THE FIRST VERY LONG BASELINE INTERFEROMETRY IMAGE OF A 44 GHz METHANOL MASER WITH THE KVN AND VERA ARRAY (KaVA)

Abstract

We have carried out the first very long baseline interferometry (VLBI)\nimaging of 44 GHz class I methanol maser (7_{0}-6_{1}A^{+}) associated with a\nmillimeter core MM2 in a massive star-forming region IRAS 18151-1208 with KaVA\n(KVN and VERA Array), which is a newly combined array of KVN (Korean VLBI\nNetwork) and VERA (VLBI Exploration of Radio Astrometry). We have succeeded in\nimaging compact maser features with a synthesized beam size of 2.7\nmilliarcseconds x 1.5 milliarcseconds (mas). These features are detected at a\nlimited number of baselines within the length of shorter than approximately 650\nkm corresponding to 100 Mlambda in the uv-coverage. The central velocity and\nthe velocity width of the 44 GHz methanol maser are consistent with those of\nthe quiescent gas rather than the outflow traced by the SiO thermal line. The\nminimum component size among the maser features is ~ 5 mas x 2 mas, which\ncorresponds to the linear size of ~ 15 AU x 6 AU assuming a distance of 3 kpc.\nThe brightness temperatures of these features range from ~ 3.5 x 10^{8} to 1.0\nx 10^{10} K, which are higher than estimated lower limit from a previous Very\nLarge Array observation with the highest spatial resolution of ~ 50 mas. The 44\nGHz class I methanol maser in IRAS 18151-1208 is found to be associated with\nthe MM2 core, which is thought to be less evolved than another millimeter core\nMM1 associated with the 6.7 GHz class II methanol maser.\n