SPLUS J210428.01−004934.2: An Ultra Metal-poor Star Identified from Narrowband Photometry*

We report on the discovery of SPLUS J210428.01−004934.2, an ultra metal-poor (UMP) star first identified from the narrowband photometry of the Southern Photometric Local Universe Survey (S-PLUS) Data Release 1, in the SDSS Stripe 82 region. Follow-up medium- and high-resolution spectroscopy (with Gemini South and Magellan-Clay, respectively) confirmed the effectiveness of the search for low-metallicity stars using the S-PLUS narrowband photometry. At [Fe/H] = −4.03, SPLUS J2104−0049 has the lowest detected carbon abundance, A(C) = +4.34, when compared to the 34 previously known UMP stars in the literature, which is an important constraint on its stellar progenitor and also on stellar evolution models at the lowest metallicities. Based on its chemical abundance pattern, we speculate that SPLUS J2104−0049 could be a bona fide second-generation star, formed from a gas cloud polluted by a single metal-free ∼ 30M⊙ star. This discovery opens the possibility of finding additional UMP stars directly from narrowband photometric surveys, a potentially powerful method to help complete the inventory of such peculiar objects in our Galaxy.