Rubidium abundances in solar metallicity stars

Abstract

Context. Rubidium is one of the few elements produced by the neutron capture s- and r-processes in almost equal proportions. Recently, a Rb deficiency ([Rb/Fe] < 0.0), amounting to a factor of about two with respect to the Sun, has been found in M dwarfs of near-solar metallicity. This stands in contrast to the close-to-solar [Sr, Zr/Fe] ratios derived in the same stars. This deficiency is difficult to understand from the point of view of observations and of nucleosynthesis. Aims. To test the reliability of this Rb deficiency, we study the Rb and Zr abundances in a sample of KM-type giant stars across a similar metallicity range, extracted from the AMBRE Project. Methods. We used high-resolution and high signal-to-noise spectra to derive Rb and Zr abundances in a sample of 54 bright giant stars with metallicities in the range of −0.6 ≲ [Fe/H] ≲ +0.4 dex, via spectral synthesis in both local and non-local thermodynamic equilibrium (LTE and NLTE, respectively). We also studied the impact of the Zeeman broadening in the profile of the Rb I at λ7800 Å line. Results. The LTE analysis also results in a Rb deficiency in giant stars, however, it is considerably lower than that obtained in M dwarfs. However, once NLTE corrections are performed, the [Rb/Fe] ratios are very close to solar (average −0.01 ± 0.09 dex) in the full metallicity range studied here. This stands in contrast to the value found for M dwarfs. The [Zr/Fe] ratios derived are in excellent agreement with those obtained in previous studies in FGK dwarf stars with a similar metallicity. We investigate the effect of gravitational settling and magnetic activity as possible causes of the Rb deficiency found in M dwarfs. Although the former phenomenon has a negligible impact on the surface Rb abundance, the presence of an average magnetic field with an intensity that is typical of that observed in M dwarfs may result in systematic Rb abundance underestimations if the Zeeman broadening is not considered in the spectral synthesis. This may explain the Rb deficiency in M dwarfs, but not fully. On the other hand, the new [Rb/Fe] and [Rb/Zr] versus [Fe/H] relationships can be explained when the Rb production by rotating massive stars and low-to-intermediate mass stars (these latter also producing Zr) are considered, without the need to deviate from the standard s-process nucleosynthesis in asymptotic giant branch stars, as suggested previously.

A detailed analysis of the Gl 486 planetary system

“…After following all the steps enumerated by Abia et al (2020), and assuming non-local thermodynamic equilibrium (NLTE) for Rb and Sr, we obtained [Rb/M] = +0.15 ± 0.12 dex, [Sr/M] = +0.03 ± 0.14 dex, [Zr/M] = +0.00 ± 0.13 dex, from which we determined the A(X) and [X/H] values in Table 7 with solar values from Lodders et al (2009). The derived [Rb,Sr,Zr/M] ratios in Gl 486 match well the corresponding [Rb,Sr,Zr,M] vs. [M/H] relationships of unevolved field stars obtained by Abia et al (2020Abia et al ( , 2021.…”

“…The determined C/O ratio, +0.54 ± 0.05 dex, becomes exactly solar with the latest revision of solar oxygen abundance with respect to the standard value of +0.10 dex (Bergemann et al 2021). Determining this ratio is vital, as almost all the available carbon in the atmospheres of M dwarfs is locked into CO and, therefore, the abun- Abia et al (2020Abia et al ( , 2021 well.…”

High-resolution spectroscopic analysis of four new chemically peculiar stars

“…[Fe/H] Figure 12. Observed [X/Fe] abundance ratios as a function of metallicity for the stars analyzed in this study (black dots with error bars) along with data for field giant stars (grey dots, taken from Gratton & Sneden 1994;Tomkin & Lambert 1999;Burris et al 2000;Mishenina & Kovtyukh 2001;Mishenina et al 2007;Johnson 2002;Luck & Heiter 2007;Zhang et al 2009;For & Sneden 2010;Santrich et al 2013;Hansen et al 2014;Ishigaki et al 2018;Forsberg et al 2019;Abia et al 2021;Takeda 2021) Reyniers et al 2004;De Smedt et al 2012van Aarle et al 2013). Triangles are data for CEMP-𝑠 (cyan), CEMP-𝑟 (magenta), and CEMP-𝑟/𝑠 (purple) stars, taken from Masseron et al (2010) and Karinkuzhi et al (2021).…”

“…Figure12. Observed [X/Fe] abundance ratios as a function of metallicity for the stars analyzed in this study (black dots with error bars) along with data for field giant stars (grey dots, taken fromGratton & Sneden 1994;Tomkin & Lambert 1999;Burris et al 2000;Mishenina & Kovtyukh 2001;Mishenina et al 2007;Johnson 2002;Luck & Heiter 2007;Zhang et al 2009;For & Sneden 2010;Santrich et al 2013;Hansen et al 2014;Ishigaki et al 2018;Forsberg et al 2019;Abia et al 2021;Takeda 2021), barium stars (green dots, taken fromAllen & Barbuy 2006;Allen & Porto de Mello 2007;Karinkuzhi et al 2018;Shejeelammal et al 2020; Roriz et al 2021a,b), CH stars (yellow diamonds, takenfrom Vanture 1992c;Pereira & Drake 2009;Pereira et al 2012;Goswami et al 2016;Purandardas et al 2019), and post-AGB stars (brown squares, taken fromVan Winckel & Reyniers 2000;Reyniers et al 2004; De Smedt et al 2012van Aarle et al 2013). Triangles are data for CEMP-𝑠 (cyan), CEMP-𝑟 (magenta), and CEMP-𝑟/𝑠 (purple) stars, taken fromMasseron et al (2010) andKarinkuzhi et al (2021).…”