| 1. |
- Hrudkova, M., et al.
(författare)
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The discovery of a planetary candidate around the evolved low-mass Kepler giant star HD 175370
- 2017
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 464:1, s. 1018-1028
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Tidskriftsartikel (refereegranskat)abstract
- We report on the discovery of a planetary companion candidate with a minimum mass M sin i = 4.6 +/- 1.0 M-Jupiter orbiting the K2 III giant star HD 175370 (KIC 007940959). This star was a target in our programme to search for planets around a sample of 95 giant stars observed with Kepler. This detection was made possible using precise stellar radial velocity measurements of HD 175370 taken over five years and four months using the coude echelle spectrograph of the 2-m Alfred Jensch Telescope and the fibre-fed echelle spectrograph High Efficiency and Resolution Mercator Echelle Spectrograph of the 1.2-m Mercator Telescope. Our radial velocity measurements reveal a periodic (349.5 +/- 4.5 d) variation with a semi-amplitude K = 133 +/- 25 ms(-1), superimposed on a long-term trend. A low-mass stellar companion with an orbital period of similar to 88 yr in a highly eccentric orbit and a planet in a Keplerian orbit with an eccentricity e = 0.22 are the most plausible explanation of the radial velocity variations. However, we cannot exclude the existence of stellar envelope pulsations as a cause for the low-amplitude radial velocity variations and only future continued monitoring of this system may answer this uncertainty. From Kepler photometry, we find that HD 175370 is most likely a low-mass red giant branch or asymptotic giant branch star.
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| 2. |
- Van de Steene, G. C., et al.
(författare)
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Herschel imaging of the dust in the Helix nebula (NGC 7293)
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 574
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Tidskriftsartikel (refereegranskat)abstract
- Aims. In our series of papers presenting the Herschel imaging of evolved planetary nebulae, we present images of the dust distribution in the Helix nebula (NGC 7293). Methods. Images at 70, 160, 250, 350, and 500 mu m were obtained with the PACS and SPIRE instruments on board the Herschel satellite. Results. The broadband maps show the dust distribution over the main Helix nebula to be u npy and predominantly present in the barrel wall. We determined the spectral energy distribution of the main nebula in a consistent way using Herschel. IRAS. and Planck flux values, The emissivity index of beta = 0.99 +/- 0.09, in combination with the carbon rich molecular chemistry of the nebula, indicates that the dust consists mainly of amorphous carbon. The dust excess emission from the central star disk is detected at 70 pm and the flux measurement agrees with previous measurement. We present the temperature and dust column density maps. The total dust mass across the Helix nebula (without its halo) is determined to be 3.5 x 10(-3) M-circle dot at a distance of 216 pc. The temperature map shows dust temperatures between 22 K and 42 K, which is similar to the kinetic temperature of the molecular gas, confirming that the dust and gas co exist in high density clumps. Archived images are used to compare the location of the dust emission in the far infrared (Herschel) with the ionized (GALEX and H-beta) and molecular (H-2) component. The different emission components are consistent with the Helix consisting of a thick walled barrel-like structure inclined to the line of sight. The radiation field decreases rapidly through the barrel wall.
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| 3. |
- 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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| 4. |
- 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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| 5. |
- Hillen, M., et al.
(författare)
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The evolved circumbinary disk of AC Herculis : a radiative transfer, interferometric, and mineralogical study
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 578
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Tidskriftsartikel (refereegranskat)abstract
- Context. Many post-asymptotic giant branch (post-AGB) stars in binary systems have an infrared (IR) excess arising from a dusty circumbinary disk. The disk formation, current structure, and further evolution are, however, poorly understood. Aims. We aim to constrain the structure of the circumstellar material around the post-AGB binary and RV Tauri pulsator AC Her. We want to constrain the spatial distribution of the amorphous and of the crystalline dust. Methods. We present very high-quality mid-IR interferometric data that were obtained with the MIDI / VLTI instrument. We analyze the MIDI visibilities and di ff erential phases in combination with the full spectral energy distribution, using the MCMax radiative transfer code, to find a good structure model of AC Her's circumbinary disk. We include a grain size distribution and midplane settling of dust self-consistently in our models. The spatial distribution of crystalline forsterite in the disk is investigated with the mid-IR features, the 69 mu m band and the 11.3 mu m signatures in the interferometric data. Results. All the data are well fitted by our best model. The inclination and position angle of the disk are precisely determined at i = 50 +/- 8 degrees and PA = 305 +/- 10 degrees. We firmly establish that the inner disk radius is about an order of magnitude larger than the dust sublimation radius. The best-fit dust grain size distribution shows that significant grain growth has occurred, with a significant amount of mm-sized grains now being settled to the midplane of the disk. A large total dust mass >= 10(-3) M-circle dot is needed to fit the mm fluxes. By assuming alpha(turb) = 0 : 01, a good fit is obtained with a small grain size power law index of 3.25, combined with a small gas / dust ratio <= 10. The resulting gas mass is compatible with recent estimates employing direct gas diagnostics. The spatial distribution of the forsterite is di ff erent from the amorphous dust, as more warm forsterite is needed in the surface layers of the inner disk. Conclusions. The disk in the ACHer system is in a very evolved state, as shown by its small gas/dust ratio and large inner hole. Mid-IR interferometry o ff ers unique constraints, complementary to mid-IR features, for studying the mineralogy in disks. A better uv coverage is needed to constrain in detail the distribution of the crystalline forsterite in the disk of ACHer, but we find strong similarities with the protoplanetary disk HD 100546.
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