Mapping Our Path through the Local Interstellar Medium: High-resolution Ultraviolet Absorption Spectroscopy of Sight Lines along the Sun’s Historical Trajectory

Abstract

Using high-resolution UV data obtained from the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, we focus on eight sight lines along the Sun’s historical trajectory to reconstruct the morphology of our most recent interstellar environment and evaluate its effects on the Sun’s heliosphere over the past five million years. Of the eight targets (each within 50 pc), we detect interstellar absorption in six, primarily Mg ii and Fe ii . We observe C ii and O i absorption in target HD 32147. In each analyzed sight line, the Local Interstellar Cloud (LIC) is observed and bears a similar column density. We detected the Blue and Aur clouds, both of which the Sun may have encountered before entering the LIC. In at least one sight line a fourth cloud is detected. Even in this compact sample, we observe variation in absorption across neighboring sight lines, reflecting the complexity of local interstellar medium (ISM) small-scale structure. We report the discovery of an astrosphere around target GJ J173, observable due to excess Ly α absorption caused by interaction between the star’s stellar wind and the ISM. Over the past ∼five million years, the Sun spent the majority of the time in a low-density plasma region. We see no evidence of any cold, dense material along the observed sight lines. We present timelines for heliospheric response assuming a range of plausible hydrogen densities (0.1 and 0.2 cm ^−3 ). In both cases, we see the Sun, and the heliosphere, traversing significant density variations in its most recent past and most immediate future.

Keywords

• Interstellar medium
• Astrosphere interstellar medium interactions
• Hubble Space Telescope
• Ultraviolet spectroscopy
• Heliosphere