GeminiFocus April 2019 - Page 11

the coordinated observations detected two non-simultaneous dips in the stellar bright- ness at two widely separated telescopes. The detections were made by the NASA In- frared Telescope Facility on Maunakea, and the Las Cumbres 1-meter telescope at the McDonald Observatory in Texas. The obser- vations could not be explained by a single object occulting a single star; moreover, previous Hubble Space Telescope (HST) data ruled out the possibility of another satellite of sufficient size to explain the second dip in stellar brightness. To test the occultation star for possible multiplicity, the team applied for Fast Turn- around time with the visiting Differential Speckle Survey Instrument (DSSI) at Gemini South. The proposal, led by Amanda Bosh of the Massachusetts Institute of Technol- ogy, was successful, and the observations were quickly processed by the DSSI instru- ment team. The resulting image, shown in Figure 1, reveals that the occultation star is indeed a double, with a separation of 250 milli-arcseconds and a brightness differen- tial of about 0.9 magnitude in the red DSSI bandpass. Figure 2 compares the original prediction for the single path of occultation by Vanth with the paths of the two occulta- tions as reconstructed from the binary star positions in the DSSI data. The reconstruc- tions fit perfectly with the observations. Once the binary nature of the occultation star was revealed by Gemini/DSSI, the two observed occultations, combined with non- detections at the other sites, allowed the team to place a tight constraint of 443 ± 10 Figure 1. Gemini South DSSI image of the star pair occulted by Vanth, a satellite of the large trans-Neptunian object Orcus. This image consists of 1,000 seconds of speckle data combined to reveal the binary pair responsible for the observed double occultation. The bright primary is at center, and the newly detected companion is at upper right (approximately 2:00 position; the other “star” at the 8:00 position is an artifact of the autocorrelation analysis used in speckle processing). [Figure reproduced from Sickafoose et al., Icarus, 319: 657, 2019.] Figure 2. The dual paths of Vanth. Left: The predicted path of Vanth’s shadow during the occultation of March 7, 2017, based on Gaia DR1 astrometry. The locations of the telescopes participating in the occultation campaign are indicated by stars. The extent of the shadow is indicated for a physical diameter of 280 km; the shadow of Orcus is off the globe. Right: The actual shadow paths of Vanth as reconstructed using the positions of the two components of the double star determined from Gemini/DSSI imaging. The brighter star was occulted along the upper path, which passed over the observing location in Texas, but was not detected at the location in California. The occultation of the fainter star occurred along a path that passed over the observing location in Hawai‘i; no occultations were detected at the locations in Chile. The paths are drawn for a Vanth diameter of 442.5 km, the size determined from these observations. [Figure reproduced from Sickafoose et al., Icarus, 319: 657, 2019.] April 2019 GeminiFocus 9