A. Mora

Chalmers University of Technology, Goeteborg, Västra Götaland, Sweden

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Publications (88)161.81 Total impact

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    ABSTRACT: (Abridged) Tentative correlations between the presence of dusty debris discs and low-mass planets have been presented. In parallel, detailed chemical abundance studies have reported different trends between samples of planet and non-planet hosts. We determine in a homogeneous way the metallicity, and abundances of a sample of 251 stars including stars with known debris discs, with debris discs and planets, and only with planets. Stars with debris discs and planets have the same [Fe/H] behaviour as stars hosting planets, and they also show a similar <[X/Fe]>-Tc trend. Different behaviour in the <[X/Fe]>-Tc trend is found between the samples of stars without planets and the samples of planet hosts. In particular, when considering only refractory elements, negative slopes are shown in cool giant planet hosts, whilst positive ones are shown in stars hosting low-mass planets. Stars hosting exclusively close-in giant planets show higher metallicities and positive <[X/Fe]>-Tc slope. A search for correlations between the <[X/Fe]>-Tc slopes and the stellar properties reveals a moderate but significant correlation with the stellar radius and as well as a weak correlation with the stellar age. The fact that stars with debris discs and stars with low-mass planets do not show neither metal enhancement nor a different <[X/Fe]>-Tc trend might indicate a correlation between the presence of debris discs and the presence of low-mass planets. We extend results from previous works which reported differences in the <[X/Fe]>-Tc trends between planet hosts and non hosts. However, these differences tend to be present only when the star hosts a cool distant planet and not in stars hosting exclusively low-mass planets.
    Astronomy and Astrophysics 02/2015; DOI:10.1051/0004-6361/201525764 · 4.48 Impact Factor
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    ABSTRACT: The study of the planet-debris disk connection can shed light on the formation and evolution of planetary systems, and may help predict the presence of planets around stars with certain disk characteristics. In preliminary analyses of the Herschel DEBRIS and DUNES surveys, Wyatt et al. (2012) and Marshall et al. (2014) identified a tentative correlation between debris and low-mass planets. Here we use the cleanest possible sample out these surveys to assess the presence of such a correlation, discarding stars without known ages, with ages < 1 Gyr and with binary companions <100 AU, to rule out possible correlations due to effects other than planet presence. In our sample of 204 FGK stars, we do not find evidence that debris disks are more common or more dusty around stars harboring high-mass or low-mass planets compared to a control sample without identified planets, nor that debris disks are more or less common (or more or less dusty) around stars harboring multiple planets compared to single-planet systems. Diverse dynamical histories may account for the lack of correlations. The data show the correlation between the presence of high-mass planets and stellar metallicity, but no correlation between the presence of low-mass planets or debris and stellar metallicity. Comparing the observed cumulative distribution of fractional luminosity to those expected from a Gaussian distribution, we find that a distribution centered on the Solar system's value fits well the data, while one centered at 10 times this value can be rejected. This is of interest in the context of future terrestrial planet characterization because it indicates that there are good prospects for finding a large number of debris disk systems (i.e. with evidence of harboring the building blocks of planets) with exozodiacal emission low enough to be appropriate targets for an ATLAST-type mission to search for biosignatures.
    The Astrophysical Journal 01/2015; 801(2). DOI:10.1088/0004-637X/801/2/143 · 6.28 Impact Factor
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    ABSTRACT: The precise mechanisms that provide the non-radiative energy for heating the chromosphere and the corona of the Sun and those of other stars constitute an active field of research. By studying stellar chromospheres one aims at identifying the relevant physical processes. Defining the permittable extent of the parameter space can also serve as a template for the Sun-as-a-star. Earlier observations with Herschel and APEX have revealed the temperature minimum of alpha Cen, but these were unable to spatially resolve the binary into individual components. With the data reported here, we aim at remedying this shortcoming. Furthermore, these earlier data were limited to the wavelength region between 100 and 870mu. In the present context, we intend to extend the spectral mapping to longer wavelengths, where the contrast between stellar photospheric and chromospheric emission becomes increasingly evident. ALMA is particularly suited to point sources, such as unresolved stars. ALMA provides the means to achieve our objectives with both its high sensitivity of the collecting area for the detection of weak signals and the high spatial resolving power of its adaptable interferometer for imaging close multiple stars. This is the first detection of main-sequence stars at a wavelength of 3mm. Furthermore, the individual components of the binary alpha CenAB are clearly detected and spatially well resolved at all ALMA wavelengths. The high S/N of these data permit accurate determination of their relative flux ratios. The previously obtained flux ratio of 0.44, which was based on measurements in the optical and at 70mu, is consistent with the present ALMA results, albeit with a large error bar. Given the distinct difference in their cyclic activity, the similarity of their submm SEDs appears surprising.
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    ABSTRACT: Debris disks, which are inferred from the observed infrared excess to be ensembles of dust, rocks, and probably planetesimals, are common features of stellar systems. As the mechanisms of their formation and evolution are linked to those of planetary bodies, they provide valuable information. The few well-resolved debris disks are even more valuable because they can serve as modelling benchmarks and help resolve degeneracies in modelling aspects such as typical grain sizes and distances. Here, we present an analysis of the HD 207129 debris disk, based on its well-covered spectral energy distribution and ensuremath Herschelensuremath<?iensuremath>?PACS images obtained in the framework of the DUNES (DUst around NEarby Stars) programme. We use an empirical power-law approach to the distribution of dust and we then model the production and removal of dust by means of collisions, direct radiation pressure, and drag forces. The resulting best-fit model contains a total of nearly 10ensuremath -2ensuremath<?supensuremath> Earth masses in dust, with typical grain sizes in the planetesimal belt ranging from 4 to 7 ensuremath ensuremathmuensuremath<?iensuremath>m. We constrain the dynamical excitation to be low, which results in very long collisional lifetimes and a drag that notably fills the inner gap, especially at 70 ensuremath ensuremathmuensuremath<?iensuremath>m. The radial distribution stretches from well within 100 AU in an unusual, outward-rising slope towards a rather sharp outer edge at about 170?190 AU. The inner edge is therefore smoother than that reported for Fomalhaut, but the contribution from the extended halo of barely bound grains is similarly small. Both slowly self-stirring and planetary perturbations could potentially have formed and shaped this disk.
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    ABSTRACT: We used HST/WFC3 observations of a sample of 26 nearby ($\le$20 pc) mid to late T dwarfs to search for cooler companions and measure the multiplicity statistics of brown dwarfs. Tightly-separated companions were searched for using a double-PSF fitting algorithm. We also compared our detection limits based on simulations to other prior T5+ brown dwarf binary programs. No new wide or tight companions were identified, which is consistent with the number of known T5+ binary systems and the resolution limits of WFC3. We use our results to add new constraints to the binary fraction of T-type brown dwarfs. Modeling selection effects and adopting previously derived separation and mass ratio distributions, we find an upper limit total binary fraction of <16% and <25% assuming power law and flat mass ratio distributions respectively, which are consistent with previous results. We also characterize a handful of targets around the L/T transition.
    The Astronomical Journal 08/2014; 148(6). DOI:10.1088/0004-6256/148/6/129 · 4.97 Impact Factor
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    ABSTRACT: We present some highlights of two ongoing investigations that deal with the dynamics of planetary systems. Firstly, until recently, observed eccentric patterns in debris disks were found in young systems. However recent observations of Gyr-old eccentric debris disks leads to question the survival timescale of this type of asymmetry. One such disk was recently observed in the far-IR by the Herschel Space Observatory around ζ2 Reticuli. Secondly, as a binary companion orbits a circumprimary disk, it creates regions where planet formation is strongly handicapped. However, some planets were detected in this zone in tight binary systems (γ Cep, HD 196885). We aim to determine whether a binary companion can affect migration such that planets are brought in these regions and focus in particular on the planetesimal-driven migration mechanism.
    Proceedings of the International Astronomical Union 06/2014; DOI:10.1017/S1743921313008363
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    ABSTRACT: The $\textit{Herschel}$ DEBRIS, DUNES and GT 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. We have fitted the flux densities measured from recent \textit{Herschel} observations with a simple two parameter ($T_{d}$, $L_{\rm IR}/L_{\star}$) black body model (or to the 3-$\sigma$ upper limits at 100 $\mu$m). From this uniform approach we calculate the fractional luminosity, radial extent, dust temperature and disc mass. We then plotted the calculated dust luminosity or upper limits against the stellar properties, e.g. effective temperature, metallicity, age, and identified correlations between these parameters. 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 $\pm$ 9 %) is consistent with the detection rate found by DUNES (20.2 $\pm$ 2.0 %). For the debris disc systems, the dust temperatures range from 20 to 80 K, and fractional luminosities ($L_{\rm IR}/L_{\star}$) between 2.4 $\times$10$^{-6}$ and 4.1 $\times$10$^{-4}$. In the case of non-detections, we calculated typical 3-$\sigma$ upper limits to the dust fractional luminosities of a few $\times10^{-6}$. 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. (abridged)
    Astronomy and Astrophysics 03/2014; 565. DOI:10.1051/0004-6361/201323058 · 4.48 Impact Factor
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    ABSTRACT: [Abridged] Debris discs around main-sequence stars indicate the presence of larger rocky bodies. The components of the nearby binary aCentauri have higher than solar metallicities, which is thought to promote giant planet formation. We aim to determine the level of emission from debris in the aCen system. Having already detected the temperature minimum, Tmin, of aCenA, we here attempt to do so also for the companion aCenB. Using the aCen stars as templates, we study possible effects Tmin may have on the detectability of unresolved dust discs around other stars. We use Herschel and APEX photometry to determine the stellar spectral energy distributions. In addition, we use APEX for spectral line mapping to study the complex background around aCen seen in the photometric images. Models of stellar atmospheres and discs are used to estimate the amount of debris around these stars. For solar-type stars, a fractional dust luminosity fd 2e-7 could account for SEDs that do not exhibit the Tmin-effect. Slight excesses at the 2.5 sigma level are observed at 24 mu for both stars, which, if interpreted to be due to dust, would correspond to fd (1-3)e-5. Dynamical disc modelling leads to rough mass estimates of the putative Zodi belts around the aCen stars, viz. <~4e-6 MMoon of 4 to 1000 mu size grains, distributed according to n a^-3.5. Similarly, for filled-in Tmin emission, corresponding EKBs could account for ~1e-3 MMoon of dust. 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 F-IR SED of aCenB is marginally consistent with the presence of a minimum temperature region in the upper atmosphere. We also show that an aCenA-like temperature minimum may result in an erroneous apprehension about the presence of dust around other stars.
    Astronomy and Astrophysics 01/2014; 563(2014). DOI:10.1051/0004-6361/201321887 · 4.48 Impact Factor
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    ABSTRACT: [abridged] Aims. Our Herschel Open Time Key Programme 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 um, 100 um, and 160 um 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.
    Astronomy and Astrophysics 12/2013; DOI:10.1051/0004-6361/201219945 · 4.48 Impact Factor
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    ABSTRACT: Imaging of debris disks has found evidence for both eccentric and offset disks. One hypothesis is that these provide evidence for massive perturbers that sculpt the observed structures. One such disk was recently observed in the far-IR by the Herschel Space Observatory around $\zeta^2$ Ret. In contrast with previously reported systems, the disk is significantly eccentric, and the system is Gyr-old. We aim to investigate the long-term evolution of eccentric structures in debris disks caused by a perturber on an eccentric orbit. Both analytical predictions and numerical N-body simulations are used to investigate the observable structures that could be produced by eccentric perturbers. The long-term evolution of the disk geometry is examined, with particular application to the $\zeta^2$ Ret system. In addition, synthetic images of the disk are produced for comparison with Herschel observations. We show that an eccentric companion can produce both the observed offsets and eccentric disks. Such effects are not immediate and we characterise the timescale required for the disk to develop to an eccentric state. For the case of $\zeta^2$ Ret, we place limits on the mass and orbit of the companion required to produce the observations. Synthetic images show that the pattern observed around $\zeta^2$ Ret can be produced by an eccentric disk seen close to edge-on, and allow us to bring additional constraints on the disk parameters of our model (disk flux, extent). We determine that eccentric planets or stellar companions can induce long-lived eccentric structures in debris disks. Observations of such eccentric structures provide potential evidence of the presence of such a companion in a planetary system. We consider the example of $\zeta^2$ Ret, whose observed eccentric disk can be explained by a distant companion at tens of AU, on an eccentric orbit ($e_p\gtrsim 0.3$).
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    ABSTRACT: Context. Typical debris discs are composed of particles ranging from several micron sized dust grains to km sized asteroidal bodies, and their infrared emission peaks at wavelengths 60-100 μm. Recent Herschel DUNES observations have identified several debris discs around nearby Sun-like stars (F, G and K spectral type) with significant excess emission only at 160 μm. Aims: We observed HIP 92043 (110 Her, HD 173667) at far-infrared and sub-millimetre wavelengths with Herschel PACS and SPIRE. Identification of the presence of excess emission from HIP 92043 and the origin and physical properties of any excess was undertaken through analysis of its spectral energy distribution (SED) and the PACS images. Methods: The PACS and SPIRE images were produced using the HIPE photProject map maker routine. Fluxes were measured using aperture photometry. A stellar photosphere model was scaled to optical and near infrared photometry and subtracted from the far-infared and sub-mm fluxes to determine the presence of excess emission. Source radial profiles were fitted using a 2D Gaussian and compared to a PSF model based on Herschel observations of α Boo to check for extended emission. Results: Clear excess emission from HIP 92043 was observed at 70 and 100 μm. Marginal excess was observed at 160 and 250 μm. Analysis of the images reveals that the source is extended at 160 μm. A fit to the source SED is inconsistent with a photosphere and single temperature black body. Conclusions: The excess emission from HIP 92043 is consistent with the presence of an unresolved circumstellar debris disc at 70 and 100 μm, with low probability of background contamination. The extended 160 μm emission may be interpreted as an additional cold component to the debris disc or as the result of background contamination along the line of sight. The nature of the 160 μm excess cannot be determined absolutely from the available data, but we favour a debris disc interpretation, drawing parallels with previously identified cold disc sources in the DUNES sample. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
    Astronomy and Astrophysics 09/2013; 557:A58. DOI:10.1051/0004-6361/201218976 · 4.48 Impact Factor
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    ABSTRACT: This work presents X-Shooter/VLT spectra of the prototypical, isolated Herbig Ae stars HD 31648 (MWC 480) and HD 163296 over five epochs separated by timescales ranging from days to months. Each spectrum spans over a wide wavelength range covering from 310 to 2475 nm. We have monitored the continuum excess in the Balmer region of the spectra and the luminosity of twelve ultraviolet, optical and near infrared spectral lines that are commonly used as accretion tracers for T Tauri stars. The observed strengths of the Balmer excesses have been reproduced from a magnetospheric accretion shock model, providing a mean mass accretion rate of 1.11 × 10 −7 and 4.50 × 10 −7 M ⊙ yr −1 for HD 31648 and HD 163296, respectively. Accretion rate variations are observed, being more pronounced for HD 31648 (up to 0.5 dex). However, from the comparison with previous results it is found that the accretion rate of HD 163296 has increased by more than 1 dex, on a timescale of ∼ 15 years. Averaged accretion luminosities derived from the Balmer excess are consistent with the ones inferred from the empirical calibrations with the emission line luminosities, indicating that those can be extrapolated to HAe stars. In spite of that, the accretion rate variations do not generally coincide with those estimated from the line luminosities, suggesting that the empirical calibrations are not useful to accurately quantify accretion rate variability.
    The Astrophysical Journal 08/2013; 776(1). DOI:10.1088/0004-637X/776/1/44 · 6.28 Impact Factor
  • Article: Herschel's
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    ABSTRACT: Infrared excesses associated with debris disk host stars detected so far peak at wavelengths around ~100 μm or shorter. However, 6 out of 31 excess sources studied in the Herschel Open Time Key Programme, DUNES, have been seen to show significant—and in some cases extended—excess emission at 160 μm, which is larger than the 100 μm excess. This excess emission has been attributed to circumstellar dust and has been suggested to stem from debris disks colder than those known previously. Since the excess emission of the cold disk candidates is extremely weak, challenging even the unrivaled sensitivity of Herschel, it is prudent to carefully consider whether some or even all of them may represent unrelated galactic or extragalactic emission, or even instrumental noise. We re-address these issues using several distinct methods and conclude that it is highly unlikely that none of the candidates represents a true circumstellar disk. For true disks, both the dust temperatures inferred from the spectral energy distributions and the disk radii estimated from the images suggest that the dust is nearly as cold as a blackbody. This requires the grains to be larger than ~100 μm, even if they are rich in ices or are composed of any other material with a low absorption in the visible. The dearth of small grains is puzzling, since collisional models of debris disks predict that grains of all sizes down to several times the radiation pressure blowout limit should be present. We explore several conceivable scenarios: transport-dominated disks, disks of low dynamical excitation, and disks of unstirred primordial macroscopic grains. Our qualitative analysis and collisional simulations rule out the first two of these scenarios, but show the feasibility of the third one. We show that such disks can indeed survive for gigayears, largely preserving the primordial size distribution. They should be composed of macroscopic solids larger than millimeters, but smaller than a few kilometers in size. If larger planetesimals were present, then they would stir the disk, triggering a collisional cascade and thus causing production of small debris, which is not seen. Thus, planetesimal formation, at least in the outer regions of the systems, has stopped before "cometary" or "asteroidal" sizes were reached.
    The Astrophysical Journal 07/2013; 772(1):32-. · 6.28 Impact Factor
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    ABSTRACT: (abridged) Infrared excesses associated with debris disk host stars detected so far peak at wavelengths around ~100{\mu}m or shorter. However, six out of 31 excess sources in the Herschel OTKP DUNES have been seen to show significant - and in some cases extended - excess emission at 160{\mu}m, which is larger than the 100{\mu}m excess. This excess emission has been suggested to stem from debris disks colder than those known previously. Using several methods, we re-consider whether some or even all of the candidates may be associated with unrelated galactic or extragalactic emission and conclude that it is highly unlikely that none of the candidates represents a true circumstellar disk. For true disks, both the dust temperatures inferred from the SEDs and the disk radii estimated from the images suggest that the dust is nearly as cold as a blackbody. This requires the grains to be larger than ~100{\mu}m, regardless of their material composition. To explain the dearth of small grains, we explore several conceivable scenarios: transport-dominated disks, disks of low dynamical excitation, and disks of unstirred primordial macroscopic grains. Our qualitative analysis and collisional simulations rule out the first two of these scenarios, but show the feasibility of the third one. We show that such disks can survive for gigayears, largely preserving the primordial size distribution. They should be composed of macroscopic solids larger than millimeters, but smaller than kilometers in size. Thus planetesimal formation, at least in the outer regions of the systems, has stopped before "cometary" or "asteroidal" sizes were reached.
    The Astrophysical Journal 06/2013; 772(1). DOI:10.1088/0004-637X/772/1/32 · 6.28 Impact Factor
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    ABSTRACT: GASPS is a far-infrared line and continuum survey of protoplanetary and young debris disks using PACS on the Herschel Space Observatory. The survey includes [OI] at 63 microns, as well as 70, 100 and 160um continuum, with the brightest objects also studied in [OI]145um, [CII]157um, H2O and CO. Targets included T Tauri stars and debris disks in 7 nearby young associations, and a sample of isolated Herbig AeBe stars. The aim was to study the global gas and dust content in a wide disk sample, systemically comparing the results with models. In this paper we review the main aims, target selection and observing strategy. We show initial results, including line identifications, sources detected, and a first statistical study. [OI]63um was the brightest line in most objects, by a factor of ~10. Detection rates were 49%, including 100% of HAeBe stars and 43% of T Tauri stars. Comparison with published dust masses show a dust threshold for [OI]63um detection of ~1e-5 M_solar. Normalising to 140pc distance, 32% with mass 1e-6 - 1e-5 M_solar, and a small number with lower mass were also detected. This is consistent with moderate UV excess and disk flaring. In most cases, continuum and line emission is spatially and spectrally unresolved, suggesting disk emission. ~10 objects were resolved, likely from outflows. Detection rates in [OI]145um, [CII]157um and CO J=18-17 were 20-40%, but [CII] was not correlated with disk mass, suggesting it arises instead from a compact envelope. [OI] detection rates in T Tauri associations of ages 0.3-4Myr were ~50%. ~2 stars were detectable in associations of 5-20Myr, with no detections in associations of age >20Myr. Comparing with the total number of young stars, and assuming a ISM-like gas/dust ratio, this indicates that ~18% of stars retain a gas-rich disk of total mass >1M_Jupiter for 1-4Myr, 1-7% keep such disks for 5-10Myr, and none remain beyond 10-20Myr.
    Publications of the Astronomical Society of the Pacific 06/2013; 125(927). DOI:10.1086/670826 · 3.23 Impact Factor
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    ABSTRACT: Around 16% of the solar-type stars in our neighbourhood show IR-excesses due to dusty debris discs. Debris discs are, strictly speaking, signatures of planetesimal systems, the building blocks of planets. Therefore, it is interesting to compare the properties of these stars with those known to harbour planets. In this contribution, we use high-resolution spectra to derive homogeneous metallicities of solar-type stars with/without debris and planets. Our results suggest that there is a trend towards higher metallicities from stars with neither discs nor planets to stars hosting giant planets. Several trends on stars hosting debris discs and giant-planets have been identified, these planets tend to be cool and their host stars tend to show low values of the dust luminosity. We also find evidence of an anticorrelation between the luminosity of the dust and the planet eccentricity. The full implications of these findings are discussed in this contribution.
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    ABSTRACT: Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their solar system's counterparts are the asteroid and Edgeworth-Kuiper belts. The DUNES survey aims at detecting extra-solar analogues to the Edgeworth-Kuiper belt around solar-type stars, putting in this way the solar system into context. The survey allows us to address some questions related to the prevalence and properties of planetesimal systems. We used {\it Herschel}/PACS to observe a sample of nearby FGK stars. Data at 100 and 160 $\mu$m were obtained, complemented in some cases with observations at 70 $\mu$m, and at 250, 350 and 500 $\mu$m using SPIRE. The observing strategy was to integrate as deep as possible at 100 $\mu$m to detect the stellar photosphere. Debris discs have been detected at a fractional luminosity level down to several times that of the Edgeworth-Kuiper belt. The incidence rate of discs around the DUNES stars is increased from a rate of $\sim$ 12.1% $\pm$ 5% before \emph{Herschel} to $\sim$ 20.2% $\pm$ 2%. A significant fraction ($\sim$ 52%) of the discs are resolved, which represents an enormous step ahead from the previously known resolved discs. Some stars are associated with faint far-IR excesses attributed to a new class of cold discs. Although it cannot be excluded that these excesses are produced by coincidental alignment of background galaxies, statistical arguments suggest that at least some of them are true debris discs. Some discs display peculiar SEDs with spectral indexes in the 70-160$\mu$m range steeper than the Rayleigh-Jeans one. An analysis of the debris disc parameters suggests that a decrease might exist of the mean black body radius from the F-type to the K-type stars. In addition, a weak trend is suggested for a correlation of disc sizes and an anticorrelation of disc temperatures with the stellar age.
    Astronomy and Astrophysics 05/2013; 555. DOI:10.1051/0004-6361/201321050 · 4.48 Impact Factor
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    ABSTRACT: Context. Our ability to extract information from the spectra of stars depends on reliable models of stellar atmospheres and appropriate techniques for spectral synthesis. Various model codes and strategies for the analysis of stellar spectra are available today. Aims. We aim to compare the results of deriving stellar parameters using different atmosphere models and different analysis strategies. The focus is set on high-resolution spectroscopy of cool giant stars. Methods. Spectra representing four cool giant stars were made available to various groups and individuals working in the area of spectral synthesis, asking them to derive stellar parameters from the data provided. The results were discussed at a workshop in Vienna in 2010. Most of the major codes currently used in the astronomical community for analyses of stellar spectra were included in this experiment. Results. We present the results from the different groups, as well as an additional experiment comparing the synthetic spectra produced by various codes for a given set of stellar parameters. Similarities and differences of the results are discussed. Conclusions. Several valid approaches to analyze a given spectrum of a star result in quite a wide range of solutions. The main causes for the differences in parameters derived by different groups seem to lie in the physical input data and in the details of the analysis method. This clearly shows how far from a definitive abundance analysis we still are.
    Astronomy and Astrophysics 11/2012; 547:A108. DOI:10.1051/0004-6361/201219142 · 4.48 Impact Factor
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    ABSTRACT: Spectra representing four cool giant stars were made available to various groups and individuals working in the area of spectral synthesis, asking them to derive stellar parameters from the data provided. The results were discussed at a workshop in Vienna in 2010. Most of the major codes currently used in the astronomical community for analyses of stellar spectra were included in this experiment. The files contain the spectra in the same form as they were made available to the experiment participants. The spectra for Star 1 and Star 2 are on the heliocentric wavelength scale (i.e. corrected for Earth motion, but not for stellar radial velocity). The spectra are normalized to the continuum by an automatic pipeline, which first divides the spectrum by a flat field response function and then removes the overall slope of the whole spectrum. These optical spectra also contain a number of telluric lines. The spectra for Star 3 and Star 4 are on the laboratory wavelength scale and the flux is normalized to the continuum. They do not contain telluric lines. Additional information which was provided to the experiment participants: ---------------------------------------------------------------- FileName Spectral resolution Signal-to-noise ratio ---------------------------------------------------------------- star1.dat 80000 463 at 871 nm star2.dat 80000 517 at 871 nm star3.dat 70000 125 star4.dat 70000 125 ---------------------------------------------------------------- (10 data files).

Publication Stats

794 Citations
161.81 Total Impact Points

Institutions

  • 2013
    • Chalmers University of Technology
      • Department of Earth and Space Sciences
      Goeteborg, Västra Götaland, Sweden
  • 2009
    • University of Hertfordshire
      • Centre for Astrophysics Research (CAR)
      Hatfield, ENG, United Kingdom
  • 2000–2009
    • Universidad Autónoma de Madrid
      • Department of Theoretical physics
      Madrid, Madrid, Spain
  • 2001–2004
    • University of Groningen
      Groningen, Groningen, Netherlands