Jurassic: A chemically anomalous structure in the Galactic halo

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.

Eight more low luminosity globular clusters in the Sagittarius dwarf galaxy

“…3) were then inspected to select the real GCs, by comparison with the CMDs of five known Sgr GCs in the region for which we have similar data (NGC 6715, Arp 2, Pal 12, Ter 7, and Ter 8). NGC 6715 is also the largest of these GCs, with a tidal radius of r t = 10 and a mean metallicity of [Fe/H] = −1.30 ± 0.12 dex (Harris 1996, edition 2010, Baumgardt & Hilker 2018Fernández-Trincado et al 2021). We note that Bellazzini et al (2008) argued that NGC 6715 coincides with the nucleus of Sgr, but it is kinematically distinguished from the nucleus.…”

Star Formation Timescales of the Halo Populations from Asteroseismology and Chemical Abundances*

“…Therefore, their Si abundance could be astrophysical rather than artificial. Fernández-Trincado et al (2020) recently suggested that there is a population of stars in the inner halo with enhanced [Si/Fe] ratios and that the population is remnants of globular clusters being disrupted.…”

Determination of Sodium Abundance Ratio from Low-Resolution Stellar Spectra and Its Applications

“…Regarding the metal-rich ([Fe/H] > -1.0) in situ pop-ulation in the Na-high and Na-extreme subgroups, we note that these may be related to the metal-rich ([Fe/H] > -0.7) population recently discovered from APOGEE data by Fernández-Trincado et al (2021). These authors claimed that this population is likely to be stellar debris from partially or fully disrupted metal-rich GCs.…”