2012MNRAS.422.3457G

We report on the detection of a metastable He I* absorption line at 3889 Å in a spectrum of ζ Ophiuchi from the Ultraviolet and Visual Echelle Spectrograph with an extremely high signal-to-noise ratio (∼3500), which was collected using the 8-m telescope UT2 at Paranal. We cannot conclusively determine whether the He I* absorption arises within the H II region or within the foreground H I clouds, which are responsible for interstellar CH, CH⁺, Na I, Ca II, K I, etc., absorption, because the radial velocities of the main mass of gas are nearly identical in both regions. The measured equivalent width (1.0±0.07 mÅ) of the line gives a column density of 1.158(± 0.081) {x} 10¹¹/cm2, when we assume the absence of saturation effects, and an oscillator strength of f= 0.0645. We estimate the ionization rate of helium to be 1.491 {x} 10^–13, if the helium relative abundance in the H I cloud (with respect to hydrogen) is solar, and 7.475 {x} 10^–14/cm2, if the helium abundance is enhanced by the nearby star.