Examinando por Autor "Mehner, A."

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    ESPRESSO highlights the binary nature of the ultra-metal-poor giant HE 0107-5240
    (EDP Sciences, 2020) Bonifacio, P.; Molaro, P.; Adibekyan, V.; Aguado, D.; Alibert, Y.; Allende Prieto, C.; Caffau, E.; Cristiani, S.; Cupani, G.; Marcantonio, P.; D'Odorico, D.; Ehrenreich, D.; Figueira, P.; Genova, R.; González Hernández, J.; Lo Curto, G.; Lovis, C.; Martins, C.; Mehner, A.; Micela, G.; Monaco, L.; Nunes, N.; Pepe, F.; Poretti, E.; Rebolo, R.; Santos, N.; Saviane, I.; Sousa, S.; Sozzetti, A.; Suarez-Mascareño, A.; Udry, S.; Zapatero-Osorio, M.
    Context. The vast majority of the known stars of ultra low metallicity ([Fe=H] >-4:5) are known to be enhanced in carbon, and belong to the 'low-carbon band' (A(C) = log(C=H) + 12 7:6). It is generally, although not universally, accepted that this peculiar chemical composition reflects the chemical composition of the gas cloud out of which these stars were formed. The first ultra-metalpoor star discovered, HE 0107-5240, is also enhanced in carbon and belongs to the 'low-carbon band'. It has recently been claimed to be a long-period binary, based on radial velocity measurements. It has also been claimed that this binarity may explain its peculiar composition as being due to mass transfer from a former AGB companion. Theoretically, low-mass ratios in binary systems are much more favoured amongst Pop III stars than they are amongst solar-metallicity stars. Any constraint on the mass ratio of a system of such low metallicity would shed light on the star formation mechanisms in this metallicity regime. Aims.We acquired one high precision spectrum withESPRESSO in order to check the reality of the radial velocity variations. In addition we analysed all the spectra of this star in the ESO archive obtained with UVES to have a set of homogenously measured radial velocities. Methods. The radial velocities were measured using cross correlation against a synthetic spectrum template. Due to the weakness of metallic lines in this star, the signal comes only from the CH molecular lines of the G-band. Results. The measurement obtained in 2018 from an ESPRESSO spectrum demonstrates unambiguously that the radial velocity of HE 0107-5240 has increased from 2001 to 2018. Closer inspection of the measurements based on UVES spectra in the interval 2001-2006 show that there is a 96% probability that the radial velocity correlates with time, hence the radial velocity variations can already be suspected from the UVES spectra alone. Conclusions.We confirm the earlier claims of radial velocity variations in HE0107-5240. The simplest explanation of such variations is that the star is indeed in a binary system with a long period. The nature of the companion is unconstrained and we consider it is equally probable that it is an unevolved companion or a white dwarf. Continued monitoring of the radial velocities of this star is strongly encouraged.
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    Spectroscopic and photometric oscillatory envelope variability during the S Doradus outburst of the luminous blue variable R71
    (EDP Sciences, 2017) Mehner, A.; Baade, D.; Groh, J.H.; Rivinius, T.; Hambsch, F.-J.; Bartlett, E.S.; Asmus, D.; Agliozzo, C.; Szeifert, T.; Stahl, O.
    Context. Luminous blue variables (LBVs) are evolved massive stars that exhibit instabilities that are not yet understood. Stars can lose several solar masses during this evolutionary phase. The LBV phenomenon is thus critical to our understanding of the evolution of the most massive stars. Aims. The LBV R71 in the Large Magellanic Cloud is presently undergoing an S Doradus outburst, which started in 2005. To better understand the LBV phenomenon, we determine the fundamental stellar parameters of R71 during its quiescence phase. In addition, we analyze multiwavelength spectra and photometry obtained during the current outburst. Methods. We analyzed pre-outburst CASPEC spectra from 1984-1997, EMMI spectra in 2000, UVES spectra in 2002, and FEROS spectra from 2005 with the radiative transfer code CMFGEN to determine the fundamental stellar parameters of the star. A spectroscopic monitoring program with VLT X-shooter since 2012 secured visual to near-infrared spectra throughout the current outburst, which is well-covered by ASAS and AAVSO photometry. Mid-infrared images and radio data were also obtained. Results. During quiescence, R71 has an effective temperature of Teff = 15 500 K and a luminosity of log(L-/L⊙) = 5.78. We determine its mass-loss rate to 4.0 × 10-6M⊙ yr-1. We present the spectral energy distribution of R71 from the near-ultraviolet to the mid-infrared during its present outburst. Semi-regular oscillatory variability in the light curve of the star is observed during the current outburst. Absorption lines develop a second blue component on a timescale of twice that length. The variability may consist of one (quasi-)periodic component with P ∼ 425/850 d with additional variations superimposed. Conclusions. R71 is a classical LBV, but this star is at the lower luminosity end of this group. Mid-infrared observations suggest that we are witnessing dust formation and grain evolution. During its current S Doradus outburst, R71 occupies a region in the HR diagram at the high-luminosity extension of the Cepheid instability strip and exhibits similar irregular variations as RV Tau variables. LBVs do not pass the Cepheid instability strip because of core evolution, but they develop comparable cool, low-mass, extended atmospheres in which convective instabilities may occur. As in the case of RV Tau variables, the occurrence of double absorption lines with an apparent regular cycle may be due to shocks within the atmosphere and period doubling may explain the factor of two in the lengths of the photometric and spectroscopic cycles.
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    VLTI-MATISSE chromatic aperture-synthesis imaging of η Carinae's stellar wind across the Br α line: Periastron passage observations in February 2020
    (Astronomy and Astrophysics, 2021-08) Weigelt, G.; Hofmann, K.-H.; Schertl, D.; Lopez, B.; Petrov, R.G.; Lagarde, S.; Berio, Ph.; Jaffe, W.; Henning, Th.; Millour, F.; Meilland, A.; Allouche, F.; Robbe-Dubois, S.; Matter, A.; Cruzalèbes, p.; Hillier, D.J.; Russell, C.M.P.; Madura, T.; Gull, T.R.; Corcoran, M.F.; Damineli, A.; Moffat, A.F.J.; Morris, P.W.; Richardson, N.D.; Paladini, C.; Schöller, M.; Mérand, A.; Glindemann, A.; Beckmann, U.; Heininger, M.; Bettonvil, F.; Bettonvil, F.; Zins, G.; Woillez, J.; Bristow, P.; Sanchez-Bermudez, J.; Ohnaka, K.; Kraus, S.; Mehner, A.; Wittkowski, M.; Hummel, C.A.; Stee, P.; Vakili, F.; Hartman, H.; Navarete, F.; Hamaguchi, K.; Espinoza-Galeas, D.A.; Stevens, I.R.; Van Boekel, R.; Wolf, S.; Hogerheijde, M.R.; Dominik, C.; Augereau, J.-C.; Pantin, E.; Waters, L.B.F.M.; Meisenheimer, K.; Varga, J.; Klarmann, L.; Gámez Rosas, V.; Burtscher, L.; Leftley, J.; Isbell, J.W.; Hocdé, V.; Yoffe, G.; Kokoulina, E.; Hron, J.; Groh, J.; Kreplin, A.; Rivinius, Th.; De Wit, W.-J.; Danchi, W.-C.; De Souza A., Domiciano; Drevon, J.; Labadie, L.; Connot, C.; Nußbaum, E.; Lehmitz, M.; Antonelli, P.; Graser, U.; Leinert, C.
    Context. Eta Carinae is a highly eccentric, massive binary system (semimajor axis ~15.5 au) with powerful stellar winds and a phase-dependent wind-wind collision (WWC) zone. The primary star, η Car A, is a luminous blue variable (LBV); the secondary, η Car B, is a Wolf-Rayet or O star with a faster but less dense wind. Aperture-synthesis imaging allows us to study the mass loss from the enigmatic LBV η Car. Understanding LBVs is a crucial step toward improving our knowledge about massive stars and their evolution. Aims. Our aim is to study the intensity distribution and kinematics of η Car's WWC zone. Methods. Using the VLTI-MATISSE mid-infrared interferometry instrument, we perform Brα imaging of η Car's distorted wind. Results. We present the first VLTI-MATISSE aperture-synthesis images of η Car A's stellar windin several spectral channels distributed across the Brα 4.052 μm line (spectral resolving power R ~ 960). Our observations were performed close to periastron passage in February 2020 (orbital phase ~ 14.0022). The reconstructed iso-velocity images show the dependence of the primary stellar wind on wavelength or line-of-sight (LOS) velocity with a spatial resolution of 6 mas (~14 au). The radius of the faintest outer wind regions is ~26 mas (~60 au). At several negative LOS velocities, the primary stellar wind is less extended to the northwest than in other directions. This asymmetry is most likely caused by the WWC. Therefore, we see both the velocity field of the undisturbed primary wind and the WWC cavity. In continuum spectral channels, the primary star wind is more compact than in line channels. A fit of the observed continuum visibilities with the visibilities of a stellar wind CMFGEN model (CMFGEN is an atmosphere code developed to model the spectra of a variety of objects) provides a full width at half maximum fit diameter of the primary stellar wind of 2.84 ± 0.06 mas (6.54 ± 0.14 au). We comparethe derived intensity distributions with the CMFGEN stellar wind model and hydrodynamic WWC models.