Published in ApJL. Preprint typeset using LATEX style emulateapj v. 04/17/13 HIGH PRECISION ABUNDANCES OF THE OLD SOLAR TWIN HIP 102152: INSIGHTS ON LI DEPLETION FROM THE OLDEST SUN* TalaWanda R. Monroe1, Jorge Melendez´ 1, Ivan´ Ram´ırez2, David Yong3, Maria Bergemann4, Martin Asplund3, Jacob Bean5, Megan Bedell5, Marcelo Tucci Maia1, Karin Lind6, Alan Alves-Brito3, Luca Casagrande3, Matthieu Castro7, Jose{Dias´ do Nascimento7, Michael Bazot8, and Fabr´ıcio C. Freitas1 Published in ApJL. ABSTRACT We present the first detailed chemical abundance analysis of the old 8.2 Gyr solar twin, HIP 102152. We derive differential abundances of 21 elements relative to the Sun with precisions as high as 0.004 dex (.1%), using ultra high-resolution (R = 110,000), high S/N UVES spectra obtained on the 8.2-m Very Large Telescope. Our determined metallicity of HIP 102152 is [Fe/H] = -0.013 ± 0.004. The atmospheric parameters of the star were determined to be 54 K cooler than the Sun, 0.09 dex lower in surface gravity, and a microturbulence identical to our derived solar value. Elemental abundance ratios examined vs. dust condensation temperature reveal a solar abundance pattern for this star, in contrast to most solar twins. The abundance pattern of HIP 102152 appears to be the most similar to solar of any known solar twin. Abundances of the younger, 2.9 Gyr solar twin, 18 Sco, were also determined from UVES spectra to serve as a comparison for HIP 102152. The solar chemical pattern of HIP 102152 makes it a potential candidate to host terrestrial planets, which is reinforced by the lack of giant planets in its terrestrial planet region. The following non-local thermodynamic equilibrium Li abundances were obtained for HIP 102152, 18 Sco, and the Sun: log (Li) = 0.48 ± 0.07, 1.62 ± 0.02, and 1.07 ± 0.02, respectively. The Li abundance of HIP 102152 is the lowest reported to date for a solar twin, and allows us to consider an emerging, tightly constrained Li-age trend for solar twin stars. Keywords: stars: abundances | Sun: abundances | planetary systems 1. INTRODUCTION in the surrounding disk, and leaving behind material low When compared to most stars with similar stellar pa- in refractory elements to later accrete onto the Sun. rameters, the Sun displays an anomalous chemical abun- In this letter, we present elemental abundance ratios dance pattern. To date, high-resolution spectroscopic as a function of condensation temperature for the ∼8.2 chemical abundance analyses offer that only ∼15% of so- Gyr old solar twin HIP 102152 (HD 197027). This 1 M lar type stars have abundance patterns similar to the Sun star (0.97 M ; current study) that is about to leave the (Mel´endezet al. 2009; Ram´ırezet al. 2009). The dis- main sequence is of interest not only because it has an parity between the Sun and other solar twins has been abundance pattern similar to the Sun, thus making it demonstrated in a number of studies (e.g., Mel´endezet an excellent older solar analog for comparative studies, al. 2009; Ram´ırez et al. 2010, 2011; Gonzalez et al. but also because its solar abundance pattern may suggest 2010a; Schuler et al. 2011), and is possibly due to the that this star is a candidate host for terrestrial planets. formation of terrestrial planets. Planetesimal and terres- We also include abundances of the younger 2.9 Gyr so- trial planet formation may have imprinted a signature in lar twin 18 Sco and examine systematic differences be- the solar composition by locking-up refractory elements tween refractory and volatile elements in this representa- tive solar twin and the older HIP 102152. Additionally, [email protected] we report Li abundances for the two stars and examine 1 Departamento de Astronomia do IAG/USP, Universidade de the tight Li-age depletion trend of solar twins studied at S~aoPaulo, Rua do Mat~ao1226, Cidade Universit´aria,05508-900 high-resolution, bringing further insights to the debate S~aoPaulo, SP, Brasil of whether low lithium is a signature of planets, or just 2 McDonald Observatory, The University of Texas at Austin, Austin, TX 78712, USA an effect of secular stellar depletion (e.g., Takeda et al. 3 Research School of Astronomy and Astrophysics, The Aus- 2007; Takeda & Tajitsu 2009; Ram´ırezet al. 2012; Bau- tralian National University, Cotter Road, Weston, ACT 2611, mann et al. 2010; Ghezzi et al. 2010; Gonzalez et al. Australia 4 Max Planck Institute for Astrophysics, Postfach 1317, 85741 2010b; Israelian et al. 2009). Garching, Germany 5 Department of Astronomy and Astrophysics, University of 2. SPECTROSCOPIC OBSERVATIONS AND REDUCTIONS Chicago, 5640 S. Ellis Ave, Chicago, IL 60637, USA 6 Institute of Astronomy, University of Cambridge, Madingley Spectra of HIP 102152 and 18 Sco were obtained Road, Cambridge, CB3 0HA, UK with the Ultra-violet and Visible Echelle Spectrograph 7 Departamento de F´ısicaTe´oricae Experimental, Universi- (UVES; Dekker et al. 2000) on the 8.2-m Very Large dade Federal do Rio Grande do Norte, 59072-970 Natal, RN, Telescope (VLT) at the ESO Paranal Observatory. Ob- Brazil 8 Centro de Astrof´ısicada Universidade do Porto, Rua das Es- servations were carried out on 30 August 2009, with air trelas, 4150-762 Porto, Portugal masses of 1.1{1.5 for HIP 102152 and 1.6{1.7 for 18 Sco. * Based on observations obtained at the European Southern Multiple exposures were obtained at three spectrograph Observatory (observing programs 083.D-0871 and 188.C-0265). configurations (six exposures per configuration) for each 2 Monroe et al. star to achieve redundancy in the observations around length splittings of the measured lines. The HFS line lists the Li doublet at λ6708A,˚ with individual exposure times used were adopted from Mel´endezet al. (2012). Addi- of 10{20 min for HIP102152 and 45{90 s for the brighter tionally, abundances of the Li resonance doublet feature 18 Sco. Spectral coverage included 3060{3870A,˚ 4800{ at λ=6708A˚ were determined using spectrum synthesis, 5770A,˚ 5850{8200A,˚ and 8430{10200A,˚ with incomplete and will be discussed below. Differential NLTE correc- coverage in the near-infrared due to gaps between echelle tions were shown to be very small in solar twins relative orders. The spectral resolving powers were R(λ/δλ) = to the Sun (∼0.001 dex) by Mel´endezet al.; therefore, 65,000 at λ3060{3870A,˚ and R = 110,000 for λ4800{ we did not take them into account in this study, except 10200A.˚ The asteroid Juno was also observed with the for Cr, Mn, and Co (e.g., Bergemann & Cescutti 2010). same spectrograph setups, and served as the solar refer- The sensitivity of our differential chemical abundances ence in our differential analysis. to the choice of model atmosphere grid has been shown Standard reductions were carried out in IRAF, in- to be negligible (Mel´endezet al.). cluding bias subtraction, flat fielding, wavelength cal- 3.2. Atmospheric Parameters ibrations, spectral order extractions, and spectra co- Abundances were first determined for the Sun using additions. Continuum normalization was performed on the standard solar atmospheric parameters of Teff =5777 the combined one-dimensional spectra in IDL. Final com- K and log g=4.44. We determined our own value of posite spectra had signal-to-noise (S/N) ratios that var- the solar microturbulent velocity (v ) by using the com- ied across the spectral orders, but had a typical S/N∼500 t −1 mon spectroscopic technique of removing any trend be- pixel . It should be noted that our spectrograph setups tween the abundances of Fe I lines and reduced equivalent allowed us to achieve S/N∼1000 pixel−1 in the contin- width. The resulting microturbulence was vt = 0.86 km uum regions about the Li feature at λ6708A.˚ s−1, although this value was individually tweaked slightly 3. ANALYSIS for the analyses of both HIP 102152 and 18 Sco. An initial EW analysis was performed for the stars us- 3.1. Chemical Abundances ing the above atmospheric parameters adopted for the Chemical abundances of most species were determined Sun. Effective temperatures (Teff ) for the program stars similarly to the methods outlined in Mel´endez et al. were initially set at the solar value and then refined (2012). Equivalent width analyses were performed using by employing excitation equilibrium on the differential the abfind driver in the local thermodynamic equilibrium abundances of the Fe I lines. That is, all trends between (LTE) line analysis code MOOG (2002 version; Sneden the differential abundances of each line with excitation 1973) and Kurucz (ATLAS9) model atmospheres, with- potential (EP) were removed. Stellar microturbulences out convective overshooting (Castelli & Kurucz 2004). were refined by removing trends between the differen- LTE abundances were determined for the following tial abundances and reduced EW. Surface gravities (log Z≤30 elements in our analysis: C, N, O, Na, Mg, Al, Si, g) were incrementally adjusted from solar to arrive at S, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn; and ionization balance between the average differential iron are included in Table 1. The line list was mostly taken abundances derived from Fe I and Fe II lines. from Mel´endezet al. (2012). Equivalent width (EW) Final atmospheric parameters for the stars are pre- measurements were first made with the automated code sented in Table 1. We determined the following values ARES (Sousa et al. 2007). Lines were measured by hand for HIP 102152: Teff = 5723±5 K, log g = 4.35±0.02 −1 −1 in both the star and Sun with the IRAF task splot when dex, and vt = 0.86±0.01 km s (∆vt = 0.0 km s for −1 the EW < 10 mA,˚ or if the species had five or fewer vt; = 0.86 km s ).