Hydrogen infrared recombination lines as a diagnostic tool for the geometry of the circumstellar material of hot stars

We have analysed the infrared hydrogen recombination lines of a sample of well studied hot massive stars observed with the Infrared Space Observatory. Our sample contains stars from several classes of objects, whose circumstellar environment is believed to be dominated by an ionized stellar wind (the Luminous Blue Variables) or by a dense disk-like geometry (Be stars and B[e] stars). We show that hydrogen infrared recombination lines can be used as a diagnostic tool to constrain the geometry of the ionized circumstellar material. The line strengths are sensitive to the density of the emitting gas. High densities result in optically thick lines for which line strengths are only dependent on the emitting surface. Low density gas produces optically thin lines which may be characterized by Menzel case B recombination. The ISO observations show that stellar winds are dominated by optically thin H I recombination lines, while disks are dominated by optically thick lines. Disks and winds are well separated in a diagnostic diagram using the Hu(14-6)/Bralpha and the Hu(14-6)/Pfgamma line flux ratios. This diagnostic tool is useful to constrain the nature of hot star environments in case they are highly obscured, for instance while they are still embedded in their natal molecular cloud.