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Planet Signatures and Effect of the Chemical Evolution of the Galactic Thin-Disk Stars

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Planet Signatures and Effect of the Chemical Evolution of the Galactic Thin-Disk Stars Astronomy & Astrophysics manuscript no. 27429_ap c ESO 2019 May 8, 2019 Planet signatures and effect of the chemical evolution of the Galactic thin-disk stars Lorenzo Spina1, Jorge Meléndez1, and Ivan Ramírez2 1 Universidade de São Paulo, IAG, Departamento de Astronomia, Rua do Mãtao 1226, São Paulo, 05509-900 SP, Brasil - e-mail: [email protected] 2 Department of Astronomy, University of Texas at Austin; 2515 Speedway, Stop C1400, Austin, TX 78712-1205, USA Received September 22, 2015; accepted October 19, 2015 ABSTRACT Context. Studies based on high-precision abundance determinations revealed that chemical patterns of solar twins are characterised by the correlation between the differential abundances relative to the Sun and the condensation temperatures (Tc) of the elements. It has been suggested that the origin of this relation is related to the chemical evolution of the Galactic disk, but other processes, associated with the presence of planets around stars, might also be involved. Aims. We analyse HIRES spectra of 14 solar twins and the Sun to provide new insights on the mechanisms that can determine the relation between [X/H] and Tc. Methods. Our spectroscopic analysis produced stellar parameters (Teff , log g, [Fe/H], and ξ), ages, masses, and abundances of 22 elements (C, O, Na, Mg, Al, Si, S, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, and Ba). We used these determinations to place new constraints on the chemical evolution of the Galactic disk and to verify whether this process alone can explain the different [X/H]-Tc slopes observed so far. Results. We confirm that the [X/Fe] ratios of all the species correlate with age. The slopes of these relations allow us to describe the effect that the chemical evolution of the Galactic disk has on the chemical patterns of the solar twins. After subtracting the chemical evolution effect, we find that the unevolved [X/H]-Tc slope values do not depend on the stellar ages anymore. However, the wide 5 1 diversity among these [X/H]-T slopes, covering a range of 4 10− dex K− , indicates that processes in addition to the chemical c ± evolution may affect the [X/H]-Tc slopes. Conclusions. The wide range of unevolved [X/H]-Tc slope values spanned at all ages by our sample could reflect the wide diversity among exo-planetary systems observed so far and the variety of fates that the matter in circumstellar disks can experience. Key words. Stars: abundances – Stars: fundamental parameters – Stars: solar-type – (Stars:) planetary systems – Galaxy: disk Galaxy: evolution 1. Introduction present in the circumstellar disks that is otherwise employed to form planets. The main motivation of this study has been a recent paper pub- lished by Nissen (2015) (hereafter, N15), who determined the These striking results openeda vivid debate related to the ori- high-precision abundances of 13 elements in addition to iron gin of the observed [X/H]-Tc slopes. In particular, several teams in a sample of 21 solar-twin spectra, revealing the existence of made great efforts to answer to the following question: is the a correlation between [X/Fe] and their stellar ages. This work [X/H]-Tc slope connected to the formation of planetary systems was written in the wake of a sequence of other important stud- aroundstars, or is it instead related to completely different mech- ies based on abundance determinations in solar twins and is a anisms, such as the chemical evolution of the Galactic thin-disk significant step forward in study the chemical patterns of solar population? González Hernández et al. (2010, 2013) claimed twins. that the abundance patterns of solar twins are not affected by arXiv:1511.01012v2 [astro-ph.SR] 4 Nov 2015 Meléndez et al. (2009) first showed that the differential the presence of rocky planets around the stars, and Adibekyan chemical abundances of 11 solar twins relative to the solar com- et al. (2014) clearly revealed a correlation between the steep- position correlate with the condensation temperatures (Tc) of the ness of the [X/H]-Tc slopes and stellar ages that is a result of elements. They also found that the Sun is depleted in refractory Galactic chemical evolution effects. The youngest stars in the elements with respect to most of the solar twins they analysed. Galactic disk are more overabundant in refractory elements with Ramírez et al. (2009) also analysed the chemical content of 22 respect to the Sun and older stars. On the other hand, additional solar twins and confirmed the existence of a positive trend in the insights by Ramírez et al. (2010)and Meléndez et al. (2012)con- correlation between [X/H] and the Tc (hereafter, [X/H]-Tc slope) firmed the existence of the [X/H]-Tc slope as a possible planet for 85% of the stars. Both Meléndez et al. (2009) and Ramírez formation signature. In agreement with this view, Gonzalez et al. et al. (2009) speculated that this peculiarity of the solar chemical (2010)found that stars with planets have more negative[X/H]-Tc pattern could be the signature of rocky planet formation that did slopes. Similarly, Maldonado et al. (2015) found that the [X/H]- not occur for the most of the solar twins. They suggested that Tc slope steepness could also be related to the architecture of the the positive [X/H]-Tc slopes observed in these stars have been planetary systems: negative slopes have been found for stars with produced by accretion onto the star of the refractory material cool giant planets, while stars with hot giant planets or low-mass Article number, page 1 of 15 A&A proofs: manuscript no. 27429_ap planets exhibit positive slopes. However, Maldonado and collab- Table 1. Planet properties orators also cautioned that the Galactic chemical evolution could affect the [X/H]-Tc slopes as well. Planet Mass Axis Period e Reference Observations of wide binary systems, whose components are [MJ ] [AU] [yr] two solar analogues, provided further indications that planet for- HD 13931 b 1.88 5.2 11.6 0.02 Howard et al. (2010) HD 95128 b 2.53 2.1 3.0 0.032 Butler & Marcy (1996) mation can play a role in setting the [X/H]-Tc slope. The compo- nents of a stellar system are most likely formedfrom the collapse HD 95128 c 0.54 3.6 6.6 0.098 Fischer et al. (2002) HD 95128 d 1.64 11.6 38.4 0.16 Gregory & Fischer (2010) of the same gaseousclump,thus theyshould share the same com- HD 106252 b 7.56 2.6 4.2 0.48 Wittenmyer et al. (2009) position. High-resolution spectra of the 16 Cyg binary system, composed of a planet-hosting star and a star without detected planets, revealed that the star with the planet is poorer in iron by 0.04 dex (Ramírez et al. 2011). Recently, Tucci Maia et al. In the context of this lively debate, N15 determined the (2014∼ ) have shown that this difference in not limited to iron, but [X/H]-Tc slopes for the solar twins of his sample that have ages involves all the refractory elements. Similar results have been spanning 8 Gyr. From these data he was able to reproduce the found for the two components of XO-2 (Teske et al. 2015; Bi- ∼ correlation between [X/H]-Tc slope values and stellar ages ob- azzo et al. 2015; Ramírez et al. 2015). We note, however, that served by Adibekyan et al. (2014), confirming that the Galactic no differences have been found within the error bars for the stars chemical evolution plays a role in setting the [X/H]-Tc slopes. of the binary systems HAT-P-1 (Liu et al. 2014) and HD 80606 On the other hand, he was unable to exclude that other processes, (Saffe et al. 2015). related to the fate of the rocky material orbiting the star during An alternative mechanism that could generate a positive the different phases of its evolution, might also affect the [X/H]- [X/H]-T slope is represented by a planet engulfment event. c Tc slopes. The migration of giant planets can induce other rocky objects The main aim of the present paper is to produce new in- to move onto unstable orbits and be accreted onto the central sights on the origin of the [X/H]-Tc slopes and verify whether star (Sandquist et al. 2002; Ida & Lin 2008; Zhou & Lin 2008). the [X/Fe]-age correlations identified by N15 can exhaustively After penetrating the stellar atmosphere, the rocky mass would explain the diversity of [X/H]-Tc slope values observed so far be rapidly dissolved and pollute the ambient matter with refrac- in solar twins. To do this, we derived high-precision abundances tory elements. Strong hints have been provided by Schuler et al. of 21 elements for a new sample of 14 solar twins to investi- (2011), who showed that stars with close-in giant planets have gate the [X/Fe]-age correlations and to deconvolve the age effect positive [X/H]-Tc slopes. Recently, Spina et al. (2014) found a from the [X/H]-Tc slopes. In Sect. 2 we describe the stellar sam- member of the Gamma Velorum cluster that is significantly en- ple, the spectroscopic observations, and the spectral reduction, riched in iron relative to the other members of the same cluster, while Sect. 3 details the spectral analysis procedure. In Sect. 4 which, as for binary stars, are also assumed to share the same we show our data and discuss our scientific results and, finally, initial compositions.
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