Six intermediate-mass stars with far-infrared excess: a search for evolutionary connections

We present the results of high-resolution spectroscopic, low-resolution spe ctrophotometric and spectropolarimetric and broad-band multicolour observat ions of four B-type stars (HD 4881, 5839, 224638 and 179218) and two A-type stars (HD 32509 and 184761) with strong far-infrared (IR) excesses. The ex cess in HD 184761, which is located at a distance of 65 pc from the Sun, wa s recognized for the first time. Double-peaked H alpha emission line profil es are found in HD 4881 and HD 5839, while HD 184761, I-ID 224638 and IID 3 2509 display no emission in H alpha. The remarkable variations observed in the H alpha profile of RD 179718 are also observed in some classical Be and Herbig Ae/Be stars. An intrinsic component of polarization is clearly pres ent in HD 179218, only an interstellar component is detected in HD 4881 and HD 224648, and HD 184761 was found to be unpolarized. Improved effective t emperatures for all six objects were derived. Parallaxes measured by the Hi pparcos satellite were used to determine positions of the stars in the HR d iagram. HD 4881 and HD 5839 are an order of magnitude more luminous than ma in-sequence stars of similar temperatures and rue most likely newly discove red classical Be stars. Study of the high-resolution IRAS maps and modellin g of the spectral energy distributions of HD 4881, HD 5839 and fro 224648 s uggest that the observed large IR excesses an caused by radiation from circ umstellar dust rather than free-free radiation or infrared cirrus, so they may be higher mass counterparts of beta Pictoris stars. HD 32509, HD 224638 and HD 184761, which have very small near-IR excesses, are probably young main-sequence stars. HD 179218, which exhibits the largest near- and far-IR excess in the sample, is an isolated pre-main-sequence Herbig Be star.