The barium to iron enrichment versus age relation of ancient disc stars

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(1)MNRAS 471, 3768–3774 (2017). doi:10.1093/mnras/stx1848. Advance Access publication 2017 July 21. The barium-to-iron enrichment versus age relation of ancient disc stars 1 Astronomisches. Institut, Ruhr-Universität Bochum, Universitätsstraße 150, D-44801 Bochum, Germany Newton Group of Telescopes, Apartado 321, E-38700 Santa Cruz de La Palma, Spain 3 Instituto de Astronomı́a, Universidad Católica del Norte, Avenida Angamos 0610, Casilla 1280, Antofagasta, Chile 4 Purple Mountain Observatory and Key Laboratory for Radio Astronomy, Chinese Academy of Sciences, 2 West Beijing Road, 210008 Nanjing, China 5 Centro de Astroingenierı́a, Facultad de Fı́sica, Instituto de Astrofı́sica, Pontificia Universidad Católica de Chile, Casilla 306, 8970117 Santiago 22, Chile 6 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany 2 Isaac. Accepted 2017 July 19. Received 2017 July 19; in original form 2017 February 21. ABSTRACT. We report an intrinsically precise relation of the barium-to-iron enrichment as a function of age for a local, volume-complete (N = 30) sample of ancient Population II (τ ≥ 12 Gyr) and intermediate-disc stars (τ 10 Gyr), which suggests a common, r-process-dominated nucleosynthesis site for both elements in the early stages of the Milky Way. Deviants from this empirical relation are to a large extent identified as formerly known or new blue straggler stars. We report in particular the striking case of the Population II star HD 159062, whose barium overabundance is difficult to explain without wind accretion of s-process material from a former asymptotic giant branch (AGB) primary that very likely survived as a white dwarf companion. The weak but significant barium enhancement that we measure for HR 3578 and 104 Tau also suggests that both may be accompanied by faint degenerate companions. If confirmed through precision astrometry or direct imaging observations, this would mean a very efficient method to uncover ancient stellar remnant companions around solar-type stars. Key words: stars: abundances – blue stragglers – stars: mass-loss – stars: Population II – stars: solar-type – white dwarfs.. 1 I N T RO D U C T I O N The photospheres of unevolved stars usually represent the chemical abundance mixture of their place and epoch of formation. Some stars, however, signify a subsequent evolution in their photospheres that can only originate from hidden stellar remnants. A well-known example of this kind are the so-called barium stars (Bidelman & Keenan 1951), where a previously more massive component in a binary system (McClure, Fletcher & Nemec 1980) evolves to an asymptotic giant branch (AGB) star, whose nucleosynthesis products are transferred to the secondary through wind accretion, as first suggested by Boffin & Jorissen (1988). Solar-type stars are mostly non-single, and in particular among Population II (τ ≥ 12 Gyr) and intermediate-disc stars (τ 10 Gyr) we recently found that at least two out of three objects appear to have a companion (Fuhrmann et al. 2017b). In view of the timescales involved with these ancient sources, a significant fraction of them is necessarily accompanied by white dwarfs. Provided these degenerates are young, they can be directly uncovered from their excess ultraviolet radiation (Gray et al. 2011), whereas degenerates with orbital periods up to several years can be inferred from their. E-mail:. klaus@ing.iac.es. period-mass relation (Rappaport et al. 1995) and the low orbital eccentricities that result from Roche lobe overflow. At larger separations up to at least 100 au the above mentioned wind accretion is an efficient source to enrich an unevolved companion with the products of a former AGB primary (e.g. Crepp et al. 2013; Matthews et al. 2014; Desidera, D’Orazi & Lugaro 2016). The white dwarf descendant can then be inferred from the abundance anomalies it caused on its companion. In this context, the barium-to-iron enrichment of solar-type stars has been studied for decades and in numerous investigations (e.g. Wallerstein 1961; Butcher 1975; Spite & Spite 1978; Steenbock 1983; Edvardsson et al. 1993; Gratton & Sneden 1994; Nissen & Schuster 1997; Chen et al. 2000; Mashonkina & Gehren 2000, Mashonkina & Gehren 2001; Mashonkina et al. 2003; Reddy et al. 2003, 2006; Allende Prieto et al. 2004; Bensby et al. 2005, 2007, 2014; Jonsell et al. 2005; Luck & Heiter 2005, 2006; Allen & Barbuy 2006a, Allen & Barbuy 2006b; Brewer & Carney 2006; Cescutti et al. 2006; Bond et al. 2008; Korotin et al. 2011; Mishenina et al. 2013; Nissen 2016; Zhao et al. 2016). These analyses mostly concur with a fairly flat [Ba/Fe] relation among ancient disc stars (e.g. Gratton & Sneden 1994, their fig. 7; Bensby et al. 2005, their fig. 11; Reddy et al. 2006, their fig. 17) and there is as well a general consensus that striking outliers likely arise from slow neutron capture (s-process). C 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Downloaded from https://academic.oup.com/mnras/article-abstract/471/3/3768/4002687 by Pontificia Universidad Catï¿½lica de Chile user on 12 June 2019. K. Fuhrmann,1,2‹ R. Chini,1,3 L. Kaderhandt,1 Z. Chen4 and R. Lachaume5,6.

(2) Barium enrichment of ancient disc stars. 2 O B S E RVAT I O N S A N D A N A LY S E S The stars considered in this work are themselves part of a larger, volume-complete sample of about 500 nearby sources (d < 25 pc), mostly solar-type stars with main-sequence effective temperatures Teff ≥ 5300 K. Detailed stellar population and multiplicity studies for this sample were recently presented in Fuhrmann et al. (2017a,b). The observations are based on high-resolution (R = 50000-65000), high signal-to-noise ratio (S/N≥200) spectra secured with the FOCES échelle spectrograph (Pfeiffer et al. 1998) in the Northern hemisphere in the years 1995 to 2007, and the FEROS (Kaufer et al. 1999) and BESO (Steiner et al. 2006) échelle spectrographs at southern declinations in the years thereafter. The échelle spectrographs cover the Ca II H&K lines and the Ca II infrared triplet in a single exposure. The local field star sample mostly consists of Population I (thin disc) and Population II (thick disc) stars. For the latter population with an age at or above 12 Gyr the stars down to spectral type F are essentially all gone to the realm of degenerates (Fuhrmann et al. 2012a). Hence, the bulk of the stars of the local sample are actually members of Population I, which came into being about 8 Gyr ago and continues to form stars until today. Apart from Populations I and II, the local field also displays a few disc stars with intermediate chemical and kinematical characteristics, and notably, intermediate ages at around 10 Gyr (cf. Bernkopf & Fuhrmann 2006). Halo stars are not present in the local sample, which is not unexpected as these sources are extremely rare (Fuhrmann 2002). In the work that follows on the barium-to-iron enrichment we will concentrate on the complete subset of the ancient Population II and intermediate-disc stars as they result from the local sample. The only Population II star not included is the giant Arcturus, a likely early-AGB star, where dredge-up processes may have altered its photospheric barium abundance. In Tables 1 and 2 we start by summarizing the basic stellar parameters of the Population II (N = 21) and intermediate-disc stars (N = 9) from our previous work; this includes also minor updates for part of the sources. For the visual binary HR 2667/8 in Table 2 we present the analyses of. 1 We use the heuristic term ‘blue straggler star’ synonymously with mass transfer systems of any kind (e.g. Roche lobe overflow, wind accretion, merger). The mass transfer leads to a stellar rejuvenation, which – among others – is observable as an age discrepancy, too high effective temperatures, or the classic too blue colours (cf. Fuhrmann et al. 2017a, fig. 4).. both its solar-type components, but will later only consider their average barium abundance. The element barium does not provide many absorption lines in the optical wavelength region. Hyperfine structure, isotopic shifts, blends from other atomic species, the underlying model atmospheres and a non-local thermodynamic equilibrium (NLTE) can affect most of these lines. Likewise, faint companions can have an impact on the derived abundances. While the choice of a particular set of lines cannot escape the latter aspect, hyperfine splitting and line blends can – at the expense of a short line list – be reduced to a minimum. The Ba II line λ6141, for instance, almost coincides with a strong Fe I line. The Ba II line λ6496, in turn, shows damping wings and some overlap with neighbouring absorption lines for part of our sample stars that render an exact placement of the continuum a source of dispersion. Along this line of arguments, the widely used Ba II line λ5853 appears to be the only choice for reliable barium abundances and can also consistently be used down to the most metal-poor sources of our sample at [Fe/H] − 0.90. As opposed to λ6496, there are no concerns with the placement of the continuum for λ5853, and at variance with λ6141, there exists only a weakly contributing Fe I line (e.g. Mashonkina & Zhao 2006), which we include in the line formation of λ5853. Since the surface gravities of the sample stars in Tables 1 and 2 are all above log g ≥ 4.0 the recent work by Korotin et al. (2015) would imply that non-LTE corrections for λ5853 are mostly less than 0.02 dex. However, to reach this level of precision, it is important that one can refer to S/N spectra of at least S/N≥200 (actually, many of our spectra exceed S/N 400.). Likewise, it is important that the basic stellar parameters are derived from α-enhanced model atmospheres as exemplified for 82 Eri in Bernkopf et al. (2012), and the barium line formation also needs to account for this. Ignoring these aspects is an immediate source of scatter that easily exceeds the above mentioned level of NLTE effects. Finally, and given that we work only with a single Ba II line, it is important that one should refer to repeated observations, which is the case for almost all stars of our sample. We summarize the derived barium abundances from Ba II λ5853 for both the Population II and intermediate-disc stars in Table 3. The LTE abundance uncertainties in column (2) are internal rms errors from repeated observations, the number of échelle spectra given in parentheses. The non-LTE corrections in column (3) refer to the calculations of Korotin et al. (2015). Note that for the mean barium abundances below only the average of the visual binary HR 2667/8 is used.. 3 E A R LY BA R I U M - T O - I RO N E N R I C H M E N T 3.1 Previous work on ancient blue straggler stars Given that two-thirds of the investigated stars are non-single (Fuhrmann et al. 2017b, tables 7 and 8) and in view of the substantial time-scales involved it is evidently no surprise that a significant fraction of the present sample is accompanied by white dwarfs. The ‘classical’ Population II blue straggler is here the F-type HR 4657 that was identified as a mass transfer system two decades ago by Fuhrmann & Bernkopf (1999) and whose white dwarf companion simultaneously solved the aspects of its high rotation, chromospheric activity, lithium depletion, low orbital eccentricity, and – in particular – a substantial age discrepancy. Later, it was also found (cf. Fuhrmann 2011) that the HR 4657 secondary fulfils the Rappaport et al. (1995) period–white dwarf mass relation. MNRAS 471, 3768–3774 (2017). Downloaded from https://academic.oup.com/mnras/article-abstract/471/3/3768/4002687 by Pontificia Universidad Catï¿½lica de Chile user on 12 June 2019. nucleosynthesis of AGB progenitor stars in binary systems (cf. Edvardsson et al. 1993, their section 6.2.8). In this work, we report an intrinsically precise barium-to-iron enrichment versus age relation for a volume-complete (N = 30) sample of ancient Population II and intermediate-disc stars. The relation we present suggests a common rapid neutron capture (rprocess) nucleosynthesis site for barium and iron when the Milky Way was in its infancy and it becomes visible once the contaminating effect of mass transfer from degenerate companions is identified and the systems discarded. While most of these blue straggler stars1 were uncovered years ago, we present here a new case for an AGB progenitor, and two other wind accretion candidate systems that warrant astrometric or direct imaging observations. If confirmed as blue stragglers, this would prove to be a very efficient method for the search of ancient stellar remnant companions around solar-type stars.. 3769.

(3) 3770. K. Fuhrmann et al.. Table 1. Basic stellar parameters of the Population II stars. For each star the second row gives 2σ error estimates, with the errors of log g, ξ t , [Fe/Mg] and BCV generally assessed as 0.1 dex, 0.2 km s−1 , 0.05 dex, and 0.05 mag, respectively. Macroturbulent velocities ζ RT are adopted from the relations in Gray (1984, 1992). The bolometric corrections are taken from Alonso et al. (1995). Uncertainties in the stellar masses are likely less than 10 per cent. Note that the nearby Population II giant Arcturus is not included in this list. HR. HD. V (mag). Teff (K). log g (cgs). [Fe/H] (dex). ξt (km s−1 ). [Fe/Mg] (dex). ζ RT (km s−1 ). vsin i (km s−1 ). Mbol (mag). BCV (mag). Mass (M ). Radius (R ). 4308. 6.544 0.005 5.152 0.005 3.489 0.005 6.645 0.005 4.256 0.005. 5683 70 5386 70 5373 80 5714 70 5490 70. 4.36 0.10 4.51 0.10 4.54 0.10 4.34 0.10 4.46 0.10. − 0.35 0.07 − 0.85 0.07 − 0.53 0.07 − 0.28 0.07 − 0.38 0.06. 0.99 0.20 0.82 0.20 0.80 0.20 0.95 0.20 0.89 0.20. − 0.32 0.05 − 0.42 0.05 − 0.31 0.05 − 0.32 0.05 − 0.36 0.05. 3.2. 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0. 4.66 0.05 5.53 0.05 5.45 0.05 4.57 0.06 5.15 0.05. − 0.16 0.05 − 0.23 0.05 − 0.23 0.05 − 0.16 0.05 − 0.20 0.05. 0.92. 1.05 0.04 0.78 0.03 0.82 0.03 1.08 0.04 0.90 0.03. 5.378 0.010 7.444 0.012 6.974 0.005 5.586 0.005 6.785 0.005. 5856 70 5320 80 5324 80 5970 70 5634 70. 4.24 0.10 4.54 0.10 4.55 0.10 4.40 0.10 4.45 0.10. − 0.79 0.07 − 0.89 0.07 − 0.73 0.07 − 0.32 0.07 − 0.40 0.07. 1.10 0.20 0.95 0.20 0.79 0.20 1.11 0.20 0.93 0.20. − 0.44 0.05 − 0.37 0.05 − 0.39 0.05 − 0.34 0.05 − 0.34 0.05. 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0. 4.30 0.05 5.68 0.10 5.58 0.05 4.41 0.05 4.92 0.06. − 0.16 0.05 − 0.25 0.05 − 0.24 0.05 − 0.13 0.05 − 0.17 0.05. 4.737 0.005 5.805 0.005 6.108 0.010 5.648 0.005 6.659 0.005. 6355 80 5910 70 6209 70 5707 70 5329 80. 4.12 0.10 4.15 0.10 4.32 0.10 4.36 0.10 4.56 0.10. − 0.25 0.07 − 0.87 0.07 − 0.75 0.07 − 0.36 0.07 − 0.69 0.07. 1.62 0.20 1.18 0.20 1.33 0.20 0.96 0.20 0.82 0.20. − 0.29 0.05 − 0.42 0.05 − 0.45 0.05 − 0.31 0.05 − 0.38 0.05. 9.7 0.7 1.0 1.0 6.7 0.7 1.0 1.0 1.5 1.0. 3.15 0.13 4.03 0.05 4.23 0.06 4.63 0.05 5.63 0.05. − 0.09 0.05 − 0.16 0.05 − 0.14 0.05 − 0.16 0.05 − 0.24 0.05. 5.377 0.005 7.219 0.005 6.793 0.005 7.177 0.020 6.512 0.005. 5739 60 5409 80 5794 70 5400 100 5815 70. 4.30 0.10 4.51 0.10 4.43 0.10 4.48 0.10 4.35 0.10. − 0.36 0.07 − 0.38 0.07 − 0.39 0.07 − 0.29 0.10 − 0.70 0.07. 0.99 0.20 0.77 0.20 1.03 0.20 0.80 0.20 1.17 0.20. − 0.39 0.05 − 0.32 0.05 − 0.35 0.05 − 0.35 0.05 − 0.37 0.05. 1.0 1.0 1.0 1.0 2.0 1.0 1.0 1.0 1.0 1.0. 4.44 0.05 5.35 0.05 4.75 0.06 5.28 0.06 4.62 0.05. − 0.15 0.05 − 0.22 0.05 − 0.15 0.05 − 0.22 0.05 − 0.16 0.05. 5.810 0.030. 5570 90. 4.45 0.10. − 0.81 0.10. 0.98 0.20. − 0.44 0.05. 1.0 1.0. 5.19 0.06. − 0.20 0.05. μ Cas A. 321. 6582. τ Cet. 509. 10700 18757 A. 82 Eri. 1008. 20794. 3018 Aa. 63077 64606 Aa 65583. 3138 A. 65907 68017 A. ν 2 Lup. 3220 Aa. 68456. 3578 A. 76932. 4657 A. 106516. 5699. 136352 144579 A. 72 Her. 6458. 157214 159062 A 165401 Aa 195987 Aa 199288. 85 Peg A. 9088. 224930. It was then particularly clear that all solar-type Population II stars must be slowly rotating at projected rotational velocities vsin i 1 km s−1 and that any chromospheric activity (unless driven by bound rotation) must be the sequel of some former mass transfer. Hence, and although we still lack the orbital elements of the inner Aa-Ab subsystem of the Population II triple star HD 165401 (Fuhrmann 2008; Chini et al. 2014), there can be no doubt that only some former mass transfer can be the source of the observed chromospheric activity (Soderblom 1985). As we shall see below, the present evidence favours a HD 165401 Ab white dwarf companion. When the survey proceeded in the Southern hemisphere, starting in 2010, clear-cut evidence for a blue straggler star with a white dwarf companion was found for the bright F-type star HR 3220 (Fuhrmann et al. 2011). Soon thereafter, we disclosed the blue MNRAS 471, 3768–3774 (2017). 2.0 2.0 3.3 2.6 3.6 1.8 1.8 4.2 2.5 6.3 4.1 5.0 3.3 1.8 3.7 2.1 3.6 2.1 3.5 2.5. 0.73 0.78 0.93 0.85 0.86 0.70 0.76 1.03 0.88 1.35 0.86 0.95 0.92 0.76 0.94 0.84 0.95 0.84 0.85 0.76. 1.16 0.04 0.75 0.04 0.78 0.03 1.07 0.04 0.95 0.03 1.68 0.11 1.30 0.04 1.07 0.04 1.05 0.04 0.76 0.03 1.14 0.04 0.84 0.03 0.97 0.04 0.87 0.04 1.02 0.04 0.85 0.04. straggler HR 3138 A that – contrary to the other three Population II systems – may lack a close companion.2 Although this could be suggestive for a merger with an ordinary low-mass source, possibly driven by its distant visual binary, HR 3138 BC, an accretion scenario of mass from a nearby open cluster appears to be conceivable as well (Fuhrmann et al. 2012b). As we will also see below, the above blue straggler stars are likely no descendants of AGB primaries. In particular, the orbital periods P = 843 d for HR 4657 (Carney et al. 2001; Griffin 2013). 2 We note, however, the recent Gaia DR1 astrometric data for HR 3138 A, whose comparatively high Hipparcos/Gaia discrepancy (DQ) value, DQ=15.64, may be tentative evidence for a close companion.. Downloaded from https://academic.oup.com/mnras/article-abstract/471/3/3768/4002687 by Pontificia Universidad Catï¿½lica de Chile user on 12 June 2019. Object.

(4) Barium enrichment of ancient disc stars. 3771. Table 2. Basic stellar parameters of the intermediate-disc stars. HR. HD. V (mag). Teff (K). log g (cgs). [Fe/H] (dex). ξt (km s−1 ). [Fe/Mg] (dex). ζ RT (km s−1 ). vsin i (km s−1 ). Mbol (mag). BCV (mag). Mass (M ). Radius (R ). 104 Tau. 1656. 32923. 4.908 0.005 6.986 0.005 5.568 0.032 6.905 0.091 6.420 0.005. 5724 60 5538 70 5812 60 5298 70 5802 70. 4.03 0.10 4.43 0.10 4.28 0.10 4.55 0.10 4.35 0.10. − 0.24 0.06 − 0.11 0.06 − 0.22 0.06 − 0.22 0.06 − 0.33 0.07. 1.11 0.20 0.85 0.20 1.07 0.20 0.79 0.20 1.02 0.20. − 0.20 0.05 − 0.16 0.05 − 0.18 0.05 − 0.17 0.05 − 0.22 0.05. 3.7. 1.0 1.0 1.0 1.0 1.0 1.0 1.8 1.0 1.0 1.0. 3.82 0.05 4.95 0.06 4.29 0.07 5.52 0.11 4.47 0.05. − 0.15 0.05 − 0.18 0.05 − 0.14 0.05 − 0.24 0.05 − 0.15 0.05. 0.98. 1.53 0.05 0.97 0.04 1.19 0.04 0.81 0.05 1.10 0.04. 7.051 0.005 4.886 0.005 6.359 0.005 6.582 0.005 5.395 0.005. 5438 70 5684 70 5664 70 5403 80 5807 70. 4.46 0.10 4.45 0.10 4.25 0.10 4.51 0.10 4.19 0.10. − 0.01 0.06 − 0.32 0.07 +0.06 0.07 − 0.09 0.07 − 0.23 0.07. 0.82 0.20 0.96 0.20 0.99 0.20 0.82 0.20 1.04 0.20. − 0.14 0.05 − 0.21 0.05 − 0.13 0.05 − 0.18 0.05 − 0.21 0.05. 2.0 1.0 1.5 1.0 1.5 1.0 1.0 1.0 1.0 1.0. 5.13 0.06 4.90 0.05 4.32 0.07 5.17 0.05 4.07 0.05. − 0.20 0.05 − 0.16 0.05 − 0.15 0.05 − 0.21 0.05 − 0.14 0.05. 40397 A 2667. 53705. 2668. 53706. 4098 A. 90508 97343. 4523 A. 102365. 5566 Aa. 131923 135204. ρ CrB Aa. 5968. 143761. Table 3. Barium-to-iron ratios of the local Population II and intermediatedisc stars. Object. [Ba/Fe]LTE (dex). NLTE (dex). Remark. 2.6 3.7 1.7 3.4 2.3 3.2 3.6 2.0 3.8. 0.91 0.98 0.85 0.96 0.89 0.90 0.99 0.89 0.97. 0.92 0.03 0.94 0.03 1.23 0.05 0.92 0.04 1.32 0.05. and P = 899 d for HR 3220 (Murdoch & Hearnshaw 1993) suggest first-ascent red giant progenitors, meaning that one may not expect any barium overabundances from the mass transfer. A plain system for an AGB star progenitor, however, recently came to our attention with the ancient G-type star HD 159062.. Population II HD 4308 μ Cas A τ Cet HD 18757 A 82 Eri HR 3018 Aa HD 64606 Aa HD 65583 HR 3138 A HD 68017 A HR 3220 Aa HR 3578 A HR 4657 Aa ν 2 Lup HD 144579 A 72 Her HD 159062 A HD 165401 Aa HD 195987 Aa HD 199288 85 Peg A. −0.09 ± 0.02 (3) −0.15 ± 0.03 (3) −0.09 ± 0.01 (4) −0.11 ± 0.01 (2) −0.15 ± 0.01 (5) −0.15 ± 0.01 (2) −0.16 ± 0.01 (2) −0.10 ± 0.01 (2) −0.12 ± 0.03 (2) −0.15 ± 0.01 (2) −0.24 ± 0.02 (3) −0.01 ± 0.02 (3) −0.25 ± 0.01 (4) −0.06 ± 0.01 (3) −0.13 ± 0.01 (2) −0.12 ± 0.01 (4) +0.40 ± 0.01 (2) −0.21 ± 0.03 (3) −0.14 ± 0.01 (2) −0.13 ± 0.00 (1) −0.16 ± 0.00 (1). −0.01 −0.01 −0.01 −0.01 −0.01 −0.00 −0.01 −0.01 −0.01 −0.01 +0.01 −0.00 +0.01 −0.01 −0.01 −0.01 −0.01 −0.01 −0.01 −0.00 −0.01. Intermediate disc 104 Tau HD 40397 A HR 2667 HR 2668 HR 4098 A HD 97343 HR 4523 A HR 5566 Aa HD 135204 ρ CrB Aa. +0.10 ± 0.01 (3) −0.07 ± 0.01 (3) −0.04 ± 0.01 (4) −0.08 ± 0.01 (2) −0.04 ± 0.01 (2) −0.06 ± 0.01 (2) −0.05 ± 0.01 (2) −0.03 ± 0.01 (2) −0.07 ± 0.01 (2) −0.01 ± 0.01 (3). −0.03 −0.01 −0.02 −0.01 −0.01 −0.01 −0.01 −0.03 −0.01 −0.02. 3.2 The striking case of HD 159062. BS BS BS cand. BS. BS BS. Likely on account of the fact that most of its space velocity is not evident from its proper motion, but stored in its radial velocity, the Population II star HD 159062 has not received much attention in the literature. In particular, it appears that the star is also not part of a precision radial velocity monitoring. An astrometric evidence for a companion has been given in the Wielen et al. (1999) Catalogue of μ-binaries,3 but the direct imaging survey by Leconte et al. (2010) did not uncover any close companion.4 However, and as can be seen in Fig. 1, HD 159062 displays a substantial barium-to-iron overabundance by more than 0.5 dex compared to 82 Eri; the latter has similar basic stellar parameters (cf. Table 1) and represents a typical Population II barium-to-iron enrichment. As we will see below, the [Ba/Fe] = +0.40 abundance for HD 159062 can hardly arise from a field anomaly at its formation epoch and must instead be the s-process nucleosynthesis product of a former AGB primary, which is to say, there must be a cool degenerate companion lurking around HD 159062.5. BS cand.. Notes. Uncertainties for the individual stars are internal rms errors from repeated observations with the number of échelle spectra given in parentheses. Blue stragglers or blue straggler candidates are flagged accordingly.. 3. http://wwwadd.zah.uni-heidelberg.de/datenbanken/dmubin/ However, in a private communication, K.-W. Hodapp has informed us about an ∼3 arcsec distant companion that was recently disclosed with Subaru HiCIAO coronographic observations. The large angular separation explains why this source was not seen in Leconte et al. (2010) with their 2 arcsec circular field of view. Whether the Subaru source is an optical or physical companion, a red dwarf or white dwarf still needs to be clarified. 5 We note in this context also the recent Gaia DR1 astrometric data for HD 159062, whose Hipparcos/Gaia discrepancy (DQ) value, DQ=11.35, appears suspicious. 4. MNRAS 471, 3768–3774 (2017). Downloaded from https://academic.oup.com/mnras/article-abstract/471/3/3768/4002687 by Pontificia Universidad Catï¿½lica de Chile user on 12 June 2019. Object.

(5) 3772. K. Fuhrmann et al.. Figure 2. Barium-to-iron ratios for the complete sample of local Population II (blue) and intermediate-disc stars (red). The open circle stars HR 4657, HD 165401, HR 3138 and HR 3220 are all formerly known blue stragglers, whereas HD 159062 is a new identification. Upon exclusion of these objects in the right-hand panel, the Population II and intermediate-disc stars with ages of 12 Gyr and 10 Gyr, respectively, each define a flat and intrinsically precise distribution. This suggests a common, r-process-dominated nucleosynthesis site for barium and iron in the early stages of the Galaxy. Two slightly discrepant stars, HR 3578 and 104 Tau, are candidates for white dwarf companions. Differences for LTE (left-hand panel) versus non-LTE abundances (right-hand panel) are weak.. 3.3 The barium-to-iron enrichment relation In the [Fe/H]–[Ba/Fe] abundance plane of Fig. 2 the Population II and intermediate-disc stars are given with blue and red circles, respectively. In the left-hand panel, the five blue stragglers discussed above (HR 4657, HD 165401, HR 3220, HR 3138, HD 159062) are explicitly labelled and given by open circles. Upon exclusion of these stars, we obtain the flat abundance distributions LTE: [Ba/Fe]Pop II = −0.126 ± 0.030dex(rms), [Ba/Fe]Interm.disc = −0.049 ± 0.021dex(rms), which also reveal two likely deviant cases, HR 3578 and 104 Tau, both explicitly flagged in the right-hand panel of Fig. 2, and discussed below in more detail. MNRAS 471, 3768–3774 (2017). Considering the non-LTE corrections in column (3) of Table 3, we have NLTE: [Ba/Fe]Pop II = −0.135 ± 0.030dex(rms), [Ba/Fe]Interm.disc = −0.063 ± 0.017dex(rms), which represents only a very small −0.01 dex abundance shift in comparison to the LTE relations. These single-valued, near-dispersionless barium-to-iron enrichment relations suggest a common, r-process-dominated nucleosynthesis site for both elements in the early Milky Way: after a rapid core-collapse supernovae production of iron and barium in lockstep during the initial τ ≥ 12 Gyr Population II formation epoch, the delayed onset of both Type Ia supernovae for the iron production and the s-process from AGB stars for the two billion years younger. Downloaded from https://academic.oup.com/mnras/article-abstract/471/3/3768/4002687 by Pontificia Universidad Catï¿½lica de Chile user on 12 June 2019. Figure 1. Comparison of the Population II stars HD 159062 (blue, S/N 300) and 82 Eri (red, S/N 500) around the Ba II line λ5853.683, also showing the Fe I lines λλ5852.227, 5853.153 and 5855.081. Both spectra have a resolution R 60 000 and are shown with an equidistant spacing λ = 20 mÅ. 82 Eri at [Ba/Fe] = −0.15 represents a typical r-process case for a Population II barium abundance, whereas HD 159062 is substantially enhanced at [Ba/Fe] = +0.40. This enrichment is difficult to explain without wind accretion of s-process material from a former AGB primary..

(6) Barium enrichment of ancient disc stars. 3.4 Barium enrichment and stellar remnant companions As pointed out above, in the case of HD 159062 it would be extremely difficult to invoke a plausible mechanism for its striking barium enhancement other than wind accretion from a former AGB star. While this work is still required and the proof essential, the existence of a stellar remnant for this system appears inescapable. With HR 3578 and 104 Tau there are, however, two stars of key importance for the above given barium-to-iron enrichment relations. For both we need to know if they simply reflect a less efficient mixing of gas, different sites of nucleosynthesis for barium and iron, or whether the early enrichment of both elements was indeed extremely well correlated and thereby allows us to unveil both stars. as wind accretion systems with degenerate companions. If true, this would have important implications and could, in addition, become an efficient means in the search for stellar remnants around ancient disc stars. For HR 3578 we note that there is evidence for a spectroscopic binary from the work of Nordström et al. (2004) and support for an astrometric binary also arises from the Gaia DR1 data of HR 3578 with a comparatively high Hipparcos/Gaia discrepancy (DQ) value DQ=22.81. Hence there seems to be a faint companion, but it remains to be seen whether this is indeed the white dwarf that Fig. 2 suggests. With an observable λ6707 lithium absorption for HR 3578 (cf. Fuhrmann et al. 2012b, fig. 4) one can infer that any transferred mass must be restricted to a few hundredths solar masses (otherwise a much less massive main-sequence progenitor would have destroyed this element), as they are typical for wind accretion. In other words, both the existence of lithium and the enhanced barium abundance on HR 3578 render a Roche lobe overflow from a first-ascent red giant progenitor very unlikely, as opposed to a scenario that involves a more distant AGB companion. Even more challenging is the case of 104 Tau. Here, a nearequal-luminosity companion was formerly claimed to exist, but this can be easily ruled out (cf. Bernkopf & Fuhrmann 2006) and one is apparently left with a single star. With a surface gravity log g = 4.03, we note that 104 Tau is the most evolved star of our sample (cf. Tables 1 and 2) and has already reached its early subgiant stage of evolution. The NLTE calculations by Korotin et al. (2015) would then imply an abundance correction of −0.03 dex for 104 Tau (cf. Table 3). Yet, and as can be seen in the right-hand panel of Fig. 2, this correction would still maintain a significant +0.13 dex abundance discrepancy for 104 Tau. It will be of interest to find out whether the forthcoming Gaia astrometric data or direct imaging (coronographic) observations may disclose here a degenerate companion. To this end, small angular separations could be a real challenge for a direct imaging search. Barium enrichment via wind accretion, however, likely works for orbital periods from about 10 to 1000 years (Han et al. 1995; Jeffries & Stevens 1996), meaning that reasonable separations for nearby systems are possible. For a recent example, we mention the discovery of a Teff, B = 4260K cool white dwarf companion separated at ρ = 0.7 arcsec from the G-type star HD 114174. The orbital period of this system is estimated to be between 154 and 881 years for eccentricities in the range 0 ≤ e ≤ 0.5 (Crepp et al. 2013; Matthews et al. 2014), whereas the G star is found to have a [Ba/Fe] = +0.24 dex overabundance (Desidera, D’Orazi & Lugaro 2016). This example particularly shows that the observational techniques for sub-arcsec AGB descendants are accessible. Translated to the distance of 104 Tau, the HD 114174 white dwarf would be at a projected separation ρ = 1.2 arcsec, and given that the barium enhancement for 104 Tau is only modest, its putative white dwarf companion could easily reside at a distance of a few arcsec. AC K N OW L E D G E M E N T S Based on observations collected at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly by the Max-Planck Institut für Astronomie and the Instituto de Astrofı́sica de Andalucı́a (CSIC). Based on data collected under the ESO/RUB - USB agreement at the Paranal Observatory and the La Silla Paranal Observatory under ESO programmes ID 092.A-9002(A) and 096.A-9009(A). We thank our reviewer for constructive comments that improved the manuscript. ZC acknowledges support MNRAS 471, 3768–3774 (2017). Downloaded from https://academic.oup.com/mnras/article-abstract/471/3/3768/4002687 by Pontificia Universidad Catï¿½lica de Chile user on 12 June 2019. intermediate-disc stars – and with plenty of time for an efficient mixing of interstellar gas – then possibly led to the small +0.07 dex step-like [Ba/Fe] increase. In this context, the time-scales, involved masses and the efficiency of the s-process continue to be a source of uncertainty, however. Earlier calculations by Mathews et al. (1992), for instance, had favoured intermediate-mass AGB stars, whereas in the recent work by Cristallo et al. (2011) low-mass AGB stars with M 2.0 M down to M 1.5 M dominate. There is also recent evidence for much higher s-process yields for even less massive M 1.0–1.5 M AGB stars (D’Orazi et al. 2009; Maiorca et al. 2011, 2012). It appears thus questionable whether the rapid Population II formation could have been affected by a slow neutron capture process if most or all of the contributing low-mass AGB stars were not in place at that epoch. While the efficiency and yields associated with the s-process require further observational and computational efforts (cf. Karakas & Lattanzio 2014, and references therein), for the discussion of Fig. 2 we note that the deviants from the Ba/Fe enrichment relations only appear to arise from self-pollution in binary systems, depending on the details of dredge-up, mixing, ejection and accretion of mass. Interestingly, both mass transfer systems with first-ascent red giant progenitors, HR 4657 and HR 3220, show their [Ba/Fe] ratio slightly depleted by about 0.1 dex. Since there is also a weak depletion for HD 165401 visible in Fig. 2, it appears likely that it may have a comparable orbital period (P ∼ 103 d) for its inner Aa-Ab subsystem. HD 159062, on the other hand, is a case for a barium overabundance through wind accretion of s-process-enriched material from a more distant AGB progenitor and both, HR 3578 and 104 Tau, appear as candidates of the same scenario. We note at this point that in view of the fact that the bulk of the local Population II expires at [Fe/H]=−1.0, the [Ba/Fe] scatter observed among more metal-poor stars is likely not related to this population and hence neither related to the Milky Way formation. Indeed, the best case for the abundance spread observed among halo stars is the lack of a homogeneous population. It is, in particular, fatal to believe that simply because they are mostly metal-poor they are necessarily very old sources. On the contrary, in almost all circumstances with halo stars we have no good age information, no information on their star formation history, on the efficiency, mixing, and environment, and in particular the involved initial mass function. For an introduction to the neutron-capture nucleosynthesis among metal-poor halo stars we refer the reader to Sneden, Cowan & Gallino (2008). With respect to the abundant Population I stars of our local sample, we intend to address their barium enrichment in a future volume-complete study. It appears also worth to consider other neutron-capture elements, such as strontium or europium.. 3773.

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