Astronomy and Astrophysics, volume 489, 1175-1182 (2008/10-3)
Detection of 6.7 GHz methanol absorption towards hot corinos.
PANDIAN J.D., LEURINI S., MENTEN K.M., BELLOCHE A. and GOLDSMITH P.F.
Abstract (from CDS):
Methanol masers at 6.7GHz have been found exclusively towards high-mass star forming regions. Recently, some Class 0 protostars have been found to display conditions similar to what are found in hot cores that are associated with massive star formation. These hot corino sources have densities, gas temperatures, and methanol abundances that are adequate for exciting strong 6.7GHz maser emission. This raises the question of whether 6.7GHz methanol masers can 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 6.7GHz methanol line towards five hot corino sources in the Perseus region using the Arecibo radio telescope. To constrain the excitation conditions of methanol, we observed thermal submillimeter lines of methanol in the NGC 1333-IRAS 4 region with the APEX telescope. We did not detect 6.7GHz emission in any of the sources, but found absorption against the cosmic microwave background in NGC 1333-IRAS 4A and NGC 1333-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 6.7GHz line is indeed strongly anti-inverted for densities lower than 106cm6-3^. We used the submillimeter observations of methanol to verify the predictions of our model for IRAS 4A by comparison with other CH3OH transitions. Our results indicate that the methanol observations from the APEX and Arecibo telescopes are consistent with dense (n∼106cm–3), cold (T∼15-30K) gas. The lack of maser emission in hot corinos and low-mass protostellar objects in general may be due to densities that are much higher than the quenching density in the region where the radiation field is conducive to maser pumping.
masers - stars: low-mass, brown dwarfs