Browsing by Subject "stars: mass-loss"

Now showing 1 - 5 of 5
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    Open Access
    Aperture synthesis imaging of the carbon AGB star R Sculptoris: Detection of a complex structure and a dominating spot on the stellar disk
    (2017) Wittkowski, M; Hofmann, K-H; Höfner, S; Le Bouquin, J B; Nowotny, W; Paladini, C; Young, J; Berger, J-P; Brunner, M; de Gregorio-Monsalvo, I; Eriksson, K; Hron, J; Humphreys, E M L; Lindqvist, M; Maercker, M; Mohamed, S; Olofsson, H; Ramstedt, S; Weigelt, G
    Aims. We present near-infrared interferometry of the carbon-rich asymptotic giant branch (AGB) star R Sculptoris (R Scl).
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    Open Access
    Near-infrared evidence for a sudden temperature increase in Eta Carinae
    (2014) Mehner, Andrea; Ishibashi, Kazunori; Whitelock, Patricia; Nagayama, Takahiro; Feast, Michael; van Wyk, Francois; de Wit, Willem-Jan
    Aims. Eta Car's ultra-violet, optical, and X-ray light curves and its spectrum suggest a physical change in its stellar wind over the last decade. It was proposed that the mass-loss rate decreased by a factor of about 2 in the last 15 years. We complement these recent results by investigating the past evolution and the current state of eta Car in the near-infrared (IR). Methods. We present JHKL photometry of eta Car obtained at SAAO Sutherland from 2004-2013 with the Mk II photometer at the 0.75-m telescope and JHKs photometry with SIRIUS at the 1.4-m IRSF telescope from 2012-2013. The near-IR light curves since 1972 are analyzed. Results. The long-term brightening trends in eta Car's JHKL light curves were discontinuous around the 1998 periastron passage. After 1998, the star shows excess emission above the extrapolated trend from earlier dates, foremost in J and H, and the blueward, cyclical progression in its near-IR colors is accelerated. The near-IR color evolution is strongly correlated with the periastron passages. After correcting for the secular trend we find that the color evolution matches an apparent increase in blackbody temperature of an optically thick near-IR emitting plasma component from about 3500 to 6000 K over the last 20 years. Conclusions. We suggest that the changing near-IR emission may be caused by variability in optically thick bremsstrahlung emission. Periastron passages play a key role in the observed excess near-IR emission after 1998 and the long-term color evolution. We thus propose as a hypothesis that angular momentum transfer (via tidal acceleration) during periastron passages leads to sudden changes in eta Car's atmosphere resulting in a long-term decrease in the mass-loss rate.
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    Open Access
    Near-infrared spectro-interferometry of Mira variables and comparisons to 1D dynamic model atmospheres and 3D convection simulations
    (2016) Wittkowski, M; Chiavassa, A; Freytag, B; Scholz, M; Höfner, S; Karovicova, I; Whitelock, P A
    Aims. We aim at comparing spectro-interferometric observations of Mira variable asymptotic giant branch (AGB) stars with the latest 1D dynamic model atmospheres based on self-excited pulsation models (CODEX models) and with 3D dynamic model atmospheres including pulsation and convection (CO5BOLD models) to better understand the processes that extend the molecular atmosphere to radii where dust can form.
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    Open Access
    On the nature of the cool component of MWC 560
    (2007) Gromadzki, M; Mikołajewska, J; Whitelock, P A; Marang, F
    Context.MWC 560 (V694 Mon) is one of the most enigmatic symbiotic system with a very active accretion-powered hot component. Such activity can be supported only by a luminous asymptotic giant branch star, i.e. a Mira or SR variable, with a high mass-loss rate. It is also a very unusual jet source because the jet axis lies practically parallel to the line of sight. Aims. The aims of our study are the determination of the evolutionary status of the cool component of MWC 560. Methods. Our methods involve analysis of near-IR JHKL and optical light curves. Results. The cool component of MWC 560 pulsates with a period of ∼340 days, and it is probably a red SR variable on the thermally pulsing AGB. The high mass-loss rate expected for such a star is sufficient to power the observed activity of the hot companion.
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    Open Access
    The Spitzer spectroscopic survey of S-type stars
    (2012) Smolders, K; Neyskens, P; Blommaert, J A D L; Hony, S; Van Winckel, H; Decin, L; Van Eck, S; Sloan, G C; Cami, J; Uttenthaler, S; Degroote, P; Barry, D; Feast, M; Groenewegen, M A T; Matsuura, M; Menzies, J; Sahai, R; van Loon, J Th; Zijlstra, A A; Acke, B; Bloemen, S; Cox, N; de Cat, P; Desmet, M; Exter, K; Ladjal, D; Østensen, R; Saesen, S; van Wyk, F; Verhoelst, T; Zima, W
    Context. S-type AGB stars are thought to be in the transitional phase between M-type and C-type AGB stars. Because the composition of the circumstellar environment reflects the photospheric abundances, one may expect a strong influence of the stellar C/O ratio on the molecular chemistry and the mineralogy of the circumstellar dust.