Examinando por Autor "Ohnaka, K."

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    Spatially resolved mid-infrared observations of the circumstellar environment of the born-again object FG Sge
    (EDP Sciences, 2022-12-01) Ohnaka, K.; Jara Bravo, B.A.
    Context. FG Sge has evolved from the hot central star of the young planetary nebula Hen 1-5 to a G-K supergiant in the last 100 yr. It is one of the three born-again objects that have been identified as of yet, and they are considered to have undergone a thermal pulse in the post-asymptotic giant branch evolution. Aims. We present mid-infrared spectro-interferometric observations of FG Sge and probe its dusty environment. Methods. FG Sge was observed with MIDI at the Very Large Telescope Interferometer at baselines of 43 and 46 m between 8 and 13 μm. Results. The circumstellar dust environment of FG Sge was spatially resolved, and the Gaussian fit to the observed visibilities results in a full width at half maximum of ~10.5 mas. The observed mid-infrared visibilities and the spectral energy distribution can be fairly reproduced by optically thick (τV ≈8) spherical dust shell models consisting of amorphous carbon with an inner radius rin of ~30 R∗ (corresponding to a dust temperature of 1100 ± 100 K). The dust shell is characterized with a steep density profile proportional to r-3.5±0.5 from the inner radius rin to (5-10) × rin, beyond which it changes to r-2. The dust mass is estimated to be ~ 7 × 10-7 M⊙, which translates into an average total mass-loss rate of ~ 9 × 10-6 M⊙ yr-1 as of 2008 with a gas-to-dust ratio of 200 being adopted. In addition, the 8-13 μm spectrum obtained with MIDI with a field of view of 200 mas does not show a signature of the polycyclic aromatic hydrocarbon (PAH) emission, which is in marked contrast to the spectra taken with the Spitzer Space Telescope six and 20 months before the MIDI observations with wide slit widths of 3″6-10″. This implies that the PAH emission originates from an extended region of the optically thick dust envelope. Conclusions. The dust envelope of FG Sge is much more compact than that of the other born-again stars' Sakurai's object and V605 Aql, which might reflect the difference in the evolutionary status. The PAH emission from the extended region of the optically thick dust envelope likely originates from the material ejected before the central star became H-deficient, and it may be excited by the UV radiation from the central star escaping through gaps among dust clumps and/or the bipolar cavity of a disk-like structure. © 2022 EDP Sciences. All rights reserved.
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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.
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    VLTI-MATISSE L - And N -band aperture-synthesis imaging of the unclassified B[e] star FS Canis Majoris
    (EDP Sciences, 2022-02-01) Hofmann, K.-H.; Bensberg, A.; Schertl, D.; Weigelt, G.; Wolf, S.; Meilland, A.; Millour, F.; Waters, L.B.F.M.; Kraus, S.; Ohnaka, K.; Lopez, B.; Petrov, R.G.; Lagarde, S.; Berio, Ph.; Allouche, F.; Robbe-Dubois, S.; Jaffe, W.; Henning, Th.; Paladini, C.; Schöller, M.; Mérand, A.; Glindemann, A.; Beckmann, U.; Heininger, M.; Bettonvil, F.; Zins, G.; Woillez, J.; Bristow, P.; Stee, P.; Vakili, F.; Van Boekel, R.; Hogerheijde, M.R.; Dominik, C.; Augereau, J.-C.; Matter, A.; Hron, J.; Pantin, E.; Rivinius, Th.; De Wit, W.-J.; Varga, J.; Klarmann, L.; Meisenheimer, K.; Gámez Rosas, V.; Burtscher, L.; Leftley, J.; Isbell, J.W.; Yoffe, G.; Kokoulina, E.; Danchi, W.C.; Cruzalèbes, P.; Domiciano De Souza, A.; Drevon, J.; Hocdé, V.; Kreplin, A.; Labadie, L.; Connot, C.; Nußbaum, E.; Lehmitz, M.; Antonelli, P.; Graser, U.; Leinert, C.
    Context. FS Canis Majoris (FS CMa, HD 45677) is an unclassified B[e] star surrounded by an inclined dust disk. The evolutionary stage of FS CMa is still debated. Perpendicular to the circumstellar disk, a bipolar outflow was detected. Infrared aperture-synthesis imaging provides us with a unique opportunity to study the disk structure. Aims. Our aim is to study the intensity distribution of the disk of FS CMa in the mid-infrared L and N bands. Methods. We performed aperture-synthesis imaging of FS CMa with the MATISSE instrument (Multi AperTure mid-Infrared SpectroScopic Experiment) in the low spectral resolution mode to obtain images in the L and N bands. We computed radiative transfer models that reproduce the L- and N-band intensity distributions of the resolved disks. Results. We present L- and N-band aperture-synthesis images of FS CMa reconstructed in the wavelength bands of 3.4-3.8 and 8.6-9.0 μm. In the L-band image, the inner rim region of an inclined circumstellar disk and the central object can be seen with a spatial resolution of 2.7 milliarcsec (mas). An inner disk cavity with an angular diameter of ~6 × 12 mas is resolved. The L-band disk consists of a bright northwestern (NW) disk region and a much fainter southeastern (SE) region. The images suggest that we are looking at the bright inner wall of the NW disk rim, which is on the far side of the disk. In the N band, only the bright NW disk region is seen. In addition to deriving the inclination and the inner disk radius, fitting the reconstructed brightness distributions via radiative transfer modelling allows one to constrain the innermost disk structure, in particular the shape of theinner disk rim. © K.-H. Hofmann et al. 2022.