristics, revealing its classification as an FS CMa-type object and constraining its physical parameters.

Contribution

It provides the first detailed long-term monitoring of CI Cam's orbital and pulsational behavior, clarifying its spectral type, pulsation modes, and evolutionary stage.

Findings

Orbital period of 19.407 days with eccentricity 0.44-0.49.

Detection of multiperiodic pulsations transitioning to a single mode.

Spectral type constrained to B0-B2 III, distance 2.5-4.5 kpc.

Abstract

We report the results of 24 years of photometric and spectroscopic monitoring of CI Cam since its outburst in 1998. In the early years of our research, we identified a system component responsible for the emission of the He II 4686 line, which moves in an elliptical orbit with a period of 19.407 days and an eccentricity from 0.44 to 0.49. Variations in optical brightness with the same period were observed, with an average amplitude of 0.04 magnitudes. The total amplitude of the He II radial velocity variations was approximately 380 kilometers per second. The equivalent width of the line varied on timescales of tens of minutes as well as with the orbital period, reaching maximum values when the companion passed the descending node of the orbit. The intensity of the He II 4686 emission has gradually increased over time. Our photometric monitoring revealed pulsations of the main B component of the CI Cam system. Between 2005 and 2009, the B star exhibited multiperiodic pulsations, however, since 2012, it pulsated in a single mode. We interpret the pulsations from 2005 to 2009 as a resonance of radial modes, with the residual stable mode being the first overtone. The pulsations are coherent over several months, with average amplitudes from 0.02 to 0.04 magnitudes in the V band. The pulsation data constrain the spectral type of the primary component of B0 to B2 III, the distance to the system from 2.5 to 4.5 kpc. The classification of the main component of CI Cam as a supergiant is ruled out due to the observed pulsation periods. CI Cam is likely in the stage after the first mass exchange and may belong to the FS CMa-type objects.