The protostellar mass limit for 6.7 GHz methanol masers I. A low-mass YSO survey

We report the results of a search for 6.7 GHz methanol masers toward low-mass young stellar objects (YSOs) and (pre)protostellar condensations with the Australia Telescope Compact Array (ATCA). Our sample consisted of 13 class 0 protostars and 44 class I YSOs as well as 66 (pre)protostellar condensations. A single detection was obtained toward NGC 2024: FIR4 in the Orion B region. This is the first detection of a 6.7 GHz methanol maser in Orion. The nature of FIR4 has been a subject of debate with some evidence suggesting that it is a very cold high-mass (pre)protostellar condensation and others arguing that it is a low-mass YSO. The discovery of a methanol maser associated with this source is inconsistent with both of these hypotheses and we suggest that FIR4 probably harbours an intermediate- or high-mass YSO. The less massive objects in our sample do not exhibit any methanol maser stronger than 400 mJy (4σ). Based on the nil detection rate toward the low-mass YSOs we can place an upper limit of 3 × 106 K on the brightness temperature of any methanol maser associated with class 0.I or II sources. These results support the hypothesis that no strong methanol masers are associated with low-mass star formation (≲ 3 M⊙).