Abstract
Detailed elemental-abundance patterns of giant stars in the Galactic halo measured by the Apache Point Observatory Galactic Evolution Experiment (APOGEE-2) have revealed the existence of a unique and significant stellar subpopulation of silicon-enhanced ([Si/Fe] ≳ +0.5) metal-poor stars, spanning a wide range of metallicities (−1.5 ≲ [Fe/H] ≲ −0.8). Stars with over-abundances in [Si/Fe] are of great interest because these have very strong silicon (28Si) spectral features for stars of their metallicity and evolutionary stage, offering clues about rare nucleosynthetic pathways in globular clusters (GCs). Si-rich field stars have been conjectured to have been evaporated from GCs, however, the origin of their abundances remains unclear, and several scenarios have been offered to explain the anomalous abundance ratios. These include the hypothesis that some of them were born from a cloud of gas previously polluted by a progenitor that underwent a specific and peculiar nucleosynthesis event or, alternatively, that they were due to mass transfer from a previous evolved companion. However, those scenarios do not simultaneously explain the wide gamut of chemical species that are found in Si-rich stars. Instead, we show that the present inventory of such unusual stars, as well as their relation to known halo substructures (including the in situ halo, Gaia-Enceladus, the Helmi Stream(s), and Sequoia, among others), is still incomplete. We report the chemical abundances of the iron-peak (Fe), the light- (C and N), the α- (O and Mg), the odd-Z (Na and Al), and the s-process (Ce and Nd) elements of 55 newly identified Si-rich field stars (among more than ∼600 000 APOGEE-2 targets), which exhibit over-abundances of [Si/Fe] as extreme as those observed in some Galactic GCs, and they are relatively well distinguished from other stars in the [Si/Fe]−[Fe/H] plane. This new census confirms the presence of a statistically significant and chemically-anomalous structure in the inner halo: Jurassic. The chemo-dynamical properties of the Jurassic structure is consistent with it being the tidally disrupted remains of GCs, which are easily distinguished by an over-abundance of [Si/Fe] among Milky Way populations or satellites.
Highlights
After the internal release of the Apache Point Observatory Galactic Evolution Experiment (APOGEE)-2+ catalogue, we discovered many more stars belonging to the Si-rich sample, which we report here as part of a larger homogeneous census
We find a star (2M22375002−1654304) in the Jurassic structure whose chemistry is consistent with a genuine second-generation globular clusters (GCs). 2M22375002−1654304 is a not carbon-enhanced metal-poor star ([Fe/H] ∼ −1.27) which has a [Mg/Fe] ratio of ∼−1 accompained by a modest enrichment in [N, Al, Si/Fe] +0.5, which is the typical signature of secondgeneration GC stars
We report on the identification of 55 new Si-rich, mildly metal-poor stars, 45 of which exhibit high [Al/Fe] +0.5, making it unlikely that dwarf galaxy satellites could have contributed the majority of these stars
Summary
The stellar content of the halo of the Milky Way (MW) is littered by a mixture of stellar debris of completely and/or partially destroyed dwarf galaxies and globular clusters (GCs) (e.g., Helmi et al 1999; Carollo et al 2007, 2010; Nissen & Schuster 2010; Fernández-Trincado et al 2013, 2015a,b, 2016a,b, 2017, 2019a,b,c, 2020a,b,c; Recio-Blanco et al 2017; Bekki 2019; Koch et al 2019; Massari et al 2019; Hanke et al 2020; Thomas et al 2020; Yuan et al 2020; Wan et al 2020; Naidu et al 2020), which preserve signatures of the Galaxy’s assembly history (see Naidu et al 2020, for a recent review). There exists a wealth of observational evidence for a possible in situ channel, especially for the inner-halo population itself, which is spatially, kinematically, and chemically distinguishable from the outer-halo population (Carollo et al 2007, 2010; Beers et al 2012; An et al 2013, 2015; An & Beers 2020) and thought to have formed, in part, from gas accreted by the MW at early times (Carollo et al 2013; Tissera et al 2014; Hawkins et al 2015; Hayes et al 2018; Fernández-Alvar et al 2018, 2019) These studies illustrate the complex formation history of the stellar halo of the MW, which may involve a mixture of stars that likely formed in situ and stellar debris, which were accreted from different structures.
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