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 ).
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 discov-
ered. 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 finding
is that the systemic motion of
2MASS J18082002–5104378
indicates that it belongs to
the thin disk component of
our Galaxy. The derived or-
bit of the system about the
center of the Milky Way has
a pericenter of about 5.6 ki-
loparsecs (kpc), an ellipticity
of 0.16, and a very low incli-
nation so that the system
10
GeminiFocus
January 2019