Bibcode
Kemp, S. N.; Bates, B.; Beckman, J. E.; Killow, C. J.; Barrena, R.; Kennedy, D. C.; Rodríguez Alamo, J.
Referencia bibliográfica
Monthly Notices of the Royal Astronomical Society, Volume 333, Issue 3, pp. 561-574.
Fecha de publicación:
7
2002
Número de citas
13
Número de citas referidas
13
Descripción
Here we make a new study of the behaviour of the NaI/KI column density
ratio in the interstellar medium, using a sample of new observations of
28 stars obtained at the Nordic Optical Telescope (NOT) in 1996 and
1997, and previously published observations (obtained by some of the
authors) of 21 stars. The sightlines cover a range of distances and
directions, including into the Galactic halo. We make use of new
observations of the NaI ultraviolet (UV) doublet for some 18 stars. This
doublet is much weaker than the NaI D doublet and so is less susceptible
to saturation effects, and it is well known that it can be used to
obtain more accurate NaI column densities with a smaller error range. We
find 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 increasing
column density, while we also find a sample of low N(NaI)/N(KI) ratio
clouds generally seen towards distant objects on high-latitude
sightlines that reach into the halo, so that the ratio decreases more
sharply at lower column densities. As the values of the ratio for these
halo clouds (10-20) bracket the cosmic Na/K abundance ratio, we suggest
that these ratios result from a harder radiation field in the lower
halo, such that the ionized fractions of NaI and KI become similar.
Clearly caution needs to be applied in using any kind of `standard
value' for the NaI/KI column density ratio.