arXiv subjects

C. Ulusoy

Publications and source records attributed to C. Ulusoy.

13 recordsLinked to original sources

arXiv2020

A comprehensive study of the eclipsing binaries V1241 Tau and GQ Dra

We present new photometric and spectroscopic observations and analyses for the eclipsing binary systems V1241 Tau and GQ Dra. Our photometric light and radial velocity curves analyses combining with the TESS light curves show that both are conventional semi-detached binary systems. Their absolute parameters are also derived. We present the $O-C$ analyses of the systems and we propose the most possible orbital period modulating mechanisms. Furthermore, Fourier analyses are applied to the photometric residual data of the systems to check for the pulsational behavior of the components. We conclude that the primary component of the system GQ Dra is a $δ$ Sct type pulsator with a dominant pulsation frequency of 18.58 d$^{-1}$ based on our $B$ filter residual light curve although it can not be justified by 30-minute cadence TESS data. No satisfactory evidence of pulsational behaviour for V1241 Tau was verified. Finally, the evolutionary tracks of the components of both systems are calculated, while their locations within evolutionary diagrams are compared with other Algol-type systems.

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arXiv2019

A study of Variability of the Marginal Am star HD 176843 observed in the Kepler field

We present results of a study of the variability of the marginal Am star HD\,176843 observed in the {\it Kepler} field. {\it Kepler} photometry and ground-based spectroscopy are used to investigate the light variations of the star. HD\,176843 is classified as a marginal Am star that shows $δ$\ Sct type pulsations. From an analysis of the {\it Kepler} time series, we find that the light curve of HD\,176843 is dominated by three modes with frequencies $f_{1}$=0.1145, $f_{2}$=0.0162 and $f_{3}$=0.1078 d$^{-1}$. The amplitude of the radial velocity variations of about 10 km/s is much more than the radial velocity errors and allows us to conclude clear radial velocity variations. Using the radial velocity data and the adopted spectra, the orbital solution of HD\,176843 is also obtained with an orbital period of 34.14 days. However, the available photometric data show no significant evidence for any possible motion in the binary system.

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arXiv2017

Asteroseismology of the {\it Kepler} target KIC\,9204718

The high precision data obtained by the {\it Kepler} satellite allows us to detect hybrid type pulsator candidates more accurately than the data obtained by ground-based observations. In this study, we present preliminary results on the new analysis of the {\it Kepler} light curve and high resolution spectroscopic observations of pulsating Am star KIC\,9204718. Our tentative analysis therefore show that the star has hybrid pulsational characteristics.

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arXiv2014

New Photometric Study of A Southern Contact Binary System: HI Puppis

We present results of new photometric observations of the contact binary system \astrobj{HI Pup} as well as the radial velocity curve of the system. Time series multicolour photometry was obtained at the South African Astronomical Observatory (SAAO) using the 1-m Cassegrain Telescope. We applied a simultaneous solution to the $BVRI$ light and radial velocity curves in order to determine the physical parameters of the system. From an analysis of the new multicolour data, the physical parameters were found to be $M_1=1.22M_{\odot}$, $M_2=0.23M_{\odot}$, $R_1=1.44R_{\odot}$, $R_2=0.67R_{\odot}$, $L_1=3.3L_{\odot}$, $L_2=0.7L_{\odot}$. Our solution confirms that HI Pup has a typical A--type W UMa binary system characteristics.

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arXiv2014

Frequency and spectrum analysis of $γ$ Doradus type Kepler target KIC 6462033

We present results of an asteroseismic study on the $γ$ Dor type {\it Kepler} target KIC\,6462033. {\it Kepler} photometry is used to derive the frequency content and principal modes. High-dispersion ground-based spectroscopy is also carried out in order to determine the atmospheric parameters and projected rotational velocity. From an analysis of the {\it Kepler} long cadence time series, we find that the light curve of KIC\,6462033 is dominated by three modes with frequencies $f_{1}$=0.92527, $f_{2}$=2.03656 and $f_{3}$=1.42972 d$^{-1}$ as well as we detect more than a few hundreds of combination terms. However, two other independent frequencies appear to have lower amplitudes in addition to these three dominant terms. No significant peaks are detected in the region $>$ 5 d$^{-1}$. We therefore confirm that KIC\,6462033 pulsates in the frequency range of $γ$ Dor type variables, and a future study will allow us to investigate modal behaviour in this star.

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arXiv2013

A search for pulsations in the HgMn star HD 45975 with CoRoT photometry and ground-based spectroscopy

The existence of pulsations in HgMn stars is still being debated. To provide the first unambiguous observational detection of pulsations in this class of chemically peculiar objects, the bright star HD 45975 was monitored for nearly two months by the CoRoT satellite. Independent analyses of the light curve provides evidence of monoperiodic variations with a frequency of 0.7572 c/d and a peak-to-peak amplitude of ~2800 ppm. Multisite, ground-based spectroscopic observations overlapping the CoRoT observations show the star to be a long-period, single-lined binary. Furthermore, with the notable exception of mercury, they reveal the same periodicity as in photometry in the line moments of chemical species exhibiting strong overabundances (e.g., Mn and Y). In contrast, lines of other elements do not show significant variations. As found in other HgMn stars, the pattern of variability consists in an absorption bump moving redwards across the line profiles. We argue that the photometric and spectroscopic changes are more consistent with an interpretation in terms of rotational modulation of spots at the stellar surface. In this framework, the existence of pulsations producing photometric variations above the ~50 ppm level is unlikely in HD 45975. This provides strong constraints on the excitation/damping of pulsation modes in this HgMn star.

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arXiv2013

Seismic investigation of the γ Dor star KIC 6462033: The first results of Kepler and ground-based follow up observations

We present the first preliminary results on the analysis of ground-based time series of the γ Dor star KIC 6462033 (TYC 3144-646-1, V = 10.83, P = 0.69686 d) as well as Kepler photometry in order to study pulsational behaviour in this star.γ Dor variables, which exhibit g-mode pulsations, are promising asteroseismic targets to understand their rich complexity of pulsational characteristics in detail. In order to achieve this goal, intensive and numerous multicolour and high resolution spectroscopic observations are also required, to complete space-based data aimed at the determination of their physical parameters.

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arXiv2013

Multi-site photometric campaign on the high amplitude δ Scuti star KIC 6382916

We present results of a multi-site photometric campaign on the high-amplitude $δ$\,Scuti star KIC\,6382916 in the {\it Kepler} field. The star was observed over a 85-d interval at five different sites in North America and Europe during 2011. {\it Kepler} photometry and ground-based multicolour light curves of KIC\,6382916 are used to investigate the pulsational content and to identify the principal modes. High-dispersion spectroscopy was also obtained in order to derive the stellar parameters and projected rotational velocity. From an analysis of the {\it Kepler} time series, three independent frequencies and a few hundred combination frequencies are found. The light curve is dominated by two modes with frequencies $f_{1}$= 4.9107 and $f_{2}$= 6.4314\,d$^{-1}$. The third mode with $f_{3}$= 8.0350\,d$^{-1}$ has a much lower amplitude. We attempt mode identification by examining the amplitude ratios and phase differences in different wavebands from multicolour photometry and comparing them to calculations for different spherical harmonic degree, $l$. We find that the theoretical models for $f_1$ and $f_2$ are in a best agreement with the observations and lead to value of l = 1 modes, but the mode identification of $f_3$ is uncertain due to its low amplitude. Non-adiabatic pulsation models show that frequencies below 6\,d$^{-1}$ are stable, which means that the low frequency of $f_1$ cannot be reproduced. This is further confirmation that current models predict a narrower pulsation frequency range than actually observed.

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arXiv2013

New β Cephei variable in the Southern open cluster NGC 6200: ALS 3728

We report the evidence of a new $β$ Cephei type variable located in the Southern open cluster \astrobj{NGC 6200}: \astrobj{ALS 3728}. Spectroscopic and photometric observations of NGC\,6200 stars were carried out at the South African Astronomical Observatory (SAAO) in 2011. It was found that \astrobj{ALS 3728} has a frequency of about 4.95 d$^{-1}$ with highest amplitude. The star also shows a remarkable stillstand phase just before its light maxima which is a typical characteristic occurring among these type of stars. Furthermore, a mode identification is applied for the dominant frequency that is calculated from the Fourier analysis.

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arXiv2013

An asteroseismic study of the Southern $β$ Cephei star \astrobj{ALS 3721}

We present the results of a new investigation aimed to identify the pulsational characteristics of the Southern $β$ Cephei star ALS 3721. Spectroscopic and multicolour photometric data were acquired at the South African Astronomical Observatory (SAAO), South Africa in 2011. Frequency analysis showed that the oscillations of ALS 3721 could be attributed to the two main frequencies with higher significance. Stellar parameters and projected rotational velocity obtained by the spectra were also used to perform photometric mode identification. In order to determine spherical harmonic degrees ($l$), a principal method was followed by comparing the observed light amplitude ratios in different passbands with those computed from non--adiabatic pulsation models. In general case, therefore, the spherical harmonic degrees corresponding of the frequencies were found in the expected $β$ Cephei range.

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arXiv2012

Mode identification in the high-amplitude δ Scuti star V2367 Cyg

We report on a multi-site photometric campaign on the high-amplitude $δ$ Scuti star V2367 Cyg in order to determine the pulsation modes. We also used high-dispersion spectroscopy to estimate the stellar parameters and projected rotational velocity. Time series multicolour photometry was obtained during a 98-d interval from five different sites. These data were used together with model atmospheres and non-adiabatic pulsation models to identify the spherical harmonic degree of the three independent frequencies of highest amplitude as well as the first two harmonics of the dominant mode. This was accomplished by matching the observed relative light amplitudes and phases in different wavebands with those computed by the models. In general, our results support the assumed mode identifications in a previous analysis of Kepler data.

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arXiv2012

New studies of a variety of Southern pulsating B stars

We present preliminary results of multi-colour photometry of Beta Cephei stars observed in the LMC and in NGC 6200. Tentative identifications of pulsation modes have been made, and a number of new B pulsators have been noted. Interesting features have also been discovered in the light curves of some of these stars.

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arXiv2010

The field high-amplitude SX Phe variable BL Cam: results from a multisite photometric campaign. II. Evidence of a binary - possibly triple - system

Short-period high-amplitude pulsating stars of Population I ($δ$ Sct stars) and II (SX Phe variables) exist in the lower part of the classical (Cepheid) instability strip. Most of them have very simple pulsational behaviours, only one or two radial modes being excited. Nevertheless, BL Cam is a unique object among them, being an extreme metal-deficient field high-amplitude SX Phe variable with a large number of frequencies. Based on a frequency analysis, a pulsational interpretation was previously given. aims heading (mandatory) We attempt to interpret the long-term behaviour of the residuals that were not taken into account in the previous Observed-Calculated (O-C) short-term analyses. methods heading (mandatory) An investigation of the O-C times has been carried out, using a data set based on the previous published times of light maxima, largely enriched by those obtained during an intensive multisite photometric campaign of BL Cam lasting several months. results heading (mandatory) In addition to a positive (161 $\pm$ 3) x 10$^{-9}$ yr$^{-1}$ secular relative increase in the main pulsation period of BL Cam, we detected in the O-C data short- (144.2 d) and long-term ($\sim$ 3400 d) variations, both incompatible with a scenario of stellar evolution. conclusions heading (mandatory) Interpreted as a light travel-time effect, the short-term O-C variation is indicative of a massive stellar component (0.46 to 1 M$_{\sun}$) with a short period orbit (144.2 d), within a distance of 0.7 AU from the primary. More observations are needed to confirm the long-term O-C variations: if they were also to be caused by a light travel-time effect, they could be interpreted in terms of a third component, in this case probably a brown dwarf star ($\geq$ 0.03 \ M$_{\sun}$), orbiting in $\sim$ 3400 d at a distance of 4.5 AU from the primary.

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