Extremely Metal-poor Stars:
Windows into the Early Universe
The only way to understand and character-
ize the first generation of stars is to look for
their direct descendants that would still be
alive today: second-generation low-mass,
low-metallicity stars. A subset of these, the
Extremely Metal-Poor (EMP; [Fe/H] < -3.0)
stars, 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 Pop. III mas-
sive star. Apart from the very low iron abun-
dance, the majority (more than 60%) of
the observed EMP stars show a very strong
molecular carbon signature in their optical
spectrum. Such high carbon abundances are
one of the expected yields of the final stages
of evolution of zero-metallicity Pop. III stars
and can help trace back the nature of the
first stars in the Universe.
Finding the Needle in the
Haystack
Figure 1.
Equatorial and Galactic
coordinate distribution
of the stars observed
with Gemini North and
Gemini South in poor
weather conditions.
Identifying such pristine objects is a chal-
lenging endeavor. EMP stars are intrinsically
rare (less than 30 stars identified to date with
[Fe/H] < -4.0) and can only be properly char-
acterized as such via spectroscopic studies.
In addition, metal-poor stars are generally
found in higher fractions in the halo popu-
lations of the Galaxy, making most of them
faint and "expensive" in terms of telescope
time. Thus, it is important to have reliable se-
lection criteria in the search for the brightest
metal-poor star candidates for high-resolu-
tion spectroscopic follow-up.
Since changes in metallicity affect the col-
ors in optical wavelengths in predictable
ways, we pre-selected a number of such
candidates from broadband or narrowband
photometry. Even though these methods
can successfully identify metal-poor star
candidates, they become more and more
uncertain as metallicities decrease. As a re-
sult, medium-resolution (R = λ/Δλ ≈ 1,500)
spectroscopy becomes a valuable tool not
only for pre-selection of targets to be fol-
lowed-up in high-resolution (R ≈ 30,000) but
also for parameter determination and stellar
population studies.
Recently, our team published two studies in
The Astronomical Journal (Placco et al., 2018;
Placco et al., 2019), aiming to increase the
inventory of EMP star candidates observed
with medium-resolution spectroscopy. We
observed these stars over the course of sev-
en semesters (from 2014A to 2017A) with a
variety of telescopes, including the Gemini
North and South telescopes, the Southern
Astrophysical Research telescope, Kitt Peak
National Observatory’s Mayall
telescope, and the European
Southern Observatory’s New
Technology Telescope. In total,
2,551 stars were observed.
We selected the (bright) candi-
dates from two sources — the
RAdial Velocity Experiment
(RAVE) and the Best & Brightest
Survey (B&B) — and used the
Gemini North and South tele-
scopes to observe 666 stars out
of the 2,551. Figure 1 shows the
distribution of equatorial and
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GeminiFocus
April 2019