ar X iv : a st ro - p h / 07 01 34 6 v 1 1 1 Ja n 20 07 Young Stellar Groups Around Herbig Ae / Be Stars : A Low - Mass YSO Census

We present NIR and MIR observations of eight embedded young stellar groups around Herbig Ae/Be stars (HAEBEs) using archived Spitzer IRAC data and 2MASS data. These young stellar groups are nearby (≤ 1 kpc) and still embedded within their molecular clouds. In order to identify the young stellar objects in our sample, we use the color-color diagram of J [3.6] vs. Ks [4.5]. The Spitzer images of our sample show that the groups around HAEBEs, spectral types earlier than B8, are usually associated with bright infrared nebulosity. Within this, there are normally 10 50 young stars distributed close to the HAEBEs (< 1 pc). Not only are there young stars around the HAEBEs, there are also young stellar populations throughout the whole cloud, some are distributed and some are clumped. The groups around the HAEBEs are sub-structures of the large young population within the molecular cloud. The sizes of groups are also comparable with those sub-structures seen in massive clusters. Young stars in groups around HAEBEs have generally larger SED slopes compared to those outside, which suggests that the young stars in groups are probably younger than the distributed systems. This might imply that there is usually a higher and more continuous star forming rate in groups, that the formation of groups initiates later, or that low mass stars in groups form slower than those outside. Finally, there is no obvious trend between the SED slopes and the distance to the HAEBEs for those young stars within the groups. This suggests that the clustering of young stars dominates over the effect of massive stars on the lowmass young stars at the scale of our study. Subject headings: stars: formation—stars: pre-main sequence—infrared: stars