GeminiFocus January 2019 - Page 11

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.] 9