Перегляд за Автор "Turner, D. G."

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    On the existence of young embedded clusters at high Galactic latitude
    (2017) Turner, D. G.; Carraro, G.; Panko, Olena O.; Панько, Олена Олексіївна; Панько, Елена Алексеевна
    Careful analyses of photometric and star count data available for the nine putative young clusters identified by Camargo et al. (2015, 2016) at high Galactic latitudes reveal that none of the groups contain early-type stars, and most are not significant density enhancements above field level. 2MASS colours for stars in the groups match those of unreddened late-type dwarfs and giants, as expected for contamination by (mostly) thin disk objects. A simulation of one such field using only typical high latitude foreground stars yields a colour-magnitude diagram that is very similar to those constructed by Camargo et al. (2015, 2016) as evidence for their young groups as well as the means of deriving their reddenings and distances. Although some of the fields are coincident with clusters of galaxies, one must conclude that there is no evidence that the putative clusters are extremely young stellar groups.
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    Spectroscopic investigations of Galactic clusters with associated cepheid variables II NGC 5662 and V CEN
    (Одеський національний університет імені І. І. Мечникова, 2018) Usenko, I. A.; Kniazev, A. Yu.; Kovtyukh, Valery V.; Mishenina, T. V.; Miroshnichenko, A. S.; Turner, D. G.; Protsyuk, Yu. I.
    We present the results of a spectroscopic and photometric investigation of 15 objects from the open cluster NGC 5662, which contains the Cepheid V Cen. Besides the Cepheid, we studied one G-supergiant, two K-giants, four B-giants, and seven main sequence stars. Radial velocities (RV), vsini, TeS, logg, and [Fe/Hj were determined using model fitting. We have derived the color-excesses, reddenings, and intrinsic colors for these stars to determine their true Tff and log g from comparison to the atmosphere models, especially for hot stars, and to determine their absolute magnitudes. RV and GAIA DR2 2018 parallax/distanee values for these stars allowed us to determine their membership in the cluster and to construct it 3D kinematic models. These parallaxes and reddenings led to the distances in a range of 700875 pc, while photometric My gave the distances of 550-660 pc. Two objects from our list were found to be non-members of the cluster, and two other objects to be background stars. All members have [Fe/Hj = —0.1 ±0.04 dex. The main sequence stars and B-giants have abundances of C, N, and Na close to the solar one, while the cool supergiants and giants show a deficit of carbon, an overabundance of nitrogen, and a small overabundance or close to the solar sodium content. Two Li-rich K-giants, HD 127733 and HD 127753, were discovered in the cluster.
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    Spectroscopic investigations of Galactic clusters with associated cepheid variables. I. Polaris cluster and UMi
    (Одеський національний університет імені І. І. Мечникова, 2018) Usenko, I. A.; Miroshnichenko, A. S.; Danford, S.; Kovtyukh, Valery V.; Turner, D. G.; Shulga, O. V.; Protsyuk, Yu. I.
    We present the results of an analysis of the spectra of nine so called Polaris cluster A􀀀FV stars obtained during 2016-2018. Radial velocities (RV) and GAIA DR 18 parallaxes-distances for these stars allow us to determine their membership in the cluster and to construct its 3D kinematic model. However Teff for four stars determined spectroscopically give cause for doubts about the GAIA DR 18 parallaxes accuracy. The comparison between their MV and distances with the those from Pecaut & Mamajek (2013) calibrations shows signi cant di erences for the objects with parallaxes less than 8 mas. The di erences in distances increase exponentially with decreasing parallax. These facts indicate an unreliability of the GAIA DR2 parallx measurements of less than 10 mas. We estimated the distance to Polaris B to be 104.2 pc.
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    Spectroscopic investigations of galactic clusters with associated Cepheid variables. III. Collinder 394 and BB SGR
    (Одеський національний університет імені І. І. Мечникова, 2019) Usenko, I. A.; Kniazev, A. Yu.; Katkov, I. Yu.; Kovtyukh, Valery V.; Mishenina, T. V.; Miroshnichenko, A. S.; Turner, D. G.
    We present the results of a spectroscopic and photometric investigation of 20 objects from the open cluster Collinder 394, which contains the Cepheid BB Sgr. Besides the Cepheid, we studied tree K-giants, four B-giants, and twelve B-A-F main sequence stars. Radial velocities (RV), v sin i, Teff, log g, were determined using spectroscopic model fitting and atmosphere models. We have derived the color-excesses, reddenings, and intrinsic colors for these stars using their Teff and log g from comparison to the atmosphere models, especially for hot stars. Proper motions, RV and GAIA DR2 2018 parallax/distance values for these stars allowed us to determine their membership in the cluster and absolute magnitudes. We found that seven stars do not belong to the cluster. The parallaxes and reddenings of 13 confidently cluster members led to the distances in a range of 630–800 pc, although the majority of them (8 objects) are located at a mean distance of 657.7±66.7 pc, and 5 objects with the Cepheid could probably belong to the cluster’s corona. All the members have [Fe/H] near 0.1 dex. The main sequence B-star No. 76 (HD 174307) has a low rotational velocity projection, and this fact allows us to determine its chemical composition to compare with that of the Cepheid BB Sgr. Its CNO and Na abundances are close to the solar one, while the Cepheid show a deficit of carbon, an overabundance of nitrogen, nearly solar oxygen, and an overabundance of sodium. Three foreground K-giants have different C abundances (from a deficit to the solar), and Na (from the solar to an overabundance) as well as a solar-like O content. The foreground F7-8 V star No. 75 (CPD −20◦7218) was revealed to be a Li-rich star, while the A7 V star No. 69 (BD −20◦5290) has an anomalous deficit of CNO with overabundances of some α, r- and s-process elements.
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    Spectroscopic investigations of galactic clusters with associated cepheid variables. IV. Collinder 220 and UW CAR
    (Одеський національний університет імені І. І. Мечникова, 2020) Usenko, I. A.; Kniazev, A. Yu.; Katkov, I. Yu.; Kovtyukh, Valery V.; Mishenina, T. V.; Miroshnichenko, A. S.; Turner, D. G.
    We present the results of a spectroscopic and photometric investigation of 18 probable members of the open cluster Collinder 220, which contains the Cepheid UW Car. Besides the Cepheid, we studied tree K-giants, two B-giants, and twelve B-A-F main sequence stars. Radial velocities (RV), v sin i, Te , and log g were determined using spectroscopic model fitting and atmosphere models. We have derived color-excesses, reddenings, and intrinsic colors for these stars using their Te and log g from comparison to the atmosphere models, especially for hot stars. Proper motions, RV, and the GAIA DR2 2018 parallaxes allowed us to determine their membership in the cluster and absolute magnitudes. We found that seven stars along with the Cepheid can be the cluster members with a high confidence. The parallaxes and reddenings of the 7 confident cluster members led to the distances in a range of 1900–2800 pc, while the other stars are probably foreground objects. All the members have [Fe/H] near 0.1 dex. The B-giant HD90435 located near the cluster’s “turn-off” point has a low projected rotational velocity, and this fact allows us to determine its chemical composition to compare with that of the Cepheid UW Car. The CNO abundances of HD90435 are nearly solar, while the Cepheid shows a deficit of carbon, an overabundance of nitrogen, a nearly solar oxygen, and an overabundance of sodium. The confident cluster member, K-supergiant CPD 􀀀57 3199, has CNO- and Na abundances close to those of UW Car.
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    Spectroscopic investigations of Polaris field stars
    (Одеський національний університет імені І. І. Мечникова, 2023) Usenko, I. A.; Miroshnichenko, A. S.; Danford, S.; Turner, D. G.; Majaess, D. J.; Balam, D. D.
    We present the results of an analysis of 28 spectra of 18 Polaris field A–G V stars obtained in 2016–2023. Derived radial velocities and Teff along with distances and reddenings from the Gaia DR3 catalog allowed us to calculate radii of the stars and compare them with calibration relationships “Specral type – Luminosity” for MS stars. As a result, radii and distances for 9 stars were found overestimated compred to those determined photometrically. Therefore, the DR3 distances are unreliable and should be revisited. Accordring to our data for these stars and their photometric distances, 15 objects belong to a possible old open cluster that is currently dissolved in the Polarisfieldatadistance∼70–110pc, whiletwoobjectsbelong to the thick disk, and one belongs to a possible another star group located at a distance of 130 pc.
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    Spectroscopic investigations of the polaris ( UMi) system: radial velocity measurements, new orbit, and companion influence for the cepheid polaris aa pulsation activity
    (Одеський національний університет імені І. І. Мечникова, 2020) Usenko, I. A.; Miroshnichenko, A. S.; Danford, S.; Kovtyukh, Valery V.; Turner, D. G.
    Thirty three spectra of the Polaris system obtained in August-December 2019 and February-April 2020 at the 0.81m telescope of the Three College Observatory (TCO, North Carolina, USA) were used to determine the radial velocities (RV) and effective temperature of the Cepheid Polaris Aa. These new data have been added to the entire Polaris system RV data collection (over 2,500 measurements) to compute a new orbit of the Polaris Aa companion. Furthermore we have used our eight observational datasets taken in 2015-2020 and eight datasets taken in 2011-2018 by Anderson (2019) to check for possible influence of the orbital motion of Polaris Ab on the Polaris Aa pulsational activity. It was found that the mean pulsational period in 2015-2020 was quite stable (3.9760.012 days), while the pulsational amplitude showed evident changes: a growth before HJD 2457350 with a following decrease. This fact could be due to the Polaris Ab passing through the periastron.
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    Spectroscopic studies of Polaris: getting back to normal life of a Cepheid?
    (Odesa I. I. Mechnikov National University, 2024) Usenko, Igor O.; Miroshnichenko, A. S.; Danford, S.; Vaidman, N. L.; Turner, D. G.; Majaess, D. J.; Protsyuk, S. V.; Усенко, Ігор Олександрович; Мірошніченко, А. С.; Данфорд, С.; Вайдман, Н. Л.; Тьорнер, Д.; Маджаєсс, Д. Дж.; Процюк, С. В.
    Based on 583 radial velocity (RV) measurements we obtained at the Three College Observatory (TCO, North Carolina, USA) in 2015–2024, (including 396 not previously analyzed from 2020–2024) as well as on 236 RV data from the German amateur R. Bücke (2011–2022), we calculated the pulsation period and amplitude of the Cepheid Polaris Aa system (α UMi). The analysis showed that the pulsation period was stable within a few minutes in 2020–2024. During this time, the pulsation amplitude was increasing and reached the 1960s level (4–6 km s−1), when its sharp decline began. Since its new growth began after Polaris Ab, the system secondary component, passed a periastron, we concluded that the observed amplitude changes were due to its orbital motion. It is clear that Polaris Aa is returning to normal pulsating activity.