arXiv subjects

S. Bilir

Publications and source records attributed to S. Bilir.

At least 109 records · Page 6Linked to original sources

arXiv2008

Luminosity-Colours relations for thin disc main-sequence stars

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 of effects 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 $χ{^2}_{min}$ 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.

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arXiv2008

New absolute magnitude calibrations for W Ursa Majoris type binaries

Parallaxes of W UMa stars in the Hipparcos catalogue have been analyzed. 31 W UMa stars, which have the most accurate parallaxes ($σ_π/π<0.15$) which are neither associated with a photometric tertiary nor with evidence of a visual companion, were selected for re-calibrating the Period--Luminosity--Color (PLC) relation of W UMa stars. Using the Lutz--Kelker (LK) bias corrected (most probable) parallaxes, periods ($0.26< P(day)< 0.87$), and colors (0.04<$(B-V)_{0}$<1.28) of the 31 selected W UMa, the PLC relation have been revised and re-calibrated. The difference between the old (revised but not bias corrected) and the new (LK bias corrected) relations are almost negligible in predicting the distances of W UMa stars up to about 100 parsecs. But, it increases and may become intolerable as distances of stars increase. Additionally, using $(J-H)_{0}$ and $(H-K_{s})_{0}$ colors from 2MASS (Two Micron All Sky Survey) data, a PLC relation working with infrared data was derived. It can be used with infrared colors in the range $-0.01<(J-H)_{0}<0.58$, and $-0.10<(H-K_{s})_{0}<0.18$. Despite {\em 2MASS} data are single epoch observations, which are not guaranteed at maximum brightness of the W UMa stars, the established relation has been found surprisingly consistent and reliable in predicting LK corrected distances of W UMa stars.

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arXiv2008

Absolute Dimensions and Apsidal Motion of the Eccentric Binary V731 Cephei

A detailed analysis of new and existing photometric, spectroscopic and spatial distribution data of the eccentric binary V731 Cep was performed. Spectroscopic orbital elements of the system were obtained by means of cross-correlation technique. According to the solution of radial velocities with UBVRcIc light curves, V731 Cep consists of two main-sequence stars with masses M$_{1}$=2.577 (0.098) M$_{\odot}$, M$_{2}$=2.017 (0.084) M$_{\odot}$, radii R$_{1}$=1.823 (0.030) R$_{\odot}$, R$_{2}$=1.717 (0.025) R$_{\odot}$, and temperatures T$_{eff1}$=10700 (200) K, T$_{eff2}$=9265 (220) K separated from each other by a=23.27 (0.29) R$_{\odot}$ in an orbit with inclination of 88$^{\circ}$.70 (0.03). Analysis of the O--C residuals yielded a rather long apsidal motion period of U=10000(2500) yr compared to the observational history of the system. The relativistic contribution to the observed rates of apsidal motion for V731 Cep is significant (76%). The combination of the absolute dimensions and the apsidal motion properties of the system yielded consistent observed internal structure parameter (log$\bar{k}_{2,obs}$=-2.36) compared to the theory (log$\bar{k}_{2,theo}$=-2.32). Evolutionary investigation of the binary by two methods (Bayesian and evolutionary tracks) shows that the system is t=133(26) Myr old and has a metallicity of [M/H]=-0.04(0.02) dex. The similarities in the spatial distribution and evolutionary properties of V731 Cep with the nearby ($ρ\sim3^{\circ}$.9) open cluster NGC 7762 suggests that V731 Cep could have been evaporated from NGC 7762.

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arXiv2008

New absolute magnitude calibrations for detached binaries

Lutz-Kelker bias corrected absolute magnitude calibrations for the detached binary systems with main-sequence components are presented. The absolute magnitudes of the calibrator stars were derived at intrinsic colours of Johnson-Cousins and 2MASS (Two Micron All Sky Survey) photometric systems. As for the calibrator stars, 44 detached binaries were selected from the Hipparcos catalogue, which have relative observed parallax errors smaller than 15% ($σ_π/π\leq0.15$). The calibration equations which provide the corrected absolute magnitude for optical and near-infrared pass bands are valid for wide ranges of colours and absolute magnitudes: $-0.18<(B-V)_{0}<0.91$, $-1.6<M_{V}<5.5$ and $-0.15<(J-H)_{0}<0.50$, $-0.02<(H-K_{s})_{0}<0.13$, $0<M_{J}<4$, respectively. The distances computed using the luminosity-colours (LCs) relation with optical (BV) and near-infrared ($JHK_{s}$) observations were compared to the distances found from various other methods. The results show that new absolute magnitude calibrations of this study can be used as a convenient statistical tool to estimate the true distances of detached binaries out of Hipparcos' distance limit.

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arXiv2008

A catalog of chromospherically active binary stars (third edition)

Chromospherically Active Binaries (CAB) catalogue have been revised and updated. With 203 new identifications, the number of CAB stars is increased to 409. Catalogue is available in electronic format where each system has various number of lines (sub-orders) with a unique order number. Columns contain data of limited number of selected cross references, comments to explain peculiarities and position of the binarity in case it belongs to a multiple system, classical identifications (RS CVn, BY Dra), brightness and colours, photometric and spectroscopic data, description of emission features (Ca II H&K, $H_α$, UV, IR), X-Ray luminosity, radio flux, physical quantities and orbital information, where each basic entry are referenced so users can go original sources.

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arXiv2008

Estimation of galactic model parameters in high latitudes with SDSS

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 ({\em SDSS}), to explore their possible variation with the Galactic longitude. The thick disc scaleheight moves from $\sim$550 pc at $120^{\circ}<l<150^{\circ}$ to $\sim$720 pc at $250^{\circ}<l<290^{\circ}$, while the thin disc scaleheight is as large as $\sim$195 pc in the anticenter direction and $\sim$15% lower at $|l|<30^\circ$. Finally, the axis ratio ($c/a$) of the halo changes from a mean value of $\sim$0.55 in the two first quadrants of the Galaxy to $\sim$0.70 at $190^{\circ}<l<300^{\circ}$. 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.

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arXiv2008

Formation and Evolution of W UMa Stars: Fallacies and Corrections

The period histograms of eclipsing binaries generated with ASAS data cannot only be interpreted by orbital evolution. The eclipse probabilities, selection effects and space distributions in the solar neighborhood should be considered before any interpretations are made. Depending upon physical dimensions (total mass and period) of the progenitor stars and the efficiency of angular momentum loss (AML) mechanism, a newly formed W UMa type binary can be at any age up to several Gyr, and evolution in the contact stage is controlled not only by angular momentum and mass loss but also by mass transfer between the component stars. Thus, mean life of contact stages should be about 1.6 Gyr. A different time scale would cause inconsistencies.

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arXiv2007

Transformations between 2MASS, SDSS and BVRI photometric systems: bridging the near infrared and optical

We present colour transformations for the conversion of the {\em 2MASS} photometric system to the Johnson-Cousins $UBVRI$ system and further into the {\em SDSS} $ugriz$ system. We have taken {\em 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 \citet{Cu03}, 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 de-reddened, 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 {\em SDSS} $ugriz$ magnitudes are saturated. The combination of these densities with those evaluated at larger distances using {\em SDSS} $ugriz$ photometry will supply accurate Galactic model parameters, particularly the local space densities for each population.

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arXiv2007

Volume limited dependent Galactic model parameters

We estimated 34 sets of Galactic model parameters for three intermediate latitude fields with Galactic longitudes l=60, l=90, and l=180, and we discussed their dependence on the volume. Also, we confirmed 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, (4,5], and (5,6], whereas for the third field a larger absolute magnitude interval is adopted, (4,10]. The limiting apparent magnitudes of star samples are g=15 and g=22.5 mag which provide space densities within distances in the line of sight 0.9 and 25 kpc. The Galactic model parameters for the thin disc are not volume dependent. However, the ones for thick disc and halo do show spectacular trends in their variations 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 axial ratio of the halo increases abruptly for the volumes where thick disc is dominant, whereas it approaches an asymptotic value gradually for larger volumes, indicating a continuous transition from disclike 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.

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arXiv2007

Spatial distribution and galactic model parameters of cataclysmic variables

The spatial distribution, galactic model parameters and luminosity function of cataclysmic variables (CVs) in the solar neighbourhood have been determined from a carefully established sample of 459 CVs. The sample contains all of the CVs with distances computed from the Period-Luminosity-Colours (PLCs) relation of CVs which has been recently derived and calibrated with {\em 2MASS} photometric data. It has been found that an exponential function fits best to the observational z-distributions of all of the CVs in the sample, non-magnetic CVs and dwarf novae, while the sech^{2} function is more appropriate for nova-like stars and polars. The vertical scaleheight of CVs is 158$\pm$14 pc for the {\em 2MASS} J-band limiting apparent magnitude of 15.8. On the other hand, the vertical scaleheights are 128$\pm$20 and 160$\pm$5 pc for dwarf novae and nova-like stars, respectively. The local space density of CVs is found to be $\sim3\times10^{-5}$ pc^{-3} which is in agreement with the lower limit of the theoretical predictions. The luminosity function of CVs shows an increasing trend toward higher space densities at low luminosities, implying that the number of short-period systems should be high. The discrepancies between the theoretical and observational population studies of CVs will almost disappear if for the z-dependence of the space density the sech^{2} density function is used.

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arXiv2007

The metallicity distributions in high-latitudes with SDSS

We present metallicities and their variations with different parameters for 36 high-latitude fields covering Galactic longitudes 0<l<360. 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 of the disc. This suggestion is supported by the metallicity variations which we find as functions of radial distance. The metallicity variation at larger vertical distances (6.5<z<9.5 kpc) is small but monotonic. Three different vertical metallicity gradients could be detected: d[M/H]/dz=-0.22(+/-0.03), d[M/H]/dz=-0.38 (+/-0.06), and d[M/H]/dz=-0.08 (+/-0.07) dex/kpc for the intervals z<3, 3<z<5, and 5<z<10 kpc, respectively. Since our data cover the whole Galactic longitude interval, the resulting metallicity gradients can be interpreted as properties of the larger-scale Galaxy. The first gradient confirms the dissipational formation of the disc at short z-distances. The steeper gradient corresponds to the transition region between different population components of the Galaxy, and finally, the lowest value provides an adequate description of the inner-halo metallicity gradient.

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arXiv2007

A new absolute magnitude calibration with 2MASS for cataclysmic variables

Using reliable trigonometric measurements, we find that the absolute magnitude of cataclysmic variables depends on the orbital period and de-reddened $(J-H)_{0}$ and $(H-K_{s})_{0}$ colours of {\em 2MASS} (Two Micron All Sky Survey) photometric system. The calibration equation covers the ranges $0.032^{d} < P_{orb} \leq 0.454^{d}$, $-0.08 < (J-H)_{0} \leq 1.54$, $-0.03 <(H-K_{s})_{0} \leq 0.56$ and $2.0 < M_{J} < 11.7$; It is based on trigonometric parallaxes with relative errors of $(σ_π/π) \leq 0.4$. By using the period-luminosity-colours (PLCs) relation, we estimated the distances of cataclysmic variables with orbital periods and {\em 2MASS} observations and compared them with distances found from other methods. We suggest that the PLCs relation can be a useful statistical tool to estimate the distances of cataclysmic variables.

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arXiv2006

Estimation of Galactic model parameters with the SDSS and metallicity distribution in two fields in the anti-centre direction of the Galaxy

We estimated the Galactic model parameters from the SDSS data reduced for two fields in the anti-centre direction of the Galaxy, $l= 180^{o}$, and symmetric relative to the Galactic plane, $b=+45^{o}$ and $b=-45^{o}$. The large size of each field, 60 deg$^{2}$, and the faint limiting apparent magnitude, g=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 scalelengths for two discs which are adopted from Jurić et al. (2005). Metallicities were evaluated by a recent calibration for SDSS, and absolute magnitudes of stars with $4<M(g)\leq8$ were derived as a function of (g-r) colour and metallicity. 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, $κ=0.45$. The metallicity distribution for unevolved G type stars with $5<M(g)\leq6$ shows three substructures relative to the distance from the Galactic plane: for $z^{*}<3$ kpc, the metallicity gradient for two fields is $d[M/H]/dz \sim -0.20(\pm0.02)$ dex kpc$^{-1}$, consistent with the formation scenario of the thin disc. For $5<z^{*}<10$ kpc, $d[M/H]/dz \sim -0.03(\pm0.001)$ dex kpc$^{-1}$ for two fields, confirming that the metallicity gradient for the halo component is close to zero. When we combine these substructures, however, we find a smooth metallicity gradient for two fields, $-0.30(\pm0.04)$ dex kpc$^{-1}$.

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arXiv2006

Estimation of Galactic model parameters in high latitudes with 2MASS

In this paper, we try to find any dependence of the Galactic structural parameters with the Galactic longitude for either the thin disc or the thick disc of the Milky Way, that would indicate to possible inhomogeneities or asymmetries in those Galactic components. Galactic model parameters obtained using a set of 36 high-latitude fields with 2MASS photometry for the different fields show that, effectively, they are Galactic longitude-dependent. The thick disc scaleheight changes from 800 pc at 150 deg < l < 230 deg to 1050 pc at |l| < 30 deg. A plausible explanation for this finding might be the effect of the flare in this Galactic component that changes the scaleheight with Galactocentric distance

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arXiv2006

Galactic longitude dependent Galactic model parameters

We present the Galactic model parameters for thin disc estimated by Sloan Digital Sky Survey (SDSS) data of 14 940 stars with apparent magnitudes $16 =220$ pc ($196\leq H \leq 234$ pc) and $ =1900$ pc ($1561\leq h \leq 2280$ 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.

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arXiv2006

Dynamical evolution of active detached binaries on log Jo - log M diagram and contact binary formation

Orbital angular momentum (Jo), systemic mass (M) and orbital period (P) distributions of chromospherically active binaries (CAB) and W Ursae Majoris (W UMa) systems were investigated. The diagrams of log Jo - log P, log M - log P and log Jo-log M were formed from 119 CAB and 102 W UMa stars. The log Jo-log M diagram is found to be most meaningful in demonstrating dynamical evolution of binary star orbits. A slightly curved borderline (contact border) separating the detached and the contact systems was discovered on the log Jo - log M diagram. Since orbital size (a) and period (P) of binaries are determined by their current Jo, M and mass ratio q, the rates of orbital angular momentum loss (dlog Jo/dt) and mass loss (dlog M/dt) are primary parameters to determine the direction and the speed of the dynamical evolution. A detached system becomes a contact system if its own dynamical evolution enables it to pass the contact border on the log Jo - log M diagram. Evolution of q for a mass loosing detached system is unknown unless mass loss rate for each component is known. Assuming q is constant in the first approximation and using the mean decreasing rates of Jo and M from the kinematical ages of CAB stars, it has been predicted that 11, 23 and 39 cent of current CAB stars would transform to W UMa systems if their nuclear evolution permits them to live 2, 4 and 6 Gyrs respectively.

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arXiv2006

Evidences on secular dynamical evolution of detached active binary orbits and contact binary formation

Evidence of secular dynamical evolution for detached active binary orbits are presented. First order decreasing rates of orbital angular momentum (OAM), systemic mass ($M=M_{1}+M_{2}$) and orbital period of detached active binaries have been determined as $\dot J/J = -3.48 \times 10^{-10}$yr$^{-1}$, $\dot M/M = -1.30 \times 10^{-10}$yr$^{-1}$ and $\dot P/P = -3.96\times 10^{-10}$yr$^{-1}$ from the kinematical ages of 62 field detached systems. The ratio of $d \log J/ d \log M = 2.68$ implies that either there are mechanisms which amplify AM loss $δ=2.68$ times with respect to isotropic AM loss of hypothetical isotropic winds or there exist external causes contributing AM loss in order to produce this mean rate of decrease for orbital periods. Various decreasing rates of OAM ($d \log J / dt$) and systemic mass ($d \log M/ dt$) determine various speeds of dynamical evolutions towards a contact configuration. According to average dynamical evolution with $δ= 2.68$, the fraction of 11, 23 and 39 per cent of current detached sample is expected to be contact system within 2, 4 and 6 Gyr respectively.

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arXiv2006

Separation of dwarf and giant stars with ROTSE-IIId

136 stars which were known to be the members of open cluster NGC 752 were observed at R band with ROTSE-IIId telescope located at the Turkish National Observatory (TUG) site. The data had been evaluated together with BV and 2MASS photometric data. A new practical method for separating dwarf and giant was described and applied. Evaluating the colour magnitude--diagrams with Padova isochrones revealed metallicity similar to the Sun and an age of 1.41 Gyr for the open cluster NGC 752.

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