We present a multiwavelength study of five methanol maser sites which are not directly associated with a strong ( > 100 mJy ) radio continuum source : G 31.28+0.06 , G 59.78+0.06 , G 173.49+2.42 ( S231 , S233IR ) , G 188.95+0.89 ( S252 , AFGL5180 ) and G 192.60-0.05 ( S255IR ) .
These radio-quiet methanol maser sites are often interpreted as precursors of ultra-compact \ion 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 ^ { \prime \prime } -34 ^ { \prime \prime } .
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 ( > 50 M _ { \odot } ) , deeply embedded ( A _ { v } > 40 mag ) and very luminous ( > 10 ^ { 4 } L _ { \odot } ) molecular clump , with L _ { total } { \propto } M _ { gas } ^ { 0.75 } .
These physical properties characterise massive star-forming clumps in earlier evolutionary phases than \ion 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 ) \ion 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 ( \beta ) range between 1.6 and 1.9 .