High‐Resolution STIS/Hubble Space Telescopeand HIRES/Keck Spectra of Three Weak MgiiAbsorbers toward PG 1634+706

High resolution optical (HIRES/Keck) and UV (STIS/HST) spectra, covering alarge range of chemical transitions, are analyzed for three single-cloud weakMgII absorption systems along the line of sight toward the quasar PG 1634+706.Weak MgII absorption lines in quasar spectra trace metal-enriched environmentsthat are rarely closely associated with the most luminous galaxies (>0.05L^*).The two weak MgII systems at z=0.81 and z=0.90 are constrained to have >=solarmetallicity, while the metallicity of the z=0.65 system is not aswell-constrained, but is consistent with >1/10th solar. These weak MgII cloudsare likely to be local pockets of high metallicity in a lower metallicityenvironment. All three systems have two phases of gas, a higher density regionthat produces narrower absorption lines for low ionization transitions, such asMgII, and a lower density region that produces broader absorption lines forhigh ionization transitions, such as CIV. The CIV profile for one system (atz=0.81) can be fit with a single broad component (b~10 km/s), but those for theother two systems require one or two additional offset high ionization clouds.Two possible physical pictures for the phase structure are discussed: one witha low-ionization, denser phase embedded in a lower density surrounding medium,and the other with the denser clumps surrounding more highly ionized gas.Comment: 32 pages, 4 figures; to appear in ApJ on May 20, 200