Fernández-Trincado, J. G.; Zamora, O.; García-Hernández, D. A.; Souto, Diogo; Dell'Agli, F.; Schiavon, R. P.; Geisler, D.; Tang, B.; Villanova, S.; Hasselquist, Sten; Mennickent, R. E.; Cunha, Katia; Shetrone, M.; Allende Prieto, C.; Vieira, K.; Zasowski, G.; Sobeck, J.; Hayes, C. R.; Majewski, S. R.; Placco, V. M.; Beers, T. C.; Schleicher, D. R. G.; Robin, A. C.; Mészáros, Sz.; Masseron, T.; García Pérez, Ana E.; Anders, F.; Meza, A.; Alves-Brito, A.; Carrera, R.; Minniti, D.; Lane, R. R.; Fernández-Alvar, E.; Moreno, E.; Pichardo, B.; Pérez-Villegas, A.; Schultheis, M.; Roman-Lopes, A.; Fuentes, C. E.; Nitschelm, C.; Harding, P.; Bizyaev, D.; Pan, K.; Oravetz, D.; Simmons, A.; Ivans, Inese I.; Blanco-Cuaresma, S.; Hernández, J.; Alonso-García, J.; Valenzuela, O.; Chanamé, J.
Bibliographical reference
The Astrophysical Journal Letters, Volume 846, Issue 1, article id. L2, 8 pp. (2017).
Description
We report the peculiar chemical abundance patterns of 11 atypical Milky
Way (MW) field red giant stars observed by the Apache Point Observatory
Galactic Evolution Experiment (APOGEE). These atypical giants exhibit
strong Al and N enhancements accompanied by C and Mg depletions,
strikingly similar to those observed in the so-called second-generation
(SG) stars of globular clusters (GCs). Remarkably, we find low Mg
abundances ([Mg/Fe] < 0.0) together with strong Al and N
overabundances in the majority (5/7) of the metal-rich ([Fe/H] ≳
-1.0) sample stars, which is at odds with actual observations of
SG stars in Galactic GCs of similar metallicities. This chemical pattern
is unique and unprecedented among MW stars, posing urgent questions
about its origin. These atypical stars could be former SG stars of
dissolved GCs formed with intrinsically lower abundances of Mg and
enriched Al (subsequently self-polluted by massive AGB stars) or the
result of exotic binary systems. We speculate that the stars
Mg-deficiency as well as the orbital properties suggest that they could
have an extragalactic origin. This discovery should guide future
dedicated spectroscopic searches of atypical stellar chemical patterns
in our Galaxy, a fundamental step forward to understanding the Galactic
formation and evolution.
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