arXiv subjects

T. Yontan

Publications and source records attributed to T. Yontan.

28 records · Page 2Linked to original sources

arXiv2016

CCD UBV Photometry and Kinematics of the Open Cluster NGC 225

We present the results of CCD $UBV$ photometric and spectroscopic observations of the open cluster NGC 225. In order to determine the structural parameters of NGC 225, we calculated the stellar density profile in the cluster's field. We estimated the probabilities of the stars being physical members of the cluster using the existing astrometric data. The most likely members of the cluster were used in the determination of the astrophysical parameters of the cluster. We calculated the mean radial velocity of the cluster as $V_{r}=-8.3\pm 5.0$ km s$^{-1}$ from the optical spectra of eight stars in the cluster's field. Using the U-B vs B-V two-colour diagram and UV excesses of the F-G type main-sequence stars, the reddening and metallicity of NGC 225 were inferred as $E(B-V)=0.151\pm 0.047$ mag and $[Fe/H]=-0.11\pm 0.01$ dex, respectively. We fitted the colour-magnitude diagrams of NGC 225 with the PARSEC isochrones and derived the distance modulus, distance and age of the cluster as $μ_{V}=9.3\pm 0.07$ mag, d=585$\pm$20 pc and $t=900\pm 100$ Myr, respectively. We also estimated the galactic orbital parameters and space velocity components of the cluster and found that the cluster has a slightly eccentric orbit of $e=0.07\pm 0.01$ and an orbital period of $P_{orb}= 255\pm 5$ Myr.

astro-ph.GA↗

arXiv2016

CCD UBV photometry of the open cluster NGC 6819

We present the results of CCD UBV observations of the open cluster NGC 6819. We calculated the stellar density profile in the cluster's field to determine the structural parameters of NGC 6819. Using the existing astrometric data, we calculated the probabilities of the stars being physical members of the cluster, and used these objects in the determination of the astrophysical parameters of NGC 6819. We inferred the reddening and metallicity of the cluster as $E(B-V)=0.130\pm0.035$ mag and $[Fe/H]=+0.051\pm 0.020$ dex, respectively, using the U-B vs B-V two-colour diagram and UV excesses of the F-G type main-sequence stars. We fit the colour-magnitude diagrams of NGC6819 with the PARSEC isochrones and derived the distance modula, distance and age of the cluster as $μ_{V}=12.22\pm 0.10$ mag, $d=2309\pm106$ pc and $t=2.4\pm0.2$ Gyr, respectively. The parameters of the galactic orbit estimated for NGC 6819 indicate that the cluster is orbiting in a slightly eccentric orbit of $e=0.06$ with a period of $P_{orb}=142$ Myr. The slope of the mass function estimated for the cluster is close to the one found for the stars in the solar neighbourhood.

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arXiv2015

A New Ranking Scheme for the Institutional Scientific Performance

We propose a new performance indicator to evaluate the productivity of research institutions by their disseminated scientific papers. The new quality measure includes two principle components: the normalized impact factor of the journal in which paper was published, and the number of citations received per year since it was published. In both components, the scientific impacts are weighted by the contribution of authors from the evaluated institution. As a whole, our new metric, namely, the institutional performance score takes into account both journal based impact and articles specific impacts. We apply this new scheme to evaluate research output performance of Turkish institutions specialized in astronomy and astrophysics in the period of 1998-2012. We discuss the implications of the new metric, and emphasize the benefits of it along with comparison to other proposed institutional performance indicators.

astro-ph.IM↗

arXiv2015

A comprehensive study of the open cluster NGC 6866

We present CCD $UBVRI$ photometry of the field of the open cluster NGC 6866. Structural parameters of the cluster are determined utilizing the stellar density profile of the stars in the field. We calculate the probabilities of the stars being a physical member of the cluster using their astrometric data and perform further analyses using only the most probable members. The reddening and metallicity of the cluster were determined by independent methods. The LAMOST spectra and the ultraviolet excess of the F and G type main-sequence stars in the cluster indicate that the metallicity of the cluster is about the solar value. We estimated the reddening $E(B-V)=0.074 \pm 0.050$ mag using the $U-B$ vs $B-V$ two-colour diagram. The distance modula, the distance and the age of NGC 6866 were derived as $μ= 10.60 \pm 0.10$ mag, $d=1189 \pm 75$ pc and $t = 813 \pm 50$ Myr, respectively, by fitting colour-magnitude diagrams of the cluster with the PARSEC isochrones. The Galactic orbit of NGC 6866 indicates that the cluster is orbiting in a slightly eccentric orbit with $e=0.12$. The mass function slope $x=1.35 \pm 0.08$ was derived by using the most probable members of the cluster.

astro-ph.GA↗

arXiv2014

CCD UBVRI photometry of NGC 6811

We present the results of CCD UBVRI observations of the open cluster NGC 6811 obtained on 18th July 2012 with the 1m telescope at the TÜBİTAK National Observatory (TUG). Using these photometric results, we determine the structural and astrophysical parameters of the cluster. The mean photometric uncertainties are better than 0.02 mag in the V magnitude and B-V, V-R, and V-I colour indices to about 0.03 mag for U-B among stars brighter than magnitude V=18. Cluster member stars were separated from the field stars using the Galaxia model of Sharma et al. (2011) together with other techniques. The core radius of the cluster is found to be $r_{c}$=3.60 arcmin. The astrophysical parameters were determined simultaneously via Bayesian statistics using the colour-magnitude diagrams V versus B-V, V versus V-I, V versus V-R, and V versus R-I of the cluster. The resulting most likely parameters were further confirmed using independent methods, removing any possible degeneracies. The colour excess, distance modulus, metallicity and the age of the cluster are determined simultaneously as E(B-V)=0.05$\pm$0.01 mag, $μ=10.06\pm0.08$ mag, [M/H]=-0.10$\pm$0.01 dex and t=1.00$\pm$0.05 Gyr, respectively. Distances of five red clump stars which were found to be members of the cluster further confirm our distance estimation.

astro-ph.GA↗

arXiv2014

Colour Transformations between BVRc and g'r'i' Photometric Systems for Giant Stars

The transformation equations from $BVR_c$ to $g'r'i'$ magnitudes and vice versa for the giants were established from a sample of 80 stars collected from Soubiran et al. (2010) with confirmed surface gravity ($2\leq \log g$ (cms$^{-2}$)$ \leq3$) at effective temperatures $4000<T_{eff} (K)<16000$. The photometric observations, all sample stars at $g'r'i'$ and 65 of them at $BVR_c$, were obtained at TÜBİTAK National Observatory (TUG) 1m (T100) telescope, on the Taurus Mountains in Turkey. The $M_V$ absolute magnitudes of the giant stars were estimated from the absolute magnitude-temperature data for the giant stars by Sung et al. (2013) using the $T_{eff}$ from the intrinsic colours considered in this study. The transformation equations could be considered to be valid through the ranges of the following magnitudes and colours involved: $7.10<V_0<14.50$, $7.30<g'_0<14.85$, $-0.20<(B-V)_0<1.41$, $-0.11<(V-R_c)_0<0.73$, $-0.42<(g'-r')_0<1.15$, and $-0.37<(r'-i')_0<0.47$ mag. The transformations were successfully applied to the synthetic $BVR_c$ data of 427 field giants in order to obtain the $g'r'i'$ magnitudes and colours. Comparisons of these data with the $g'r'i'$ observations of giants in this study show that the mean residuals and standard deviations lie within [-0.010, 0.042] and [0.028, 0.068] mag, respectively.

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arXiv2013

The early-type near-contact binary system V337 Aql revisited

The close binary V337 Aql consists of two early B-type components with an orbital period of 2.7339 d. New multi-band photometric observations of the system together with published radial velocities enabled us to derive the absolute parameters of the components. The simultaneous light and radial velocity curves solution yields masses and radii of \emph{M$_{1}$}=17.44$\pm$0.31 \emph{M$_\odot$} and \emph{R$_{1}$}=9.86$\pm$0.06 \emph{R$_\odot$} for the primary and \emph{M$_{2}$}=7.83$\pm$0.18 \emph{M$_\odot$} and \emph{R$_{2}$}=7.48$\pm$0.04 \emph{R$_\odot$} for the secondary component. Derived fundamental parameters allow us to calculate the photometric distance as 1355$\pm$160 pc. The present analysis indicates that the system is a near-contact semi-detached binary, in which a primary star is inside its Roche lobe with a filling ratio of 92 percent and the secondary star fills its Roche lobe. From \emph{O-C} data analysis, an orbital period decrease was determined with a rate of -7.6 $\times$ 10$^{-8}$ yr$^{-1}$. Kinematic analysis reveals that V337 Aql has a circular orbit in the Galaxy and belongs to a young thin-disc population.

astro-ph.SR↗

arXiv2013

A New Absolute Magnitude Calibration for Red Clump Stars

We present an M_V absolute magnitude calibration including the B-V colour and [Fe/H] metallicity for the red clump stars in the globular and open clusters with a wide range of metallicities: M_V = 0.627(0.104)(B-V)o+0.046(0.043)[Fe/H]+0.262(0.111). The calibration equation is valid in the ranges 0.42<(B-V)o<1.20 mag, -1.55<[Fe/H]<+0.40 dex and 0.43<M_V<1.03 mag. We found that the consistencies in the comparisons of the distances estimated from the calibration equation in this study both with the distances obtained from trigonometric parallaxes and spectrophotometric analysis demonstrate that reliable precise absolute magnitudes for the clump giants can be estimated from the calibration formula.

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arXiv2012

Research Performance of Turkish Astronomers in the Period of 1980-2010

We investigated the development of astronomy and astrophysics research productivity in Turkey in terms of publication output and their impacts as reflected in the Science Citation Index (SCI) for the period 1980-2010. It includes 838 refereed publications, including 801 articles, 16 letters, 15 reviews, and six research notes. The number of papers were prominently increased after 2000 and the average number of papers per researcher is calculated as 0.89. Total number of received citations for 838 papers is 6938, while number of citations per papers is approximately 8.3 in 30 years. Publication performance of Turkish astronomers and astrophysicists was compared with those of seven countries that have similar gross domestic expenditures on research and development, and members of Organization for Economic Co-operation and Development (OECD). Our study reveals that the output of astronomy and astrophysics research in Turkey has gradually increased over the years.

astro-ph.IM↗

arXiv2012

Local stellar kinematics from RAVE data: III. Radial and Vertical Metallicity Gradients based on Red Clump Stars

We investigate radial and vertical metallicity gradients for a sample of red clump stars from the RAdial Velocity Experiment (RAVE) Data Release 3. We select a total of 6781 stars, using a selection of colour, surface gravity and uncertainty in the derived space motion, and calculate for each star a probabilistic (kinematic) population assignment to a thin or thick disc using space motion and additionally another (dynamical) assignment using stellar vertical orbital eccentricity. We derive almost equal metallicity gradients as a function of Galactocentric distance for the high probability thin disc stars and for stars with vertical orbital eccentricities consistent with being dynamically young, e_v<=0.07, i.e. d[M/H]/dR_m = -0.041(0.003) and d[M/H]/dR_m = -0.041(0.007) dex/kpc. Metallicity gradients as a function of distance from the Galactic plane for the same populations are steeper, i.e. d[M/H]/dz_{max} = -0.109(0.008) and d[M/H]/dz_{max} = -0.260(0.031) dex/kpc, respectively. R_m and z_{max} are the arithmetic mean of the perigalactic and apogalactic distances, and the maximum distance to the Galactic plane, respectively. Samples including more thick disc red clump giant stars show systematically shallower abundance gradients. These findings can be used to distinguish between different formation scenarios of the thick and thin discs.

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