Chemically Peculiar A and F Stars with Enhanced s-process and Iron-peak Elements: Stellar Radiative Acceleration at Work

Abstract

We present 15,000 metal-rich ([Fe/H]>−0.2 dex) A and F stars whose surface abundances deviate strongly from solar abundance ratios and cannot plausibly reflect their birth material composition. These stars are identified by their high [Ba/Fe] abundance ratios ([Ba/Fe]>1.0 dex) in the LAMOST DR5 spectra analyzed by Xiang et al. They are almost exclusively main-sequence and subgiant stars with T eff 6300 K. Their distribution in the Kiel diagram (T effg log ) traces a sharp border at low temperatures along a roughly fixed-mass trajectory (around 1.4 M e ) that corresponds to an upper limit in convective envelope mass fraction of around 10 −4 . Most of these stars exhibit distinctly enhanced abundances of iron-peak elements (Cr, Mn, Fe, Ni) but depleted abundances of Mg and Ca. Rotational velocity measurements from GALAH DR2 show that the majority of these stars rotate slower than typical stars in an equivalent temperature range. These characteristics suggest that they are related to the so-called Am/Fm stars. Their abundance patterns are qualitatively consistent with the predictions of stellar evolution models that incorporate radiative acceleration, suggesting they are a consequence of stellar internal evolution, particularly involving the competition between gravitational settling and radiative acceleration. These peculiar stars constitute 40% of the whole population of stars with mass above 1.5 M e , affirming that "peculiar" photospheric abundances due to stellar evolution effects are a ubiquitous phenomenon for these intermediate-mass stars. This large sample of Ba-enhanced, chemically peculiar A/F stars with individual element abundances provides the statistics to test more stringently the mechanisms that alter the surface abundances in stars with radiative envelopes. Unified AstronomyThesaurus concepts: Am stars (33); Barium stars (135); Chemically peculiar stars (226); Stellar types (1634); Heavy metal stars (705); Chemical abundances (224); Stellar abundances (1577); Stellar diffusion (1593); Stellar processes (1623); Stellar evolution (1599); Radiative processes (2055); S-process (1419)

The behaviour of lithium at high metallicity in the Milky Way

“…Importantly, for most of the elements, the dispersion in abundances among the seven Li-rich stars is in good agreement with (and, for some elements, lower than) the spread in initial abundances depicted by G-type stars. The only elements for which the seven stars deviate from the bulk are Ca, Cu, and Ba, with one star also showing a slight underabundance in C. The Ba overabundances observed in five of the seven stars are, however, lower than the minimum value assumed by Xiang et al (2020) to characterise metal-rich AmFm stars; additionally, for this element, NLTE effects could potentially be important (Mashonkina et al 2020) and may blur the comparison with the abundances derived for cool stars. This analysis thus tends to indicate that the seven Li rich stars in GALAH bin 4 have relatively normal abundances of heavy elements.…”

Stellar labels for hot stars from low-resolution spectra - I. the HotPayne method and results for 330,000 stars from LAMOST DR6

“…We note that such stars are prevalence in the hot stars regime; it is suggested that a high fraction of A stars could be chemically peculiar (e.g. Gray et al 2016;Qin et al 2019;Xiang et al 2020), and this is why their high number density constitutes a prominent stripe in the T eff −log g plane. Such artefact reveals one of the limitations of the current model, which tends to yield incorrect log g for these chemically peculiar stars.…”

“…17 shows that at the cool temperature side (T eff 10000 K), there is a large fraction of stars with super-solar metallicity, forming the high-[Fe/H] tail of the [Fe/H] distribution. Many of these super-solar metallicity stars are probably AmFm stars, which have been demonstrated to contributed a large fraction (∼ 40%) of the A/F stars with mass higher than 1.4M in the LAMOST sample (Xiang et al 2020).…”

“…Particularly, it has long been known that "chemically peculiar" stars are common among hot stars (e.g. Renson & Manfroid 2009;Gray et al 2016;Qin et al 2019;Chojnowski et al 2020;Xiang et al 2020;Paunzen et al 2021), which provide good constraints on these stellar element transport processes (e.g. Talon et al 2006;Vick et al 2010;Michaud et al 2015).…”

Reliable stellar abundances of individual stars with the MUSE integral-field spectrograph

“…For example, the LAMOST [Fe/H] estimates can be rebost down to −2.5 dex (see Fig. 10 of Conroy et al 2019); DD-Payne can also generate reasonable [Ba/Fe] stars with 1 < [Ba/Fe] < 3 dex and 𝑇 eff > 7000K (Xiang et al 2020).…”