The physical and chemical properties of prestellar cores , especially massive ones , are still far from being well understood due to the lack of a large sample .
The low dust temperature ( < 14 K ) of Planck cold clumps makes them promising candidates for prestellar objects or for sources at the very initial stages of protostellar collapse .
We have been conducting a series of observations toward Planck cold clumps ( PCCs ) with ground-based radio telescopes .
In general , when compared with other star forming samples ( e.g .
infrared dark clouds ) , PCCs are more quiescent , suggesting that most of them may be in the earliest phase of star formation .
However , some PCCs are associated with protostars and molecular outflows , indicating that not all PCCs are in a prestellar phase .
We have identified hundreds of starless dense clumps from the mapping survey with the Purple Mountain Observatory ( PMO ) 13.7-m telescope .
Follow-up observations suggest that these dense clumps are ideal targets to search for prestellar objects .