Chemical evolution of ice and gas from molecular clouds to protostars
We present observations toward stars behind molecular clouds (back ground stars) and toward massive protostars in order to study the chemical evolution in molecular clouds before and during star formation. Infrared ab sorption spectroscopy with the Spitzer Space Telescope toward background stars shows that complex ices exist toward lines of sight not associated with star formation. In addition to solid H2O, CO, and CO2, we ﬁnd evidence for HCOOH and a tentative identiﬁcation of NH+ 4 . We also ﬁnd that the 6.0 µm H2O band changes when mixed with CO2 in high concentrations. These results give the initial composition of solid material prior to star formation. Once the star formation process ensues, the icy grain mantles sublimate due to heating from the protostar. We present gas-phase, infrared absorption spectroscopy using TEXES, a high-resolution spectrograph, toward the mas sive protostars NGC 7538 IRS 1 and AFGL 2591. While we only detect two molecules (C2H2 and HCN) toward AFGL 2591, NGC 7538 IRS 1 shows a very rich mid-infrared spectrum with absorption from seven molecules (C2H2, HCN, CH3, HNCO, NH3, CH4, and CS). We present the ﬁrst infrared detection of interstellar HNCO as well as the ﬁrst detection of CH3 in dense gas. Sublimation of icy mantles can explain the observed enhancement of molecules formed on grains such as CH4, NH3, and HNCO. Other species such as C2H2 are also enhanced though its formation is less certain. It is possible that C2H2 also forms on grains though no evidence of solid C2H2 has been found to date. Daughter molecules such as CH3 are also found in high abundance toward NGC 7538 IRS 1. The gas observations presented in this work could be trac ing material in a disk or material that, at some point in time, was part of a circumstellar disk. By combining observations of gas and ice compositions, we can begin to understand the chemical evolution from quiescent molecular clouds to protostars.