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[71.24] Interstellar Material towards eta UMa

P.C. Frisch (U. Chicago), E.B. Jenkins (Princeton U.), D.E. Welty (U. Chicago)

The star eta UMa (B3 V, vsini=205 km s^-1, d=31 pc, l=101^o, b=+65^o) samples nearby interstellar gas in a high latitude direction relatively devoid of material. IMAPS, Hubble GHRS Echelle, and ground based optical data are combined to present a comprehensive picture of the interstellar material (ISM) in this direction. Two main components dominate: the blue-shifted component which appears to be ionized, and the dominant, red-shifted, component which exhibits a low electron density (~0.2 cm^-3). However, the Mg^o/Mg⁺ ratio and C⁺ fine-structure lines yield different ionizations, depending on the adopted temperature, similar to differences found in the diffuse material towards 23 Ori (Welty et al. 1999). The IMAPS and GHRS data give C, N, O, and Fe column densities, which form the basis for calculating the gas-to-dust mass ratio for the main component using a ``missing mass'' calculation combined with an assumed reference abundance (Frisch et al. 1999). Comparing the eta UMa value with other diffuse cloud values then further constrains uncertainties in N(H^o) values for this sightline.

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