Astronomy & Astrophysics manuscript no. Palaciosetal2015_WGB c ESO 2021 August 12, 2021 New determination of abundances and stellar parameters for a set of weak G-band stars ⋆ A. Palacios1, G. Jasniewicz1, T. Masseron2, F. Thévenin3, J. Itam-Pasquet1, and M. Parthasarathy4 1 LUPM UMR 5299 CNRS/UM, Université de Montpellier, CC 72, F-34095 Montpellier Cedex 05, France 2 Institute of Astronomy - University of Cambridge, Madingley Road, Cambridge CB3 0HA, United Kingdom 3 Laboratoire Lagrange, UMR 7293, OCA, CS 34229, F-06304 Nice Cedex 4, France 4 Indian Institute of Astrophysics, Koramangala, Bangalore - 560034, India Received, accepted ABSTRACT Context. Weak G-band (wGb) stars are a very peculiar class of red giants; they are almost devoided of carbon and often present mild lithium enrichment. Despite their very puzzling abundance patterns, very few detailed spectroscopic studies existed up to a few years ago, which prevented any clear understanding of the wGb phenomenon. We recently proposed the first consistent analysis of published data for a sample of 28 wGb stars and were able to identify them as descendants of early A-type to late B-type stars, although we were not able to conclude on their evolutionary status or the origin of their peculiar abundance pattern. Aims. Using new high-resolution spectra, we present the study of a new sample of wGb stars with the aim of homogeneously deriving their fundamental parameters and surface abundances for a selected set of chemical species that we use to improve our insight on this peculiar class of objects. Methods. We obtained high-resolution and high signal-to-noise spectra for 19 wGb stars in the southern and northern hemisphere that we used to perform consistent spectral synthesis to derive their fundamental parameters and metallicities, as well as the spectroscopic abundances for Li, C, 12C/13C, N, O, Na, Sr, and Ba. We also computed dedicated stellar evolution models that we used to determine the masses and to investigate the evolutionary status and chemical history of the stars in our sample. Results. We confirm that the wGb stars are stars with initial masses in the range 3.2 to 4.2 M⊙ . We suggest that a large fraction could be mildly evolved stars on the subgiant branch currently undergoing the first dredge-up, while a smaller number of stars are more probably in the core He burning phase at the clump. After analysing their abundance pattern, we confirm their strong nitrogen enrichment anti-correlated with large carbon depletion, characteristic of material fully processed through the CNO cycle to an extent not known in evolved intermediate-mass stars in the field and in open clusters. However, we demonstrate here that such a pattern is very unlikely owing to self-enrichment. Conclusions. In the light of the current observational constraints, no solid self-consistent pollution scenario can be presented either, leaving the wGb puzzle largely unsolved. Key words. Stars : late-type - Stars : evolution - Stars : abundances 1. Introduction scopic CO data and those determined using features of the CH (G-band). Lambert & Sawyer (1984) confirmed the pres- The weak G-band (wGb) stars are G and K giants whose spec- ence of lithium in some of the wGb stars, adding yet another tra show very weak or absent G-band of the molecule CH at complexity to these peculiar objects. It is only in the past few 4300Å. The first one, HD18474, was discovered in the north- years that the interest for these objects has been reawakened ern sky by Bidelman (1951) and is the prototype of this class with the papers by Palacios et al. (2012) (hereafter Paper I) and of stars. Later Bidelman & MacConnell (1973) established a ◦ Adamczak & Lambert (2013) (hereafter AL13). list of 34 wGb stars in the southern sky (declination -83 < In paper I, we homogeneously reanalysed spectroscopic δ < +6◦). These stars are rare among the population of G-K arXiv:1512.04417v1 [astro-ph.SR] 14 Dec 2015 archival data for a sample of wGb stars, and used dedicated stel- giants in the Galaxy. The ratio of wGb stars among G-K gi- lar evolution models to establish their mass range. They appear ants in the Bright Star Catalogue is less than 0.3%. This small to be the descendantsof 3 M⊙ to4.5M⊙ stars, now located in the sample of stars was mainly studied in the late 1970s and early crowded Hertzsprung-Russell diagram region of the red clump, 1980s (see e.g. Sneden et al. 1978; Rao 1978; Cottrell & Norris which makes their evolutionary status ambiguous (red giant or 1978; Parthasarathy & Rao 1980; Day 1980; Parthasarathy et al. core helium burning stars). This result is confirmed by AL13 in 1984). Later Sneden & Pilachowski (1984) found an excellent their new spectroscopic study of a large sample of 24 southern agreement between carbon abundances derived from spectro- wGb stars. They establish that wGb stars present a strong depletion in car- Send offprint requests to: A. Palacios: ana.palacios AT umontpellier.fr ⋆ bon that is about a factor of 20 larger than for normal giants Based on data collected at La Silla Observatory (ESO, Chile), pro- 12 13 gram identifier ID 089.D-0189(A), and at the Telescope Bernard Lyot and that the C/ C ratio approaches the CN-cycle equilibrium (USR5026) operated by the Observatoire Midi-Pyrénées, Université de value. In addition, they emphasize a strong N overabundance Toulouse (Paul Sabatier), Centre National de la Recherche Scientifique anti-correlated with the C underabundance, which indicates that of France. the atmospheres of wGb stars have been processed through the Article number, page 1 of 19 A&A proofs: manuscript no. Palaciosetal2015_WGB Table 1. Basic data and atmospheric parameters of weak G-band stars. For rotational velocities υ sini, the typical error is 2km.s−1 HDNo T eT logg elogg log(L/L⊙) elog(L/L⊙) ξturb eξturb υ sini colour (K) (K) (dex) (dex) (dex) (dex) km.s−1 km.s−1 km.s−1 coding HD18474 5198 38 2.65 0.03 2.11 0.08 1.54 0.03 < 5 black HD49960 5030 32 2.61 0.05 2.09 0.09 1.89 0.05 < 5 red HD56438 5037 163 2.75 0.01 1.94 0.09 1.15 0.11 < 5 green HD67728 4827 121 2.28 0.13 2.42 0.16 1.87 0.07 13 blue HD78146 4734 96 2.13 0.03 2.55 0.09 1.74 0.05 < 5 cyan HD82595 4995 79 2.28 0.01 2.47 0.08 1.62 0.04 < 5 magenta HD91805 5247 15 2.56 0.04 2.21 0.08 1.63 0.03 < 5 orange HD94956 5131 75 2.76 0.30 1.99 0.31 1.74 0.04 < 5 lightblue HD102851 4991 41 2.68 0.26 2.01 0.27 1.91 0.05 < 5 reddishbrown HD119256 4984 54 2.64 0.07 2.04 0.10 1.71 0.04 < 5 lightpink HD120170 5127 38 2.76 0.06 1.97 0.09 1.46 0.03 < 5 lightbrown HD120213 4577 46 1.95 0.26 2.71 0.27 1.70 0.05 < 5 lightgreen HD124721 5107 61 2.64 0.11 2.10 0.13 1.58 0.04 < 5 olivegreen HD146116 4920 33 2.03 0.41 2.73 0.42 2.04 0.05 < 5 violet HD165462 5078 110 2.45 0.13 2.30 0.15 1.72 0.04 11 darkred HD165634 5114 45 2.55 0.08 2.19 0.11 1.54 0.03 < 5 limegreen HD166208 5177 52 2.81 0.04 1.94 0.08 1.92 0.05 < 5 goldenyellow HD204046 4984 90 2.51 0.37 2.20 0.38 1.76 0.04 < 5 purple HD207774 5125 33 2.79 0.42 1.96 0.43 1.85 0.04 < 5 grey CN-cycle and probably also the ON-cycle. These abundance means of ThArNe exposures using standard processing tools anomalies are often accompanied by high lithium abundances, available at ESO. similar to those of Li-rich K giants (A(Li) ≥ 1.4 dex). In addition, two northern wGb stars, HD18474 and HD166208, The concurrence of high lithium content and CN(O) cycled ma- were observed in service mode at the Observatoire du Pic du terial is extremely puzzling since these nuclei are issued from Midi, France, with the NARVAL spectrograph mounted on the exclusive regions. The interpretation of the abundance pattern Bernard Lyot 2.0m telescope. The NARVAL instrument consists of wGb stars differs in AL13 and Paper I, and both studies are of a bench-mounted cross-dispersed échelle spectrograph, inconclusive. fibre-fed from a Cassegrain-mounted polarimeter unit (Aurière The goal of this paper is to perform a new homogeneous 2003). It was used in its non-polarimetric mode; it provided abundance analysis of a large sample of wGb stars with up-to- almost complete spectral coverage from ∼ 375 to ∼ 1050nm at date model atmospheres and line-lists, and to compare them a resolution of 75000 in a single spectrogram spread over 40 or- with the predictions of dedicated stellar evolution models in ders, . The NARVAL data were reduced using the data reduction order to clarify the origin of the peculiarities of wGb stars. To software Libre-ESpRIT, written and provided by Donati et al. this end we have conducted new spectroscopic observations ofa (1997) from the LATT (Observatoire Midi-Pyrénées). sample of 19 wGb stars, mainly in the southern hemisphere. The observation log is given in Appendix (Table A.1). We present the sample and the observations in § 2, and the spectral analysis applied to retrieve the atmospheric parameters and abundances in § 3; we also compare our results to AL13 3.