Through a combi-
nation of Director’s
Discretionary
Time
and Fast Turnaround
programs at Gemini
North, a team of as-
tronomers 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 mi-
cron) spectrum in Se-
mesters 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, nucleosynthesis, changes
in ionization, and velocities 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. Evidence 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,
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-
January 2019 / 2018 Year in Review
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
mass range of Pop III stars remains a subject
of debate, but recent simulations suggest a
lower limit of about 10 solar masses (M B ).
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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