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Öğe Estimation of galactic model parameters and metalicity distribution in intermediate latitudes with SDSS(Elsevier Science Bv, 2010) Yaz, E.; Karaali, S.We estimated the galactic model parameters for a set of 20 intermediate-latitude fields with galactic longitudes 0 degrees <= 1 <= 100 degrees and 160 degrees <= 1 <= 240 degrees, included in the currently available Data Release 6 (DR6) of the Sloan Digital Sky Survey (SDSS), to explore their possible variation with galactic longitude. The local space densities of the thick disc and halo are almost the same for all fields, <(n(2)/n(1))> = 6.52% and <(n(3)/n(1))> = 0.35%, respectively, a result different than the one cited for high latitude fields. The thin disc's scaleheight is 325 pc in the galactic centre and it changes to 369 pc in the third quadrant, which confirms the existence of disc flare, whereas the thick disc scaleheight is as large as 952 pc at galactic longitude I = 20 degrees and 10% lower at I = 160 degrees, which confirms the existence of the disc long bar in the direction I similar or equal to 27 degrees. Finally, the variation of the axis ratio of the halo with galactic longitude is almost flat, <(c/a)> = 0.56, except a slight minimum and a small maximum in the second and third quadrants, respectively, indicating an effect of the long bar which seems plausible for a shallow halo. We estimated the metallicities of unevolved G-type stars and discussed the metallicity gradient for different vertical distances. The metallicity gradient is d[M/H]/dz similar to -0.30 dex kpc(-1) for short distances, confirming the formation of this region of the Galaxy by dissipational collapse. However, its change is steeper in the transition regions of different galactic components. The metallicity gradient: is almost zero for inner halo (5 < z < 10 kpc), indicating a formation of merger or accretion of numerous fragments such as dwarf galaxies. (C) 2009 Elsevier B.V. All rights reserved.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 Galactic longitude dependent galactic model parameters(Elsevier Science Bv, 2006) Bilir, S.; Karaali, S.; Ak, S.; Yaz, E.; Hamzaoglu, E.We present the Galactic model parameters for thin disc estimated by Sloan Digital Sky Survey (SDSS) data of 14 940 stars with apparent magnitudes 16 < g(o) <= 21 in six intermediate latitude fields in the first Galactic quadrant. Star/galaxy separation was performed by using the SDSS photometric pipeline and the isodensity contours in the (g - r)(o) - (r - i)(o) two colour diagram. The separation of thin disc stars is carried out by the bimodal distribution of stars in the (g - r)o histogram, and the absolute magnitudes were evaluated by a procedure presented in the literature (Bilir, S., Karaali, S., Tuncel, S. 2005. AN 326, 321). Exponential density law fits better to the derived density functions for the absolute magnitude intervals 8 < M(g) <= 9 and 11 < M(g) <= 12, whereas sech/sech(2) laws are more appropriate for absolute magnitude intervals 9 < M(g) <= 10 and 10 < M(g) <= 11. We showed that the scaleheight and scatelength are Galactic longitude dependent. The average values and ranges of the scaleheight and the scalelength are < H > = 220 pc (196 <= H <= 234 pc) and < H > = 1900 pc (1561 <= h <= 2280 pc) respectively. This result would be useful to explain different numerical values claimed for those parameters obtained by different authors for the fields in different directions of the Galaxy. (c) 2006 Elsevier B.V. All rights reserved.Öğe An Improved Metallicity Calibration with UBV Photometry(Csiro Publishing, 2011) Karaali, S.; Bilir, S.; Ak, S.; Yaz, E.; Coskunoglu, B.We used the data of 701 stars covering the colour index interval 0.32 < B - V <= 1.16, with metallicities -1.76 <= [Fe/H] <= +0.40 dex. The data were taken from the PASTEL catalogue and estimated metallicity-dependent guillotine factors, which provide a more accurate metallicity calibration. We reduced the metallicities of 11 different authors to the metallicities of Valenti & Fischer (2005), and thus obtained a homogeneous set of data which increased the accuracy of the calibration, i.e. [Fe/H] = -4.316d delta(2)(0.6) - 3.557 delta(0.6) + 0.105. Comparison of the metallicity residuals for two sets of data based on the metallicity-dependent guillotine factors with the ones obtained via metal-free guillotine factors shows that metallicities estimated by means of the new guillotine factors are more accurate than the other ones. This advantage can be used in the metallicity gradient investigation of the Galactic components, i.e. thin disc, thick disc, and halo.Öğe Local stellar kinematics from RAVE data - I. Local standard of rest(Oxford Univ Press, 2011) Coskunoglu, B.; Ak, S.; Bilir, S.; Karaali, S.; Yaz, E.; Gilmore, G.; Seabroke, G. M.We analyse a sample of 82 850 stars from the RAdial Velocity Experiment (RAVE) survey, with well-determined velocities and stellar parameters, to isolate a sample of 18 026 high-probability thin-disc dwarfs within 600 pc of the Sun. We derive space motions for these stars, and deduce the solar space velocity with respect to the local standard of rest. The peculiar solar motion we derive is in excellent agreement in radial U-circle dot and vertical W-circle dot peculiar motions with other recent determinations. Our derived tangential peculiar velocity, V-circle dot, agrees with very recent determinations, which favour values near 13 km s-1, in disagreement with earlier studies. The derived values are not significantly dependent on the comparison sample chosen, or on the method of analysis. The local Galaxy seems very well dynamically relaxed, in a near symmetric potential.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 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 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, WeinheimÖğe Transformations between the WISE, 2MASS, SDSS and BVRI photometric systems - I. Transformation equations for dwarfs(Wiley-Blackwell, 2011) Bilir, S.; Karaali, S.; Ak, S.; Dagtekin, N. D.; Onal, O.; Yaz, E.; Coskunoglu, B.We present colour transformations for the conversion of the W1 and W2 magnitudes of the Wide-field Infrared Survey Explorer (WISE) photometric system to the Johnson-Cousins BVRI, Sloan Digital Sky Survey gri and Two-Micron All-Sky Survey JHK(s) photometric systems, for dwarfs. The W3 and W4 magnitudes were not considered due to their insufficient signal-to-noise ratio (S/N). The coordinates of 825 dwarfs along with their BVRI, gri and JHK(s) data taken from Bilir et al. were matched with the coordinates of stars in the preliminary data release of the WISE, and a homogeneous dwarf sample with high S/N had been obtained using the following constraints: (1) the data were dereddened; (2) giants were identified and excluded from the sample; (3) sample stars were selected according to data quality; (4) transformations were derived for subsamples of different metallicity ranges; and (5) transformations were two-colour-dependent. These colour transformations, coupled with known absolute magnitudes at shorter wavelengths, can be used in space-density evaluation for the Galactic (thin and thick) discs, at distances larger than the ones evaluated with JHK(s) photometry.Öğe Volume-limited dependent Galactic model parameters(Csiro Publishing, 2007) Karaali, S.; Bilir, S.; Yaz, E.; Hamzaoglu, E.; Buser, R.We estimate 34 sets of Galactic model parameters for three intermediate-latitude fields with Galactic longitudes l = 60 degrees, l = 90 degrees, and l= 180 degrees, and we discussed their dependence on the volume. Also, we confirm the variation of these parameters with absolute magnitude and Galactic longitude. The star samples in two fields are restricted with bright and unit absolute-magnitude intervals, 4 < M-g <= 5, and 5 < M-g <= 6, whereas for the third field (l= 60 degrees) a larger absolute-magnitude interval is adopted, 4 < M-g <= 10. The limiting apparent magnitudes of the star samples are g(0) = 15 and g(0) = 22.5 mag, which provide space densities within distances in the line of sight of similar to 0.9 and 25 kpc. The Galactic model parameters for the thin disc are not volume dependent. However, the ones for the thick disc and halo do show spectacular trends in their variation with volume, except for the scalelength of the thick disc. The local space density of the thick disc increases, whereas the scaleheight of the same Galactic component decreases monotonically. However, both model parameters approach asymptotic values at large distances. The relative local space density of the halo estimated by fitting the density laws to the space densities evaluated for all volumes is constant, except for the small ones. However it is absolute-magnitude and Galactic-longitude dependent. The axial ratio of the halo increases abruptly for the volumes where a thick disc is dominant, whereas it approaches an asymptotic value gradually for larger volumes, indicating a continuous transition from a disc-like structure to a spherical one at the outermost region of the Galaxy. The variation of the Galactic model parameters with absolute magnitude can be explained by their dependence on the stellar luminosity, whereas the variation with volume and Galactic longitude at short distances is a bias in analysis.
