Using the 13.7 m telescope of the Purple Mountain Observatory （PMO）, a survey of the 3 = 1 - 0 lines of CO and its isotopes was carried out on 98 methanol maser sources in January 2008. Eighty-five sources have infrared counterparts within one arcmin. In the survey, except for 43 sources showing complex or multiple-peak profiles, almost all the ^13CO line profiles of the other 55 sources have large line widths of 4.5km s^-1 on average and are usually asymmetric. Fifty corresponding Infrared Astronomical Satellite （IRAS） sources of these 55 sources have Lbdl larger than 10^3 L⊙, which can be identified as possible high-mass young stellar sources. Statistics show that the ^13CO line widths correlate with the bolometric luminosity of the associated IRAS sources. Here, we also report the mapping results of two sources; IRAS 06117＋1350 and IRAS 07299-1651. Two cores were found in IRAS 06117＋1350 and one core was detected in IRAS 07299-1651. The northwest core in IRAS 06117＋1350 and the core in IRAS 07299-1651 can be identified as precursors of UC HII regions or high-mass protostellar objects （HMPOs）. The southeast core of IRAS 06117＋1350 has no infrared counterpart, seeming to be at a younger stage than the pre-UC HII phase.
We present modeling work on three young stellar objects that are promis-ing targets for future high-resolution observations to investigate circumstellar disk evolution. The currently available data comprise the spectral energy distribution from optical to millimeter wavelengths which allow constraining the structure of the cir-cumstellar disk using self-consistent radiative transfer models. The results suggest that the assumption of well-mixed dust and gas leads to overestimation of flux in the far-infrared. Observational and theoretical arguments suggest that an overall decrease in far-infrared excess can be explained by dust settling towards the midplane. A new disk model is hence employed to take the effect of dust sedimentation into account. The extended model satisfactorily reproduces all existing observations. The three tar-gets studied here therefore deserve follow-up observations to reveal the evolutionary state of their protoplanetary disks.
