| 1. |
- Koumpia, E., et al.
(författare)
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Optical and near-infrared observations of the Fried Egg Nebula: Multiple shell ejections on a 100 yr timescale from a massive yellow hypergiant
- 2020
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 635
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Tidskriftsartikel (refereegranskat)abstract
- Context. The fate of a massive star during the latest stages of its evolution is highly dependent on its mass-loss rate and geometry and therefore knowing the geometry of the circumstellar material close to the star and its surroundings is crucial. Aims. We aim to provide insight into the nature (i.e. geometry, rates) of mass-loss episodes, and in particular, the connection between the observed asymmetries due to the mass lost in a fast wind or during a previous, prodigious mass-losing phase. In this context, yellow hypergiants offer a good opportunity to study mass-loss events. Methods. We analysed a large set of optical and near-infrared data in spectroscopic and photometric, spectropolarimetric, and interferometric (GRAVITY/VLTI) modes, towards the yellow hypergiant IRAS 17163-3907. We used X-shooter optical observations to determine the spectral type of this yellow hypergiant and we present the first model-independent, reconstructed images of IRAS 17163-3907 at these wavelengths tracing milli-Arcsecond scales. Lastly, we applied a 2D radiative transfer model to fit the dereddened photometry and the radial profiles of published diffraction-limited VISIR images at 8.59 μm, 11.85 μm, and 12.81 μm simultaneously, adopting a revised distance determination using Gaia Data Release 2 measurements. Results. We constrain the spectral type of IRAS 17163-3907 to be slightly earlier than A6Ia (Teffâ ∼â 8500 K). The interferometric observables around the 2 μm window towards IRAS 17163-3907 show that the Brγ emission appears to be more extended and asymmetric than the Naâ » I and the continuum emission. Interestingly, the spectrum of IRAS 17163-3907 around 2 μm shows Mgâ » II emission that is not previously seen in other objects of its class. In addition, Brγ shows variability in a time interval of four months that is not seen towards Naâ » I. Lastly, in addition to the two known shells surrounding IRAS 17163-3907, we report on the existence of a third hot inner shell with a maximum dynamical age of only 30 yr. Conclusions. The 2 μm continuum originates directly from the star and not from hot dust surrounding the stellar object. The observed spectroscopic variability of Brγ could be a result of variability in the mass-loss rate. The interpretation of the presence of Naâ » I emission at closer distances to the star compared to Brγ has been a challenge in various studies. To address this, we examine several scenarios. We argue that the presence of a pseudo-photosphere, which was traditionally considered to be the prominent explanation, is not needed and that it is rather an optical depth effect. The three observed distinct mass-loss episodes are characterised by different mass-loss rates and can inform theories of mass-loss mechanisms, which is a topic still under debate both in theory and observations. We discuss these in the context of photospheric pulsations and wind bi-stability mechanisms.
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| 2. |
- Koumpia, Evgenia, et al.
(författare)
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Tracing a decade of activity towards a yellow hypergiant. The spectral and spatial morphology of IRC+10420 at au scales
- 2022
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 515:2, s. 2766-2777
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Tidskriftsartikel (refereegranskat)abstract
- The fate of a massive star during the latest stages of its evolution is highly dependent on its mass-loss history and geometry, with the yellow hypergiants (YHGs) being key objects. We present near-IR interferometric observations of the famous YHG IRC+10420 and blue spectra taken between 1994 and 2019. Our 2.2-mu m GRAVITY/VLTI observations attain a spatial resolution of similar to 5 stellar radii and spatially resolve the hot emission in the K-band tracing the gas via Na i doublet emission and the Br gamma emission. Our geometric modelling reveals a compact neutral zone (Na i) which is slightly larger than the continuum but within an extended Br gamma emitting region. Our study confirms an hour-glass geometry of the wind, but we find no signature of a companion at 7-800 au separations at the contrast limit of our observations (3.7 mag at 3 sigma) to explain this geometry. We report an evolution of the ejecta over 7 yr, and constrain the opening angle of the hour-glass to be <10 degrees. Lastly, we present the first blue optical spectra of IRC+10420 since 1994. The multi-epoch data indicate that the spectral type, and thus temperature, of the object has essentially remained constant during the intervening years. Therefore, the observed increase in temperature of 2000 K in less than two decades prior to 1994 is now halted. This suggests that this YHG has 'hit' the White Wall in the HR-diagram preventing it from evolving blue-wards, and will likely undergo a major mass-loss event in the near future.
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