An Ultra Metal-poor Star Near the Hydrogen-burning Limit*

Abstract

ApJ in pressABSTRACT It is unknown whether or not low-mass stars can form at low metallicity. While theoretical simulations of Population III (Pop III) star formation show that protostellar disks can fragment, it is impossible for those simulations to discern if those fragments survive as low-mass stars. We report the discovery of a low-mass star on a circular orbit with orbital period P = 34.757 ± 0.010 days in the ultra metalpoor (UMP) single-lined spectroscopic binary system 2MASS J18082002-5104378. The secondary star 2MASS J18082002-5104378 B has a mass M 2 = 0.14 +0.06 −0.01 M , placing it near the hydrogenburning limit for its composition. The 2MASS J18082002-5104378 system is on a thin disk orbit as well, making it the most metal-poor thin disk star system by a considerable margin. The discovery of 2MASS J18082002-5104378 B confirms the existence of low-mass UMP stars and its short orbital period shows that fragmentation in metal-poor protostellar disks can lead to the formation and survival of low-mass stars. We use scaling relations for the typical fragment mass and migration time along with published models of protostellar disks around both UMP and primordial composition stars to explore the formation of low-mass Pop III stars via disk fragmentation. We find evidence that the survival of low-mass secondaries around solar-mass UMP primaries implies the survival of solar-mass secondaries around Pop III primaries with masses 10 M M * 100 M . If true, this inference suggests that solar-mass Pop III stars formed via disk fragmentation could survive to the present day.

Be and O in the ultra metal-poor dwarf 2MASS J18082002-5104378: the Be–O correlation

“…In this case, a highly significant peak, which has a false alarm probability less than 0.001, is apparent corresponding to a period of 34.7538 days. This period is almost identical to that obtained by Schlaufman et al (2018) using Keplerian fits to their high resolution data. We decided to fit a Keplerian orbit to our radial velocities measurements based on high resolution spectra, keeping the value of the period fixed.…”

“…We used version 1.3 of the program velocity (Wichmann et al 2003). Our preferred solution is summarised in Table 2 and is very close to that found by Schlaufman et al (2018) except for the angle of the periastron, the time of passage at periastron and the eccentricity. We did not run a Monte Carlo to estimate errors, since this orbit is certainly preliminary.…”

“…Schlaufman et al (2018) confirmed the binary nature of 2MASS J1808-5104. Using 17 radial velocity measurements from spectra obtained with MIKE at the Magellan telescope, and three measurements obtained from the UVES R580 spectra (which we also used), Schlaufman et al (2018) were able to determine the orbital parameters for this system. They also gathered 31 epochs of radial velocity measurements obtained from low resolution spectra using GMOS-S on the Gemini South telescope.…”

“…The Gaia photometry and the reddening are listed in Table 3. We note that following Schlaufman et al (2018), the mass of the secondary must be very low (M 2 = 0.14M ⊙ ) and thus its contribution to the total flux is negligible. In Fig.…”

“…They also gathered 31 epochs of radial velocity measurements obtained from low resolution spectra using GMOS-S on the Gemini South telescope. Since we have independent measurements of the radial velocities (the UVES R580 spectra and a new epoch from our UVES R760 spectrum), we decided to redetermine the orbital parameters of the system combining our measurements with those of Schlaufman et al (2018).…”