Browsing by Subject "Stars"

Now showing 1 - 7 of 7

Open Access
Analysis of the most precise light curves of HAT-P-36 detrended from spot signals
(Fundacja Astronomii Polskiej im. Mikolaja Kopernika,Copernicus Foundation for Polish Astronomy, 2021-11-22) Yalçınkaya, S.; Baştürk, Ö.; El Helweh, Fadel; Esmer, E. M.; Yörükoğlu, O.; Yılmaz, M.; Şenavcı, H.V.; Kılıçoğlu, T.; Selam, S. O.
We study the most precise light curves of the planet-host HAT-P-36 that we obtained from the ground primarily with a brand-new 80 cm telescope very recently installed at Ankara University Kreiken Observatory of Turkey and also from the space with Transiting Exoplanet Survey Satellite. The main objective of the study is to analyze the Transit Timing Variations (TTV) observed in the hot-Jupiter type planet HAT-P-36 b, a strong candidate for orbital decay, based on our own observations as well as that have been acquired by professional and amateur observers since its discovery. HAT-P-36 displays out-of-transit variability as well as light curve anomalies during the transits of its planet due to stellar spots. We collected and detrended all the complete transit light curves we had access to from these anomalies, which we modeled with EXOFAST and measured the mid-transit times forming a homogeneous data set for a TTV analysis. We found an increase in the orbital period of HAT-P-36 b at a rate of 0.014 s per year from the best fitting quadratic function, which is only found in the TTV constructed by making use of the mid-transit times measured from detrended light curves, against an expectation of an orbital decay based on its parameters. We refined the values of these system parameters by modeling the Spectral Energy Distribution of the host star, its archival radial velocity observations from multiple instruments, and most precise transit light curves from the space and the ground covering a wide range of wavelengths with EXOFASTV2. We also analyzed the out-of-transit variability from TESS observations to search for potential rotational modulations through a frequency analysis. We report a statistically significant periodicity in the TESS light curve at 4.22 +/- 0.02 d, which might have been caused by instrumental systematics but should be tracked in the future observations of the target.
Open Access
Ginga observations of XI820 — 303 in the globular cluster NGC 6624
(Oxford University Press, 1993) Ercan, E. N.; Cruise, A. M.; Kellett, B. J.; Saygili, K.
We present the first spectral observations obtained by Ginga of the low-mass X-ray binary source X1820-303 located in the globular cluster NGC6624. Two twocomponent spectral models are found to provide good fits to the data: a bremsstrahlung component plus a blackbody, or a power law with exponential cut-off plus a blackbody. In both cases, inclusion of an iron line at 6.7 keV improves the fits. After considering the importance of Comptonization for the spectral and temporal behaviour, we select the power law plus blackbody model. This model then provides simple explanations for many of the observed features present in our data. The model requires a scattering cloud surrounding the neutron star, with a Thomson optical depth of~ 7. By studying the variation in the amphtude of the 685-s binary period with source luminosity, it is possible to gain some insight into the source geometry. This reveals that our line of sight to X1820 - 303 is close to some ‘critical’ angle. We suggest that gravitationally induced shocks in the rim of the accretion disc cause the height of the rim to vary with azimuthal angle, resulting in the quasi-sinusoidal X-ray orbital modulations. Superposed on this are random ‘dips’ which result from chaotic, timevarying structures in the disc which, principally, only cover the blackbody component of the spectrum.
Open Access
Homogeneous transit timing analyses of 10 exoplanet systems
(Oxford University Press, 2022-03-07) Baştürk, Ö.; Esmer, E. M.; Yalçınkaya, S.; Torun, Ş.; Mancini, L.; Helweh, Fadel El; Karamanlı, Ertuğrul; Southworth, J.; Aliş, S.; Wünsche, A.; Tezcan, F.; Aladağ, Y.; Aksaker, N.; Tunç, Ege; Davoudi, F.; Fişek, S.; Bretton, M.; Evans, D. F.; Yeşilyaprak, C.; Yılmaz, M.; Tezcan, C. T.; Yelkenci, K.
We study the transit timings of 10 exoplanets in order to investigate potential transit timing variations in them. We model their available ground-based light curves, some presented here and others taken from the literature, and homogeneously measure the mid-transit times. We statistically compare our results with published values and find that the measurement errors agree. However, in terms of recovering the possible frequencies, homogeneous sets can be found to be more useful, of which no statistically relevant example has been found for the planets in our study. We corrected the ephemeris information of all 10 planets we studied and provide these most precise light elements as references for future transit observations with space-borne and ground-based instruments. We found no evidence for secular or periodic changes in the orbital periods of the planets in our sample, including the ultra-short period WASP-103 b, whose orbit is expected to decay on an observable time-scale. Therefore, we derive the lower limits for the reduced tidal quality factors (Q) for the host stars based on best-fitting quadratic functions to their timing data. We also present a global model of all available data for WASP-74 b, which has a Gaia parallax-based distance value ∼25 per cent larger than the published value.
Open Access
Quantum canonical transformations in star-product formalism
(IOP, 2013) Dereli, T.; Hakioğlu, Tuğrul; Temen, A.
We study construction of the star-product version of three basic quantum canonical transformations which are known as the generators of the full canonical algebra. By considering the fact that star-product of c-number phase-space functions is in complete isomorphism to Hilbert-space operator algebra, it is shown that while the constructions of gauge and point transformations are immediate, generator of the interchanging transformation deforms this isomorphism. As an alternative approach, we study all of them within the deformed form. How to transform any c-number function under linear-nonlinear transformations and the intertwining method are shown within this argument as the complementary subjects of the text.
Open Access
Rossby waves in astrophysics
(Springer, 2021-01-19) Zaqarashvili, T. V.; Albekioni, M.; Ballester, J. L.; Bekki, Y.; Biancofiore, Luca; Birch, A. C.; Dikpati, M.; Gizon, L.; Gurgenashvili, E.; Heifetz, E.; Lanza, A. F.; McIntosh, S. W.; Ofman, L.; Oliver, R.; Proxauf, B.; Umurhan, O. M.; Yellin-Bergovoy, R.
Rossby waves are a pervasive feature of the large-scale motions of the Earth’s atmosphere and oceans. These waves (also known as planetary waves and r-modes) also play an important role in the large-scale dynamics of different astrophysical objects such as the solar atmosphere and interior, astrophysical discs, rapidly rotating stars, planetary and exoplanetary atmospheres. This paper provides a review of theoretical and observational aspects of Rossby waves on different spatial and temporal scales in various astrophysical settings. The physical role played by Rossby-type waves and associated instabilities is discussed in the context of solar and stellar magnetic activity, angular momentum transport in astrophysical discs, planet formation, and other astrophysical processes. Possible directions of future research in theoretical and observational aspects of astrophysical Rossby waves are outlined.
Open Access
Ultralow-threshold up-converted lasing in oligofluorenes with tailored strong nonlinear absorption
(Royal Society of Chemistry, 2015) Guzelturk, B.; Kanibolotsky, A.L.; Orofino-Pena, C.; Laurand, N.; Dawson, M.D.; Skabara P.J.; Demir, Hilmi Volkan
Nonlinear optical response in organic semiconductors has been an attractive property for many practical applications. For frequency up-converted lasers, to date, conjugated polymers, fluorescent dyes and small organic molecules have been proposed but their performances have been severely limited due to the difficulty in simultaneously achieving strong nonlinear optical response and high performance optical gain. In this work, we show that structurally designed truxene-based star-shaped oligofluorenes exhibit strong structure-property relationships enabling enhanced nonlinear optical response with favorable optical gain performance. As the number of fluorene repeat units in each arm is increased from 3 to 6, these molecules demonstrate a two-photon absorption cross-section as high as 2200 GM, which is comparable to that of linear conjugated polymers. Tailored truxene oligomers with six fluorene units in each arm (T6) show two-photon absorption pumped amplified spontaneous emission with a threshold as low as 2.43 mJ cm-2, which is better than that of the lowest reported threshold in organic semiconductors. Furthermore, we show a frequency up-converted laser using the newly designed and synthesized star-shaped oligomer T6 with a threshold as low as 3.1 mJ cm-2, which is more than an order of magnitude lower than that of any conjugated polymer. Thus, these oligomers with enhanced nonlinear optical properties are highly attractive for bio-integrated applications such as photodynamic therapy and in vivo bio-sensing. © The Royal Society of Chemistry 2015.
Open Access
Yeşilçam in letters : a "cinema event" in 1960s Turkey from the perspective of an audience discourse
(2002) Mutlu, Dilek Kaya
This study examines “Ye§il9am event” in Turkish cinema from the perspective of an audience discourse that is reconstituted from the audience letters published in popular cinema magazines Sinema and Perde. Through a study of the letters, it is observed that neither Ye§il9am cinema that marks the 1960s can be reduced to a film industry nor the social experience of it could be evaluated in terms of a cinema-audience relation consisting merely of movie going and film viewing. The topics and the forms of expressions in the letters suggest that Ye§il9am was a “cinema event” diffused throughout everyday life especially through social images of stars and that the audiences had an important role in its (re)production and circulation outside movie theatre.