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
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