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Methanol masers at 6.7 GHz have been found exclusively towards high-mass starforming regions. Recently, some Class 0 protostars have been found to displayconditions similar to what are found in hot cores that are associated withmassive star formation. These hot corino sources have densities, gastemperatures, and methanol abundances that are adequate for exciting strong 6.7GHz maser emission. This raises the question of whether 6.7 GHz methanol maserscan be found in both hot corinos and massive star forming regions, and if not,whether thermal methanol emission can be detected. We searched for the 6.7 GHzmethanol line towards five hot corino sources in the Perseus region using theArecibo radio telescope. To constrain the excitation conditions of methanol, weobserved thermal submillimeter lines of methanol in the NGC1333-IRAS 4 regionwith the APEX telescope. We did not detect 6.7 GHz emission in any of thesources, but found absorption against the cosmic microwave background inNGC1333-IRAS 4A and NGC1333-IRAS 4B. Using a large velocity gradient analysis,we modeled the excitation of methanol over a wide range of physical parameters,and verify that the 6.7 GHz line is indeed strongly anti-inverted for densitieslower than 10^6 cm^-3. We used the submillimeter observations of methanol toverify the predictions of our model for IRAS 4A by comparison with other CH3OHtransitions. Our results indicate that the methanol observations from the APEXand Arecibo telescopes are consistent with dense (n ~ 10^6 cm^-3), cold (T ~15-30 K) gas. The lack of maser emission in hot corinos and low-massprotostellar objects in general may be due to densities that are much higherthan the quenching density in the region where the radiation field is conduciveto maser pumping.

Detection of 6.7 GHz methanol absorption towards hot corinos