A&A 579, A52 (2015) Astronomy DOI: 10.1051/0004-6361/201526269 & c ESO 2015 Astrophysics High-precision abundances of elements in solar twin stars Trends with stellar age and elemental condensation temperature, P. E. Nissen Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark e-mail: [email protected] Received 7 April 2015 / Accepted 28 April 2015 ABSTRACT Context. High-precision determinations of abundances of elements in the atmospheres of the Sun and solar twin stars indicate that the Sun has an unusually low ratio between refractory and volatile elements. This has led to the suggestion that the relation between abundance ratios, [X/Fe], and elemental condensation temperature, TC, can be used as a signature of the existence of terrestrial planets around a star. Aims. HARPS spectra with S/N 600 for 21 solar twin stars in the solar neighborhood and the Sun (observed via reflected light from asteroids) are used to determine very precise (σ ∼ 0.01 dex) differential abundances of elements in order to see how well [X/Fe] is correlated with TC and other parameters such as stellar age. Methods. Abundances of C, O, Na, Mg, Al, Si, S, Ca, Ti, Cr, Fe, Ni, Zn, and Y are derived from equivalent widths of weak and medium-strong spectral lines using MARCS model atmospheres with parameters determined from the excitation and ionization bal- ance of Fe lines. Non-LTE effects are considered and taken into account for some of the elements. In addition, precise (σ 0.8Gyr) stellar ages are obtained by interpolating between Yonsei-Yale isochrones in the log g – Teff diagram. Results. It is confirmed that the ratio between refractory and volatile elements is lower in the Sun than in most of the solar twins (only one star has the same [X/Fe]-TC distribution as the Sun), but for many stars, the relation between [X/Fe] and TC is not well defined. For several elements there is an astonishingly tight correlation between [X/Fe] and stellar age with amplitudes up to ∼0.20 dex over an age interval of eight Gyr in contrast to the lack of correlation between [Fe/H] and age. While [Mg/Fe] increases with age, the s-process element yttrium shows the opposite behavior meaning that [Y/Mg] can be used as a sensitive chronometer for Galactic evolution. The Na/Fe and Ni/Fe ratios are not well correlated with stellar age, but define a tight Ni–Na relation similar to that previously found for more metal-poor stars albeit with a smaller amplitude. Furthermore, the C/O ratio evolves very little with time, although [C/Fe] and [O/Fe] change by ∼0.15 dex. Conclusions. The dependence of [X/Fe] on stellar age and the [Ni/Fe]– [Na/Fe] variations complicate the use of the [X/Fe]-TC rela- tion as a possible signature for the existence of terrestrial planets around stars. The age trends for the various abundance ratios provide new constraints on supernovae yields and Galactic chemical evolution, and the slow evolution of C/O for solar metallicity stars is of interest for discussions of the composition of exoplanets. Key words. stars: abundances – stars: fundamental parameters – stars: solar-type – planetary systems – Galaxy: disk – Galaxy: evolution 1. Introduction stars find significant variations of the [X/Fe]-TC slope with only about 15% of the stars having a slope similar to that of the Sun. In an important paper based on very precise determinations of abundances in the Sun and 11 solar twin stars, Meléndez et al. The works of Meléndez et al. (2009)andRamírezetal. (2009) have triggered several papers investigating relations be- (2009) found that the Sun has a lower ratio between refrac- / tory and volatile elements than most of the solar twins. They tween [X Fe]-TC slopes and the existence of planets around suggest that this may be explained by the sequestration of re- stars (Gonzalez et al. 2010; González Hernández et al. 2010, fractory elements in terrestrial planets and that the slope of abun- 2013; Schuler et al. 2011; Adibekyan et al. 2014; Maldonado dances relative to iron, [X/Fe]1, as a function of elemental con- et al. 2015). The results obtained are not very conclusive, partly because the stars included span wider ranges in Teff,logg, densation temperature, TC, in a solar-composition gas (Lodders / 2003) can be used as a signature of the existence of terres- and [Fe H] than the solar twins in Meléndez et al. (2009)mak- ing the abundance ratios less precise and introducing a depen- trial planets around a star. This idea was supported by Ramírez / et al. (2009), who in a study of abundances of 64 solar-type dence of the derived [X Fe]-TC slopes on possible corrections for Galactic evolution effects. Thus, González Hernández et al. Based on data products from observations made with ESO (2013) obtain both positive and negative slopes for ten stars with Telescopes at the La Silla Paranal Observatory under programs given detected super-Earth planets, and Adibekyan et al. (2014)find in Table 1. evidence that the slope depends on stellar age. Furthermore, Tables 2 and 3 are available in electronic form at Schuler et al. (2011) show that the [X/Fe]-TC slope depends crit- http://www.aanda.org ically on whether only refractory elements (TC > 900 K) are 1 For two elements, X and Y, with number densities NX and NY, considered or whether volatile elements (TC < 900 K) are also [X/Y] ≡ log(NX/NY)star − log(NX/NY)Sun. included. Article published by EDP Sciences A52, page 1 of 15 A&A 579, A52 (2015) Sequestration in terrestrial planets is not the only possible of FGK stars, a sample of solar twins was selected to have pa- explanation of the low abundances of refractory elements in the rameters that agree with those of the Sun within ±100 K in Teff, Sun. Önehag et al. (2014) found that the [X/Fe]-TC trend for ±0.15 dex in log g,and±0.10 dex in [Fe/H]. Listed in Table 1, solar-type stars belonging to the open cluster M 67 agrees with 21 of these stars have HARPS spectra with S/N ≥ 600 after the trend for the Sun and they suggest that the gas of the proto- combination of spectra available in the ESO Science Archive. cluster was depleted in refractory elements by formation and The HARPS spectra have a resolution of R 115 000 cleansing of dust before the stars formed. According to this, (Mayor et al. 2003). Individual spectra for a given star were the Sun may have formed in a dense stellar environment un- combined after correction for Doppler shifts and then normal- like most of the solar twins. Similar scenarios are discussed by ized with the IRAF continuum task using cubic splines with a / Gaidos (2015), who suggests that [X Fe]-TC correlations can be scale length of ∼100 Å in the blue spectral region (3800–5300 Å) explained by dust-gas segregation in circumstellar disks rather and ∼15 Å in the red region (5350–6900 Å), for which the than the formation of terrestrial planets. archive spectra are more irregular. The S/N values given in Additional information on the connection between abun- Table 1 were estimated from the rms variation of the flux in dance ratios in stars and the occurrence of planets can be ob- continuum regions for the 4700–6900 Å spectral region, which tained from studies of wide binaries for which the two compo- contains the spectral lines applied in this study. By including nents have similar Teff and log g values. Dust-gas segregation spectra obtained from 2011 to 2013 (programs 183.C-0972 and ff before planet formation is likely to a ect the chemical com- 188.C-0265), the S/N is in most cases significantly higher than position of the components in the same way. In the case of that of the HARPS spectra applied in previous studies of solar / 16 Cyg A B, for which the secondary component has a detected twin stars (e.g., González Hernández et al. 2010, 2013). For the ff planet of at least 1.5 Jupiter mass (MJ), there is a di erence of stars in Table 1,theS/N ranges from 600 to 2400 with a median / ff [Fe H](A–B) 0.04 dex (Ramírez et al. 2011)andadi erence value of 1000. / in the [X Fe]-TC slope for refractory elements (Tucci Maia et al. The IRAF splot task was used to measure equivalent 2014) that can be explained by accretion of elements in the rocky widths (EWs) of spectral lines by Gaussian fitting relative to core of the giant planet around 16 Cyg B. Furthermore, the two pseudo-continuum regions lying within 3 Å from the line mea- components of XO-2, which are hosts of different planets, have sured. These regions do not necessarily represent the true con- an abundance difference of ∼0.05 dex for refractory elements tinuum, but care was taken to use the same continuum windows (Teske et al. 2015; Damasso et al. 2015). On the other hand, Liu in all stars, so that differences in EW between stars are precisely et al. (2014) did not detect any abundance differences between measured given that the same instrument and resolution were ap- the two components of HAT-P-1, for which the secondary has a plied for obtaining spectra. In this way differential abundances 0.5 M planet. J may be obtained with a precision better than 0.01 dex as shown Recent precise determinations of abundances in four so- by Bedell et al.