Widespread Ethanol in Galactic Center Clouds

Abstract

We present observations of ethanol (C2H5OH) toward molecular clouds in Sgr A, Sgr B2 and associated to the thermal features (Arc and the Sickle) and the Non-Thermal Radio Arc (NTRA) in the Galactic Center. Ethanol emission is much weaker or it is even not detected toward some molecular clouds in both complexes, in particular those with radial velocities between 70 and 120 km s−1. While most of the clouds associated with thermal features do not show ethanol emission, that associated with the NTRA does show emission. Comparison between the column densities of 13CO, CS and C2H5OH shows that the 13CO/CS abundance ratio is fairly constant in all molecular clouds, while the abundance of ethanol changes by more than one order of magnitude. The fractional abundance of ethanol in most of the clouds with radial velocities between 0 and 70 km s−1 in Sgr A and Sgr B2 is relatively high, of few 10−8. The C2H5OH abundance decreases by more than one order of magnitude (~ 10−9) in the clouds associated with the thermal features. The large abundance of C2H5OH in gas phase indicates that ethanol has been formed on grains and released to gas phase by shocks which also heats the gas. This picture also explains the low C2H5OH abundance in the thermal features where heating is dominated by the UV radiation from massive evolved stars. According with current chemical models, evaporation of C2H5OH in the Sgr A and Sgr B2 clouds should have occurred nearly simultaneously and very recently, just in ~ 105 years. Two possible scenarios could account for the time scale implied from the ethanol evaporation, large scale shocks related to infall of matter toward the Galactic Center and/or shocks produced by evolved massive stars formed in a starburst ~ 107 years ago.