Yazar "Bilir, S." seçeneğine göre listele
Listeleniyor 1 - 14 / 14
Sayfa Başına Sonuç
Sıralama seçenekleri
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 Estimation of galactic model parameters with the Sloan Digital Sky Survey and the metallicity distribution in two fields in the anti-centre direction of the Galaxy(Wiley-V C H Verlag Gmbh, 2007) Ak, S.; Bilir, S.; Karaali, S.; Buser, R.We estimated the galactic model parameters from the Sloan Digital Sky Survey (SDSS) data reduced for two fields in the anti-centre direction of the Galaxy, l = 180 degrees, and symmetric relative to the galactic plane, b = +45 degrees (north field) and b = -45 degrees (south field). The large size of each field, 60 deg(2), and the faint limiting apparent magnitude, go 22, give us the chance to determine reliable parameters for three components, thin and thick discs and halo, in the north and south hemispheres of the Galaxy, except the scale lengths for two discs which are adopted from Juric et al. (2005). Metallicities were evaluated by a recent calibration for SDSS, and absolute magnitudes of stars with 4 < M(g) < 8 were derived as a function of (g - r)(0) colour and metallicity. A chi(2) method was employed to fit the analytical density laws to the observational-based space densities with the addition constraint of producing local densities consistent with those derived from Hipparcos. Conspicuous differences could not be detected between the corresponding galactic model parameters for the thin disc of north and south fields, and our results are consistent. with the ones in the literature. The same case is valid for the halo, especially the axis ratios for two fields are exactly equal, K = 0.45, and close to the one of Juric et al. (2005). However, we revealed differences between the scale heights and local space densities for the thick disc of the north and south fields. The metallicity distribution for unevolved G type stars with 5 < M(g) < 6 shows three substructures relative to the distance from the galactic plane: for z* < 3 kpc, the metallicity gradient for two fields is d[M/H]/dz similar to -0.20(+/- 0.02) dex kpc(-1), consistent with the formation scenario of the thin disc. For 5 < z* < 10 kpc, d[M/H]/dz similar to -0.03(+/- 0.001) dex. kpc(-1) for two fields, confirming that the metallicity gradient for the halo component is close to zero. However, the tendency of the metallicity for stars with 3 < z* < 5 kpc, corresponding to the transition region from thick disc to halo is different. For the north field d[M/H]/dz = -0.36(+/- 0.12) dex kpc(-1), whereas it is half of this value for the south field, d[M/H]/dz = -0.18(+/- 0.01) dex kpc(-1). The origin of this conflict is probably due to the different structure of the thick disc in opposite latitudes of the Galaxy. When we combine these substructures, however, we find a smooth metallicity gradient for two fields, -0.30(+/- 0.04) dex kpc(-1). (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim.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 Identification of field dwarfs and giants in the second Radial Velocity Experiment Data Release(Wiley-Blackwell, 2011) Bilir, S.; Karaali, S.; Ak, S.; Onal, O.; Coskunoglu, B.; Seabroke, G. M.The second Radial Velocity Experiment (RAVE) Data Release (DR2) derives log g values. However, we present a simpler and cleaner method of identifying dwarfs and giants by using only magnitudes, which does not require spectroscopic analysis. We confirm the method presented in an earlier paper, with which we estimate the number of dwarfs and giants using their positions in the JV two-magnitude diagram, by applying it to RAVE DR2. It is effective in estimating the number of dwarfs and giants at JH > 0.4 compared to RAVEs log g values. For JH= 0.4, where dwarfs and subgiants show a continuous transition in the J magnitude histogram, we used the Besancon Galaxy model predictions to statistically isolate giants. The percentages of giants for red stars and for the whole sample are 85 and 34 per cent, respectively. If we add the subgiants, the percentage of evolved stars for the whole sample increases to 59 per cent. For the first time in the literature, we have analysed the effect of CHISQ on RAVEs log g values (CHISQ is the penalized 2 from RAVEs technique of finding an optimal match between the observed spectrum and synthetic spectra to derive stellar parameters). Neither the CHISQ values nor the signal-to-noise ratio bias the RAVE log g values. Therefore, the method of identifying dwarfs and giants using the two-magnitude diagram has been verified against an unbiased data set.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 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, 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 A universal vertical stellar density distribution law for the Galaxy(Springer, 2009) Karaali, S.; Hamzaoglu, E.; Bilir, S.We reduced the observational logarithmic space densities in the vertical direction up to 8 kpc from the galactic plane, for stars with absolute magnitudes (5,6], (6,7] and [5,10] in the fields #0952+5245 and SA114, to a single exponential density law. One of three parameters in the quadratic expression of the density law corresponds to the local space density for stars with absolute magnitudes in question. There is no need of any definition for scaleheights or population types. We confirm with the arguments of non-discrete thin and thick discs for our Galaxy and propose a single structure up to several kiloparsecs from the galactic plane. The logarithmic space densities evaluated by this law for the ELAIS field fit to the observational ones. Whereas, there are considerable offsets for the logarithmic space densities produced by two sets of classical galactic model parameters from the observational ones, for the same field.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.
