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Spectroscopic Study of Solar Twins and Analogues⋆⋆⋆⋆⋆⋆

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Spectroscopic Study of Solar Twins and Analogues⋆⋆⋆⋆⋆⋆ A&A 574, A124 (2015) Astronomy DOI: 10.1051/0004-6361/201425000 & c ESO 2015 Astrophysics Spectroscopic study of solar twins and analogues,, Juliet Datson1, Chris Flynn2,3,4, and Laura Portinari1 1 Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland e-mail: [email protected] 2 Centre for Astrophysics and Supercomputing, Swinburne University of Technology, VIC 3122, Australia 3 Finnish Centre for Astronomy with ESO, University of Turku, 21500 Piikkio, Finland 4 Department of Physics and Astronomy, University of Sydney, NSW 2006, Australia Received 16 September 2014 / Accepted 17 December 2014 ABSTRACT Context. Many large stellar surveys have been and are still being carried out, providing huge amounts of data, for which stellar phys- ical parameters will be derived. Solar twins and analogues provide a means to test the calibration of these stellar catalogues because the Sun is the best-studied star and provides precise fundamental parameters. Solar twins should be centred on the solar values. Aims. This spectroscopic study of solar analogues selected from the Geneva-Copenhagen Survey (GCS) at a resolution of 48 000 pro- vides effective temperatures and metallicities for these stars. We test whether our spectroscopic parameters, as well as the previous photometric calibrations, are properly centred on the Sun. In addition, we search for more solar twins in our sample. Methods. The methods used in this work are based on literature methods for solar twin searches and on methods we developed in previous work to distinguish the metallicity-temperature degeneracies in the differential comparison of spectra of solar analogues versus a reference solar reflection spectrum. Results. We derive spectroscopic parameters for 148 solar analogues (about 70 are new entries to the literature) and verify with a-posteriori differential tests that our values are well-centred on the solar values. We use our dataset to assess the two alternative calibrations of the GCS parameters; our methods favour the latest revision. We show that the choice of spectral line list or the choice of asteroid or time of observation does not affect the results. We also identify seven solar twins in our sample, three of which are published here for the first time. Conclusions. Our methods provide an independent means to differentially test the calibration of stellar catalogues around the values of a well-known benchmark star, which makes our work interesting for calibration tests of upcoming Galactic surveys. Key words. stars: abundances – stars: fundamental parameters – stars: solar-type 1. Introduction to determine precise and accurate physical parameters for the observed stars. The methods used to provide these vary from Large stellar surveys of the Milky Way are becoming increas- survey to survey because they are based on different wavelength ingly important in astronomy. Whether it is data from past regimes and types of data (photometry or spectroscopy), which missions such as the Two Micron All-Sky Survey (2MASS, ipparcos makes it particularly important to ensure that these catalogues Skrutskie et al. 1995), H (Perryman et al. 1997; are properly calibrated. van Leeuwen 2007), the Sloan Digital Sky Survey (SDSS, York In previous work Datson et al. (2012, 2014,D12and et al. 2000), the Geneva-Copenhagen-Survey (GCS, Nordström D14 hereafter) have shown that by using a sample of photomet- et al. 2004; Holmberg et al. 2007, 2009), or the RAdial Velocity rically chosen solar analogue stars and applying spectroscopic Experiment (RAVE, Steinmetz 2003), or from current and future / methods that are based on solar twin searches, it is possible to surveys such as HERMES GALAH (Freeman 2010), the Gaia- independently test the calibration of a catalogue around the val- ESO Survey (GES, Gilmore et al. 2012)orGaia(Munari 2003)– ues of a benchmark star by a purely differential analysis. This they all have one thing in common: the wish to better understand star needs to have well-known parameters and be included in the the Milky Way and through it, galaxy evolution in general. observations. So far, in D12 and D14 this was achieved by using To address questions about and problems of the chemical the Sun, that is, a solar reflection spectrum of an asteroid like evolution of galaxies, the stellar mass and luminosity functions, Ceres, as a reference solar spectrum. the star formation history of the Milky Way, dynamical evolu- By doubling the sample size of D12, we improve the accu- tion within the disc, radial migration of stars, etc., it is necessary racy of our analysis and test whether the results we obtained in that paper change with higher number of targets, meaning Based on observations made with ESO Telescopes at the La Silla ff Observatory under programme ID 077.D-0525 and 090.D-0133. that we test whether the previously found o sets in the GCS-III Table 1 is also available at the CDS via anonymous ftp to changed. The larger sample size also allows us to search for cdsarc.u-strasbg.fr (130.79.128.5)orvia more solar twins in the data and inspect the reanalysis of the http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/574/A124 Geneva-Copenhagen Survey (C11, Casagrande et al. 2011)by Full Table 5 is only available at the CDS via anonymous ftp to dividing the sample with method of stellar parameter determina- cdsarc.u-strasbg.fr (130.79.128.5)orvia tion (infrared-flux method versus colour calibrations) to test the http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/574/A124 two calibrations separately. Article published by EDP Sciences A124, page 1 of 12 A&A 574, A124 (2015) moog (Sneden 1973) is a very widely used tool to determine Distribution of stellar parameters in the whole sample spectroscopic stellar parameters, therefore we also used it here to 16 25 T Sun = 5777K [Fe/H ]Sun =0.0dex determine the characteristics of the sample stars. One source of 14 20 12 error and discussion is the choice of spectral lines used for the s s 10 nt 15 nt elemental abundance determination. While the chosen lines are u u 8 expected to be weak, unblended, and isolated lines on the lin- Co 10 Co 6 4 5 ear part of the curve of growth, the exact list always varies (e.g. 2 0 0 Meléndez et al. 2012; Porto de Mello et al. 2014). The choice of 5400 5600 5800 6000 −0.2 −0.1 0.0 0.1 0.2 T eff(GCS) [Fe/H] (GCS) lines matters, and a careful selection of lines is key to a precise 35 16 (b-y) = 0. 410 Sun M V , Sun =4.83mag analysis (Sousa et al. 2014; Jofré et al. 2014). For this reason, 14 30 we chose four different line lists here, all selected by the respec- 12 25 s 10 s 20 nt nt tive authors to fulfil the above-mentioned requirements, to test u 8 u 15 whether our conclusions on the solar zero point of a given cat- Co 6 Co 10 alogue calibration and solar twin determination are reliable or 4 2 5 depend on the line list. 0 0 0.36 0.38 0.40 0.42 0.44 0.46 4.2 4.4 4.6 4.8 5.0 5.2 5.4 Another source of systematic error could be the specific (b-y) (GCS) M V (GCS) choice of the reflected solar spectrum. Typically, one or sev- eral of the brightest asteroids and/or Jovian moons are used (e.g. Fig. 1. Distribution of stellar parameters in the combined FEplus sam- top two panels Ramírez et al. 2014a; Nissen et al. 2014; Meléndez et al. 2014). ple. The show values from the GCS in the shaded his- tograms and the values from our own spectroscopic determinations as We took the opportunity of taking a total of 15 spectra of the as- the dashed histograms. The bottom two panels show values from the teroids Ceres (8) and Vesta (7) at several different times during GCS for (b − y)andHipparcos for MV. Vertical black lines show our our three-night run to test for significant differences in using one adopted solar values of all four parameters. We quote a solar (b − y) asteroid or the other. colour that we found in previous work (D12 and D14) and not the lower The structure of the paper is as follows: in Sect. 2 we present value by Holmberg et al. (2006). the data. Section 3 provides new spectroscopic parameters for our sample stars and compares them with literature values. In seemstobeanoffset of −0.10 dex in metallicity for solar-type Sect. 4 we search for more solar twins in our new sample, and in stars in the GCS, therefore we expanded the metallicity criterium Sect. 5 we reassess the calibration of GCS-III versus its reanaly- to −0.25 < [Fe/H] < 0.15, allowing more metal-poor stars into sis (C11), including a possible dependency of our analysis on the the sample to better bracket the solar values. Taking into account chosen line list. Section 6 is a test on asteroid spectra stability, the observability of the targets during the run and choosing the and we conclude in Sect. 7. brightest targets, this resulted in 88 stars, 70 of which were ob- served. There is no overlap with the FE12 sample. 2. Observations and data The data for this sample were taken between 21−24 December 2012 with the FEROS instrument on the MPG/ESO Our data consist of two samples of spectra, taken with the Fiber- 2.2 m telescope on La Silla.
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