ATLASGAL. 3mm class I masers

The new detections increase the number of known 104.3GHz masers from five to nine. The 95 GHz class I methanol maser is generally stronger than the 84 GHz maser counterpart. We find nine sources showing class I methanol masers, but no SiO emission, indicating that class I methanol masers might be the only signpost of protostellar outflow activity in extremely embedded objects at the earliest evolutionary stage.

Class I methanol masers that are associated with sources that show SiO line wings are more numerous and stronger than those without such wings. The total integrated intensity of class I methanol masers is well correlated with the integrated intensity and velocity coverage of the SiO (2-1) emission. The properties of class I methanol masers are positively correlated with the bolometric luminosity, clump mass, and peak H_2 column density of their associated clumps, but are uncorrelated with the luminosity-to-mass ratio, dust temperature, and mean H_2_ volume density.

We suggest that the properties of class I masers are related to shocks traced by SiO. Based on our observations, we conclude that class I methanol masers at 84 and 95GHz can trace a similar evolutionary stage to the H_2O maser, and appear prior to 6.7 and 12.2GHz methanol and OH masers. Despite their small number, the 104.3GHz class I masers appear to trace a shorter and more evolved stage compared to the other class I masers.