This paper describes recent studies of the FS CMa-type objects, a group of stars showing the B[e] phenomenon defined in 2007. The objects exhibit strong emission-line spectra with both permitted and forbidden lines suggesting the presence of a B-type star as well as strong IR excesses due to radiation of circumstellar dust. These properties are hard to explain in the framework of the evolution of single stars with luminosities between ~300 and ~30,000 L_⊙ typical of most B-type stars. We explore the hypothesis that the gaseous-and-dusty envelopes of FS CMa objects are due to either earlier or ongoing mass transfer between the binary system components. It is hard to detect the secondary components in these systems because of veiling and distortions by the circumstellar matter because of the relative faintness of the companions. Nevertheless, we detected regular radial velocity variations of the spectral lines in MWC 728, 3 Pup, and AS 386 and we found absorption lines typical of cool stars in the spectra of MWC 645, AS 174, and several other objects. The diversity of the secondary components in FS CMa objects is discussed in the context of non-conservative binary evolution.

We present the results of optical and near-IR spectroscopy and multicolor photometry of the emission-line star MWC 645, which exhibits the B[e] phenomenon. The presence of positionally variable absorption lines of a cool star detected for the first time indicates that the object is a binary system. Using a combination of the photometric and spectroscopic data as well as the Gaia EDR3 distance (D = 6.5 ± 0.9 kpc), we disentangled the components’ contributions and estimated their surface temperatures and luminosities (18, 000 ± 2000 K and 4250 ± 250 K, log L/L _⊙ = 4.0 ± 0.5 and 3.1 ± 0.3 for the hot and cool components, respectively). Quasi-cyclic short-period (months) and long-period (∼4 yr) photometric variations were detected in both optical and near-IR regions, and are most likely due to variable circumstellar extinction. Our analysis suggests that MWC 645 is a member of the FS CMa group. The object’s properties can be described by an evolutionary model of a close binary system that currently undergoes a nonconservative mass transfer between intermediate-mass stars (e.g., 7 M _⊙ + 2.8 M _⊙).

HD 327083 is a binary system that consists of two supergiant components and exhibits the B[e] phenomenon. In this paper, we report the determination of a new set of the system’s fundamental parameters using a combination of photometric and spectroscopic data as well as the Gaia EDR3 distance. We found that the orbital period of the system is 107.68 ± 0.02 days. The spectral line content implies the effective temperatures of ≈7000 K and 25,400 ± 1400 K, while the photometric variations are consistent with the radii of ≈106 R _⊙ and ≈10 R _⊙ for the cool and hot components, respectively. The absorption lines of the cool component show a radial velocity semiamplitude of 48.3 ± 1.7 km s⁻¹, similar to that of the emission lines that originate around the hot component. The inclination of the system to the line of sight is 47 − 20 + 17 °. Modeling of the system's evolutionary history suggests that the components have masses of ∼12.5 M _⊙ and currently undergo mass transfer between them. This configuration, which takes in heating of the surface of the cool component by the radiation from the hot one, can reproduce the photometric and spectroscopic data and is in agreement with previous infrared observations of the circumbinary disk. The results of this study further confirm the hypothesis that the reason for the presence of the B[e] phenomenon in most objects is a consequence of the evolution of various binary systems.

HD327083 is a member of a small group of supergiants exhibiting the B[e] phenomenon. It was found to be a binary system with an early-B and an early-F supergiant components. However the fundamental and orbital parameters of the system were not accurately known. We determined a new set of the system parameters that include the orbital period and the components’ masses using a combination of photometric and spectroscopic data. A new orbital period of 107.7 days was found from both the spectral line positional variations and the visual light curve. Absorption lines of the cool component show a radial velocity semi-amplitude of 48.3 kms −1 , similar to that of emission lines that originate around the hot component. The system shows partial eclipses. We estimated the components’ masses to be nearly equal and close to 6-8 M ? . The masses turned out to be smaller then the evolutionary masses that may be a consequence of a recent mass-transfer.

This study reports on high-angular-resolution observations of the massive star-forming region G328.2551−0.5321 obtained with the Atacama Large Millimeter/submillimeter Array at a wavelength of 0.89 mm. The innermost core structure is investigated by targeting high-excitation transitions of methanol, sulfur dioxide, and its sulfur-34 isotopologue. Analysis of the molecular emission reveals a compact and dense methanol component located at the continuum peak, which is surrounded by more extended emission from sulfur-bearing molecules. Kinematic analysis demonstrates a coherent velocity gradient along the major axis of the system. Position–velocity diagrams exhibit a characteristic butterfly morphology, which is consistent with the presence of a Keplerian rotation. Numerical model fitting provides dynamical mass estimates of approximately twenty solar masses for the central object, confirming the existence of a massive, rotating disk-like structure. Rotational diagram analysis of methanol transitions yields a rotational temperature of 187.9 ± 23.7 Kelvin and a total column density of 4.27 × 10¹⁶ cm⁻², indicating the presence of a hot molecular core. These findings suggest that a high-mass protostar is actively accreting within a compact disk-envelope system embedded in a hot core environment. The results contribute to the understanding of the physical conditions and gas dynamics in the early stages of massive star formation.

We present high–angular resolution 1.3 mm continuum and molecular line observations toward the highmass star-forming region G350.29+0.12, using CH₃CN and CH₃¹³CN rotational transitions. The continuum emission resolves two main cores: a bright, compact northern core G350.29+0.12 A and a weaker southern core G350.29+0.12 B. Core A exhibits six compact substructures embedded within more extended emission. M K-components of the CH3CN J = 14→13 and CH3 13CN J = 14→13 transitions are detected, with the emission arising primarily from core A. The CH3CN moment 0 maps show that the integrated intensities peak at the main continuum position, indicating that the molecular emission traces warm and dense gas. The moment 1 maps reveal a pronounced velocity gradient of ∼2 km s-1 across core A, oriented from northwest to southeast, while the moment 2 maps show K-dependent variations in velocity dispersion. Position–velocity diagrams further indicate organized rotational motions, exhibiting a compact velocity structure with a central peak and systematic gradients. Rotational temperature analysis yields = 360.6 ± 34.8 K for CH3CN, tracing the hottest and densest gas and Trot = 138 ± 45 K for CH3 13CN, consistent with cooler and more extended material. Together, these results demonstrate that G350.29+0.12 A is a rotating hot molecular core undergoing active high-mass star formation.