The Line Emission Terahertz Observatory (LETO): Exploring the lifecycle of the ISM and the origins of water
The Interstellar Medium (ISM) is the reservoir of baryonic matter from which stars and planetary systems are formed. It is also the repository of the material that is expelled at the end of the stellar evolutionary cycle feeding the baryonic matter reservoir. These evolutionary phases in the ISM together form a complex interplay driving planet and star formation and thus the evolution of our own Milky Way as well as galaxies at low and high redshifts. The design of the Line Emission Terahertz Observatory (LETO) has been optimized to investigate the impact of the ISM on star formation on galactic and extragalactic scales, study the processes that transform gas clouds into stars and planetary systems, and trace the flow of water in the ISM. To achieve these goals, LETO will carry out deep velocity-resolved wide-area spectroscopic observations of key FIR lines in the ISM covering an area of approximately 900 square degrees of the Galactic Plane. To complement our local view LETO will map a large sample of about 200 nearby galaxies in addition to surveys of Galaxies at Cosmic Noon. To shed light on the planet formation process, LETO will study the physical and chemical properties (especially gas mass) of numerous proto-planetary disks and stellar cores through pointed observations of the HD and H2O lines. LETO is a powerful FIR mission building on rich European heritage. To satisfy the requirements for sensitivity, resolving power and mapping speed, LETO utilizes a 3.5m class mirror and several bands with sensitive state-of-the-art multi-pixel heterodyne arrays. The bands together will cover the wavelength range from 56 to 666 micron and with the heterodyne receivers and backends high resolving power spectroscopy a set of key FIR atomic, ionic, and molecular lines can be studied in great detail. The mission is one of several selected for further study in the context of the ESA M8 call.