Is the Solar System Entering a Nearby Interstellar Cloud?

A model, based on different observations of the local interstellar medium, indicates the presence of a very close interstellar cloud in front of the Scorpius-Ophiuchus association (almost in the direction of the galactic center) approaching the solar system from a distance of about 0.03 pc at a velocity of about 15-20 km s. These observations are as follows: 1. The strong gradient of the hydrogen density in the interstellar medium deduced from current observations of this gas inside the solar system («H ~ 0.1 cm ) and in front of many nearby stars in the anticenter direction («h ^ 0.01 cm). 2. The anisotropy of the UV flux (around 950 Â) from the brightest and closest O and B stars. 3. The important variation of the deuterium to hydrogen ratio, which ranges from 2 x 10“ in the a Cen A direction to 4 x 10“ in the a Aur direction. A mechanism based on a selective radiation pressure effect that acts on deuterium atoms and not on hydrogen atoms explains satisfactorily the large spread in the deuterium abundance in the local interstellar medium. The operation of this mechanism requires that the geometrical configuration remain stable for approximately 10 years. This requirement implies the existence of a nearby interstellar cloud. Possible candidates do exist in the proper sky direction. One candidate, extensively discussed, presents a persistent interstellar absorption pattern over an angle of 40° related to a high column density of hydrogen (Ah ~ 10 cm“). Another with a AH ~ 10 20 cm“ column density, although covering only a few degrees in the sky, would lead to the same consequence. The presence of such candidates is not at all contradicted by the interstellar reddening and absorption line observations. Other implications of the presence of such a close cloud are presented. Subject headings: deuterium — interstellar: matter — solar system: general I find that if the results that have been presented to us this afternoon are correct, I say if they are correct, then a hitherto unknown body must exist in the vicinity of the solar system. Fred Hoyle, The Black Cloud