The nearby interstellar material in the general direction towards M 22.

Abstract

High-resolution optical spectra of interstellar absorption lines are used to examine reddening variations across the globular cluster M22 which are produced by the nearby low-velocity gas. Fifteen cluster stars together with three stars within 1.3 deg of the cluster were observed in the Na D lines and in some case also the K I (7699 A) line. Optical and ultraviolet spectra of closer stars which lie within several degrees of M22 confirm that a rapid increase in gas-column density occurs at a distance of approximately 100 pc, but the material is considerably more extensive in thickness than has been suggested previously. Although the current schematic models for the Loop I and local bubbles describe in a general way some of the observed features of this local interstellar material, they cannot account directly for the properties and thickness of the gas determined in the present study.