Some Observations Related to the Origin and Evolution of the Local Bubble/Local ISM
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I discuss some recent observations of the Local Interstellar medium (ISM) that are related to its history and temperature structure. I focus on three topics: (i) the abundance pattern of interstellar deuterium and metals, (ii) highly charged ion data, (iii) soft X-ray data.
Deuterium has been unambiguously shown to correlate almost linearly with refractory metals, confirming that interstellar grains are a “reservoir” of deuterium, and release it into the gaseous phase jointly with metals when the gas is shocked and heated. By interpreting the observed level of deuterium with respect to the abundance patterns of metals and oxygen, these data give some clues to the event, which gave rise to the expanding Gould belt. As a matter of fact abundance data seem to be linked to the belt, and the observed trends suggest an explosive origin, rather than a collision with an external cloud made of unastrated material.
X-rays and high ions trace hot gas and interfaces between hot and cool gas. However absorption lines of high ions show highly complex characteristics and no relationships have been established yet between their detected columns and the existence of hot-cool gas interfaces. Adding to the complexity, the X-ray emission through charge-exchange reactions between highly charged solar wind ions and neutrals plays a significant role, calling for modifications of the global picture of the LISM. In addition to the ubiquitous contamination of background data by locally emitted X-rays, there are also potential distant charge transfer (CX) X-ray emissions from clouds-hot gas interfaces.
There is a strong need for high quality, high spectral resolution X-ray data, because X-ray lines emitted after charge-transfer neutralization of helium-like ions bear a clear signature of the charge transfer process, if present, and allow to disentangle thermal and CX emission. More precise density and velocity distributions of the local ISM are also needed to take full advantage of the X-ray and high ion data and build a consistent picture of the Local Cavity (LC) and its surroundings. As an example of these requirements I discuss the case of the North Polar Spur for which there may be some evidence for CX emission.
KeywordsISM X-ray background
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