In Search of an Interface between Warm and Hot Gas within the Local Bubble

TL;DR

This study used UV spectra from the Hubble Space Telescope to search for ion absorption features indicating an interface between warm and hot gas in the Local Bubble, but found no such features, suggesting possible suppression mechanisms.

Contribution

First observational attempt to detect ionized gas interfaces in the Local Bubble using UV spectra, providing upper limits and exploring reasons for non-detections.

Findings

No detection of Si IV, C IV, N V ions in the observed sight lines.

Upper limit for C IV column density is below expectations for interface regions.

Possible suppression of conductive layers due to magnetic shielding or cloud interactions.

Abstract

We have examined UV spectra recorded by the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope for three stars, HD32309, 41 Ari, and $\eta$~Tel, that are located well inside the boundary of the Local Hot Bubble in our search for absorption features of Si IV, C IV, and N V that could reveal the presence of an interface between the local warm ($T\sim 7000$ K) neutral medium and a more distant hot ($T\sim 10^6$ K) interstellar medium. In all cases, we failed to detect such ions. Our most meaningful upper limit is that for log N(C IV)< 11.86 toward HD32309, which is below the expectation for a sight line that penetrates either a conductive/evaporative interface or a turbulent mixing layer. We offer conjectures on the reasons for these negative results in terms of either a suppression of a conductive layer caused by the shielding of the local cloud by other clouds, which may make it more difficult for us to sense discrete absorption features from gases at intermediate temperatures, or by the presence of a tangential magnetic field at most locations on the surface of the local cloud.