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Using Solar Twins to Explore the Planet–Star Connection with Unparallelled Precision

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Using Solar Twins to Explore the Planet–Star Connection with Unparallelled Precision Astronomical Science Using Solar Twins to Explore the Planet–Star Connection with Unparallelled Precision Jorge Meléndez1 natural connection between stars and requisite milestone 0.01 dex precision Jacob L. Bean2 planets is expected to arise at these early (Melendez et al., 2009), by performing Megan Bedell2 stages. The formation of rocky planets a strictly differential analysis between Iván Ramírez3 requires refractory elements, i.e., those Solar twins and the Sun, opening new Martin Asplund4 that condense at high temperatures windows on the study of the planet–star Stefan Dreizler5 (about 1500 K), which are found in the connection. Alan Alves-Brito6 inner protoplanetary disc. In contrast, Lorenzo Spina1 giant planets are rich in volatile elements Luca Casagrande4 that can condense only at low tempera- Solar twins and planet signatures TalaWanda Monroe7 tures (< 200 K) and are found in the outer Marcelo Tucci Maia1 region of the proto­Solar nebula. As the Solar twins are stars with physical char- Fabrício Freitas1 process of planet formation and the last acteristics (effective temperature, surface stages of star formation are coeval, gravity and chemical composition) similar chemical signatures of planet formation to the Sun. With stellar atmospheres so 1 Departamento de Astronomia, Instituto can be imprinted on the outer zones of similar to the Sun, many systematic de Astronomia, Geofísica e Ciências stars, because the late-accreted gas effects that plague chemical abundance Atmosféricas, Universidade de São could be depleted in some chemical ele- analyses are removed when each spec- Paulo, Brazil ments by the sequestration of the differ- tral line is analysed differentially between 2 Department of Astronomy and Astro- ent chemical elements needed to form the Solar twin and the Sun, allowing a physics, University of Chicago, USA planets. precision of 0.01 dex to be achieved. Our 3 McDonald Observatory and Department precision has been tested by performing of Astronomy, University of Texas at The most-studied signature of a relation differential abundances in the Sun using Austin, USA between stars and planets is the higher two different asteroids (Bedell et al., 4 Research School of Astronomy and frequency of close-in giant planets 2014), achieving an element-to-element Astrophysics, The Australian National around metal-rich stars (Gonzalez, 1997). scatter of only 0.006 dex (Figure 1). We University, Australia Stars with metallicities higher than the have also studied Solar abundances at 5 Institut für Astrophysik, Universität Sun are observed to have a higher different Solar latitudes, achieving an Göttingen, Germany chance of hosting a close-in giant planet, agreement of 0.005 dex for all elements, 6 Instituto de Fisica, Universidade Federal reflecting the trend that giant planet for- and 0.002 dex for the refractory elements do Rio Grande do Sul, Porto Alegre, mation is enhanced in discs of higher (Kiselman et al., 2011). Brazil metallicity. Planet engulfment events can 7 Space Telescope Science Institute, also enhance the stellar metallicity, as Our improved 0.01 dex precision led Baltimore, USA predicted, for example, by Laughlin & to a breakthrough in 2009, when we Adams (1997) and probably observed in showed that the chemical composition one star of the γ Velorum cluster by of the Sun is anomalous when compared This year marks the 20th anniversary of Spina et al. (2014). Iron is used as a proxy to the average composition of Solar the first definitive detection of an exo- for stellar metallicity because it has many twins. The Sun appears to be deficient in planet orbiting a Sun-like star by Mayor lines in the spectra of Solar­type stars, elements that are abundant in the Earth and Queloz (1995). Almost 2000 exo- and it is thus easier to estimate its con- and other rocky material in the Solar planets have been discovered since this tent. About a decade after the discovery System (Meléndez et al., 2009; Ramírez breakthrough, but many fundamental of the correlation between giant-planet et al., 2009). Importantly, there is a robust questions remain open despite the frequency and iron abundance, it was correlation between the abundance enormous progress: How common are suggested from observational studies anomalies and the dust condensation analogues of the Solar System? How do that there is no significant correlation temperature in the proto­Solar nebula planets form and evolve? What is the between Neptune-mass planets and the (see Figure 2). This strongly suggests a relationship between stars and planets? iron abundance of the host star (e.g., relationship with the formation of terres- We are observing stars that are near- Sousa et al., 2008; Ghezzi et al., 2010). trial planets in the Solar System, as perfect matches to the Sun to provide verified quantitatively by Chambers (2010), new insights into the above questions, For more than a decade after the land- who showed that the deficiency of thus exploring the planet–star connec- mark discovery by Gonzalez (1997), no refractory elements in the Sun could be tion with unprecedented precision. other firm relations are known for ele- eradicated by adding back a few Earth ments other than iron. This is because masses of rocky material with the abun- the most optimistic minimum uncertain- dance pattern of the Earth and meteorites. The relationship between stars and ties in chemical abundance analyses planets are 0.05 dex (Asplund, 2005), which are Other recent work by our group used the too high to convincingly detect the effect binary pair of Solar twins 16 Cyg to Stars and planets are formed from the of rocky planets such as the Earth (pre- search for signatures left over from giant same natal cloud and the two processes dicted to be only a few times 0.01 dex). planet formation (Ramírez et al., 2011; occur on similar timescales, and thus a Our group was the first to achieve the Tucci Maia et al., 2014). The secondary 28 The Messenger 161 – September 2015 σ (volatiles) = 0.011 dex 0.04 µ = 0.001 dex 0.10 σ = 0.006 dex σ (refractories) = 0.007 dex N 0.08 S 0.02 a C ) st Ve s> X, 0.06 P in A O 0.00 tw Na – V s Ni r re O Mg Si K Fe Cu Mn Ce C 0.04 Zn Si Sc Ti Mn Co ola X, A Ca K Al S Cr –0.02 Na Zn Mg V Cu – <s 0.02 Ca un Co Y Ti –0.04 e] (S 0.00 Cr Ni /F [X 5 10 15 20 25 30 Δ –0.02 Ba Z Sc Zr Figure 1. Chemical abundance differences between –0.04 the Solar abundances obtained through reflected Al light off the asteroids Ceres and Vesta (Bedell et al., 2014), show an element-to-element scatter of only –0.06 0.006 dex. 0 5001000 1500 Tcond (K) star (16 Cyg B) has a giant planet, while to exploit these advantages, in 2011, we Figure 2. Chemical abundance differences between no planet has been found around the pri- started a four-year Large Programme the Sun and the average of 11 Solar twins (Meléndez et al., 2009). The refractory elements in the Sun are mary star (16 Cyg A). Naïvely both stars (188.C­0265) to use the High Accuracy deficient compared to the volatile elements, perhaps should have the same chemical composi- Radial velocity Planetary Searcher due to the formation of the rocky planets in the Solar tion, as they were born from the same (HARPS) to search for planets around System. natal cloud; however, the stars have a Solar twins. We also acquired high­reso- distinct chemical composition, with the lution, high signal­to­noise (S/N) spectra star hosting the giant planet being more at the 6.5-metre Magellan Telescope to The HARPS observations are taken with deficient in both volatile and refractory determine the stellar parameters and pre- a total integration time of about 15 min- elements, perhaps because they were cise chemical composition of our sample utes, so that oscillations from the stars used to form the giant planet. A trend Solar twins. are averaged to below 1 m s–1. Detailed with condensation temperature has been suggested (Tucci Maia et al., 2014), and Figure 3. Abundance it could be the signature of a rocky core differences between the in the giant planet (see Figure 3). Abun- 0.06 16 Cyg A – 16 Cyg B binary Solar twins 16 Cyg A and 16 Cyg B. dance differences have been also found Sc in the binary pair of stellar twins XO­2, The star without planets Mg V is enhanced in both vol- where both stars in the pair host planets, 0.05 atiles and refractories, providing important clues about planet i.e., the star with a Ti formation (Ramírez et al., 2015; Teske et FeNi detected giant planet ) Si Ca (16 Cyg B) is deficient in al., 2015; Biazzo et al., 2015). ex 0.04 Mn those elements, proba- (d Al O Cu Cr ) Co bly because they were used to form the giant –B Searching for planets around Solar twins (A Na planet 16 Cyg Bb. The ] 0.03 trend with condensation /H C temperature (Tcond) could The synergy between accurate (0.01 dex) [X S be a signature of the Δ chemical composition determination rocky core of the giant that can be achieved in Solar twins and 0.02 planet. precise (1 m s–1) radial velocities to char- Zn acterise planets around these stars, allows us to study the planet–star con- 0.01 nection at an unprecedented level of detail. Furthermore, the use of the Sun as a standard is a key part of our project, 0.00 0 5001000 1500 as the Sun is the only known star that T (K) hosts a planet where life thrives.
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