Mode identification from time-resolved spectroscopy of the pulsating whitedwarf G29-38

We have used time-resolved spectroscopy to measure the colour dependence of pulsation amplitudes in the DAV white dwarf G29-38. Model atmospheres pred ict that mode amplitudes should change with wavelength in a manner that dep ends on the spherical harmonic degree l of the mode. This dependence arises from the convolution of mode geometry with wavelength-dependent limb darke ning. Our analysis of the six largest normal modes detected in Keck observa tions of G29-38 reveals one mode with a colour dependence different from th e other five, permitting us to identify the l-value of all six modes and to test the model predictions. The Keck observations also show pulsation ampl itudes that are unexpectedly asymmetric within absorption lines. We show th at these asymmetries arise from surface motions associated with the non-rad ial pulsations (which are discussed in detail in a companion paper). By inc orporating surface velocity fields into line profile calculations, we are a ble to produce models that more closely resemble the observations.