The Local Interstellar Medium

The density of the interstellar medium can vary widely, from over 100 atoms per cm³ to 0.001 atoms per cm³. Such a variation would result in a change in the ramscoop radius by a factor of 1000!

What is the typical interstellar medium like? Especially around G-class stars like our own? For example, it is not likely than an interstellar ship would pass through a hot HII region powered by an OB association: they are rare and would not support habitable planets.

Instead we focus on the local interstellar medium (LISM).

Methods:

1) Soft x-ray emission.

The LISM is very thin (0.05 atoms cm^-3) and very hot (1,000,000 K). This is atypical of the ISM –but just how atypical is unknown. Spectrum of the soft x-rays yields temperature, and surface brightness, density.

Density and temperature yield a pressure consistent with rest of ISM, so an equilibrium phase.

2) Absorbtion.

Not easy to do with precision. Even for nearby stars, optical depth for H Lyman-alpha line is 10⁵ – 10⁶. Absorption lines for metals (e.g. Ca, Mg) not reliable for high precision because of variance in abundancies.

Linsky et al use deuterium Lyman-alpha line. Optical depth for nearby stars is approximately 1. D/H ratio approx 1.5 x 10^-5 consistently and well-studied because of significance to primordial nucleosynthesis.

3) Backscatter of solar UV from Local Interstellar Cloud ("Local Fluff").

Local maps

Maps of Local "Clouds" (within a few parsecs)

"Local Fluff" is 0.3 atoms cm^-3 and 7000 K

Defined by H column 10¹⁹ cm^-2.

Ranges in distance from 30 pc to 200 pc. Volume that of sphere with radius 100 pc. Most of ISM in Local Bubble is 0.001 – 0.05 atoms cm^-3 and 1 million K.

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