New Hubble Space Telescope spectra of QSO PG 1718+4807: No evidence for strong deuterium absorption

The Z(abs) similar to 0.701 absorption system toward QSO PG 1718 + 4807 is the only example of a QSO absorption system which might have a deuterium/hy drogen ratio approximately 10 times the value found toward other QSOs. We h ave obtained new Space Telescope Imaging Spectrograph spectra from the Hubb le Space Telescope of the Ly alpha and Lyman limit regions of the system. T hese spectra give the redshift and velocity dispersion of the neutral hydro gen, which produces most of the observed absorption. The Ly alpha line is t oo narrow to account for all of the observed absorption. It was previously known that extra absorption is needed on the blue side of the main H I near the expected position of deuterium. We do not find evidence in the current data that the extra absorption is entirely deuterium and find that it is m ore likely that some of the extra absorption is contaminating H. Until new data can be found that can independently constrain the line parameters of t he potential contaminating H, it will not be possible to measure D/H in thi s system. Some uncertainty persists because we have a low signal-to-noise r atio and the extra absorption-be it deuterium or hydrogen-is heavily blende d with the Ly alpha absorption from the main hydrogen absorption.