PROPERTIES OF GALACTIC FS CMA TYPE OBJECTS, A NEW GROUP OF DUST-FORMING SYSTEMS

RESUMEN El recientemente definido grupo galáctico de FS CMa abarca ∼ 40 objetos no muy luminosos que amplían significativamente el rango de luminosidades de formación de estrellas de polvo caliente. Estos objetos exhiben espectros de emisión de línea extremadamente fuertes y excesos IR, que son indicativos de envolturas compactas de polvo. Estos son muy probablemente sistemas binarios que experimentan actualmente, o han experimentado recientemente, una fase de rápido intercambio de masa, asociado con la formación de polvo. La compleja estructura del ambiente circunestelar vela significativamente las estrellas subyacentes y requiere la investigación de múltiples técnicas, en la cual la interferometría debe desempeñar un papel importante. ABSTRACT The recently defined Galactic FS CMa group comprises ∼40 not very luminous objects that significantly expand the luminosity range of hot dust-forming stars. These objects exhibit extremely strong emission-line spectra and IR excesses, which are indicative of compact dust envelopes. They are most likely binary systems that currently undergo or have recently undergone a phase of a rapid mass exchange, associated with dust formation. Complex structure of the circumstellar environments significantly veils the underlying stars and requires multitechnique investigation, in which interferometry should play an important role. Key Words: circumstellar matter -stars: early-type -stars: emission-line, Be This is the fourth and the last talk on objects with the B[e] phenomenon. Since basic information about it has already been presented (see papers by M. Borges Fernandes and M. Kraus in this proceedings), I will directly focus on the new subgroup of FS CMa stars, previously known as unclassified B[e] stars. It consists of ∼ 40 Galactic objects that exhibit very strong emission-line spectra and a steep decrease of the IR flux at λ ≥ 10 − 30 µm . Until recently, the nature and evolutionary state of the majority of these objects were considered controversial. Their luminosity range (2.5 ≤ log L/L ≤ 4.5) indicates that the stars are not very massive, while the location on the Hertzsprung-Russell diagram (typically within the main sequence) suggests that they are not extremely evolved. The shape of the IR spectral energy distribution in combination with a lack of a visible nebulosity suggest that circumstellar (CS) dust is compactly distributed. It has probably been formed only recently or is being formed now. Our initial results, presented in the group defining papers (see above), show that a noticeable fraction of the objects (∼ 30%) are either recognized or suspected binary systems. Binarity is usually manifested by the presence of absorption lines, typical of late-type stars (e.g., Li I 6708Å and Ca I 6717 A). In some cases, individual emission lines exhibit a different behavior compared to the rest of them. For example, a traveling He II 4686Å emission line in the spectrum of CI Cam Apart from the fact that the FS CMa objects are interesting as a still poorly studied group of emissionline stars, two important reasons to further investigate them can be highlighted here. The first one is to reveal mechanisms responsible for the very strong emission-line spectra. The second one is to reveal the origin and location of the CS dust. The strong emission-line spectra of FS CMa objects are not expected from single stars, because the current theory of stellar evolution predicts relatively weak stellar winds for main-sequence stars in the mentioned luminosity range. Their Hα line equiva-10