| 1. |
- Kraus, Stefan, et al.
(författare)
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Planet Formation Imager (PFI) : Science vision and key requirements
- 2016
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Ingår i: Optical and Infrared Interferometry and Imaging V. - : SPIE. - 9781510601932 ; 9907
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Konferensbidrag (refereegranskat)abstract
- The Planet Formation Imager (PFI) project aims to provide a strong scientific vision for ground-based optical astronomy beyond the upcoming generation of Extremely Large Telescopes. We make the case that a breakthrough in angular resolution imaging capabilities is required in order to unravel the processes involved in planet formation. PFI will be optimised to provide a complete census of the protoplanet population at all stellocentric radii and over the age range from 0.1 to ∼100 Myr. Within this age period, planetary systems undergo dramatic changes and the final architecture of planetary systems is determined. Our goal is to study the planetary birth on the natural spatial scale where the material is assembled, which is the "Hill Sphere" of the forming planet, and to characterise the protoplanetary cores by measuring their masses and physical properties. Our science working group has investigated the observational characteristics of these young protoplanets as well as the migration mechanisms that might alter the system architecture. We simulated the imprints that the planets leave in the disk and study how PFI could revolutionise areas ranging from exoplanet to extragalactic science. In this contribution we outline the key science drivers of PFI and discuss the requirements that will guide the technology choices, the site selection, and potential science/technology tradeoffs.
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| 2. |
- Wallström, Sofia, 1988, et al.
(författare)
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ALMA Compact Array observations of the Fried Egg nebula: Evidence for large-scale asymmetric mass-loss from the yellow hypergiant IRAS 17163-3907
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 597, s. A99, pp. 1-10
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Tidskriftsartikel (refereegranskat)abstract
- Yellow hypergiants are rare and represent a fast evolutionary stage of massive evolved stars. That evolutionary phase is characterised by a very intense mass loss, the understanding of which is still very limited. Here we report ALMA Compact Array observations of a 50??-mosaic toward the Fried Egg nebula, around one of the few Galactic yellow hypergiants IRAS 17163-3907. The emission from the 12CO J = 2-1 line, H30? recombination line, and continuum is imaged at a resolution of ~8??, revealing the morphology of the molecular environment around the star. The continuum emission is unresolved and peaks at the position of the star. The radio recombination line H30? shows unresolved emission at the star, with an approximately Gaussian spectrum centered on a velocity of 21 ± 3km s-1 with a width of 57 ± 6km s-1. In contrast, the CO 2-1 emission is complex and decomposes into several components beyond the contamination from interstellar gas in the line of sight. The CO spectrum toward the star is a broad plateau, centered at the systemic velocity of +18 km s-1 and with an expansion velocity of 100 ± 10km s-1. Assuming isotropic and constant mass-loss, we estimate a mass-loss rate of 8 ± 1.5 × 10-5M? yr-1. At a radius of 25?? from the star, we detect CO emission associated with the dust ring previously imaged by Herschel. The kinematics of this ring, however, is not consistent with an expanding shell, but show a velocity gradient of vsys ± 20km s-1. In addition, we find a puzzling bright feature radially connecting the star to the CO ring, at a velocity of +40 km s-1 relative to the star. This spur feature may trace a unidirectional ejection event from the star. Our ACA observations reveal the complex morphology around IRAS 17163 and illustrate the breakthroughs that ALMA will bring to the field of massive stellar evolution.
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| 3. |
- 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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| 4. |
- Wheelwright, H. E., et al.
(författare)
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The close Be star companion of β Cephei
- 2009
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 497, s. 487-495
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Tidskriftsartikel (refereegranskat)abstract
- Context: The prototype of the β Cephei class of pulsating stars, β Cep, rotates relatively slowly, and yet displays episodic Hα emission. Such behaviour is typical of a rapidly rotating, classical Be star. For some time this posed a contradiction to our understanding of the Be phenomena as rapid rotation is thought to be a prerequisite for the characteristic emission phases of Be stars. Recent work has demonstrated that the Hα emission is in fact due to a close companion (separation ≈ 0.25 arcsec) of the star. This resolves the apparent enigma if this close companion is indeed a classical Be star, as has been proposed. Aims: We aim to test the hypothesis that this close companion is a valid Be star by determining properties such as its spectral type and v sin i. Methods: We employed the technique of spectroastrometry to investigate the close binary system. Using the spectroastrometric signatures observed, we split the composite binary spectra into its constituent spectra in the B band (4200-5000 Å) and R band (6200-7000 Å). Results: The spectroastrometrically split spectra allow us to estimate spectral types of the binary components. We find that the primary of the close binary system has a spectral type of B2III and the secondary a spectral type of B5Ve. From the relationship between mass and spectral type, we determine the masses of the binary components to be M_pri = 12.6 ± 3.2 Msun and M_sec = 4.4 ± 0.7 Msun respectively. The spectroastrometric data allow some constraint on the orbit, and we suggest a moderate revision to the previously determined orbit. We confirm that the primary of the system is a slow rotator (v sin i = 29+43-29 km s-1), while the secondary rotates significantly faster, at a v sin i = 230 ± 45 km s-1 Conclusions: We show that the close companion to the β Cephei primary is certainly a valid classical Be star. It has a spectral type of B5Ve and is a relatively fast rotator. We confirm that the β Cephei system does not contradict our current understanding of classical Be stars.
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| 5. |
- 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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| 6. |
- Wallström, Sofia, 1988, et al.
(författare)
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Investigating the nature of the Fried Egg nebula: CO mm-line and optical spectroscopy of IRAS 17163–3907
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 574:A139
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Tidskriftsartikel (refereegranskat)abstract
- Through CO mm-line and optical spectroscopy, we investigate the properties of the Fried Egg nebula IRAS 17163–3907, which has recently been proposed to be one of the rare members of the yellow hypergiant class. The CO J = 2−1 and J = 3−2 emission arises from a region within 20'' of the star and is clearly associated with the circumstellar material. The CO lines show a multi-component asymmetrical profile, and an unexpected velocity gradient is resolved in the east-west direction, suggesting a bipolar outflow. This is in contrast with the apparent symmetry of the dust envelope as observed in the infrared. The optical spectrum of IRAS 17163–3907 between 5100 and 9000 Å was compared with that of the archetypal yellow hypergiant IRC+10420 and was found to be very similar. These results build on previous evidence that IRAS 17163–3907 is a yellow hypergiant.
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