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A study of the behaviour of the NaI/KI column density ratio in the interstellar medium using the Na ultraviolet doublet
Here we make a new study of the behaviour of the NaI/KI column densityratio in the interstellar medium, using a sample of new observations of28 stars obtained at the Nordic Optical Telescope (NOT) in 1996 and1997, and previously published observations (obtained by some of theauthors) of 21 stars. The sightlines cover a range of distances anddirections, including into the Galactic halo. We make use of newobservations of the NaI ultraviolet (UV) doublet for some 18 stars. Thisdoublet is much weaker than the NaI D doublet and so is less susceptibleto saturation effects, and it is well known that it can be used toobtain more accurate NaI column densities with a smaller error range. Wefind an average N(NaI)/N(KI) ratio from the NaI UV data of about 90,which is rather higher than that found previously by Hobbs and Lequeux.The Na UV-KI ratio shows a small increase in value with increasingcolumn density, while we also find a sample of low N(NaI)/N(KI) ratioclouds generally seen towards distant objects on high-latitudesightlines that reach into the halo, so that the ratio decreases moresharply at lower column densities. As the values of the ratio for thesehalo clouds (10-20) bracket the cosmic Na/K abundance ratio, we suggestthat these ratios result from a harder radiation field in the lowerhalo, such that the ionized fractions of NaI and KI become similar.Clearly caution needs to be applied in using any kind of `standardvalue' for the NaI/KI column density ratio.
| The structure of the interstellar gas towards stars in the globular cluster M 10.
Observations are presented of high resolution interstellar Na D and K I(λ7699 A) line profiles towards 3 stars in the globular cluster M10 and 2 field stars within 5deg of the cluster. The gas which isobserved towards all the programme stars is dominated by a strongcomponent at a velocity =~-0.7km/s; the bulk of this gas is shown to lieat a distance <~360 pc. Comparison is made with gas properties foradjacent high-latitude molecular clouds (Penprase 1993). Thelow-velocity gas apparently extends to distances well beyond themolecular clouds in this direction; there is, however, no evidence foradditional gas components in the region between the nearby gas and thedistant cluster. These observations extend our survey of the large-scalestructure of the nearby gas in the general direction l=~10+/-5degfurther into the northern Galactic hemisphere and we review the gascolumn density and velocity structure over the latitude range -25deg to+25deg.
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