First direct measurement of acceleration in the outflow of a nova: U Scorpii (1999)

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 H alpha emission line, obtained with the Space T elescope Imaging Spectrograph camera on board the Hubble Space Telescope, s hows a bright emission component moving across the spectrum at a rate of +0 .16 +/- 0.02 Angstrom in 30 minutes. This is consistent with a clump of emi tting 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 collimate d bipolar outflow. Due to projection effects, this implies an actual accele ration of the clump relative to the star of a greater than or similar to +2 5 m s(-2).