1999ApJ...522L.121L

We report the first direct measurement of acceleration in the outflow of a nova, U Sco, observed 5 days after the start of an outburst. Spectroscopic time series of the intense Hα emission line, obtained with the Space Telescope Imaging Spectrograph camera on board the Hubble Space Telescope, shows a bright emission component moving across the spectrum at a rate of +0.16±0.02 Å in 30 minutes. This is consistent with a clump of emitting material having a line-of-sight acceleration a_r=+4.1±0.5 m.s^–2. The data suggest that this clump resides within a moderately collimated bipolar outflow. Due to projection effects, this implies an actual acceleration of the clump relative to the star of a≳+25 m.s^–2.