2008ApJ...672..984V

Interstellar dust grain models are not sufficiently constrained by UV extinction curves to allow researchers to distinguish between them. By testing grain models in the X-ray regime and applying elemental abundance constraints, we show to what extent the models can reproduce the observables in these regimes and whether they are capable of doing so while respecting the abundance limits. We tested the MRN and WD grain models using the UV extinction and X-ray halos along the line of sight toward X Per. Both models provide reasonable fits to the UV extinction and X-ray halos but cannot do so while respecting the elemental abundance constraints. Furthermore, the abundances and N_H required to reproduce the observables in these two regimes are not consistent with each other, reflecting the fact that X-ray regime constraints were not taken into account when the models' grain size distributions were constructed, and thus the models are incomplete. Both MRN and WD underestimate the hydrogen column density N_H when the standard model parameters for R_V=3.1 are used. The problem is mitigated when model parameters derived from fitting the UV extinction are used, but this does not necessarily lead to agreement between the two regimes.