Measuring the Local ISM Along the Sight Lines of the Two Voyager Spacecraft with HST/STIS

As the Sun travels around the galactic center in its 220 million-year orbit, it passes through multiple clouds of gas, dust, and plasma. These clouds are the interstellar medium, or colloquially: the substance between stars. The interstellar medium contains the "repository" of building blocks necessary for star formation, and is replenished by stars at the end of their lifetimes as they deposit material either in the form of a supernova or a planetary nebula. Much of the interstellar medium consists of low-density, partially-ionized gas coupled with dust. The majority of the LISM clouds consist of warm to hot neutral or ionized gas, though there are areas of cool gas. The volume of interplanetary space through which the Voyagers have spent much of their mission traveling is known as the heliosphere. It can be thought of as a giant bubble enclosing the Sun and planets, protecting the Solar System from high energy cosmic rays. This bubble is formed by the solar wind interacting with the gas and magnetic fields of the interstellar medium. When the solar wind moves out from the Sun, it eventually comes into contact with much cooler interstellar plasma, creating the boundary known as the heliopause. The heliopause is considered to be the boundary of the heliosphere. Voyager 1 measured a large increase in interstellar plasma density after crossing the heliopause in 2012. Voyager 2 has not yet crossed this boundary, but is predicted to within the next five years. The local interstellar medium (LISM) forms the outer boundary for the heliosphere, and its behavior dictates the behavior of the Sun and heliosphere. In August 2012, Voyager 1 crossed the heliopause, becoming the first human-made object to exit the Solar System. This milestone signifies the beginning of an important new era for local interstellar medium (LISM) discoveries. We present measurements of the structure and composition of the LISM by using high-resolution Hubble Space Telescope spectra of nearby stars that lie along the same lines of sight as the respective paths of the Voyager spacecraft. We provide a comprehensive inventory of LISM absorption in the near-ultraviolet (2600-2800Å) and the far-ultraviolet (1200-1500Å). The LISM absorption profiles are used to make comparisons between each pair of closely spaced (<15°) sight lines. With these fits, we can make measurements of the physical properties of the LISM, including temperature, turbulence, electron density, and dust composition. As both HST and Voyager reach the end of their lifetimes, we now have the opportunity to synthesize their respective independent and complementary observations, combining in-situ measurements with the shortest possible line-of-sight measurements to provide an unprecedented study of the galactic ISM surrounding the Sun.