The stratospheric terahertz observatory (STO): An LDB experiment to investigate the life cycle of the interstellar medium

The Stratospheric Terahertz Observatory (STO) is a balloon-borne, 0.8-meter telescope designed to investigate the structure of the interstellar medium and the life cycle of interstellar clouds. In its first long duration flight, STO will use two, 4-beam HEB receiver arrays to survey part of the Galactic Plane in the [C II] line at 158 microns (the brightest spectral line in the Galaxy) and the [N II] line at 205 microns (a tracer of the star formation rate). At ~1' angular resolution and < 1 km/s velocity resolution, STO will detect every interstellar cloud with AV > 0.3 in the surveyed region, and, through excitation and kinematic diagnostics provided by [C II] and [N II] line emission, will illustrate how atomic and molecular clouds are formed and dispersed in the Galaxy. STO will make 3dimensional maps of the structure, dynamics, turbulence, energy balance, and pressure of the Milky Way's Interstellar Medium (ISM), as well as the star formation rate. In future flights, STO will observe the important far-infrared lines of [O I], [N II], and HD. I. SCIENCE GOALS AND OBJECTIVES STO will provide a comprehensive understanding of the inner workings of our Galaxy by exploring the connection between star formation and the life cycle of interstellar clouds. We will study the formation of molecular clouds from diffuse atomic gas, the feedback of high mass star formation on the lives of atomic and molecular clouds, and the effect of these processes upon the global structure and evolution of the Galaxy. The detailed understanding of star formation and evolution of stars and gas in the Galaxy is directly relevant to star formation in other galaxies. The nature of the feedback mechanism of massive star formation with its interstellar environment is pivotal to the evolution of galaxies. In its first flight, STO addresses the following high priority goals: 1. Determine the life cycle of Galactic interstellar gas. 2. Study the creation and disruption of star-forming clouds in the Galaxy. 3. Determine the parameters that affect the star formation rate in a galaxy. 4. Provide templates for star formation and stellar/interstellar feedback in other galaxies. STO will utilize two heterodyne receiver arrays to produce a total of eight ~1' pixels in the focal plane, each with 1024 spectral channels. In the first long duration (10-14 day) flight, STO will map a 35 square degree area (-20 > l > -55; |b| < 1, see Figures 1 and 2) spanning the Molecular Ring, the Crux-Scutum-Centaurus spiral arm, and at least one interarm region. Two deeper, 1/2 square degree maps will be performed within the larger survey in both arm and interarm regions. STO has the sensitivity to detect and the ability to resolve spectrally and spatially all Giant Molecular Clouds (GMCs), all significant HII regions, and all cold neutral medium (CNM) atomic clouds with AV > 0.3 mag in the surveyed region. The STO heterodyne receivers provide sub-km/s velocity discrimination and sufficient bandwidth to detect and resolve line emission from every Galactic cloud in the surveyed region. The data product will be a high fidelity database of spatially and velocity resolved far-infrared [C II] 158 micron and [N II] 205 micron fine-structure line emission in the Galaxy. 19th International Symposium on Space Terahertz Technology, Groningen, 28-30 April 2008