Structure of the local interstellar medium and the line of sight to α Oph

Abstract

The precise physical state and spatial distribution of interstellar matter within ≃100 pc of the Sun has been the subject of intense speculation because of the implications its knowledge would have on interstellar medium theory^1,2, soft X-ray and EUV transparency^3,4 and, possibly, for very nearby objects, on solar and terrestrial evolution^5,6. The general consensus holds that the local interstellar medium out to ≃100 pc from the Sun is exceptionally clear of any condensation of density ≥0.1 cm⁻³. Presumably, this is due to the effect of a supernova or stellar wind blast wave sweeping through the local interstellar medium in the relatively recent past^7,8. I point out here that a surprising number of independent lines of observational evidence unambiguously imply the existence of a large, moderately dense (n_H≥1 cm⁻³), cool (T≃70 K) cloud of interstellar gas lying between the Sun and α Oph, an A5V star located 16.7±0.8 pc away⁹ in the direction l = 32°, b = 23°.