Evaluating the Morphology of the Local Interstellar Medium: Using New Data to Distinguish Between Multiple Discrete Clouds and a Continuous Medium

Abstract

Ultraviolet and optical spectra of interstellar gas along the lines of sight to nearby stars have been interpreted by and previous studies as a set of discrete warm, partially ionized clouds each with a different flow vector, temperature, and metal depletion. Recently, Gry & Jenkins (2014) have proposed a fundamentally different model consisting of a single cloud with nonrigid flows filling space out to 9 parsecs from the Sun that they propose better describes the local ISM. Here we test these fundamentally different morphological models against the spatially unbiased Malamut et al. (2014) spectroscopic data set, and find that the multiple cloud morphology model provides a better fit to both the new and old data sets. The detection of three or more velocity components along the lines of sight to many nearby stars, the presence of nearby scattering screens, the observed thin elongated structures of warm interstellar gas, and the likely presence of strong interstellar magnetic fields also support the multiple cloud model. The detection and identification of intercloud gas and the measurement of neutral hydrogen density in clouds beyond the Local Interstellar Cloud could provide future morphological tests.

The Interface between the Outer Heliosphere and the Inner Local ISM: Morphology of the Local Interstellar Cloud, Its Hydrogen Hole, Strömgren Shells, and 60Fe Accretion*

“…Other nearby interstellar clouds have values of ξ as large as 3.6 km s −1 , about half of the cloud's sound speed. Gry & Jenkins (2014) have proposed that the multicloud scenario can be replaced by a single interstellar cloud with velocity gradients, but Redfield & Linsky (2015) have argued that the data are more accurately fit with multiple clouds each with its own velocity vector.…”

Atomic gas in debris discs

“…In addition to the UCL, LCC and US shells, one must also consider the structure of the local interstellar medium including the Local Interstellar Cloud (LIC; Redfield & Linsky 2015). The LIC is a warm, low density interstellar cloud which is itself located within a hot Local Cavity in the interstellar medium which extends to approximately 50 pc (Welsh et al 2010).…”

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

“…The kinematic model of the LISM by Redfield & Linsky (2008) includes 15 clouds moving with distinct but similar bulk velocity vectors. These component clouds are individual comoving structures of partially ionized gas identified by common velocities across large patches of the sky (Redfield & Linsky 2015). Most LISM clouds consist of warm to hot neutral or ionized gas maintained by photoionization.…”