GeminiFocus April 2019 - Page 10

John Blakeslee Science Highlights Speckle imaging with DSSI at Gemini South resolves double dip mystery during occultation by Orcean moon Vanth; GNIRS spectra constrain mass of the highest redshift highly magnified quasar; and high-definition GeMS/GSAOI data reveal the age of an ancient, dust-obscured star cluster deep in the Milky Way’s bulge. Vanth Surprises with Double Dip During Occultation The sizes and surface compositions of trans-Neptunian objects (TNOs) are notoriously dif- ficult to study. As seen from Earth, the largest TNO has a maximum angular size of about 0.1 arcsecond; more typical ones are unresolved at 0.01 arcsecond or smaller. Except for the two TNOs that have been visited by spacecraft, the most direct measurements of TNO sizes come from stellar occultations. Consequently, planetary scientists exercise great vigi- lance in taking advantage of these rare opportunities. One such opportunity occurred on March 7, 2017. Based on ground-based astrometry, it was thought that an occultation of a magnitude V = 14.6 star by the large TNO Orcus would be viewable from parts of the Pacific and the Americas on that date. With an esti- mated diameter in excess of 900 kilometers (km), Orcus likely meets the shape criteria for a dwarf planet. Like Pluto, it is in a 3:2 orbital resonance with Neptune, has a semi-major axis of 39 astronomical units, and a high eccentricity. It has one large satellite named Vanth, which orbits with a period of 9.5 days. With the availability of astrometry from the Gaia space mission, it became clear that Vanth, rather than Orcus, would be the one tracing a path of occultation across the Earth’s surface on the predicted date. In anticipation of this event, an international team of occultation-chasers led by Amanda Sickafoose of the South African Astronomical Observatory organized a monitoring cam- paign with five telescopes located in Hawai‘i, California, Texas, and Chile. To their surprise, 8 GeminiFocus April 2019