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Öğe A Silent Revolution in Fundamental Astrophysics(Iop Publishing Ltd, 2022) Eker, Zeki; Soydugan, Faruk; Bakis, Volkan; Bilir, Selcuk; Steer, IanArbitrariness in the zeropoint of bolometric corrections is a paradigm that is nearly a century old and leads to two more paradigms. Bolometric corrections must always be negative, and the bolometric magnitude of a star ought to be brighter than its V magnitude. Both were considered valid before the IAU 2015 General Assembly Resolution B2, a revolutionary document that supersedes all three aforementioned paradigms. The purpose of this article is to initiate new insight into and a new understanding of the fundamental astrophysics and present new capabilities to obtain standard and more accurate stellar luminosities and gain more from accurate observations in the era after Gaia. The accuracy gained will aid in advancing stellar structure and evolution theories and also Galactic and extragalactic research, observational cosmology, and searches for dark matter and dark energy.Öğe Aktif Kromosferli Çift Yıldızların Kinematik Yaş ve Dinamik Davranışları(Türk Astronomi Derneği, 2020) Tüysüz, Mehmet; Soydugan, Faruk; Bilir, Selçuk; Demircan, Osman; Eker, ZekiGeri tayf türünden bileşen(ler)e sahip ve kromosferik aktivite belirteci gösteren ayrık ya da yarı-ayrık sistemler aktif kromosferli çift yıldızlar (CAB) olarak bilinir. Bu çalışmada kütle, yarıçap ve sıcaklık değerleri bilinen ve bileşenleri anakol yıldızlarından oluşan Güneş civarındaki CAB'lar kinematik özellikleri dikkate alınarak mercek altına alındı. Örnekteki sistemler, açısal momentumlarına göre alt gruplara ayrılarak ortalama kinematik yaşları hesaplandı. Grupların ortalama kinematik yaşlarıyla, ortalama açısal momentum, ortalama kütle ve ortalama dönem değişimleri incelendi. Ayrıca aktif ve aktif olmayan geri tayf türüne sahip (F veya daha geri) ayrık ve anakol bileşenli sistemlerin kinematik yaş ile dinamik davranışları birlikte ele alınarak, aktivitenin dinamik evrim üzerine etkileri tartışıldı.Öğe Çift Yıldızların Kinematik ve Dinamik Özellikleri: Açısal Momentum ve Yörünge Dönemi Evrimi(Türk Astronomi Derneği, 2020) Soydugan, Faruk; Bilir, Selçuk; Soydugan, Esin; Eker, Zeki; Demircan, Osman; Tüysüz, MehmetBu çalışmada, yaklaşık 1200 örten çift yıldızın temel parametrelerini içeren veri tabanları oluşturulmuş ve ek olarak 29 örten çift yıldızın analizleri yapılmış ve veri tabanları zenginleştirilmiştir. Verileri toplanan tüm örten çift yıldızların temel parametrelerinin dağılımları incelenmiş ve sistemlerin anakol bileşenleri için kütle-ışıtma bağıntıları üretilmiştir. Ayrık çift yıldızlar için farklı kütle aralıklarında dört ayrı kütle-ışıtma bağıntısı belirlenmiştir. Farklı tür çift yıldız grupları için kinematik analizler yapılarak alt-grup yaşları belirlenmiştir.Yapılan dinamik analizler sonucunda, türler arasındaki geçişlerin olabileceği görülmüştür. Oluşturulan büyük ölçekli veri tabanları yanında, çift yıldızların temel parametre dağılım ve ilişkileriyle özellikle toplam kütle, yörünge açısal momentumu ve yörünge döneminin yaş ile değişimi incelenerek tartışılmıştır.Öğe Fundamentals of Stars: A Critical Look at Mass-Luminosity Relations and Beyond(Istanbul University, 2024) Eker, Zeki; Soydugan, Faruk; Bilir, SelçukDevelopments on various relations among stellar variables such as the main sequence empirical mass-luminosity (MLR), massradius (MRR) and mass-effective temperature (MTR) relations were reviewed. Conceptual changes in their understanding and usages were discussed. After its discovery, MLR was treated as one of the fundamental secrets of the cosmos. Differences between fundamental laws and statistical relations were used to understand long-term developments of MLR, MRR and MTR. Developments show a break point, initiated by Andersen (1991), in the line of progress. Before the break when reliable data were limited, MLR and MRR were calibrated using M, L, and R of binary components of all kinds visual, spectroscopic, and eclipsing for two purposes: i) to obtain mean mass, mean luminosity, and mean radius, ii) to estimate M and R of single stars. By the time of the break, the number of solutions from detached double-lined eclipsing binaries (DDEB) giving accurate M and R within a few percent levels are increased. Parameters from very close, semi-detached, and contact binaries were excluded for refinement, however, MLR and MRR diagrams were found insufficient to derive MLR and MRR functions because the dispersions are not only due to random observational errors but also due to chemical composition and age differences. Then, a new trend was adopted by replacing classical MLR and MRR with empirical M and R predicting relations. Thus, the purpose one was suppressed also because the new trend found a fruitful application in determining M and R of exoplanet hosting single stars.Öğe Kinematics of Cataclysmic Variables in the Solar Neighborhood in the Gaia Era(Iop Publishing Ltd, 2025) Canbay, Remziye; Ak, Tansel; Bilir, Selçuk; Soydugan, Faruk; Eker, ZekiUsing high-precision astrometric data from Gaia DR3 and updated systemic velocities from the literature, the kinematical properties of cataclysmic variables (CVs) were investigated. By constraining the data according to the total space velocity error and Galactic population class, a reliable sample of data was obtained. Nonmagnetic CVs located in the thin disk have been found to have a total space velocity dispersion of sigma(nu) = 46.33 +/- 4.23 km s(-1), indicating that the thin disk CVs with a mean kinematical age of tau = 3.95 +/- 0.75 Gyr are much younger than the local thin disk of the Galaxy with tau similar to 6-9 Gyr. Total space velocity dispersions of nonmagnetic CVs belonging to the thin disk component of the Galaxy were found to be sigma(nu) = 47.67 +/- 3.94 and sigma(nu) = 44.43 +/- 4.33 km s(-1) for the systems below and above the orbital period gap, respectively, corresponding to kinematical ages of tau = 4.19 +/- 0.71 and tau = 3.61 +/- 0.74 Gyr. gamma velocity dispersions of the thin disk CVs below and above the gap were obtained sigma(gamma) = 27.52 +/- 2.28 and sigma(gamma) = 25.65 +/- 2.44 km s(-1), respectively. This study also shows that the orbital period is decreasing with increasing age, as expected from the standard theory. The age-orbital period relation for nonmagnetic thin disk CVs was obtained as dP/dt = -2.09(+/- 0.22) x 10(-5) s yr(-1). However, a significant difference could not be found between the gamma velocity dispersions of the systems below and above the gap, which were calculated to be sigma(gamma) = 27.52 +/- 2.28 and sigma(gamma) = 25.65 +/- 2.44 km s(-1), respectively.Öğe On the zero point constant of the bolometric correction scale(Oxford University Press, 2021) Eker, Zeki; Bakiş, Volkan; Soydugan, Faruk; Bilir, SelçukArbitrariness attributed to the zero-point constant of the V-band bolometric corrections (BCV) and its relation to ‘bolometric magnitude of a star ought to be brighter than its visual magnitude’ and ‘bolometric corrections must always be negative’ was investigated. The falsehood of the second assertion became noticeable to us after IAU 2015 General Assembly Resolution B2, where the zero-point constant of bolometric magnitude scale was decided to have a definite value CBol(W) = 71.197 425 ... . Since the zero-point constant of the BCV scale could be written as C2 = CBol − CV, where CV is the zero-point constant of the visual magnitudes in the basic definition BCV = MBol − MV = mbol − mV, and CBol > CV, the zero-point constant (C2) of the BCV scale cannot be arbitrary anymore; rather, it must be a definite positive number obtained from the two definite positive numbers. The two conditions C2 > 0 and 0 < BCV < C2 are also sufficient for LV < L, a similar case to negative BCV numbers, which means that ‘bolometric corrections are not always negative’. In sum it becomes apparent that the first assertion is misleading causing one to understand bolometric corrections must always be negative, which is not necessarily true.Öğe Standard stellar luminosities: what are typical and limiting accuracies in the era after Gaia?(Oxford University Press, 2021) Eker, Zeki; Soydugan, Faruk; Bilir, Selçuk; Bakiş, VolkanMethods of obtaining stellar luminosities (L) have been revised and a new concept, standard stellar luminosity, has been defined. In this paper, we study three methods: (i) a direct method from radii and effective temperatures; (ii) a method using a mass–luminosity relation (MLR); and (iii) a method requiring a bolometric correction. If the unique bolometric correction (BC) of a star extracted from a flux ratio (fV/fBol) obtained from the observed spectrum with sufficient spectral coverage and resolution are used, the third method is estimated to provide an uncertainty (ΔL/L) typically at a low percentage, which could be as accurate as 1 per cent, perhaps more. The typical and limiting uncertainties of the predicted L of the three methods were compared. The secondary methods, which require either a pre-determined non-unique BC or MLR, were found to provide less accurate luminosities than the direct method, which could provide stellar luminosities with a typical accuracy of 8.2–12.2 per cent while its estimated limiting accuracy is 2.5 per cent.Öğe Updated MS Luminosity-Radius-Temperature-Mass Relations for Solar Neighborhood Galactic Disk Stars(Amer Inst Physics, 2018) Eker, Zeki; Bakis, Volkan; Bilir, Selcuk; Soydugan, Faruk; Steer, Ian; Soydugan, Esin; Bakis, HicranConventional mass- luminosity (M- L) relations are defined using 509 nearby main- sequence (MS) stars within the Solar neighborhood. A smooth mass- radius (M- R) relation was fixed from radii of MS stars appeared in literature having masses within the limits 0.179- 1.5 M. Similarly, a smooth mass- effective temperature (M- Teff) relation was derived for the main- sequence stars having masses within the limits 1.5- 31 M using their effective temperatures available. The omitted parts of M- R and M- Teff relations in the range from 0.179 to 31 M were computed according to Stephan- Boltzmann law, so inter- related M- L, M- R and M- Teff were achieved.Öğe Uzun dönemli RS CVn çift yıldızlarında ışıkkürenin ve renkkürenin aktivitesi(2008) Erdem, Ahmet; Demircan, Osman; Buddıng, Edwin; Eker, Zeki; Çiçek, Caner; Soydugan, Faruk; Soydugan, Esin[Abstract Not Available]
