The S2 Stream: the shreds of a primitive dwarf galaxy.*

Abstract

The S2 stream is a kinematically cold stream that is plunging downwards through the Galactic disc. It may be part of a hotter and more diffuse structure called the Helmi stream. We present a multi-instrument chemical analysis of the stars in the metal-poor S2 stream using both high- and low-resolution spectroscopy, complemented with a re-analysis of the archival data to give a total sample of 62 S2 members. Our high-resolution program provides α-elements (C, Mg, Si, Ca and Ti), iron-peak elements (V, Cr, Mn, Fe, Ni), n-capture process elements (Sr, Ba) and other elements such as Li, Na, Al, and Sc for a subsample of S2 objects. We report coherent abundance patterns over a large metallicity spread (∼1 dex) confirming that the S2 stream was produced by a disrupted dwarf galaxy. The combination of S2’s α-elements displays a mildly decreasing trend with increasing metallicity which can be tentatively interpreted as a “knee” at [Fe/H]<−2. At the low metallicity end, the n-capture elements in S2 may be dominated by r-process production however several stars are Ba-enhanced, but unusually poor in Sr. Moreover, some of the low-[Fe/H] stars appear to be carbon-enhanced. We interpret the observed abundance patterns with the help of chemical evolution models that demonstrate the need for modest star-formation efficiency and low wind efficiency confirming that the progenitor of S2 was a primitive dwarf galaxy.

Dynamically Tagged Groups of Very Metal-poor Halo Stars from the HK and Hamburg/ESO Surveys

“…It has also been associated with 4 RPE stars. This could be another strong indication that, in the very low-metallicity regime, the progenitor of the HStr experienced enrichment in neutron-capture elements pre-dominantly via the r-process, in accord with other recent results from chemical-abundance analyses (Roederer et al 2010;Aguado et al 2020).…”

“…Three of these stars have very similar [Eu/Fe] ratios ([Eu/Fe] ≈ +0.40; r-I), and the other is more enriched in r-process elements ([Eu/Fe] = +1.07; r-II). The excess of RPE stars in this substructure is also consistent with recent spectroscopic efforts (Roederer et al 2010;Aguado et al 2020). In both works, the authors argued that the enrichment in neutron-capture elements of the member stars of the HStr was dominated by the r-process, based on their [Sr/Ba] patterns at the VMP end.…”

“…The discovery of such a group, with apparently disconnected blobs in velocity space, is a demonstration of the power of searches in the integrals-of-motion space. More recent studies focused on the chemical aspects of this substructure (Roederer et al 2010;Aguado et al 2020), confirming its ancient nature from its low typical metallicity (−3.0 [Fe/H] −1.5), its profile in [α/Fe] (with a "knee" at [Fe/H] ≈ −2.0), and predominance of the r-process contribution to the enrichment of neutron-capture elements ([Sr/Ba] 0.0) in the low-metallicity regime. However, the complicated story of the HStr and its progenitor is still under scrutiny (and debate; see, e.g., Koppelman et al 2019b;Naidu et al 2020), and its characterization is far from being complete.…”

The R-Process Alliance: Chemo-Dynamically Tagged Groups of Halo $r$-Process-Enhanced Stars Reveal a Shared Chemical-Evolution History

“…[Fe/H] −1.4; its average metallicity is [Fe/H] = −2.0, consistent with Aguado et al (2021) and others in the literature (e.g., Myeong et al 2018a,b). Additionally, the [Fe/H] dispersion of ≈ 0.5 dex found for CDTG-15 leads to a CDF value of 0.71 (commensurate with random draws from the full RPE Sample), consistent with its progenitor being a dwarf galaxy with extended star formation (see Koppelman et al 2019b).…”

“…Limberg et al (2020) has also suggested a connection between this stream and RPE stars based on their dynamics. It is striking that spectroscopic studies of this structure had already claimed that its stars were enriched in neutron-capture elements, primarily via the r-process, based on the close conformity among the abundances of Ba and many elements in the rare-earth domain to the scaled-solar r-process pattern (Roederer et al 2010) or their [Sr/Ba] profiles (Aguado et al 2021).…”