Reddening and the extinction law at high Galactic latitude

We present near-infrared (JHKL) photometry of 103 southern stars located behind translucent interstellar clouds at high Galactic latitude. Our data are combined with visual photometry and spectral type information from the literature in a detailed analysis of the wavelength dependence of interstellar extinction by dust in these high-latitude clouds. We investigate the shape of the near-infrared extinction curve and evaluate the total line-of-sight extinction ( AV) and ratio of total-to-selective extinction ( RV) in each line of sight. Sources of uncertainty in RV arising from photometric errors and spectral classification errors are carefully assessed and quantified as functions of the line-of-sight reddening. We detect appreciable differences in our results compared with the average extinction curve for dust in the diffuse interstellar medium ( ISM) close to the Galactic plane. Assuming a power-law form in the near-infrared, we find the mean for high-latitude clouds to be well described by [A(lambda)] proportional to lambda(-2.3), somewhat steeper than that for the diffuse ISM ([A(lambda)] proportional to lambda(-1.8)). Our sample includes a substantial number of lines of sight with low R-V values (47% with R-V < 2.8) relative to the diffuse ISM average of 3.05 +/- 0.15. We conclude that many high-latitude clouds have enhanced abundances of relatively small grains.