The Gemini telescopes helped identify low-metallicity stars by
gathering medium-resolution spectroscopic GMOS data for 666 bright
stars under poor weather conditions. These data provide a unique
opportunity to explore the chemical evolution of the Milky Way and look
at the enrichment of star-forming gas clouds in the early Universe.
Note: Below are highlights from an article published in the April 2019 issue of GeminiFocus by Principal Investigator Vinicius Placco of the University of Notre Dame. The original papers were published in The Astrophysical Journal and The Astronomical Journal.
Low-metallicity stars (stars with less than 1% of the elements
heavier than Hydrogen and Helium than the Sun contains) are the Rosetta
Stones of stellar astrophysics. Encoded in the atmosphere of these
low-mass, long-lived relics are the signatures of the processes by which
the first chemical elements were cooked up, perhaps as early as a few
tens of millions of years after the Big Bang. These stars are believed
to be the direct descendants of the first stars to be born in the
Universe, which were massive and short-lived. Hence, second-generation,
low-metallicity stars are the artifacts which help us write the ancient
chemical history of the Universe.
In particular, the Extremely Metal-Poor stars (EMP - with iron
abundances of 1/1,000 of the solar value) are believed to carry in their
atmospheres the chemical fingerprints of the evolution of as few as one
first-generation massive star. EMP stars are intrinsically rare (less
than 30 stars identified to date with iron abundances of 1/10,000 of the
solar value) and the majority (more than 60%) show a very strong
molecular carbon signature in their optical spectrum. The
low-metallicity and strong carbon also affect the colors of these stars
in optical wavelengths. Taking advantage of this, we were able to
preselect bright candidates from two public stellar databases — the
RAdial Velocity Experiment (RAVE) and the Best & Brightest Survey
(B&B). We then used the Gemini Multi-Object Spectrograph (GMOS;
North and South) with the B600 gratings and 1-arcsecond slits to obtain
the spectra of 666 stars (see Figure 1).
In total, seven GMOS Poor Weather programs were executed (three in
the North and four in the South) spanning four semesters (from 2015A to
2016B). Those programs had 310 hours of allocated time, and, assuming
the 666 targets took 222 hours of observing time, the efficiency was
around 72%, meaning that only 28% of the already poor weather was lost,
which is a great accomplishment for the program and the Observatory.
The spectra gathered at Gemini/GMOS are of sufficient quality
(signal-to-noise ratios and spectral resolution) to allow for the
determination of stellar atmospheric parameters: effective temperature,
surface gravity, metallicity, and carbon abundance. A subset of these
stars were then re-observed in higher-resolution, so it was possible to
determine their full chemical abundance pattern. There is already a
study published based on the extremely metal-poor star J2005-3057, first
identified at Gemini (Cain et al., 2018). We are also gathering
high-resolution data for the most carbon-enhanced stars identified by
Gemini and the results are also promising. In the near future, such
bright stars will be perfect targets for high-resolution spectroscopic
follow-up with GHOST, which will be a great asset in pushing these
efforts forward.
Source: Gemini Observatory
