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1446273 
Journal Article 
THE PROTOCLUSTER G18.67+0.03: A TEST CASE FOR CLASS I CH3OH MASERS AS EVOLUTIONARY INDICATORS FOR MASSIVE STAR FORMATION 
Cyganowski, CJ; Brogan, CL; Hunter, TR; Zhang, Q; Friesen, RK; Indebetouw, R; Chandler, CJ 
2012 
760 
2 
WOS:000311103000001 
We present high angular resolution Submillimeter Array and
Karl G. Jansky Very Large Array observations of the massive protocluster G18.67+0.03. Previously
targeted in maser surveys of GLIMPSE Extended Green Objects (EGOs), this cluster contains three
Class I CH3OH maser sources, providing a unique opportunity to test the proposed role of Class I
masers as evolutionary indicators for massive star formation. The millimeter observations reveal
bipolar molecular outflows, traced by (CO)-C-13(2-1) emission, associated with all three Class I
maser sources. Two of these sources (including the EGO) are also associated with 6.7 GHz Class II
CH3OH masers; the Class II masers are coincident with millimeter continuum cores that exhibit hot
core line emission and drive active outflows, as indicated by the detection of SiO(5-4). In these
cases, the Class I masers are coincident with outflow lobes, and appear as clear cases of
excitation by active outflows. In contrast, the third Class I source is associated with an
ultracompact (UC) H II region, and not with Class II masers. The lack of SiO emission suggests
that the 13CO outflow is a relic, consistent with its longer dynamical timescale. Our data show
that massive young stellar objects (MYSOs) associated only with Class I masers are not
necessarily young and provide the first unambiguous evidence that Class I masers may be excited
by both young (hot core) and older (UC H II) MYSOs within the same protocluster. 
ISM: individual objects (G18.67+0.03); ISM: jets and outflows; ISM: molecules; masers; stars: formation; techniques: interferometric 
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