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 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 .