Non-emission line young stars of intermediate mass

Abstract

We present optical spectra of four intermediate mass candidate young stellar objects that have often been classified as Herbig Ae/Be stars. Typical Herbig Ae/Be emission features are not present in the spectra of these stars. Three of them, HD 36917, HD 36982 and HD 37062 are members of the young Orion Nebula Cluster (ONC). This association constrains their ages to be < 1 Myr. The lack of appreciable near infrared excess in them suggests the absence of hot dust close to the central star. But they do possess significant amounts of cold and extended dust component as revealed by the large excess emission observed at far infrared wavelengths. Fractional infrared luminosity $L_{ir}/L_{\star}$ and the dust masses computed from IRAS fluxes are systematically lower than that found for Herbig Ae/Be stars but higher than those for Vega-like stars. These stars may then represent the youngest examples of Vega phenomenon known so far. In contrast, the other star in our sample, HD 58647, is more likely to be a classical Be star as evident from the low $L_{ir}/L_{\star}$, scarcity of circumstellar dust, low polarization, presence of H$\alpha$ emission and near infrared excess and far infrared spectral energy distribution consistent with free-free emission similar to other well known classical Be stars.