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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