Mapping Our Path Through the Local Interstellar Medium: High Resolution Ultraviolet Absorption Spectroscopy of Sight Lines Along the Sun's Historical Trajectory

The local interstellar medium (LISM) is a complex environment, comprised of a suite of interstellar clouds extending tens of parsecs and surrounding the nearest stars. As our solar system journeyed through the Milky Way, the Sun's heliosphere will have expanded and contracted, responding to variations in the density of interstellar gas and dust. Thus, the properties of the LISM are critically important in understanding the recent interaction between the Sun and its surrounding interstellar environment. Using high-resolution UV data obtained from the Space Telescope Imaging Spectrograph aboard 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 environments and evaluate its effects on the Sun's heliosphere over the past ~5 million years. Of the eight targets, each of which is within 50 pc, we see interstellar absorption in almost every one, primarily Mg II and Fe II. We observe CII and OI absorption in our highest signal to noise target HD32147. In each analyzed sight line, the LIC cloud 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, and in at least one sight line a fourth cloud is detected. In three of the targets, we were able to observe the Lyman-alpha profile. For sight line GJ173, this enabled measurements of additional physical properties of our past environment, such as depletion, temperature, turbulent velocity, and ionization structure. In addition, we report the discovery of an astrosphere around target GJ173, observable due to excess Lyman-alpha absorption caused by interaction between the star's stellar wind and ISM. Our observations provide the best opportunity to characterize the interstellar properties just exterior to the heliosphere and estimate the heliospheric response to interstellar environments in our recent past and for our immediate future. Even in this small sample, we observe variation in density, reflecting the complexity of small-scale structure within the LISM. Over the past ~5 million years, the Sun spent the majority of its time in a region of low density plasma. ~160,000 years ago, it moved into a higher density region of LISM clouds, potentially resulting in a series of heliospheric expansions and contractions.