Barium & related stars and their white-dwarf companions II.\n Main-sequence and subgiant stars

Abstract

Barium (Ba) dwarfs and CH subgiants are the less-evolved analogues of Ba and\nCH giants. They are F- to G-type main-sequence stars polluted with heavy\nelements by a binary companion when the latter was on the Asymptotic Giant\nBranch (AGB). This companion is now a white dwarf that in most cases cannot be\ndirectly detected. We present a large systematic study of 60 objects classified\nas Ba dwarfs or CH subgiants. Combining radial-velocity measurements from\nHERMES and SALT high-resolution spectra with radial-velocity data from CORAVEL\nand CORALIE, we determine the orbital parameters of 27 systems. We also derive\ntheir masses by comparing their location in the Hertzsprung-Russell diagram\nwith evolutionary models. We confirm that Ba dwarfs and CH subgiants are not at\ndifferent evolutionary stages and have similar metallicities, despite their\ndifferent names. Additionally, Ba giants appear significantly more massive than\ntheir main-sequence analogues. This is likely due to observational biases\nagainst the detection of hotter main-sequence post-mass-transfer objects.\nCombining our spectroscopic orbits with the Hipparcos astrometric data, we\nderive the orbital inclinations and the mass of the WD companion for four\nsystems. Since this cannot be done for all systems in our sample yet (but\nshould be with upcoming Gaia data releases), we also analyse the mass-function\ndistribution of our binaries. We can model this distribution with very narrow\nmass distributions for the two components and random orbital orientation on the\nsky. Finally, based on BINSTAR evolutionary models, we suggest that the orbital\nevolution of low-mass Ba systems can be affected by a second phase of\ninteraction along the Red Giant Branch of the Ba star, impacting on the\neccentricities and periods of the giants.\n