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- Marshall, J. P., et al.
(författare)
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Correlations between the stellar, planetary, and debris components of exoplanet systems observed by Herschel
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 565, s. A15-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Stars form surrounded by gas- and dust-rich protoplanetary discs. Generally, these discs dissipate over a few (3-10) Myr, leaving a faint tenuous debris disc composed of second-generation dust produced by the attrition of larger bodies formed in the protoplanetary disc. Giant planets detected in radial velocity and transit surveys of main-sequence stars also form within the protoplanetary disc, whilst super-Earths now detectable may form once the gas has dissipated. Our own solar system, with its eight planets and two debris belts, is a prime example of an end state of this process. Aims. The Herschel DEBRIS, DUNES, and GI' programmes observed 37 exoplanet host stars within 25 pc at 70, 100, and 160 mu m with the sensitivity to detect far-infrared excess emission at flux density levels only an order of magnitude greater than that of the solar system's Edgeworth-Kuiper belt. Here we present an analysis of that sample, using it to more accurately determine the (possible) level of dust emission from these exoplanet host stars and thereafter determine the links between the various components of these exoplanetary systems through statistical analysis. Methods. We have fitted the flux densities measured from recent Herschel observations with a simple two parameter (T-d, L-IR/L-*) black-body model (or to the 3 sigma upper limits at 100 mu m). From this uniform approach we calculated the fractional luminosity, radial extent and dust temperature. We then plotted the calculated dust luminosity or upper limits against the stellar properties, e.g. effective temperature, metallicity, and age, and identified correlations between these parameters. Results. A total of eleven debris discs are identified around the 37 stars in the sample. An incidence of ten cool debris discs around the Sun-like exoplanet host stars (29 +/- 9%) is consistent with the detection rate found by DUNES (20.2 +/- 2.0%). For the debris disc systems, the dust temperatures range from 20 to 80 K, and fractional luminosities (L-IR/L-*) between 2.4 x10(-6) and 4.1 x10(-4). In the case of non-detections, we calculated typical 3 sigma upper limits to the dust fractional luminosities of a few x10(-6). Conclusions. We recover the previously identified correlation between stellar metallicity and hot-Jupiter planets in our data set. We find a correlation between the increased presence of dust, lower planet masses, and lower stellar metallicities. This confirms the recently identified correlation between cold debris discs and low-mass planets in the context of planet formation by core accretion.
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| 2. |
- Ertel, S., et al.
(författare)
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Potential multi-component structure of the debris disk around HIP 17439 revealed by Herschel/DUNES
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 561, s. Article no. A114-
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Tidskriftsartikel (refereegranskat)abstract
- Context. The dust observed in debris disks is produced through collisions of larger bodies left over from the planet/planetesimal formation process. Spatially resolving these disks permits to constrain their architecture and thus that of the underlying planetary/planetesimal system. Aims. Our Herschel open time key program DUNES aims at detecting and characterizing debris disks around nearby, sun-like stars. In addition to the statistical analysis of the data, the detailed study of single objects through spatially resolving the disk and detailed modeling of the data is a main goal of the project. Methods. We obtained the first observations spatially resolving the debris disk around the sun-like star HIP 17439 (HD23484) using the instruments PACS and SPIRE on board the Herschel Space Observatory. Simultaneous multi-wavelength modeling of these data together with ancillary data from the literature is presented. Results. A standard single component disk model fails to reproduce the major axis radial profiles at 70 mu m, 100 mu m, and 160 mu m simultaneously. Moreover, the best-fit parameters derived from such a model suggest a very broad disk extending from few au up to few hundreds of au from the star with a nearly constant surface density which seems physically unlikely. However, the constraints from both the data and our limited theoretical investigation are not strong enough to completely rule out this model. An alternative, more plausible, and better fitting model of the system consists of two rings of dust at approx. 30 au and 90 au, respectively, while the constraints on the parameters of this model are weak due to its complexity and intrinsic degeneracies. Conclusions. The disk is probably composed of at least two components with different spatial locations (but not necessarily detached), while a single, broad disk is possible, but less likely. The two spatially well-separated rings of dust in our best-fit model suggest the presence of at least one high mass planet or several low-mass planets clearing the region between the two rings from planetesimals and dust.
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| 3. |
- Wiegert, Joachim, 1985, et al.
(författare)
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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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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 563, s. A102-
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Tidskriftsartikel (refereegranskat)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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