| 1. |
- Amanullah, Rahman, et al.
(author)
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Diversity in extinction laws of Type Ia supernovae measured between 0.2 and 2 μm
- 2015
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 453:3, s. 3300-3328
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Journal article (peer-reviewed)abstract
- We present ultraviolet (UV) observations of six nearby Type Ia supernovae (SNe Ia) obtained with the Hubble Space Telescope, three of which were also observed in the near-IR (NIR) with Wide-Field Camera 3. UV observations with the Swift satellite, as well as ground-based optical and NIR data provide complementary information. The combined data set covers the wavelength range 0.2-2 mu m. By also including archival data of SN 2014J, we analyse a sample spanning observed colour excesses up to E(B - V) = 1.4 mag. We study the wavelength-dependent extinction of each individual SN and find a diversity of reddening laws when characterized by the total-to-selective extinction R-V. In particular, we note that for the two SNe with E(B - V) greater than or similar to 1 mag, for which the colour excess is dominated by dust extinction, we find R-V = 1.4 +/- 0.1 and R-V = 2.8 +/- 0.1. Adding UV photometry reduces the uncertainty of fitted R-V by similar to 50 per cent allowing us to also measure R-V of individual low-extinction objects which point to a similar diversity, currently not accounted for in the analyses when SNe Ia are used for studying the expansion history of the Universe.
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| 2. |
- Liseau, René, 1949, et al.
(author)
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alpha Centauri A in the far infrared - First measurement of the temperature minimum of a star other than the Sun
- 2013
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 549
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Journal article (peer-reviewed)abstract
- Context. Chromospheres and coronae are common phenomena on solar-type stars. Understanding the energy transfer to these heated atmospheric layers requires direct access to the relevant empirical data. Study of these structures has, by and large, been limited to the Sun thus far.Aims. The region of the temperature reversal can be directly observed only in the far infrared and submillimetre spectral regime. We aim at determining the characteristics of the atmosphere in the region of the temperature minimum of the solar sister star alpha Cen A. As a bonus this will also provide a detailed mapping of the spectral energy distribution, i.e. knowledge that is crucial when searching for faint, Kuiper belt-like dust emission around other stars.Methods. For the nearby binary system alpha Cen, stellar parameters are known with high accuracy from measurements. For the basic model parameters T-eff, log g and [Fe/H], we interpolate stellar model atmospheres in the grid of Gaia/PHOENIX and compute the corresponding model for the G2 V star alpha Cen A. Comparison with photometric measurements shows excellent agreement between observed photospheric data in the optical and infrared. For longer wavelengths, the modelled spectral energy distribution is compared to Spitzer-MIPS, Herschel-PACS, Herschel-SPIRE, and APEX-LABOCA photometry. A specifically tailored Uppsala model based on the MARCS code and extending further in wavelength is used to gauge the emission characteristics of alpha Cen A in the far infared.Results. Similar to the Sun, the far infrared (FIR) emission of alpha Cen A originates in the minimum temperature region above the stellar photosphere in the visible. However, in comparison with the solar case, the FIR photosphere of alpha Cen A appears marginally cooler, T-min similar to T-160 (mu m) = 3920 +/- 375 K. Beyond the minimum near 160 mu m, the brightness temperatures increase, and this radiation very likely originates in warmer regions of the chromosphere of alpha Cen A.Conclusions. To the best of our knowledge, this is the first time a temperature minimum has been directly measured on a main-sequence star other than the Sun.
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| 3. |
- Romeo, Alessandro, 1961, et al.
(author)
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The effective stability parameter for two-component galactic discs: is 1/Q ~ 1/Q_stars + 1/Q_gas ?
- 2011
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In: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 416:2, s. 1191-1196
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Journal article (peer-reviewed)abstract
- The Wang-Silk approximation, Q-1≈Q-1stars+Q-1gas, is frequently used for estimating the effective Q parameter in two-component discs of stars and gas. Here we analyse this approximation in detail, and show how its accuracy depends on the radial velocity dispersions and Toomre parameters of the two components. We then propose a much more accurate but still simple approximation for the effective Q parameter, which further takes into account the stabilizing effect of disc thickness. Our effective Q parameter is a natural generalization of Toomre's Q, and as such can be used in a wide variety of contexts, e.g. for predicting star formation thresholds in galaxies or for measuring the stability level of galactic discs at low and high redshifts.
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| 4. |
- Wiegert, Joachim, 1985, et al.
(author)
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94 Ceti: a triple star with a planet and dust disc
- 2016
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In: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 462:2, s. 1735-1748
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Journal article (peer-reviewed)abstract
- 94 Ceti is a triple star system with a circumprimary gas giant planet and far-infrared excess. Such excesses around main sequence stars are likely due to debris discs, and are considered as signposts of planetary systems and, therefore, provide important insights into the configuration and evolution of the planetary system. Consequently, to learn more about the 94 Ceti system, we aim to model the dust emission precisely to fit its observed spectral energy distribution and to simulate its orbital dynamics. We interpret our APEX bolometric observations and complement them with archived Spitzer and Herschel bolometric data to explore the stellar excess and to map out background sources in the fields. Dynamical simulations and 3D radiative transfer calculations were used to constrain the debris disc configurations and model the dust emission. The best-fitting dust disc model for 94 Ceti implies a circumbinary disc around the secondary pair, limited by dynamics to radii smaller than 40 au and with a grain-size power-law distribution of ~a^-3.5 . This model exhibits a dust-to-star luminosity ratio of 4.6+/-0.4*10^-6. The system is dynamically stable and N-body symplectic simulation results are consistent with semi-analytical equations that describe orbits in binary systems. In the observations, we also find tentative evidence of a circumtertiary ring that could be edge-on.
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| 5. |
- Wiegert, Joachim, 1985
(author)
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Circumstellar dust emission from nearby Solar-type stars
- 2016
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Doctoral thesis (other academic/artistic)abstract
- Far-infrared excess above the photosphere of a star indicates the presence of a circumstellar dust disc which is a sign-post for extrasolar planets, and was first detected in the mid 1980s. Dust discs are intricately connected to planets and planetesimals, give insights in the dynamics and evolution of the system, and are also useful for future exoplanet-observations. This thesis is aimed at modelling dust emission of nearby Solar-type stars, and is partly involved with the Herschel key programme DUNES (DUst around NEarby Stars). It includes detailed studies on a few nearby stars, and results from a coherent re-reduction of the combined datasets of the original DUNES catalogue and 55 DEBRIS-observed sources (Disc Emission via a Bias-free Reconnaissance in the Infrared/Sub-millimetre).Based on observations with Herschel and Spitzer of the nearby binary alpha Centauri (G2 V and K1 V), an upper limit on the fractional luminosity (dust-to-star) of circumstellar dust was determined to a few 10^-5 (Paper I). Both stars exhibit detectable temperature minima at wavelengths around 100-300um due to a chromospheric temperature inversion akin to that of the sun. The resulting flux difference, when compared to stellar photospheric models, is equivalent to dust emission with a fractional luminosity of
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| 6. |
- Wiegert, Joachim, 1985
(author)
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Extrasolar Kuiper and asteroid belts Modelling far-infrared dust emission
- 2014
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Licentiate thesis (other academic/artistic)abstract
- The first detections of circumstellar dust emission were announced in the mid 1980s. Direct observations of the edge-on disc of beta Pictoris provided evidence that the dust was part of possible planetary systems. About a decade later, in 1995, the first confirmed extrasolar planet around a main sequence star (51 Pegasi b) was announced.The aims here are to study the dynamics and evolution of planetary systems, in which both dust and planets are connected. The aim is also to put the solar system, with its combination of small and large planets, and rings of planetesimals, into a wider context by comparing it withother systems.To do this we must map out planetary systems around solar-like stars in the solar neighbourhood, through observations and precise modelling.The nearest solar-like neighbour is alpha Centauri. This is a binary star with possibilities for planets. We have been able to set upper limits on circumstellar dust emission for these stars to fractional luminosities of a few 10^−5. We have also used the primary star, alpha Centauri A, as a template to better understand how the far-infrared spectrum of solar-like stars behaves. In particular we look at how the chromospheric temperature inversion in the stellar atmosphere will affect dust emission estimates of other stars. We found with the spectrum of alpha Cen A, that a lackof detection of a temperature minimum in other stars could in reality account for dust emission with a fractional luminosity of 2*10^−7.We are continuing the work on three additional nearby solar-like stars, where one is a binary star with a giant planet. All of these stars already have confirmed dust emission, but may require additional modelling. This is an ongoing project and the results are pending.
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| 7. |
- Wiegert, Joachim, 1985, et al.
(author)
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How dusty is alpha Centauri? : Excess or non-excess over the infrared photospheres of main-sequence stars
- 2014
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 563, s. A102-
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Journal article (peer-reviewed)abstract
- Context. Debris discs around main-sequence stars indicate the presence of larger rocky bodies. The components of the nearby, solar-type binary alpha Centauri have metallicities that are higher than solar, which is thought to promote giant planet formation. Aims. We aim to determine the level of emission from debris around the stars in the alpha Cen system. This requires knowledge of their photospheres. Having already detected the temperature minimum, T-min, of alpha Cen A at far-infrared wavelengths, we here attempt to do the same for the more active companion alpha Cen B. Using the alpha Cen stars as templates, we study the possible effects that T-min may have on the detectability of unresolved dust discs around other stars. Methods. We used Herschel-PACS, Herschel-SPIRE, and APEX-LABOCA photometry to determine the stellar spectral energy distributions in the far infrared and submillimetre. In addition, we used APEX-SHeFI observations for spectral line mapping to study the complex background around alpha Cen seen in the photometric images. Models of stellar atmospheres and of particulate discs, based on particle simulations and in conjunction with radiative transfer calculations, were used to estimate the amount of debris around these stars. Results. For solar-type stars more distant than alpha Cen, a fractional dust luminosity f(d) equivalent to L-dust/L-star similar to 2 x 10(-7) could account for SEDs that do not exhibit the T-min effect. This is comparable to estimates of f(d) for the Edgeworth-Kuiper belt of the solar system. In contrast to the far infrared, slight excesses at the 2.5 sigma level are observed at 24 mu m for both alpha Cen A and B, which, if interpreted as due to zodiacal-type dust emission, would correspond to f(d) similar to (1-3) x 10(-5), i.e. some 10(2) times that of the local zodiacal cloud. Assuming simple power-law size distributions of the dust grains, dynamical disc modelling leads to rough mass estimates of the putative Zodi belts around the alpha Cen stars, viz. less than or similar to 4 x 10(-6) M-(sic) of 4 to 1000 mu m size grains, distributed according to n(a) proportional to a(-3.5). Similarly, for filled-in T-min emission, corresponding Edgeworth-Kuiper belts could account for similar to 10(-3) M-(sic) of dust. Conclusions. Our far-infrared observations lead to estimates of upper limits to the amount of circumstellar dust around the stars alpha Cen A and B. Light scattered and/or thermally emitted by exo-Zodi discs will have profound implications for future spectroscopic missions designed to search for biomarkers in the atmospheres of Earth-like planets. The far-infrared spectral energy distribution of alpha Cen B is marginally consistent with the presence of a minimum temperature region in the upper atmosphere of the star. We also show that an alpha Cen A-like temperature minimum may result in an erroneous apprehension about the presence of dust around other, more distant stars.
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