Exotic clouds in the local interstellar medium

TL;DR

Recent radio observations reveal the presence of unusual cold, diffuse clouds within the Local Bubble, challenging traditional views of the interstellar medium and suggesting complex multi-phase coexistence.

Contribution

This paper reports the discovery of cold, diffuse clouds inside the Local Bubble, highlighting their properties and potential implications for interstellar medium dynamics.

Findings

Existence of cold, low-density clouds within the Local Bubble.

Cold clouds have AU-scale sizes and are likely long-lived.

Co-existence of warm and cold phases suggests complex ISM dynamics.

Abstract

The neutral interstellar medium (ISM) inside the Local Bubble (LB) has been known to have properties typical of the warm neutral medium (WNM). However, several recent neutral hydrogen (HI) absorption experiments show evidence for the existence of at least several cold diffuse clouds inside or at the boundary of the LB, with properties highly unusual relative to the traditional cold neutral medium. These cold clouds have a low HI column density, and AU-scale sizes. As the kinematics of cold and warm gas inside the LB are similar, this suggests a possibility of all these different flavors of the local ISM belonging to the same interstellar flow. The co-existence of warm and cold phases inside the LB is exciting as it can be used to probe the thermal pressure inside the LB. In addition to cold clouds, several discrete screens of ionized scattering material are clearly located inside the LB. The cold exotic clouds inside the LB are most likely long-lived, and we expect many more clouds with similar properties to be discovered in the future with more sensitive radio observations. While physical mechanisms responsible for the production of such clouds are still poorly understood, dynamical triggering of phase conversion and/or interstellar turbulence are likely to play an important role.