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Edinburgh Research Explorer Edinburgh Research Explorer The S2 Stream Citation for published version: Aguado, DS, Myeong, GC, Belokurov, V, Evans, NW, Koposov, SE, Prieto, CA, Lanfranchi, GA, Matteucci, F, Shetrone, M, Sbordone, L, Navarrete, C, Hernández, JIG, Chanamé, J, Arriba, LPD & Yuan, Z 2021, 'The S2 Stream: the shreds of a primitive dwarf galaxy', Monthly Notices of the Royal Astronomical Society , vol. 500, no. 1, pp. 889-910. https://doi.org/10.1093/mnras/staa3250 Digital Object Identifier (DOI): 10.1093/mnras/staa3250 Link: Link to publication record in Edinburgh Research Explorer Published In: Monthly Notices of the Royal Astronomical Society General rights Copyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. Take down policy The University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorer content complies with UK legislation. If you believe that the public display of this file breaches copyright please contact [email protected] providing details, and we will remove access to the work immediately and investigate your claim. Download date: 09. Oct. 2021 MNRAS 000, 1–21 (2020) Preprint 23 July 2020 Compiled using MNRAS LATEX style file v3.0 The S2 Stream: the shreds of a primitive dwarf galaxy.⋆ David S. Aguado1†, G. C. Myeong2,1, Vasily Belokurov1, N. W. Evans1, Sergey E. Koposov3,1,4, Carlos Allende Prieto5,6, Gustavo A. Lanfranchi7, Francesca Matteucci8,9,10, Matthew Shetrone11, Luca Sbordone12, Camila Navarrete12,13, Jonay I. Gonz´alez Hern´andez5,6, Julio Chanam´e14,13, Luis Peralta de Arriba1, Zhen Yuan15 1Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK 2Harvard Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA 3McWilliams Center for Cosmology, Carnegie Mellon University, 5000 Forbes Ave, 15213 4Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK 5Instituto de Astrof´ısica de Canarias, V´ıa L´actea, 38205 La Laguna, Tenerife, Spain 6Universidad de La Laguna, Departamento de Astrof´ısica, 38206 La Laguna, Tenerife, Spain 7N´ucleo de Astrof´ısica Teo´rica, Universidade Cidade de S˜ao Paulo, R. Galv˜ao Bueno 868, Liberdade, 01506-000, S˜ao Paulo, SP, Brazil 8Dipartimento di Fisica, Sezione di Astronomia, Universit`a di Trieste, via G.B. Tiepolo 11, I-34131, Trieste, Italy 9I.N.A.F. Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, I-34131, Trieste, Italy 10I.N.F.N. Sezione di Trieste, via Valerio 2, I-34134 Trieste, Italy 11University of California Observatories, University of California Santa Cruz, Santa Cruz, CA 95064, USA 12ESO - European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile 13Millennium Institute of Astrophysics, Av. Vicu˜na Mackenna 4860, 782-0436 Macul, Santiago, Chile 14Instituto de Astrof´ısica, Pontificia Universidad Cat´olica de Chile, Av. Vicu˜na Mackenna 4860, 782-0436 Macul, Santiago, Chile 15Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, 80 Nandan Road, Shanghai 200030, China Accepted XXX. Received YYY; in original form ZZZ ABSTRACT We present a multi-instrument chemical analysis of the stars in the metal-poor S2 halo 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-process elements (Sr, Ba) and other elements such us 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 S2’s α-elements display a mildly decreasing trend with increasing metallicity which can be interpreted as a “knee” at [Fe/H]< −2. However, even at the high end of [Fe/H], S2’s [α/Fe] ratios do not climb down from the halo plateau, signaling prehistoric enrichment pattern with minimal SN Ia contribution. At the low arXiv:2007.11003v1 [astro-ph.GA] 21 Jul 2020 metallicity end, the n-capture elements in S2 are dominated by r-process production: several stars are Ba-enhanced but unusually extremely 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. Key words: stars: abundances – Galaxy: evolution – Galaxy: formation – Galaxy: Halo – Galaxy: Kinematics and Dynamics ⋆ Based on observations made with the ESO Very Large Tele- scope at the La Silla Paranal Observatory; with Gran Telescopio de Canarias at the Observatorio Roque de los Muchachos– Isla © 2020 The Authors 2 D. Aguado et al. 1 INTRODUCTION Some success has been reported for the highest surface brightness stream – that emanating from a number of In the current cosmological structure formation theory, Milky Way globular clusters (e.g., Odenkirchen et al. 2009; dwarf galaxies are not the main building blocks of big- Kuzma et al. 2015; Ishigaki et al. 2016; Belokurov et al. ger systems, although they do represent a good fraction of 2006; Sollima et al. 2011; Myeong et al. 2017b), yet fluffier the bricks or mortar from which the stellar halos of galax- sub-structures were not easy to tame (Casey et al. 2013, ies like our own are constructed (e.g. Johnston et al. 1996; 2014; Navarrete et al. 2015). The key to success turned out Helmi & White 1999; Bullock & Johnston 2005; Bell et al. to be astrometry, as was beautifully demonstrated in the 2008; Cooper et al. 2010; Font et al. 2011; Belokurov 2013; pioneering work of Helmi et al. (1999) who used Hipparcos Deason et al. 2016). It was therefore to great surprise that (Perryman et al. 1997) proper motions complemented with upon a careful inspection, the chemical abundance pat- ground-based measurements to identify a clump of stars with terns of the surviving high-luminosity (so called Classical) peculiar angular momenta passing not far from the Sun. Out dwarfs did not match that of the Galactic stellar halo (see of 12 possible members, 3 stars had positive vertical velocity Shetrone et al. 2001, 2003; Venn et al. 2004; Helmi et al. while 9 had negative vertical velocity. Helmi (2008) later pro- 2006; Kirby et al. 2009; Tolstoy et al. 2009). The arrival vided a simulation that suggested an interpretation of the of the ultra-faint dwarfs (UFDs) provided a glimmer of data as the partially phase-mixed debris of a heavily dis- hope as they appeared to contain significantly more metal- rupted satellite. The two separate velocity groups could be poor stars compared to their more luminous counterparts on different wraps of the now destroyed satellite, one moving (see e.g. Kirby et al. 2008; Simon et al. 2010; Koposov et al. downward, one upward, with respect to the Galactic plane. 2015; Walker et al. 2016). While at low [Fe/H] a much bet- The fact that this small stellar clump stood out from the ter match is indeed observed between the UFDs and the bulk of the Galaxy due to its high velocity allowed a sig- stellar halo in terms of the α-elements and carbon (see nificantly more robust membership identification (see e.g. e.g. Feltzing et al. 2009; Frebel et al. 2010b; Norris et al. Chiba & Beers 2000; Re Fiorentin et al. 2005; Kepley et al. 2010c; Gilmore et al. 2013; Salvadori et al. 2015; Spite et al. 2007). Building upon these analyses and further refining the 2018; Yoon et al. 2018), the neutron-capture abundances kinematic selection of the stream members, Roederer et al. are clearly out of kilter: the bulk of the UFDs are under- (2010) carried out a high-resolution spectroscopic follow-up enhanced in Ba and Sr compared to the halo, yet there are campaign of the stars in the halo structure identified by also surprising examples of extreme over-abundance (see e.g. Helmi et al. (1999). Theirs is the first and only example of Koch et al. 2008b, 2013; Roederer & Kirby 2014; Ji et al. a detailed chemical reconstruction of a low-mass disrupted 2016c; Roederer et al. 2016; Hansen et al. 2017; Ji et al. sub-system. 2019). The arrival of the unprecedented high-quality Naturally, the question of figuring out the physical astrometry from the Gaia space observatory conditions at the epoch of formation for the stars in the (Gaia Collaboration et al. 2016) has completely changed halo is best addressed by studying the actual progeni- the game of searching for the halo sub-structure. The tors, i.e. those unfortunate satellites that not only were last three years have witnessed an avalanche of discov- accreted by the Milky Way but were also taken apart eries of disrupted Galactic satellites in the Solar vicinity by the Galactic tides at some distant past. As they have (Helmi et al. 2017; Myeong et al. 2017a; Belokurov et al. breathed their last, presently these galaxies can only by 2018; Myeong et al. 2018c,a,b; Koppelman et al. 2018; scrutinized by identifying their debris scattered around the Myeong et al. 2019; Borsato et al. 2020; O’Hare et al. 2020; Galactic halo, piecing these shreds together and apply- Yuan et al. 2020) and further out from the Sun (Ibata et al. ing numerical models to their re-assembled cadavers. Even 2018; Malhan et al.
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