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

Abstract

The B[e] phenomenon is observed in a wide range of stars at various evolutionary stages. Its nature remains uncertain. The B[e] phenomenon is defined as the simultaneous presence of low-excitation forbidden line emission and strong infrared excess in the spectra of early-type stars. Here, we present new spectral observations of a representative of this class: the star CI Cam. A monitoring campaign was carried out for the He II 4686 Å emission line, which serves as an indicator of binarity in this system. The aim was to detect variations in this line not only due to orbital motion but also those associated with the pulsations of the system’s primary component, the B[e] star. Two maxima in the equivalent width were detected over the pulsation period, during which the equivalent width increased by a factor of three. We refine the classification of CI Cam, assigning it to the FS CMa group of B[e] stars by all criteria, and we refer the secondary component of the system to a group of recently discovered “stripped” stars.

1. Introduction

The goal of this study was to investigate the nature of the B[e] phenomenon using the unique object CI Cam, which has been a focal point of research for more than 25 years. A comprehensive description of our observations and data reduction was published in [1]. The classification of CI Cam has changed multiple times since its study began in 1932 [2]. First, the spectrum of the star was classified as a peculiar Be star with strong emission lines. It has further been identified as a B0V + G8II, as a symbiotic star, a Herbig Be star, and a supergiant B[e] star (sgB[e]). The recent consensus seems to be that it is a B[e] supergiant. This identification was established after its famous outburst in X-rays, optics and radio in 1998. Of course, that identification is due to the great uncertainty of its distance (2 to 17 kpc) for many years before the Gaia flyby. Here, we cite several key publications that, along with their referenced literature, best illustrate the evolving understanding of the nature of CI Cam—from its initial observations to current research. These include:

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multiwavelength (optical, infrared, and radio) observations of CI Cam, identified as the optical counterpart to the transient X-ray source XTE J0421+560 [3];

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results of UBVR photometry and medium-resolution spectroscopy from 1998–2001, as well as evidence of stratification in the system’s envelope during the outburst [4];

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spectroscopic observations of the candidate sgB[e]/X-ray binary CI Cam both during the outburst and from data 34 years prior [5];

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radio imaging obtained within one year after the outburst [6];

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observations of high X-ray variability on timescales of days, complemented by high-resolution optical spectroscopy [7];

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and continued monitoring of strong emission lines variability from the 1998 outburst up to the present day [8].

Our own observations of CI Cam have continued for 25 years in a quiescent state following the major outburst in 1998, which occurred across the entire electromagnetic spectrum. In total, we collected 16,000 observations. The light curve in the V filter is shown in Figure 1. It reveals both long-term cyclic brightness variations—visible in all filters—with a possible period exceeding 1000 days, as well as short-term variability. Additionally, there is a general trend of increasing brightness over time. Our observations revealed a well-defined brightness variation with a period of 19.407 days (Figure 6e in [1]).

Almost simultaneously with this variability, we discovered that the helium emission line He II 4686 Å shifts by up to 400 km s⁻¹ relative to nearby lines, with the magnitude of these shifts depending on the photometric phase [9]. Other spectral lines in the quiescent state do not show significant radial velocity variations. However, the He II-line shifts by up to 8 Å, which is clearly visible in the spectrum. This confirmed the binary nature of the system. The helium line was one of the strongest emission features during the outburst.

After the outburst, we observed a gradual increase in the equivalent width (EW) of the He II emission, coinciding with the enhancement of the system brightness. This trend, along with the rapid variability within a single night, is shown in Figures 3e and 5 in [1]. The radial velocity curve of the He II line, stacked with a period of 19.407 days, is shown in Figure 2a (source: Figure 6 from [1]).

The total amplitude of the radial velocity variations exceeds 500 km s⁻¹. The velocity curve has a large scatter, reaching 200 km s⁻¹ in some phases of the orbit. Apparently, the HeII line velocities have a periodic orbital component with a half-amplitude of 190 km s⁻¹ and real fast variations superimposed on it. The hot secondary component orbits in an elliptical orbit with an eccentricity of 0.49, and the EW maximum of the He II line is observed near the descending node of the orbit. Rapid changes in the radial velocity are accompanied by changes in the equivalent widths and shapes of the line profiles on a time scale of 10 min. The emission line can be weak and temporally asymmetric. During the quiescent period after the 1998 outburst, it was absent in 12% of all spectra. We made special observations on the 1-m SAO Zeiss telescope with the UAGS spectrograph (4.2 Å resolution) on four nights of He II-line invisibility between 19 and 22 February 2025 with long exposures of 7200–18,000 s and recorded the line with a S/N ratio of 2–2.5. This highlights its strong variability and explains why some researchers have not always detected it [7,8].

In 2008, we discovered a connection between CI Cam’s rapid night-to-night brightness variability and its multiperiodic pulsations [10,11]. CI Cam became the first known pulsating B[e] star; the second was HD 50138 (V743 Mon) [12]. Initially, three pulsation modes were present. Since 2012, the star has pulsated in a single mode [13] with a period of 0.4062 days. We identified this stable wave as a radial mode corresponding to the first overtone. In 2009, we observed the appearance of absorption components in the He I 4713 Å emission profile, forming reverse P Cygni profiles, possibly linked to the propagation of pulsation waves in the B star’s envelope [14]. A continuous series of the Transiting Exoplanet Survey Satellite (TESS) observations [15] provides further insight. The TESS light curve, shown in Figure 8 of [1], clearly reveals both orbital brightness variations and pulsations. These confirm the pulsations of the B star and the orbit of the He II emission source, based on space-based precision data.

Given that CI Cam is a B[e] star exhibiting pulsations, we applied Eddington’s formula for the pulsation constant, along with Christy’s computations [16] using the pulsation constant of the first overtone, to refine the classification of CI Cam’s primary [1]. The pulsating component is of spectral type B0–B2 III, with a luminosity of 47,000 L_☉ and a mass of 12–22 M_☉, while the hot secondary component, responsible for the He II 4686 Å line, has a mass of approximately 0.98–1.77 M_⊙. Given that the absolute magnitude of B0–B2 III stars ranges from −3^m.7 to −4^m.9, we estimate the distance to be 2.5–4.5 kpc. This range is consistent with the latest Gaia EDR3 release, which gives a distance of 4.1 kpc with an uncertainty of −0.2/+0.3 kpc [17].

2. Observations

In binary and multiple systems, the He II 4686 Å line can exhibit Doppler shifts due to the motion of the emitting source. We observe such shifts in CI Cam. The variability of the He II 4686 Å line’s intensity in this system is influenced by several factors—acting individually or in combination—including changes in the mass transfer rate, temperature fluctuations in the accretion region, variations in stellar wind density, turbulence and shock waves, and geometric effects that alter the visibility of different emitting regions. Our goal was to study not only the variability of the He II emission line’s parameters associated with orbital motion, but also their changes under the influence of other factors, such as the pulsations of the primary component, the B star.

To investigate the passage of the secondary component through the wind envelope of the primary star, we conducted high-resolution spectroscopic observations near the descending node of its orbit. At this orbital phase (0.932 relative to periastron), the helium line reaches its maximum equivalent width. At this point, the interaction velocity with the B star’s envelope exceeds 200 km s⁻¹, and the interaction surface is directed toward the observer. We successfully used orbital ephemerides to plan and coordinate the observations, which were carried out during the night of 31 October 2023, covering phases from 0.909 to 0.927 relative to periastron.

For this purpose, we used the Main Stellar Spectrograph (MSS) of the BTA telescope [18]. The MSS is designed for stellar and star-forming object spectroscopy, capable of observing sources with apparent magnitude V up to 14 mag in spectroscopic mode and 12 mag in spectropolarimetric mode. It is equipped with circular polarization analyzers and a 2k × 2k CCD detector. The spectra were recorded in the wavelength range λ 4423–4979 Å with a spectral resolution of ~0.3 Å. Individual exposure times were 3600 s. For each frame, the slicer spectra from both circular polarizations were combined and processed using the MIDAS package. The data reduction was performed within the MIDAS context LONG. The wavelength calibration accuracy was 0.005 Å. The signal-to-noise ratio (S/N) at the peak intensity of the He II 4686 Å line ranged from 6 to 12 per pixel. The total monitoring duration was approximately 9.8 h, matching the pulsation period of 9.84 h. During this interval, the secondary component traveled about seven million kilometers in its orbit along the line of sight. The orbit inclination is estimated of about 67°. The resulting spectra are shown in Figure 3.

3. Results

In the high-resolution spectra, the He II 4686 Å line exhibited a heliocentric radial velocity of approximately −260 km s⁻¹. In Figure 2, the measured radial velocities and equivalent widths are marked with orange circles, and the orbital phase of the observations is indicated by a large vertical arrow. Two maxima in the equivalent width were observed during the monitoring period, with values varying by a factor of three, as shown in Figure 4.

For comparison with the helium line behavior, we used two check lines: the weak forbidden [Fe III] 4701 Å (the average EW = −0.28 Å) and the three-times stronger Fe II 4731 Å (the average EW = −0.89 Å). These lines are located very close to the helium line under study; instrumental errors affect them in the same way as the helium line. These iron lines are of a different nature to the helium line. They are formed in the wind envelope of the main component and show only small changes in their velocity. The wind velocity varies over many years with an amplitude of no more than 12 km s⁻¹ ([1], Figure 3f). The MSS spectrograph is aperture-based, and by using these lines for processing, we could correct for the effects of sky background and passing clouds. However, it was not needed, because the sky was clear on this night.

In our case, the primary component of the binary system was a pulsating star whose pulsations can generate shock waves in the stellar atmosphere. The observed variations in the equivalent width of the He II emission line, attributed to the secondary component, may be driven by modulation of the physical conditions in the vicinity of the line-forming region under the influence of pulsations of the primary. These pulsations involve changes in luminosity, temperature, and radiative pressure, which can affect the structure and ionization balance within the accretion stream or in irradiated regions of the secondary. For instance, periodic variations in the UV flux associated with pulsation phases may alter the ionization state of helium, leading to observable changes in the He II-line’s equivalent width. We assume that the observed variability in the He II line resulted from interactions between its radiation source and two pulsation waves, with the total interaction surface oriented toward the observer. Moreover, the radiation source likely had an extended spatial structure.

In [1], ten arguments were presented in support of the hypothesis that the secondary component is a white dwarf. Only a brief remark is made that the object could possibly be of a different nature. This remark led to the idea of conducting a dedicated test.

We proceeded from the understanding that the secondary component penetrates the envelope of the B-type star at a high velocity of approximately 200 km s⁻¹. Therefore, the effect of its interaction with the surrounding medium depends strongly on its size; that is, the cross-sectional area of the object perpendicular to the direction of orbital motion. A white dwarf has a radius comparable to that of the Earth, and thus its interaction cross-section would be minimal. We anticipated, however, that the component could interact with pulsation-driven shock waves in the stellar envelope, even if the orbital passage occurred at a distance of approximately 1.5 stellar radii from the B star’s photosphere.

This single night of observations yielded several unexpected discoveries.

First, we found that the periodicity of interaction with the shock waves was half the pulsation period. We interpret this effect as a consequence of the propagation of shock fronts in the gravitational field of the massive B star, which slows their motion and leads to a frequency increase at larger distances.

Second, the experiment showed that the most pronounced variations occurred in the He II emission, not in the hydrogen lines—despite the fact that hydrogen dominates in the infalling counter-streaming flow. This suggests that the secondary has a helium envelope that becomes ionized by the hydrogen-rich inflow. During orbital passage, hydrogen accumulates in the helium envelope, potentially triggering thermonuclear explosions akin to the 1998 outburst.

Third, we realized that an unusual mode of accretion is probably at work here—what we refer to as “raking accretion”. The best theoretical description of such accretion mode is given in [19], where the mass accretion rate Ṁ = πR²ρV, R is the radius of the accretor, ρ is the density of the surrounding medium, and V—the velocity of star relative the medium. At the high velocity V, this accretion rate does not depend on the accretor’s mass, and carries information about the physical cross-section area πR² of the accreting object. In principle, it can be used to infer the radius of the accretor.

Fourth, the interaction between the secondary component and the B star’s envelope leads to such intense heating of its leading surface that its bolometric luminosity increases by as much as 1800 solar luminosities. This observational result effectively rules out the white dwarf hypothesis. The most plausible remaining candidate is a “stripped” Of-type subdwarf. It is even possible that we are observing the undressing process in real time, as the envelope of the secondary component appears to continue to be lost during its intense interaction with the massive B star. Simultaneously, the envelope can fill up with hydrogen during the accretion process.

This single-night experiment has raised numerous new questions for future investigations, particularly concerning the behavior of the He II emission at other orbital phases, and why it can appear either weak or exceptionally bright at the same orbital phase under different circumstances. These questions now serve as a key driver of our ongoing observational campaign.

4. Discussion

The observational data reliably revealed the variability of the equivalent helium emission line widths. These variations occur with increasing values by a factor of three and with a frequency that is approximately half of the pulsation period of the main component.

We know that the B-star’s envelope is dense and optically thick (the receding wind flows are screened in the profiles). Therefore, in such an envelope, there are conditions for the propagation of pulsation shock waves. We observed and described significant changes in the radial velocities of Fe II and [N II] wind lines, which indicated changes in transparency of the medium [1].

What do we know about the nature of the secondary component from past studies?

This component is responsible for the large 1998 outburst, which had energetics comparable with those of powerful classical novae. The outburst occurred across all wavelengths—from gamma to radio—and was accompanied by an envelope ejection, indicating that the companion is a white dwarf: a degenerate star. It was found at the elliptical orbit with the period of 19.407 day and ellipticity of 0.49, and the equivalent widths of He II-line change with a large amplitude when the secondary passes periastron.

Given the total bolometric luminosity of the B-type primary ~47,000 L_☉ (V ~ 11^m.5; E(B − V) = 0^m.96; A_V = 2^m.96; d = 4200 pc; M_V = −4^m.14; M_bol = −6^m.94), the orbital period is clearly detectable in photometric data, with a 4% amplitude in brightness. As the secondary passes through the dense envelope of the B star near periastron, the system’s bolometric luminosity increases by approximately 1800 L_⊙. Such a large change in luminosity contradicts the hypothesis of a white dwarf, because the surface of a typical white dwarf is too small to radiate such a luminosity.

Our spectroscopic observations showed no evidence of a double-peaked He II 4686 Å emission profile, indicating that the secondary does not host an accretion disk. Instead, we observed signatures of material capture from the B star’s envelope by the leading surface of the compact component. When this interaction coincided with pulsation waves, the helium emission increased by a factor of three.

Considering the relative velocity of ~200 km s⁻¹, the recurrence of interaction every 19 days, and He II-line profiles exclude presence of a structure with a velocity larger than 50 km s⁻¹, the formation of a stable accretion disk is unlikely. This supports the conclusion that the secondary is not a normal white dwarf. With an estimated mass between 0.98 and 1.77 M_⊙ [1] and a bolometric luminosity of at least −3^m.4, the companion is most likely a hot subdwarf, which accretes hydrogen-rich material directly from the B star’s envelope. Taking into account the outburst of 1998, which showed the greatest increase in the intensity of the ionized helium line, we can assume that secondary component is a helium-rich subdwarf having a degenerate core and an extended He-envelope. With the gradual accretion, effectively ‘’raking” material from the primary could have triggered the 1998 thermonuclear outburst. On the Hertzsprung–Russell diagram (HR), the secondary component falls into the recently proposed category of “naked” stars [20], stripped of extended envelopes through binary interaction. However, it possesses a degenerate core but lacks any internal energy source, whether in an envelope or at the center. These findings provide insights into stellar evolution and binary interactions.

It should be noted that in 2007, CI Cam was included among the top ten out of 30 stars proposed by Miroshnichenko for the FS CMa group [21]. Miroshnichenko [21] singled out a group of stars from the general class of stars with the B[e]-phenomenon and named it after the main prototype of this group, the star FS CMa. This group includes most of the stars that were previously known as unclassified B[e] stars. Not being supergiants, the dust-forming B[e] stars (FS CMa stars) share the same observational and physical classification criteria. The CI Cam system satisfies all criteria characteristic of the FS CMa group:

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hydrogen emission lines that are significantly stronger than those in the spectra of Be stars, Herbig Ae/Be stars, and normal supergiants;

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a large infrared excess peaking at 10–30 µm and declining steeply at longer wavelengths;

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location outside star-forming regions;

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a typically fainter and degenerate secondary companion;

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an effective temperature of the hot companion ≥ 30,000 K (consistent with an Of-type star);

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and a hot companion luminosity of log(L/L_⊙) ≈ 3.25.

In Miroshnichenko’s series of papers—especially those with titles like “Towards Understanding the B[e] Phenomenon”—he emphasizes that FS CMa are binary systems undergoing or having recently undergone mass transfer. He suggests that the circumstellar matter responsible for the strong emission features comes not from the fast rotation of the B-type primary (as in Be stars) but from past or ongoing binary interaction. In these papers, Miroshnichenko does not always lock down the exact nature of the secondary but stresses the need for binarity and past interaction to explain the system’s features. His works suggest a diversity among the secondaries, meaning they are not all the same type across the FS CMa class, but the defining characteristic is the presence of a mass-exchanging binary, not a single star.

Confirming the past evolution in the binary system is the recent serendipitous discovery of an [O III]-emission shell around CI Cam, made by Fesen et al. in 2024 using narrow passband emission-line filters [22]. Understanding such systems has broader implications for binary evolution, including the formation pathways of hot subdwarfs, the triggers of thermonuclear events, and the mechanisms regulating angular momentum loss.

5. Conclusions

Our high-resolution spectroscopic observations have revealed several key findings:

Significant He II 4686 Å variability was detected, closely linked to both the orbital phase and the primary star’s pulsation. The strong correlation between the He II 4686 Å emission intensity and the orbital phase, along with a double-wave modulation pattern matching the pulsation period, supports the presence of pulsation-driven shock waves. These findings offer new insight into the complex variability of CI Cam and underscore the potential role of pulsation-enhanced mass transfer in evolved massive binaries.

The observed interaction between the compact companion and the dense, extended envelope of the B[e] primary indicates an accretion mechanism that differs from conventional disk-fed models. Instead, the companion appears to gain mass through direct impact with the envelope material, resulting in substantial energy release. Notably, this “raking” mode of accretion—where the accretor moves through the dense envelope—has rarely been addressed in compact binary studies, making CI Cam an important test case for this underexplored regime.

Our detection of periodic variability at half the pulsation period suggests modulation by shock waves within the circumstellar environment, further reinforcing the role of stellar pulsations in shaping mass transfer dynamics.

Classification of CI Cam within the FS CMa group emphasizes its relevance as a prototype for studying binary mass transfer and non-standard evolutionary pathways.

Our results strongly indicate that the secondary component is not a typical white dwarf but a hot helium subdwarf with a degenerate core—likely an sdO star evolving toward a DO-type white dwarf.

Limitations of the UBVRI photometric system prevent reliable detection and temperature estimation of the hot component. These measurements require observations in the far-ultraviolet and soft X-ray ranges, ideally from space-based observatories. Far-UV spectroscopy, in particular, could provide critical information about the hot component’s properties.

Future directions include orbital phase-resolved observations, high-cadence spectroscopic monitoring synchronized with pulsation cycles, and multi-wavelength campaigns to characterize accretion-driven variability across the electromagnetic spectrum. These efforts will help refine models of pulsation-enhanced mass transfer and improve our understanding of the final evolutionary states of massive binary systems.