Asteroid 4 Vesta: Simultaneous visible and ultraviolet IUE observations

Abstract

On 6–7 October 1990, 16 long-wavelength primary camera (LWP) spectra were recorded with the long-wavelength primary spectrograph of the International Ultraviolet Explorer (IUE) to obtain the resolved UV lightcurve of asteroid 4 Vesta. Simultaneously, the fine error sensor (FES) of the telescope was used to obtain Vesta's near-B lightcurve. Deep LWP and short-wavelength primary camera (SWP) spectra were also collected to establish the reflectivity curve of Vesta in the 1700–3200 Å region. The FES data show that the rotation period is of the order of 5 hr; combined with an old observation obtained in similar geometrical conditions, we derive a new, accurate value of the rotation period, 0.22258840 ± 8 × 10 −8 day. Thus, our observations were obtained during three consecutive revolutions of the asteroid about its axis. The UV and optical lightcurves have one single minimum and one single maximum. The shape of the UV reflectivity curves is constant with rotational phase at the 3–5% level between 2600 and 3000 Å. The UV spectrum is redder than that of the Sun. The asteroid albedo increases linearly with wavelength from 2300 to 3000 Å, and this characteristic probably holds over the entire 2000–3500 Å range; its value near 2700 Å is 7.5% and it does not exceed ≈4% at 2000 Å. The intrinsic amplitude of Vesta's lightcurve is of the order of 10% and does not vary with wavelength from UV to near-IR. The present observations are compared with existing data to develop a self-consistent picture of Vesta's surface properties. No true large-scale and large-amplitude absorptions are observed in the 0.2–2.5 μm range. Small-amplitude colored features are observed at all phases: they seem to be part of a progressive and extended color change that renders one hemisphere slightly bluer than the other.