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Far-ultraviolet Dust Extinction and Molecular Hydrogen in the diffuse Milky Way Interstellar Medium

Presentation #143.01 in the session Dust — iPoster Session.

Published onJun 29, 2022
Far-ultraviolet Dust Extinction and Molecular Hydrogen in the diffuse Milky Way Interstellar Medium

We aim to compare variations in the dust extinction curve over the full UV range (912-3000 A), with the gas-to-dust ratio and the H2 content of gas along diffuse Milky Way sightlines. This way, we can learn more about how local ISM and dust properties affect each other. We combine an existing sample of 75 UV extinction curves based on IUE and FUSE data, with atomic and molecular column densities. The H2 column densities are based on Lyman-Werner absorption band models fitted to the FUSE data. These models were originally used to correct the extinction curves in the far-UV, but were not published before. HI column densities were compiled from the literature. For 23 stars, new HI columns were derived by modeling the Lyman alpha absorption line, using archived IUE and HST/STIS data. The H2 column shows a linear relationship with the extinction at wavelengths near 1000 A, or A(1000), which is in the dissociation band of H2. There is no such relationship between the atomic column and A(1000). The total H column scales with the optical extinction A(V) or reddening E(B-V) as expected from other literature, but not with A(1000). The H2 fraction correlates positively with the strength of the far-UV rise, and the width of the 2175 A bump. For these reasons, certain small dust grains that absorb most efficiently in the far-UV, are found to coincide with molecular gas, indicating that the formation and/or survival of these grains and H2 are related. UV-to-optical extinction ratios, such as A(1000)/A(V) and A(2175)/A(V), show a positive correlation with the gas-to-extinction ratio N(H)/A(V). This trend explains several lower-than-average N(H)/A(V) ratios in the sample. Therefore, dust evolution processes in the diffuse ISM that increase the dust mass relative to the gas, also appear to decrease the ratio of small to large grains.

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