The orbit and size of (87) Sylvia’s Romulus from the 2015 apparition

• Romulus, one of two moons around asteroid (87) Sylvia, has been imaged.
• The 3.5 m telescope used adaptive optics and a laser to image the moon.
• This telescope is the smallest to ever image a moon of an asteroid from the ground.
• From the moon’s orbit, the mass and density of asteroid (87) Sylvia was found.
• The relative lightcurve between the two suggests an elongated figure for the moon.

Using the US Air Force’s Starfire Optical Range 3.5 meter telescope with adaptive optics and a laser guidestar, we obtained 68 images of asteroid (87) Sylvia and its satellite Romulus over 6 nights in March and May of 2015. Adding an additional 3 images from earlier observations on one night in November 2012, we are able to derive a circular (but not an eccentric) orbit for Romulus, leading to a density for Sylvia of 1.37 ± 0.04 gm/cm3. Extending the time base to 14 years by combining our data with previous observations from Keck, HST, and the VLT reported in the literature, we can fit for a new circular orbit and change the density estimate slightly to 1.35 ± 0.04 gm/cm3. By fitting a ratio of two Fourier series to the measured magnitude difference between Sylvia (V=12.5) and Romulus, which ranged from 4.1 to 5.0 in the J-band (λ=1.2μm), and modeling both as triaxial ellipsoids, we are able to derive prolate spheroid equatorial diameters for Romulus of 41(±27) × 30(±16) km. This assumes that Romulus is rotating synchronously with its 3.64 d orbital period. However, decomposing the differential lightcurve between Sylvia and Romulus reveals a much shorter 7.96 hr rotational period, leading to more elongated prolate spheroid diameters of 82(±7) × 21(±2) km. As far as we know, our 3.5 m telescope is the smallest ground-based telescope to ever image any asteroid’s moon.