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Öğe Estimation of Galactic model parameters in high latitudes with SDSS(Csiro Publishing, 2008) Bilir, S.; Cabrera-Lavers, A.; Karaali, S.; Ak, S.; Yaz, E.; Lopez-Corredoira, M.We estimated the Galactic model parameters for a set of 36 high-latitude fields included in the currently available Data Release 5 (DR 5) of the Sloan Digital Sky Survey (SDSS), to explore their possible variation with the Galactic longitude. The thick disc scaleheight moves from similar to 550 pc at 120< l < 150 degrees to similar to 720 pc at 250 < l < 290 degrees, while the thin disc scaleheight is as large as similar to 195 pc in the anticenter direction and similar to 15% lower at vertical bar l vertical bar < 30 degrees. Finally, the axis ratio (c/a) of the halo changes from a mean value of similar to 0.55 in the two first quadrants of the Galaxy to similar to 0.70 at 190 < l < 300 degrees. For the halo, the reason for the dependence of the model parameters on the Galactic longitude arises from the well known asymmetric structure of this component. However, the variation of the model parameters of the thin and thick discs with Galactic longitude originates from the gravitational effect of the Galactic long bar. Moreover, the excess of stars in quadrant I (quadrant III) over quadrant IV (quadrant II) is in agreement with this scenario.Öğe Luminosity-colour relations for thin-disc main-sequence stars(Wiley-Blackwell, 2008) Bilir, S.; Karaali, S.; Ak, S.; Yaz, E.; Cabrera-Lavers, A.; Coskunoglu, K. B.In this study we present the absolute magnitude calibrations of thin-disc main-sequence stars in the optical (M(V)), and in the near-infrared (M(J)). Thin-disc stars are identified by means of Padova isochrones, and absolute magnitudes for the sample are evaluated via the newly reduced Hipparcos data. The obtained calibrations cover a large range of spectral types: from A0 to M4 in the optical and from A0 to M0 in the near-infrared. Also, we discuss the effects of binary stars and evolved stars on the absolute magnitude calibrations. The usage of these calibrations can be extended to the estimation of galactic model parameters for the thin disc individually, in order to compare these parameters with the corresponding ones estimated by X(min)(2) statistics (which provides galactic model parameters for thin and thick discs and halo simultaneously) to test any degeneracy between them. The calibrations can also be used in other astrophysical researches where distance plays an important role in that study.Öğe The metallicity distributions in high-latitudes with SDSS(Elsevier Science Bv, 2007) Ak, S.; Bilir, S.; Karaali, S.; Buser, R.; Cabrera-Lavers, A.We present metallicities and their variations with different parameters for 36 high-latitude fields covering Galactic longitudes < I <= 360 degrees. The metallicities for relatively short vertical distances (z < 2.5 kpc) show systematic fluctuations with Galactic longitude, similar to those of the thick-disc scaleheight, which may be interpreted as indicating a common origin, viz., the flare effect : I the disc (Bilir et al., 2007, MNRAS). This suggestion is supported by the metallicity variations which we find as functions of radial stance. The metallicity variation at larger vertical distances (6.5 < z <= 9.5 kpc) is small but monotonic. Three different vertical metallicity graents could be detected: d[M vertical bar H]dz = - 0.22( +/- 0.03), d[M vertical bar H]vertical bar dz = -0.38( +/- 0.06), and d[M vertical bar H]dz = -0.08(+/- 0.07) dex kpc(-1) for the,tervals z < 3, 3 <=, z < 5, and 5 <=, z < 10 kpc, respectively. Since our data cover the whole Galactic longitude interval, the resulting metlicity gradients can be interpreted as properties of the larger-scale Galaxy. The first gradient confirms the dissipational formation of the sc at short z-distances. The steeper gradient corresponds to the transition region between different population components of the Galcy, and finally, the lowest value provides an adequate description of the inner-halo metallicity gradient. 2007 Elsevier B.V. All rights reserved.Öğe SDSS absolute magnitudes for thin-disc stars based on trigonometric parallaxes(Wiley-Blackwell Publishing, Inc, 2009) Bilir, S.; Karaali, S.; Ak, S.; Coskunoglu, K. B.; Yaz, E.; Cabrera-Lavers, A.We present a new luminosity-colour relation based on trigonometric parallaxes for thin-disc main-sequence stars in Sloan Digital Sky Survey (SDSS) photometry. We matched stars from the newly reduced Hipparcos catalogue with the ones taken from Two-Micron All-Sky Survey (2MASS) All-Sky Catalogue of Point Sources, and applied a series of constraints, i.e. relative parallax errors (sigma pi/pi <= 0.05), metallicity (-0.30 <= [M/H] = 0.20 dex), age (0 <= t <= 10 Gyr) and surface gravity (logg > 4), and obtained a sample of thin-disc main-sequence stars. Then, we used our previous transformation equations (Bilir et al. 2008a) between SDSS and 2MASS photometries and calibrated the M(g) absolute magnitudes to the (g - r)(0) and (r - i)(0) colours. The transformation formulae between 2MASS and SDSS photometries along with the absolute magnitude calibration provide space densities for bright stars which saturate the SDSS magnitudes.Öğe Transformations between 2MASS, SDSS and BVRI photometric systems(Blackwell Publishing, 2008) Bilir, S.; Ak, S.; Karaali, S.; Cabrera-Lavers, A.; Chonis, T. S.; Gaskell, C. M.We present colour transformations for the conversion of the Two Micron All Sky Survey (2MASS) photometric system to the Johnson-Cousins UBVRI system and further into the Sloan Digital Sky Survey (SDSS) ugriz system. We have taken SDSS gri magnitudes of stars measured with the 2.5-m telescope from SDSS Data Release 5 (DR5), and BVRI and JHK(s) magnitudes from Stetson's catalogue and Cutri et al., respectively. We matched thousands of stars in the three photometric systems by their coordinates and obtained a homogeneous sample of 825 stars by the following constraints, which are not used in previous transformations: (1) the data are dereddened, (2) giants are omitted and (3) the sample stars selected are of the highest quality. We give metallicity, population type and transformations dependent on two colours. The transformations provide absolute magnitude and distance determinations which can be used in space density evaluations at short distances where some or all of the SDSS ugriz magnitudes are saturated. The combination of these densities with those evaluated at larger distances using SDSS ugriz photometry will supply accurate Galactic model parameters, particularly the local space densities for each population.Öğe Transformations between the 2MASS, SDSS, and BVI photometric systems for late-type giants(Wiley-V C H Verlag Gmbh, 2010) Yaz, E.; Bilir, S.; Karaali, S.; Ak, S.; Coskunoglu, B.; Cabrera-Lavers, A.We present colour transformations from Two Micron All Sky Survey (2MASS) photometric system to Johnson-Cousins system and to Sloan Digital Sky Survey (SDSS) system for late-type giants and vice versa. The giant star sample was formed using surface gravity constraints (2 < log g <= 3) to Cayrel de Strobel et al.'s (2001) spectroscopic catalogue. 2MASS, SDSS and Johnson-Cousins photometric data was taken from Cutri et al. (2003), Ofek (2008), and van Leeuwen (2007), respectively. The final sample was refined applying the following steps: (1) the data were dereddened, (2) the sample stars selected are of the highest photometric quality. We give two-colour dependent transformations as a function of metallicity as well as independent of metallicity. The transformations provide absolute magnitudes and distance determinations which can be used in space density evaluations at relatively short distances where some or all of the SDSS magnitudes of late-type giants are saturated. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
