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Öğe An astrometric and spectroscopic study of the ? Scuti variable HD 21190 and its wide companion CPD-83A°A 64B(Oxford Univ Press, 2017) Niemczura, E.; Scholz, R. -D.; Hubrig, S.; Jaervinen, S. P.; Schoeller, M.; Ilyin, I.; Alicavus, F. KahramanAlthough pulsations of delta Scuti type are not expected among Ap stars from a theoretical point of view, previous observations of the known delta Scuti star HD 21190 indicated a spectral classification F2 III SrEuSi:, making it the most evolved Ap star known. Our atmospheric chemical analysis based on recent High Accuracy Radial velocity Planet Searcher observations confirms the presence of chemical peculiarities in HD 21190. This star is also the only target known to host a magnetic field along with its delta Scuti pulsation properties. Using an astrometric analysis, we show that HD 21190 forms a physical binary system with the companion CPD -83A degrees A 64B. The presented astrometric and spectroscopic study of the binary components is important to understand the complex interplay between stellar pulsations, magnetic fields and chemical composition.Öğe High-resolution spectroscopy and abundance analysis of ? Scuti stars near the ? Doradus instability strip(Oxford Univ Press, 2017) Kahraman Alicavus, F.; Niemczura, E.; Polinska, M.; Helminiak, K. G.; Lampens, P.; Molenda-Zakowicz, J.; Ukita, N.delta Scuti stars are remarkable objects for asteroseismology. In spite of decades of investigations, there are still important questions about these pulsating stars to be answered, such as their positions in log T-eff-log g diagram, or the dependence of the pulsation modes on atmospheric parameters and rotation. Therefore, we performed a detailed spectroscopic study of 41 delta Scuti stars. The selected objects are located near the gamma Doradus instability strip to make a reliable comparison between both types of variables. Spectral classification, stellar atmospheric parameters T-eff, log g, xi) and v sin i values were determined. The spectral types and luminosity classes of stars were found to be A1-F5 and III-V, respectively. The T-eff ranges from 6600 to 9400 K, whereas the obtained log g values are from 3.4 to 4.3. The v sin i values were found between 10 and 222 km s(-1). The derived chemical abundances of delta Scuti stars were compared to those of the non-pulsating stars and gamma Doradus variables. It turned out that both delta Scuti and gamma Doradus variables have similar abundance patterns, which are slightly different from the non-pulsating stars. These chemical differences can help us to understand why there are non-pulsating stars in classical instability strip. Effects of the obtained parameters on pulsation period and amplitude were examined. It appears that the pulsation period decreases with increasing T-eff. No significant correlations were found between pulsation period, amplitude and v sin i.Öğe Spectroscopic survey of ? Doradus stars - I. Comprehensive atmospheric parameters and abundance analysis of ? Doradus stars(Oxford Univ Press, 2016) Alicavus, F. Kahraman; Niemczura, E.; De Cat, P.; Soydugan, Esin; Kolaczkowski, Z.; Ostrowski, J.; Telting, J. H.We present a spectroscopic survey of known and candidate gamma Doradus stars. The high-resolution, high signal-to-noise spectra of 52 objects were collected by five different spectrographs. The spectral classification, atmospheric parameters (T-eff, log g, xi), vsin i and chemical composition of the stars were derived. The stellar spectral and luminosity classes were found between G0-A7 and IV-V, respectively. The initial values for T-eff and log g were determined from the photometric indices and spectral energy distribution. Those parameters were improved by the analysis of hydrogen lines. The final values of T-eff, log g and xi were derived from the iron lines analysis. The T-eff values were found between 6000 K and 7900 K, while log g values range from 3.8 to 4.5 dex. Chemical abundances and vsin i values were derived by the spectrum synthesis method. The vsin i values were found between 5 and 240 km s(-1). The chemical abundance pattern of gamma Doradus stars were compared with the pattern of non-pulsating stars. It turned out that there is no significant difference in abundance patterns between these two groups. Additionally, the relations between the atmospheric parameters and the pulsation quantities were checked. A strong correlation between the vsin i and the pulsation periods of gamma Doradus variables was obtained. The accurate positions of the analysed stars in the Hertzsprung-Russell diagram have been shown. Most of our objects are located inside or close to the blue edge of the theoretical instability strip of gamma Doradus.Öğe Spectroscopy of hot ? Doradus and A-F hybrid Kepler candidates close to the hot border of the ? Scuti instability strip(Oxford Univ Press, 2020) Alicavus, F. Kahraman; Poretti, E.; Catanzaro, G.; Smalley, B.; Niemczura, E.; Rainer, M.; Handler, G.If gamma Dor-type pulsations are driven by the convective blocking mechanism, a convective envelope at a sufficient depth is essential. There are several hot gamma Dor and hybrid star candidates in which there should not be an adequate convective envelope to excite the gamma Dor-type oscillations. The existence of these hot objects needs an explanation. Therefore, we selected, observed, and studied 24 hot gamma Dor and hybrid candidates to investigate their properties. The atmospheric parameters, chemical abundances, and vsin i values of the candidates were obtained using medium-resolution (R = 46 000) spectra taken with the Fibre-fed Echelle Spectrograph instrument mounted at the Nordic Optical Telescope. We also carried out frequency analyses of the Kepler long- and short-cadence data to determine the exact pulsation contents. We found only five bona fide hot gamma Dor and three bona fide hot hybrid stars in our sample. The other 16 stars were found to be normal gamma Dor, delta Sct, or hybrid variables. No chemical peculiarity was detected in the spectra of the bona fide hot gamma Dor and hybrid stars. We investigated the interplay between rotation and pulsational modes. We also found that the hot gamma Dor stars have higher Gaia luminosities and larger radii compared to main-sequence A-F stars.
