Chemically peculiar A and F stars with enhanced s-process and iron-peak elements: stellar radiative acceleration at work

Abstract

We present $\gtrsim 15,000$ 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. (2019). They are almost exclusively main sequence and subgiant stars with $T_{\mathrm{eff}}\gtrsim 6300$K. Their distribution in the Kiel diagram ($T_{\mathrm{eff}}$--$\log g$) traces a sharp border at low temperatures along a roughly fixed-mass trajectory (around $1.4M_{\odot })$ 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$M_{\odot }$, 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.

21 pages, 17 figures, accepted for publication in ApJ

References in corpus (24)

• The Dartmouth Stellar Evolution Database
• The GALAH Survey: Scientific Motivation
• The Dawes Review 2: Nucleosynthesis and stellar yields of low and intermediate-mass single stars
• The Chemical Composition of an Extrasolar Minor Planet
• Determining stellar macroturbulence using asteroseismic rotational velocities from Kepler
• Atomic Diffusion and Mixing in Old Stars I. VLT/FLAMES-UVES Observations of Stars in NGC 6397
• The GALAH Survey: Observational Overview and Gaia DR1 companion
• The LAMOST Stellar Parameter Pipeline at Peking University --- LSP3
• Mapping metallicity variations across nearby galaxy disks
• LAMOST Spectroscopic Survey of the Galactic Anti-centre (LSS-GAC): target selection and the first release of value-added catalogues
• LAMOST Spectroscopic Survey of the Galactic Anticentre (LSS-GAC): the second release of value-added catalogues
• Chemical Abundances of Main-Sequence, Turnoff, Subgiant and red giant Stars from APOGEE spectra II: Atomic Diffusion in M67 Stars
• Late stages of the evolution of A-type stars on the main sequence: comparison between observed chemical abundances and diffusion models for 8 Am stars of the Praesepe cluster
• Chemical composition of A and F dwarf members of the Coma Berenices open cluster
• Chemical composition of A and F dwarfs members of the Pleiades open cluster
• Prospects for chemically tagging stars in the Galaxy
• Measuring 14 elemental abundances with R=1,800 LAMOST spectra
• A Large Sample of Am Candidates from LAMOST Data Release 1
• Metallic-Line Stars Identified from Low Resolution Spectra of LAMOST DR5
• DA white dwarfs from the LSS-GAC survey DR1: the preliminary luminosity and mass functions and formation rate
• Modelling the distributions of white dwarf atmospheric pollution: a low Mg abundance for accreted planetesimals?
• Four new HgMn stars: HD 18104, HD 30085, HD 32867, HD 53588
• Chemical mixing in low mass stars: I. Rotation against atomic diffusion including radiative accelerations
• Discovery of s-process enhanced stars in the LAMOST survey

Cited by in corpus (4)

• StarHorse results for spectroscopic surveys + Gaia DR3: Chrono-chemical populations in the solar vicinity, the genuine thick disk, and young-alpha rich stars
• Overview of the LAMOST survey in the first decade
• Ba-enhanced dwarf and subgiant stars in the LAMOST Galactic surveys
• Chemically Peculiar Stars in the Open Cluster Stock 2