Перегляд за Автор "Danford, S."

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  • Ескіз
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    Effective temperature and radial velocity of the small-amplitude cepheid polaris (a UMi) in 2015
    (Одеський національний університет імені І. І. Мечникова, 2016) Usenko, I. A.; Kovtyukh, Valery V.; Miroshnichenko, A. S.; Danford, S.
    We present the results of an analysis of 21 spectra of UMi (Polaris) obtained in September – December 2015. Frequency analysis shows an increase of the pulsation period up to 8.6 min in comparison to the 2007 observational set. The radial velocity amplitude comes to 4.16 kms􀀀1, and it approximately twice the one found in 2007. The average Teff = 6017 K, and it is close to the value determined for the 2001–2004 set. Therefore Polaris moves to the red edge of the Cepheid instability strip (CIS).
  • Ескіз
    Документ
    Effective temperature and radial velocity of the small{amplitude cepheid polaris (α UMi) in 2015
    (Одеський національний університет імені І. І. Мечникова, 2016) Usenko, I. A.; Kovtyukh, Valery V.; Miroshnichenko, A. S.; Danford, S.
    We present the results of an analysis of 21 spectra of UMi (Polaris) obtained in September – December 2015. Frequency analysis shows an increase of the pulsation period up to 8.6 min in comparison to the 2007 observational set. The radial velocity amplitude comes to 4.16 kms􀀀1, and it approximately twice the one found in 2007. The average Teff = 6017 K, and it is close to the value determined for the 2001–2004 set. Therefore Polaris moves to the red edge of the Cepheid instability strip (CIS).
  • Ескіз
    Документ
    Pulsational activity of The Small-amplitude Cepheid polaris (a UMi) in 2016-2017
    (Одеський національний університет імені І. І. Мечникова, 2017) Usenko, I. A.; Kovtyukh, Valery V.; Miroshnichenko, A. S.; Danford, S.
    We present the results of an analysis of 49 spectra of а UMi (Polaris) obtained during August – December 2016 and January – March 2017. Frequency analysis displays an unexpected decrease of the pulsational period up to 17.3 min in comparison to the 2015 observational set. The radial velocity amplitude was reduced to 3.43 kms-1 in 2016 and to 3.31 kms-1 in the beginning of 2017 in comparison with 4.16 kms-1 in 2015. This result is also unexpected, because during the last decade a gradual amplitude growth has been observed. The average Teff = 6021 K determined from the 2016–2017 data is close to the values determined for the 2001–2015 set.
  • Ескіз
    Документ
    Pulsational activity of the small–amplitude Cepheid Polaris (α UMi) in 2018-2019
    (Одеський національний університет імені І. І. Мечникова, 2019) Usenko, I. A.; Miroshnichenko, A. S.; Danford, S.; Kovtyukh, Valery V.
    We present the results of an analysis of 20 spectra of α UMi (Polaris) obtained in September 2018 – January 2019 using 0.81 m telescope of the Three College Observatory (TCO), North Carolina, USA. Frequency analysis displays an increase of the pulsational period up to 3.68 min in comparison to the August - December 2017 - January - May 2018 observational sets, and it come to 3.973216 days. The systemic velocity (γ – velocity) is equal to −11.75 km s−1 . The radial velocity amplitude rose to 3.50 km s−1 and actually returned to the previous values of 3.43, 3.31, and 3.81 km s−1 , respectively, found from the August - December 2016, January - March 2017 and August - December 2017 sets. The radial velocity’s amplitude growth tendency still remains. The average Teff = 6051±22 K shows a growth toward the value found from the 2005 – 2007 observational sets and probably has a tendency toward the Teff growth. The joint TCO and Hermes radial velocity measurements during the last four years show the changes of Polaris’ pulsational amplitude.
  • Ескіз
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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 open cluster Collinder 394 – new results
    (Одеський національний університет імені І. І. Мечникова, 2021) Usenko, I. A.; Kniazev, A. Yu.; Miroshnichenko, A. S.; Danford, S.; Kovtyukh, Valery V.; Mishenina, Tamara V.; Мішеніна, Тамара Василівна; Мишенина, Тамара Васильевна
    Confident main-sequence (MS) members of the Collinder 394 open cluster are perfect objects to check the correctness of their distances, obtained from the GAIA GR2 (2018) catalogue. The differences in the distances to the open cluster Collinder 394, determined by photometry and from the GAIA parallaxes have raised doubts about the correctness of the latter. Therefore we used spectroscopically determined Teff and log gvalues for these stars from Usenko et al. (2019) and tried to solve the inverse problem: determine radii of these stars using the derived distances and calibrations “Teff-radius” for MS stars and compare with similar ones. For this purpose we used the calibrations from Torres et al. (2010), based on the nearest MS eclipsing binaries and compilations for MS stars from Mamajek (2018). As a result, we obtained relationships that connect Teff, log g, radii, masses, and distances for the confident Collinder 394 MS stars. We have confirmed the correctness of the GAIA DR2 (2018) distances for these stars and determined their radii and masses. The latter estimates turned out to be close to those of the evolutionary masses calculated by the PARSEC models.
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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.
  • Ескіз
    Документ
    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.