Figure 2.
Near-infrared K-band
image (right) of Comet
2I/Borisov taken on
November 30, 2019,
with FLAMINGOS-2 at
Gemini South when
the comet was 2.05 AU
from the Sun, compared
to an optical V-band
image (left) taken a
week earlier (when it was
2.12 AU from the Sun)
at the Nordic Optical
Telescope. Unlike in the
optical image, the comet
appears pointlike in K.
Image reproduced from
Lee et al., Research
Notes of the American
Astronomical Society,
3:184, 2019
On November 13th, 2I/Borisov crossed into
the Southern Hemisphere, and the most
recent Gemini observations of it have been
made from Cerro Pachón. In a study pub-
lished in the Research Notes of the Ameri-
can Astronomical Society, Chien-Hsiu Lee
(NSF’s National Optical-Infrared Astronomy
Research Laboratory) and collaborators ana-
lyze 2.2-micron (μm) K-band images of the
comet obtained at Gemini South with FLA-
MINGOS-2 in late November. As shown in
Figure 2, the comet appears point-like at 2.2
μm, unlike at optical wavelengths where the
appearance is dominated by the extended
coma. Assuming that the K-band light is re-
flected directly by the nucleus, and adopt-
ing an albedo of 7% at this wavelength, the
study derives an equivalent radius of 1.5 ki-
lometers (km), similar to previous estimates.
A higher albedo would translate into a more
diminutive nucleus.
Gemini has also observed 2I/Borisov spec-
troscopically, in both the optical and NIR. A
study led by Bin Yang (European Southern
Observatory) used NIR spectra from the
Gemini Near-InfraRed Spectrometer (GNIRS)
at Gemini North, as well as from NASA’s In-
frared Telescope Facility, to search for diag-
nostic absorption features of water ice. The
data show a moderately red, featureless
spectrum in the NIR similar to D-type aster-
oids, 1I/‘Oumuamua, and many Solar System
comets. No water ice absorption features
were detected, and spectral modeling indi-
cated that large ice grains must comprise no
28
GeminiFocus
more than 10% of the coma cross-section.
Thus, the ice grains are likely confined to the
region of the nucleus. The study has been
accepted for publication in Astronomy & As-
trophysics Letters, and a preprint is available
online.
The GNIRS observations were taken on Sep-
tember 24th when 2I/Borisov was still 2.6
Astronomical Units (AU) from the Sun. It will
be interesting to see how the spectrum has
evolved as the comet reached its perihelion
distance of 2.0 AU in December and began
its long journey back to interstellar space.
The observations continue, and we are sure
to see more highlights from this first inter-
stellar comet before it’s gone for good.
GPI Imaging of Debris Disks in
Scorpius-Centaurus
The Gemini Planet Imager (GPI) has been
cranking out the results from Gemini South
for the past six years, including a demo-
graphical analysis, published last year in
The Astronomical Journal, of large exoplan-
ets and brown dwarf companions from the
first 300 stars observed in the GPI Exoplanet
Survey (GPIES). The GPIES program also in-
cluded a disk campaign, with the goal of
discovering debris disks around young stars
and characterizing the structure present in
spatially resolved scattered-light images. In
a study recently accepted for publication in
The Astronomical Journal, the GPIES team
January 2020 / 2019 Year in Review