2005A&A...429..945M

We present a multiwavelength study of five methanol maser sites which are not directly associated with a strong (>100mJy) radio continuum source: G31.28+0.06, G59.78+0.06, G173.49+2.42 (S231, S233IR), G188.95+0.89 (S252, AFGL5180) and G192.60-0.05 (S255IR). These radio-quiet methanol maser sites are often interpreted as precursors of ultra-compact HII regions or massive protostar sites. In this work, the environment of methanol masers is probed from mid-IR to millimetre wavelengths at angular resolutions of 8"-34". Spectral energy distribution (SED) diagrams for each site are presented, together with mass and luminosity estimates. Each radio-quiet maser site is always associated with a massive (>50M_☉), deeply embedded (A_v>40mag) and very luminous (>10⁴L_☉) molecular clump, with L_total∝M_gas^0.75. These physical properties characterise massive star-forming clumps in earlier evolutionary phases than HII regions. In addition, colder gas clumps seen only at mm-wavelengths are also found near the methanol maser sites. These colder clumps may represent an even earlier phase of massive star formation. These results suggest an evolutionary sequence for massive star formation from a cold clump, seen only at mm wavelengths, evolving to a hot molecular core with a two-component SED with peaks at far-IR and mid-IR wavelengths, to an (ultra-compact) HII region. Alternatively, the cold clumps might be clusters of low-mass YSOs, in formation near the massive star-forming clusters. Finally, the values of the dust grain emissivity index (β) range between 1.6 and 1.9.