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The Zn^+^/Na^0^ interstellar ratio
We study the behaviour of the interstellar R_n_=n(Zn^+^)/n(Na^0^) ratioin ionization equilibrium conditions. The ratio is very sensitive tovariations of the physical state of the gas and is only marginallyaffected by variations of interstellar depletion. We find logR_n_ {in}[=~-2,=~0] dex in cold molecular clouds, logR_n_ {in} about [0,+1] dexin cold atomic gas, and logR_n_ {in} about [+1,+3] dex in warm gas. Wecompare the theoretical R_n_ ratio with the column-density ratioR_N_=N(ZnII)/N(NaI) measured in a sample of 48 interstellar lines ofsight. The comparison supports the validity of the assumption ofionization equilibrium and indicates that R_N_ is a good indicator ofthe physical state of the gas. The few deviations from ionizationequilibrium that we find are all located in the general direction ofβ CMa, where studies of the local ISM revealed the presence of anextended (=~ 2x10^2^ pc) tunnel of hot gas. Finally, we report theresults of a correlation analysis logR_N_-logN(NaI) and logR_N_-logN(H).The correlations that we derive imply the presence of a mechanismlinking the physical state of interstellar clouds to their columndensity. A practical consequence of the correlation with N(NaI) is thata NaI measurement alone is sufficient to estimate the physical state ofthe gas in many situations of interest.
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