Figure 2.
Measured radial velocities
of 2MASS J18082002–
5104378 A from Gemini/
GMOS-S compared to the
best-fit Keplerian orbit,
derived from the high-
dispersion MIKE data. The
GMOS-S observations
span 31 epochs over a
13-month period from
June 2016 to July 2017.
[Figure reproduced from
Schlaufman et al., ApJ,
867: 98, 2018.]
In a recent study published in The Astro-
physical Journal, Kevin Schlaufman of Johns
Hopkins University and two collaborators
discovered the lowest mass UMP star known.
The star is an invisible companion to 2MASS
J18082002–5104378 A, a star measured by
Meléndez et al. (A&A, 585: L5, 2016) to have a
metallicity [Fe/H] ≈ −4.1 dex, placing it with-
in the UMP category. Schlaufman and col-
laborators report the results of an extensive
spectroscopic campaign including 14 obser-
vations with the Magellan Inamori Kyocera
Echelle (MIKE) high-resolution spectrograph
on the Magellan Clay Telescope and 31 ob-
servations with the Gemini Multi-Object
Spectrograph at Gemini South, both in Chile.
“Gemini was critical to this discovery, as its
flexible observing modes enabled weekly
check-ins on the system over six months,”
said Schlaufman. The velocities derived from
the Gemini data are shown in Figure 2.
The spectroscopic data show that 2MASS
J18082002–5104378 is a spectroscopic bina-
ry with a circular orbit and a well-determined
period of 34.76 days. The primary star (des-
ignated A) has a derived mass of 0.76 M B ,
which is typical for UMP stars, while the best-
fit mass for the secondary (designated B) is
only 0.14 M B , or about 0.05 M B above the
hydrogen-burning limit for this metallicity.
Assuming that 2MASS J18082002–5104378
B has the same composition as the primary,
it is by far the lowest mass UMP star yet dis-
covered. Moreover, because of its low mass
and metallicity, it has the smallest quantity
of metals of any known star, roughly the
same amount of heavy elements as con-
tained in the planet Mercury.
Put another way, if 2MASS
J18082002–5104378 B had
formed entirely from primor-
dial material (hydrogen and
helium), it could achieve its
current metallicity by swal-
lowing the smallest planet in
our Solar System.
Another interesting find-
ing is that the systemic mo-
tion of 2MASS J18082002–
5104378 indicates that it
belongs to the thin disk
component of our Galaxy.
The derived orbit of the sys-
tem about the center of the
Milky Way has a pericenter
of about 5.6 kiloparsecs
(kpc), an ellipticity of 0.16,
and a very low inclination
so that the system never
wanders more than 0.13 kpc
from the Galactic plane. This
makes the binary the most
metal-poor star system yet
discovered within the thin
24
GeminiFocus
January 2019 / 2018 Year in Review