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

Abstract

ABSTRA C T We present the results of high-resolution spectroscopic, low-resolution spectrophotometric and spectropolarimetric and broad-band multicolour observations of four B-type stars (HD 4881, 5839, 224648 and 179218) and two A-type stars (HD 32509 and 184761) with strong far-infrared (IR) excesses. The excess in HD 184761, which is located at a distance of 65 pc from the Sun, was recognized for the first time. Double-peaked Ha emission line profiles are found in HD 4881 and HD 5839, while HD 184761, HD 224648 and HD 32509 display no emission in Ha. The remarkable variations observed in the Ha profile of HD 179218 are also observed in some classical Be and Herbig Ae/Be stars. An intrinsic component of polarization is clearly present 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 temperatures for all six objects were derived. Parallaxes measured by the Hipparcos satellite were used to determine positions of the stars in the HR diagram. HD 4881 and HD 5839 are an order of magnitude more luminous than main-sequence stars of similar temperatures and are most likely newly discovered classical Be stars. Study of the high-resolution IRAS maps and modelling of the spectral energy distributions of HD 4881, HD 5839 and HD 224648 suggest that the observed large IR excesses are caused by radiation from circumstellar dust rather than free‐free radiation or infrared cirrus, so they may be higher mass counterparts of b Pictoris stars. HD 32509, HD 224648 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.