The Search for H⁻in Astrophysical Environments

Abstract

The negative ion H− is widely understood to be important in many astrophysical environments, including the atmospheres of late-type stars like the Sun. However, the ion has never been detected spectroscopically outside the laboratory. A search for the far-ultraviolet autodetaching transitions of H− in interstellar and circumstellar matter seems to be the best hope for directly detecting this ion. We undertook a highly sensitive search using data from the FUSE instrument. We concentrated on two types of sight lines: planetary nebulae, where model calculations suggest a sufficient abundance of H− to be determined, and translucent clouds, where H− might form on dust grains as an intermediate step in molecular hydrogen formation. Upper limits on H− abundances were set.