New Terahertz Methanol Spectroscopy for HIFI on the Herschel Mission

Summary form only given. Methanol is an important interstellar molecule that was first discovered in interstellar space over 30 years ago. Because of the internal rotational motion which is often called the torsion, the methanol spectrum is extremely dense and complicated, representing a significant challenge for theoretical global modeling. Previous successful theoretical modeling efforts (Li-Hong Xu and Hougen, 1995) included the first two torsional states (vt = 0 and 1) up to Jmax = 20, where vt and J are the torsional and overall rotational quantum numbers, respectively. The global modeling results were used for a microwave review containing line positions and intensities up to 1 THz targeted for the radio-astronomy community nearly 10 years ago (Li-Hong Xu and Lovas, 1997) With new telescopes and space missions, however, there are increased demands for methanol data both in terms of quality (prediction accuracy for line position and intensity) and quantity (frequency coverage). What was once considered a good and adequate database no longer meets the needs arising from today's ever growing astronomy and space community. As an initial demonstration, new THz methanol spectra have been recorded at JPL which not only fill in gaps between the MW and FIR and connect to states not previously observed, but also have important applications for the Herschel mission. Methanol data will be used first for calibration of the HIFI instrument on the ground prior to launch and later will be very significant in helping astronomers to remove the widespread methanol "grass" from the interstellar spectra in order to uncover weak THz lines of new molecular species in the interstellar medium.