  We report on the successful science verification phase of a new observing
mode at the Keck interferometer, which provides a line-spread function width
and sampling of 150km/s at K'-band, at a current limiting magnitude of K'~7mag
with spatial resolution of lam/2B ~2.7mas and a measured differential phase
stability of unprecedented precision (3mrad at K=5mag, which represents 3uas on
sky or a centroiding precision of 10^-3). The scientific potential of this mode
is demonstrated by the presented observations of the circumstellar disk of the
evolved Be-star 48Lib. In addition to indirect methods such as multi-wavelength
spectroscopy and polaritmetry, the here described spectro-interferometric
astrometry provides a new tool to directly constrain the radial density
structure in the disk. We resolve for the first time several Pfund emission
lines, in addition to BrGam, in a single interferometric spectrum, and with
adequate spatial and spectral resolution and precision to analyze the radial
disk structure in 48Lib. The data suggest that the continuum and Pf-emission
originates in significantly more compact regions, inside of the BrGam emission
zone. Thus, spectro-interferometric astrometry opens the opportunity to
directly connect the different observed line profiles of BrGam and Pfund in the
total and correlated flux to different disk radii. The gravitational potential
of a rotationally flattened Be star is expected to induce a one-armed density
perturbation in the circumstellar disk. Such a slowly rotating disk oscillation
has been used to explain the well known periodic V/R spectral profile
variability in these stars, as well as the observed V/R cycle phase shifts
between different disk emission lines. The differential line properties and
linear constraints set by our data lend support to the existence of a
radius-dependent disk density perturbation.
