Setting the scene for Gaia and LAMOST Proceedings IAU Symposium No. 298, 2013 S. Feltzing, G. Zhao, N. A. Walton & P. A. Whitelock, eds. c  International Astronomical Union 2014 doi:10.1017/S1743921313006200 Peculiar Metal Poor Stars as Guides to Key Processes in the Early Halo Johannes Andersen 1,2 and Birgitta Nordstr¨ om 1 1 Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100, Copenhagen, Denmark email: ja,birgitta@nbi.ku.dk 2 Nordic Optical Telescope, Apartado 474, Santa Cruz de La Palma, Spain email: ja@not.iac.es Abstract. Large photometric or spectroscopic surveys are used to sort stars into populations and define the main trends that characterise them, as diagnostics of their origin. Stars falling off the trends defined by the ‘normal’ stars are called ‘peculiar’ and typically eliminated in discussions of Galactic structure and evolution. In our programme on extremely metal-poor halo giants, we have recently focused on the small subgroup that is strongly enhanced in r- process elements, asking whether the chemical peculiarity is intrinsic to these stars or due to local surface pollution caused by mass transfer from a binary companion. Precise radial-velocity monitoring over several years turns out to disprove the binary hypothesis and has led to new insight in the processes of chemical enrichment in the early Galactic halo. An ongoing analogous programme on carbon-enhanced metal-poor giants is briefly described at the end. Keywords. Galaxy: formation, Galaxy: halo, stars: abundances, Stars: carbon, Stars: chemically peculiar, Stars: abundances, Binaries: general 1. Introduction In surveys designed to investigate Galactic structure, a few simple photometric or spectroscopic criteria are typically used to sort a large initial sample of stars into main classes or populations. Representative subsamples are then examined in more detail to delineate evolutionary trends and unravel the underlying physical mechanism(s). Stars with non-standard parameter combinations are labelled as ‘peculiar’ and rejected as irrelevant to discussions of Galactic structure, although they may be interesting in the context of stellar astrophysics. Binaries (∼30% of all stars), chemically peculiar stars such as Am or Ap stars, or large-amplitude pulsating variables are typical examples. Much of our own recent research has focused on the class of extremely metal-poor (EMP) halo stars ([Fe/H] < -2.5) detected in the objective-prism surveys of Beers et al. (1985) and Beers et al. (1992) and discussed and classified by Beers & Christlieb (2005). The abundance patterns of these stars were first explored with 4m-class telescopes by McWilliam et al. (1995) and Ryan et al. (1996), later in the large, systematic and homogeneous “First Stars” project led by Roger Cayrel and based on observations with the 8m ESO Very Large Telescope (VLT) and its high-resolution spectrograph UVES. The precise elemental abundance relations for “normal” EMP stars resulting from this work have been reported in a series of papers, notably Cayrel et al. (2004), Hill et al. (2002) and Bonifacio et al. (2009), and have been discussed extensively in the context of chemical evolution models for the early Galactic halo. Here, however, we aim to discuss recent work on some of the chemically peculiar stars that were purposely left aside in the systematic programme. 59 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921313006200 Downloaded from https://www.cambridge.org/core. IP address: 18.206.13.133, on 31 May 2020 at 22:08:03, subject to the Cambridge Core terms of use, available at