Microstructure of the Local Interstellar Cloud and the identification of the Hyades cloud

We analyze high-resolution UV spectra of the Mg II h and k lines for 18 mem bers of the Hyades to study inhomogeneity along these proximate lines of si ght. The observations were taken by the Space Telescope Imaging Spectrograp h (STIS) instrument on board the Hubble Space Telescope (HST). Three distin ct velocity components are observed. All 18 lines of sight show absorption by the Local Interstellar Cloud (LIC); 10 stars show absorption by an addit ional cloud, which we name the Hyades Cloud; and one star exhibits a third absorption component. The LIC absorption is observed at a lower radial velo city than predicted by the LIC velocity vector derived by Lallement & Berti n in 1992 and Lallement et al. in 1995 [upsilon (predicted)(LIC) - upsilon (observed)(LIC) = 2.9 +/- 0.7 km s(-1)], which may indicate a compression o r deceleration at the leading edge of the LIC. We propose an extension of t he Hyades Cloud boundary based on previous HST observations of other stars in the general vicinity of the Hyades, as well as groundbased Ca II observa tions. We present our fits of the interstellar parameters for each absorpti on component. The availability of 18 similar lines of sight provides an exc ellent opportunity to study the inhomogeneity of the warm, partially ionize d local interstellar medium (LISM). We find that these structures are rough ly homogeneous. The measured Mg II column densities do not vary by more tha n a factor of 2 for angular separations of less than or similar to8 degrees , which at the outer edge of the LIC correspond to physical separations of less than or similar to0.6 pc.