Our Stellar Neighborhood: Analysis of the Local Interstellar Medium using Absorption Spectroscopy

The Local Interstellar Medium (LISM), a region extending about 100 parsecs and in which the Sun is currently immersed, can only be studied using UV/optical absorption features against bright background stars. Perhaps in the future, in-situ measurements will be possible (e.g., the Voyager spacecraft or Breakthrough Starshot-style missions). Using high-resolution observations with the Space Telescope Imaging Spectrograph (STIS) on-board the Hubble Space Telescope (HST), we have analyzed several very nearby sight lines to measure physical properties of the LISM. The data used in this study is a part of the Advanced Spectral Library (ASTRAL) Project, an HST Large Treasury Project, in which we have analyzed the spectra of thirteen nearby stars. LISM absorption features in these stellar spectra reveal key information about the abundances, electron density, temperature, and turbulence in the intervening gas. We have fit ion transitions in the near-UV for Mg II, Fe II, C II, D I, Si II, N I, and O I. These absorption features provide direct measurements of the radial velocity, Doppler broadening parameter, and the column density along the line of sight. The presence of multiple local minima in the deep and narrow ISM profile is evidence of multiple clouds moving at different radial velocities. Included in our data set is the Centauri sight line. We provide a detailed analysis of these new observations and a comparison with previous HST observations that were observed more than 20 years ago. A discussion of the physical properties along this line of sight is provided within the context of the Breakthrough Starshot mission. We analyze and propose a new dust grain size distribution based on Weingartner & Draine (2001), which results in ~4% chance of a collision with a grain large enough to destroy the entire craft for 1000 StarChips.