We have carried out the first very long baseline interferometry ( VLBI ) imaging of 44 GHz class I methanol maser ( 7 _ { 0 } - 6 _ { 1 } A ^ { + } ) associated with a millimeter core MM2 in a massive star-forming region ( catalog IRAS 18151 $ - $ 1208 ) with KaVA ( KVN and VERA Array ) , which is a newly combined array of KVN ( Korean VLBI Network ) and VERA ( VLBI Exploration of Radio Astrometry ) .
We have succeeded in imaging compact maser features with a synthesized beam size of 2.7 milliarcseconds \times 1.5 milliarcseconds ( mas ) .
These features are detected at a limited number of baselines within the length of shorter than \approx 650 km corresponding to 100 M \lambda in the uv -coverage .
The central velocity and the velocity width of the 44 GHz methanol maser are consistent with those of the quiescent gas rather than the outflow traced by the SiO thermal line .
The minimum component size among the maser features is \sim 5 mas \times 2 mas , which corresponds to the linear size of \sim 15 AU \times 6 AU assuming a distance of 3 kpc .
The brightness temperatures of these features range from \sim 3.5 \times 10 ^ { 8 } to 1.0 \times 10 ^ { 10 } K , which are higher than estimated lower limit from a previous Very Large Array observation with the highest spatial resolution of \sim 50 mas .
The 44 GHz class I methanol maser in ( catalog IRAS 18151 $ - $ 1208 ) is found to be associated with the MM2 core , which is thought to be less evolved than another millimeter core MM1 associated with the 6.7 GHz class II methanol maser .