An Ultraviolet Initiative for Hubble in Cycle 21

Neill Reid, inr@stsci.edu & Ken Sembach, sembach@stsci.edu

Introduction

Earth’s atmosphere is essentially completely opaque to radiation at wavelengths shorter than 315 nm. Astronomers are therefore dependent on space missions for observations at those wavelengths. Starting with Orbiting Astronomical Observatory 2, and running through Copernicus, International Ultraviolet Observatory, Extreme Ultraviolet Explorer, Hubble, Far Ultraviolet Spectroscopic Explorer, and Galaxy Evolution Explorer, ultraviolet (UV) observations from orbiting observatories have played a central role in mapping the distribution and composition of warm and hot gas in the local universe, probing the characteristics of energetic phenomena, revealing the properties of nebulae and remnants of stellar evolution, studying stellar winds and chromospheres, and sampling planetary atmospheres inside and outside the solar system.

With a few limited exceptions, Hubble is the only current mission capable of undertaking detailed observations above the Earth’s atmosphere in the 90–320 nm UV range. Observations with four Hubble instruments—Cosmic Origins Spectrograph (COS), Space Telescope Imaging Spectrograph (STIS), Advanced Camera for Surveys (ACS), and Wide-Field Camera 3 (WFC3)—are currently the cornerstone of modern astrophysical investigations at those wavelengths. Imaging observations can provide information on how star-formation regions interact with the local interstellar medium (Figure 1). Spectroscopic observations are crucial to mapping the distribution of metals in gaseous media (Figure 2). These capabilities will not be available forever. STIS and ACS are single-string instruments, and COS sensitivity, while expected to be excellent for at least another five years, will eventually degrade. New missions targeting UV wavelengths are not yet on the horizon.