Mapping local interstellar medium with diffuse interstellar bands

Abstract

ABSTRACT With the use of the data from archives, we studied the correlations between the equivalent widths of four diffuse interstellar bands (4430, 5780, 5797, 6284 $\mathring{\rm A}$) and properties of the target stars (colour excess values, distances, and Galactic coordinates). Many different plots of the diffuse interstellar bands and their maps were produced and further analysed. There appears to be a structure in the plot of equivalent widths of 5780  $\mathring{\rm A}$ DIB (and 6284 $\mathring{\rm A}$ DIB) against the Galactic x-coordinate. The structure is well defined below ∼ 150 m$\mathring{\rm A}$ and within |x| < 250 pc, peaking around x = 170 pc. We argue that the origin of this structure is not a statistical fluctuation. Splitting the data in the Galactic longitude into several subregions improve or lower the well-known linear relation between the equivalent widths and the colour excess, which was expected. However, some of the lines of sight display drastically different behaviour. The region within 150° < l < 200° shows scatter in the correlation plots with the colour excess for all of the four bands with correlation coefficients R < 0.58. We suspect that the variation of physical conditions in the nearby molecular clouds could be responsible. Finally, the area 250° < l < 300° displays (from the statistical point of view) significantly lower values of equivalent widths than the other regions – this tells us that there is either a significant underabundance of carriers (when compared with the other regions) or that this has to be a result of an observational bias.