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Metallicity and Absolute Magnitude Calibrations for FG Type Main Metallicity and absolute magnitude calibrations for F-G type main-sequence stars in the Gaia era M. C¸elebi 1 • S. Bilir 2 • S. Ak 2 • T. Ak2 • Z. F. Bostancı2 • T. Yontan 1 Abstract In this study, photometric metallicity and were preferred and a multi-variable second order equa- absolute magnitude calibrations were derived using F- tion was used. Calibrations are valid for main-sequence G spectral type main-sequence stars in the Solar neigh- stars in the metallicity and absolute magnitude ranges bourhood with precise spectroscopic, photometric and −2 < [Fe/H] < 0.5 dex and 2.5 < MV < 6 mag, re- Gaia astrometric data for UBV photometry. The sam- spectively. The mean value and standard deviation of ple consists of 504 main-sequence stars covering the the differences between original and estimated values temperature, surface gravity and colour index inter- for the metal abundance and absolute magnitude are vals 5300 < Teff < 7300 K, log g > 4 (cgs) and h∆[Fe/H]i =0.00 ± 0.11 dex and h∆MV i =0.00 ± 0.22 0.3 < (B − V )0 < 0.8 mag, respectively. Stars with mag, respectively. In this work, it has been shown that relative trigonometric parallax errors σπ/π ≤ 0.01 were more precise iron abundance and absolute magnitude preferred from Gaia DR2 data for the estimation of values were obtained with the new calibrations, com- their MV absolute magnitudes. In order to obtain cali- pared to previous calibrations in the literature. brations, (U − B)0 and (B − V )0 colour indices of stars Keywords stars: abundances, stars: metallicity cali- M. C¸elebi bration, stars: distance 1Istanbul University, Institute of Graduate Studies in Science, Programme of Astronomy and Space Sciences, 34116, Beyazıt, Istanbul, Turkey 1 Introduction Corresponding Author: [email protected] S. Bilir Stars with different luminosity, if they have precise pho- 2Istanbul University, Faculty of Science, Department of Astron- tometric, spectroscopic and astrometric data, play an omy and Space Sciences, 34119 University, Istanbul, Turkey important role to understand the structure, formation and evolution of the Milky Way. Specifically, F and arXiv:1910.01111v1 [astro-ph.GA] 2 Oct 2019 S. Ak G type main-sequence stars are used in Galactic ar- 2Istanbul University, Faculty of Science, Department of Astron- chaeology surveys as they are long-lived objects and omy and Space Sciences, 34119 University, Istanbul, Turkey they contain the chemical composition of the molecule cloud in which they formed. These stars are often T. Ak considered in the study of metallicity gradients and 2 Istanbul University, Faculty of Science, Department of Astron- age-metallicity relations in the Galaxy disc, as well omy and Space Sciences, 34119 University, Istanbul, Turkey as in testing the scenarios related to the formation Z. F. Bostancı of the Galaxy (Sales et al. 2009; Robin et al. 2014; 2Istanbul University, Faculty of Science, Department of Astron- Helmi et al. 2018). omy and Space Sciences, 34119 University, Istanbul, Turkey Today, data with high resolution and high signal- to-noise ratio are preferred to investigate the chemical T. Yontan composition of stars. Spectroscopic and astrometric 1Istanbul University, Institute of Graduate Studies in Science, data from large spectroscopic surveys such as RAVE Programme of Astronomy and Space Sciences, 34116, Beyazıt, (Steinmetz et al. 2006), APOGEE (Allende Prieto et al. Istanbul, Turkey 2008), SEGUE (Yanny et al. 2009), GES (Gilmore et al. 2 2012), LAMOST (Zhao et al. 2012), GALAH (de Silva et al.[Fe/H] < +0.40 dex, respectively, in the UBV photo- 2015), BRAVA (Kunder et al. 2012) and Gaia (Gaia Collaborationmetric system.et al. Furthermore, there are several stud- 2018) are more valuable ones. Thanks to such spectro- ies on photometric metallicity calibration developed for scopic surveys, radial velocities of stars as well as atmo- different photometric systems (Walraven & Walraven spheric model parameters are calculated. The metallic- 1960; Str¨omgren 1966; Cameron 1985; Laird et al. 1988; ity, which is one of these parameters, has an important Buser & Fenkart 1990; Trefzger et al. 1995). The most role in the investigation of the chemical evolution of our recent photometric metallicity calibration in the UBV Galaxy. system is presented by Tun¸cel G¨u¸ctekin et al. (2016). The metallicity of stars can be determined by spec- In their last study, Tun¸cel G¨u¸ctekin et al. (2017) ob- tral analysis as well as by using photometric methods. tained a photometric metallicity calibration for the Metallicity obtained from spectral methods gives more SDSS photometric system using the transformation for- accurate results for the stars in the Solar neighbour- mulae of Chonis & Gaskell (2008). hood, although sensitivity in metallicity decreases as In the investigation of chemical evolution of the SNR becomes lower towards the fainter stars at large Milky Way, distances of stars as well as their iron abun- distances. On the other hand, fainter objects may have dance must be known. Distances of the stars in the accurate photometric data since they can be observed vicinity of the Sun is calculated by trigonometric par- at higher SNRs, i.e. Sloan Digital Sky Survey (SDSS, allax method, while photometric parallax is preferred York et al. 2000). Thus, the atmospheric model pa- for the more distant stars. Using the ground-based rameters calculated from spectra of nearby stars can trigonometric data of nearby stars, Laird et al. (1988) be combined with accurate photometric data to deter- calibrated differences in absolute magnitudes (∆MV ) of mine the metallicity of fainter stars. the field stars and Hyades stars with the same colour in- The photometric metallicity estimation method has dex, according to their UV excesses (δ0.6) and (B −V )0 been used by many researchers since 1950s. Roman colour index of the stars. Using similar procedure and (1955) calculated the ultra-violet (UV) excess of about Hipparcos data (ESA 1997), Karaali et al. (2003b) and 600 F and G spectral type stars with weak metal- Karata¸s& Schuster (2006) developed absolute mag- lic lines by marking them in the U − B × B − V nitude calibrations in the UBV photometric system. two-colour diagram. The author found that UV ex- Karaali et al. (2005), who established the metallicity cesses of the stars change in the range of 0 to 0.2 mag and absolute magnitude calibrations in UBV photo- and the stars with the largest UV excesses have the metric system, have moved their calibrations to the largest space velocities. In the past, many researchers SDSS system using new photometric transformation have explained the UV excess with the “blanketing equations. Bilir et al. (2005) and Juric et al. (2008) model” (Schwarzschild et al. 1955; Sandage & Eggen adapted the photometric parallax method to the SDSS 1959; Wallerstein 1962), in which the stars with the system and produced calibrations to estimate the dis- same metal abundance but different B −V colours con- tances of the main-sequence stars. Bilir et al. (2008, tain different UV excesses in the U − B × B − V two- 2009) calibrated trigonometric parallaxes of the main- colour diagram. Sandage (1969) introduced a guillo- sequence stars in the re-reduced Hipparcos catalogue tine factor to normalize UV excesses to the one for (van Leeuwen 2007) to UBV, SDSS and 2MASS, and (B − V )0 = 0.6 mag and then calculated the normal- obtained absolute magnitude relations for the main- ized values for different B − V colours. Carney (1979) sequence stars in the range of A and M types, which explained the normalized UV excesses of the main- are located in the Galactic disc. Tun¸cel G¨u¸ctekin et al. sequence stars within 100 pc according to the iron abun- (2016, 2017) calibrated absolute magnitudes of 168 F-G dance by a quadratic equation. Karaali et al. (2003a) dwarfs and the re-reduced Hipparcos parallaxes to UBV considered 77 main-sequence stars covering colour and and SDSS photometric systems. iron abundance intervals 0.37 < (B − V ) < 1.07 0 Thanks to Gaia satellite data, it is aimed to cre- mag and −2.70 < [Fe/H] < +0.26 dex, respectively, ate a three-dimensional map of more than one billion and calculated their UV excesses to the iron abun- objects in our Galaxy and beyond, and to investigate dance by a cubic equation. Karata¸s& Schuster (2006) the origin and subsequent evolution of the Milky Way. described UV excesses of the F and G spectral type Gaia DR2 release includes astrometric data for 1.7 bil- main-sequence stars in the Solar neighbourhood accord- lion sources observed in 22 months from the beginning ing to the iron abundance by a quadratic equation. of the Gaia mission (Gaia Collaboration et al. 2018). Karaali et al. (2011) obtained a more accurate photo- It covers a large magnitude range with significant un- metric metallicity calibration using 701 main-sequence certainties in the trigonometric parallaxes. For exam- stars covering colour index and iron abundance in- ple, astrometric uncertainties of the stars with G = 15, tervals 0.32 < (B − V )0 < 1.16 mag and −1.76 < 3 17, 20 and 21 mag reach 0.04, 0.1, 0.7 and 2 mas in 100 parallax, respectively. This shows that uncertainties 80 increase towards fainter stars. It is important to exam- 60 ine stars in different populations for understanding the 40 Cumulative (%) structure and evolution of our Galaxy. Therefore, we 20 0 need accurate photometric parallax relations obtained 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 with calibration of sensitive photometric data to the Gaia astrometric data. In this study, we obtain photo- 103 metric metallicity and absolute magnitude calibrations 102 using F and G spectral type main-sequence stars in the N Solar neighbourhood.
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