2014ApJ...789L...1M

We have carried out the first very long baseline interferometry (VLBI) imaging of a 44 GHz class I methanol maser (7₀-6₁ A ⁺) associated with a millimeter core MM2 in a massive star-forming region 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 milliarcsecondsx1.5 milliarcseconds (mas). These features are detected at a limited number of baselines within the length of shorter than ~ 650 km corresponding to 100 Mλ 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 ∼5 masx2 mas, which corresponds to the linear size of ∼15 AUx6 AU assuming a distance of 3 kpc. The brightness temperatures of these features range from ∼3.5x10⁸ to 1.0x10¹⁰ K, which are higher than the estimated lower limit from a previous Very Large Array observation with the highest spatial resolution of ∼50 mas. The 44 GHz class I methanol maser in 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.