Evolutionary status of isolated B[e] stars

Aims. We study a sample of eight B[e] stars with uncertain evolutionary status to shed light on the origin of their circumstellar dust. Methods. We performed a diagnostic analysis on the spectral energy distribution beyond infrared wavelengths, and conducted a census of neighboring region of each target to ascertain its evolutionary status. Results. In comparison to pre-main sequence Herbig stars, these B[e] stars show equally substantial excess emission in the near-infrared, indicative of existence of warm dust, but much reduced excess at longer wavelengths, so the dusty envelopes should be compact in size. Isolation from star-forming regions excludes the possibility of their pre-main sequence status. Six of our targets, including HD 50138, HD 45677, CD−24 5721, CD−49 3441, MWC 623, and HD 85567, have been previously considered as FS CMa stars, whereas HD 181615/6 and HD 98922 are added to the sample by this work. We argue that the circumstellar grains of these isolated B[e] stars, already evolved beyond the pre-main sequence phase, should be formed in situ. This is in contrast to Herbig stars, which inherit large grains from parental molecular clouds. It has been thought that HD 98922, in particular, is a Herbig star because of its large infrared excess, but we propose it being in a more evolved stage. Because dust condenses out of stellar mass loss in an inside-out manner, the dusty envelope is spatially confined, and anisotropic mass flows, or anomalous optical properties of tiny grains, lead to the generally low line-of-sight extinction toward these stars