Interstellar Extinction, Polarization, and Grain Alignment in the High‐Latitude Molecular Cloud toward HD 210121

We present near-infrared photometry and optical and near-infrared polarimetry for the line of sight toward HD 210121, obscured by the high-latitude cloud DBB 80. From the data, we determine remarkably small values of the ratio of total to selective extinction, RV = 2.1 ± 0.2, and the wavelength of maximum polarization, λmax = 0.38 ± 0.03 μm, as compared to average values in the diffuse Galactic interstellar medium. These values nevertheless follow the linear relationship between RV and λmax seen closer to the Galactic plane. Our results imply an abundance of small grains, which we interpret as evidence for lack of grain growth in this high-latitude cloud. This is consistent with previous observations of high molecular abundances and extremely steep far-ultraviolet extinction toward the star and also with the presence of mid-infrared emission in the foreground cloud. We show that populations of small grains responsible for extinction and polarization in DBB 80 respond to prevailing physical conditions in a correlated way. Based on its extremely short value of λmax, we predict that the line of sight to HD 210121 will be highly polarized in the ultraviolet.

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