ern sky before dawn. This
fortuitous
circumstance
provided an opportunity
to follow SN 2017eaw con-
tinuously from May until
December, before it be-
came too low in the west-
ern sky to observe from
Maunakea.
Through a combination
of Director’s Discretionary
Time and Fast Turnaround
programs at Gemini North,
a team of astronomers led
by Jeonghee Rho of the
SETI Institute and Gemini’s
own Tom Geballe were
able to follow the evolution of SN 2017eaw’s
near-infrared (0.84-2.52 micron) spectrum
in Semesters 2017A, 2017B, and 2018A. The
first nine of these spectra, obtained with the
Gemini Near-InfraRed Spectrometer in 2017,
are shown in Figure 1. They are a gold mine
of information on the abundances, nucleo-
synthesis, changes in ionization, and veloci-
ties of the ejecta, but the main goal of the
observations was to study the formation of
carbon monoxide (CO) at wavelengths from
2.0-2.5 μm. CO is a powerful coolant, which
aids in making dust formation possible; its
presence is detected by day 124 based on
the sharp increase in signal near 2.30 μm. Ev-
idence of dust also begins at day 124, based
on the flattening of the continuum slope
longward of 2.1 μm.
The resulting study, published in ApJ Letters,
used the spectra to estimate the CO mass
produced by SN 2017eaw and found that
the results qualitatively matched models for
a progenitor star of roughly 15 solar masses.
However, the dust production was observed
at earlier times than predicted. Fits to the
continuum indicate that the temperature
of the dust emitting at 2.1-2.5 μm is roughly
1,300 K and that the dust is mainly graphitic,
January 2019
which can condense at higher temperatures
than amorphous carbon. The team contin-
ued to monitor the evolution of SN 2017eaw
throughout much of 2018, both spectro-
scopically with GNIRS and photometrically
using the Near-InfraRed Imager and spec-
trometer. Thus, we have more to learn from
the latest pyrotechnics displayed by this
nearby galaxy.
Discovery of the Lowest Mass
Ultra Metal-poor Star
The properties of extremely metal-poor
(EMP; with a metal to hydrogen ratio [Fe/H]
< −3.0 dex), ultra metal-poor (UMP, [Fe/H] <
− 4.0 dex) and hyper metal-poor (HMP, [Fe/H]
< −5.0 dex) stars provide information on the
early chemical enrichment of our Galaxy
and the products of the first generations of
stars in the Universe. Because gas composed
entirely of primordial elements cannot cool
efficiently, only high-mass protostellar cores
have sufficient gravity to overcome their in-
ternal pressures and collapse to form stars.
Thus, the first generation (Pop III) of stars in
the early Universe are believed to have had
high masses and short lifetimes. The exact
GeminiFocus
Figure 1.
Gemini/GNIRS spectra
of SN 2017eaw obtained
from 22 to 205 days post
explosion, in time order
from top to bottom. The
prominent emission
and absorption lines
are listed. The spectra
have been scaled to
give a uniform vertical
spacing. The gray shaded
region indicates the
wavelengths at which CO
emission is present; the
flattening of the long-
wavelength continuum
at 124 days and later
is the signature of dust
production.
[Figure reproduced from
Rho et al., ApJ, 864: L20,
2018.]
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