Villanueva de la Cañada, Spain
Villanueva de la Cañada, Spain

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Lohne T.,Friedrich - Schiller University of Jena | Augereau J.-C.,Joseph Fourier University | Ertel S.,University of Kiel | Marshall J.P.,Autonomous University of Madrid | And 18 more authors.
Astronomy and Astrophysics | Year: 2012

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 Herschel/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 10 -2 Earth masses in dust, with typical grain sizes in the planetesimal belt ranging from 4 to 7 μm. 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 μm. 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. © 2012 ESO.

Podio L.,Institute Of Planetologie Et Dastrophysique Of Grenoble | Podio L.,Kapteyn Institute | Kamp I.,Kapteyn Institute | Flower D.,Durham University | And 9 more authors.
Astronomy and Astrophysics | Year: 2012

Context. Observations of the atomic and molecular line emission associated with jets and outflows emitted by young stellar objects provide sensitive diagnostics of the excitation conditions, and can be used to trace the various evolutionary stages they pass through as they evolve to become main sequence stars. Aims. To understand the relevance of atomic and molecular cooling in shocks, and how accretion and ejection efficiency evolves with the evolutionary state of the sources, we will study the far-infrared counterparts of bright optical jets associated with Class I and II sources in Taurus (T Tau, DG Tau A, DG Tau B, FS Tau A+B, and RW Aur). Methods. We have analysed Herschel/PACS observations of a number of atomic ([O i]63 μm, 145 μm, [C ii]158 μm) and molecular (high-J CO, H 2O, OH) lines, collected within the open time key project GASPS (PI: W. R. F. Dent). To constrain the origin of the detected lines we have compared the obtained FIR emission maps with the emission from optical-jets and millimetre-outflows, and the measured line fluxes and ratios with predictions from shock and disk models. Results. All of the targets are associated with extended emission in the atomic lines; in particular, the strong [O≠i] 63 μm≠emission is correlated with the direction of the optical jet/mm-outflow. The line ratios suggest that the atomic lines can be excited in fast dissociative J-shocks occurring along the jet. The molecular emission, on the contrary, originates from a compact region, that is spatially and spectrally unresolved, and lines from highly excited levels are detected (e.g., the o-H 2O 8 18-7 07 line, and the CO J = 36-35 line). Disk models are unable to explain the brightness of the observed lines (CO and H 2O line fluxes up to 10 -15-6 × 10 -16 W m -2). Slow C-or J-shocks with high pre-shock densities reproduce the observed H 2O and high-J CO lines; however, the disk and/or UV-heated outflow cavities may contribute to the observed emission. Conclusions. Similarly to Class 0 sources, the FIR emission associated with Class I and II jet-sources is likely to be shock-excited. While the cooling is dominated by CO and H 2O lines in Class 0 sources, [O i] becomes an important coolant as the source evolves and the environment is cleared. The cooling and mass loss rates estimated for Class II and I sources are one to four orders of magnitude lower than for Class 0 sources. This provides strong evidence to indicate that the outflow activity decreases as the source evolves. © 2012 ESO.

Eiroa C.,Autonomous University of Madrid | Marshall J.P.,Autonomous University of Madrid | Mora A.,ESA ESAC Gaia SOC | Krivov A.V.,Friedrich - Schiller University of Jena | And 25 more authors.
Astronomy and Astrophysics | Year: 2011

We present Herschel PACS 100 and 160 μm observations of the solar-type stars α Men, HD 88230 and HD 210277, which form part of the FGK stars sample of the Herschel open time key programme (OTKP) DUNES (DUst around NEarby S tars). Our observations show small infrared excesses at 160 μm for all three stars. HD 210277 also shows a small excess at 100 μm, while the 100 μm fluxes of α Men and HD 88230 agree with the stellar photospheric predictions. We attribute these infrared excesses to a new class of cold, faint debris discs. Both ? Men and HD 88230 are spatially resolved in the PACS 160 μm images, while HD 210277 is point-like at that wavelength. The projected linear sizes of the extended emission lie in the range from ∼115 to ≤250 AU. The estimated black body temperatures from the 100 and 160 ?m fluxes are ≈22 K, and the fractional luminosity of the cold dust is L dust/Lz.ast ∼ 10?6, close to the luminosity of the solar-system's Kuiper belt. These debris discs are the coldest and faintest discs discovered so far around mature stars, so they cannot be explained easily invoking "classical" debris disc models. © 2011 ESO.

Krivov A.V.,Friedrich - Schiller University of Jena | Eiroa C.,Autonomous University of Madrid | Lohne T.,Friedrich - Schiller University of Jena | Marshall J.P.,Autonomous University of Madrid | And 25 more authors.
Astrophysical Journal | Year: 2013

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. © 2013. The American Astronomical Society. All rights reserved.

Marshall J.P.,Autonomous University of Madrid | Krivov A.V.,Friedrich - Schiller University of Jena | Del Burgo C.,National Institute of Astrophysics, Optics and Electronics | Eiroa C.,Autonomous University of Madrid | And 17 more authors.
Astronomy and Astrophysics | Year: 2013

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. © ESO, 2013. © 2013 Author(s).

Maldonado J.,Autonomous University of Madrid | Eiroa C.,Autonomous University of Madrid | Villaver E.,Autonomous University of Madrid | Montesinos B.,CSIC - National Institute of Aerospace Technology | Mora A.,ESA ESAC Gaia SOC
Astronomy and Astrophysics | Year: 2012

Context. Around 16% of the solar-like stars in our neighbourhood show IR-excesses due to dusty debris discs and a fraction of them are known to host planets. Determining whether these stars follow any special trend in their properties is important to understand debris disc and planet formation. Aims. We aim to determine in a homogeneous way the metallicity of a sample of stars with known debris discs and planets. We attempt to identify trends related to debris discs and planets around solar-type stars. Methods. Our analysis includes the calculation of the fundamental stellar parameters T eff, log g, microturbulent velocity, and metallicity by applying the iron ionisation equilibrium conditions to several isolated Fe i and Fe ii lines. High-resolution échelle spectra (R ∼ 57 000) from 2, 3 m class telescopes are used. Our derived metallicities are compared with other results in the literature, which finally allows us to extend the stellar samples in a consistent way. Results. The metallicity distributions of the different stellar samples suggest that there is a transition toward higher metallicities from stars with neither debris discs nor planets to stars hosting giant planets. Stars with debris discs and stars with neither debris nor planets follow a similar metallicity distribution, although the distribution of the first ones might be shifted towards higher metallicities. Stars with debris discs and planets have the same metallicity behaviour as stars hosting planets, irrespective of whether the planets are low-mass or gas giants. In the case of debris discs and giant planets, the planets are usually cool,-semimajor axis larger than 0.1 AU (20 out of 22 planets), even 65% have semimajor axis larger than 0.5 AU. The data also suggest that stars with debris discs and cool giant planets tend to have a low dust luminosity, and are among the less luminous debris discs known. We also find evidence of an anticorrelation between the luminosity of the dust and the planet eccentricity. Conclusions. Our data show that the presence of planets, not the debris disc, correlates with the stellar metallicity. The results confirm that core-accretion models represent suitable scenarios for debris disc and planet formation. These conclusions are based on a number of stars with discs and planets considerably larger than in previous works, in particular stars hosting low-mass planets and debris discs. Dynamical instabilities produced by eccentric giant planets could explain the suggested dust luminosity trends observed for stars with debris discs and planets. © 2012 ESO.

Maldonado J.,National institute for astrophysics | Eiroa C.,Autonomous University of Madrid | Villaver E.,Autonomous University of Madrid | Montesinos B.,CSIC - National Institute of Aerospace Technology | Mora A.,ESA ESAC Gaia SOC
Astronomy and Astrophysics | Year: 2015

Context. Tentative correlations between the presence of dusty circumstellar debris discs and low-mass planets have recently been presented. In parallel, detailed chemical abundance studies have reported different trends between samples of planet and non-planet hosts. Whether these chemical differences are indeed related to the presence of planets is still strongly debated. Aims. We aim to test whether solar-type stars with debris discs show any chemical peculiarity that could be related to the planet formation process. Methods. We determine in a homogeneous way the metallicity, [Fe/H], and abundances of individual elements of a sample of 251 stars including stars with known debris discs, stars harbouring simultaneously debris discs and planets, stars hosting exclusively planets, and a comparison sample of stars without known discs or planets. High-resolution échelle spectra (R ∼ 57 000) from 2-3 m class telescopes are used. Our methodology includes the calculation of the fundamental stellar parameters (Teff, log? g, microturbulent velocity, and metallicity) by applying the iron ionisation and equilibrium conditions to several isolated Fe I and Fe II lines, as well as individual abundances of C, O, Na, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, and Zn. Results. No significant differences have been found in metallicity, individual abundances or abundance-condensation temperature trends between stars with debris discs and stars with neither debris nor planets. Stars with debris discs and planets have the same metallicity behaviour as stars hosting planets, and they also show a similar 〈[X/Fe]〉 - TC trend. Different behaviour in the 〈[X/Fe]? - TC trends 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. The statistical significance of the derived slopes is low, however, probably because of the wide range of stellar parameters of our samples. Stars hosting exclusively close-in giant planets behave in a different way, showing 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 a weak correlation with the stellar age, which remain even if Galactic chemical evolution effects are considered. No correlation between the 〈[X/Fe]〉 - TC slopes and the disc/planet properties are found. Conclusions. The fact that stars with debris discs and stars with low-mass planets do not show either metal enhancement or 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 based mainly on solar analogues with reported differences in the 〈[X/Fe]〉 - TC trends between planet hosts and non-hosts to a wider range of parameters. 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. The interpretation of these differences as a signature of planetary formation should be considered with caution since moderate correlations between the TC-slopes with the stellar radius and the stellar age are found, suggesting that an evolutionary effect might be at work. © ESO, 2015.

Maldonado J.,Autonomous University of Madrid | Mora A.,ESA ESAC Gaia SOC. | Montesinos B.,CSIC - National Institute of Aerospace Technology | Villaver E.,Autonomous University of Madrid | Eiroa C.,Autonomous University of Madrid
Journal of Physics: Conference Series | Year: 2011

The question of the origin and evolution of planetary systems is of fundamental importance for astrophysics. Dusty debris discs are signatures of planetary systems and, therefore, constitute valuable tools to provide new light in our understanding of how planetary systems form and evolve. We present the first results of a spectroscopic programme of a sample of stars with debris discs. High-resolution echelle spectra are used to determine metallicities and abundances. Properties of stars with debris discs, are compared with those of stars hosting planets, as well as 'normal' stars.

Tilling I.,University of Edinburgh | Woitke P.,University of Vienna | Woitke P.,Astronomy Technology Center | Woitke P.,University of St. Andrews | And 21 more authors.
Astronomy and Astrophysics | Year: 2012

We present detailed model fits to observations of the disc around the Herbig Ae star HD 163296. This well-studied object has an age of ∼4 Myr, with evidence of a circumstellar disc extending out to ∼540 AU. We use the radiation thermo-chemical disc code ProDiMo to model the gas and dust in the circumstellar disc of HD 163296, and attempt to determine the disc properties by fitting to observational line and continuum data. These include new Herschel/PACS observations obtained as part of the open-time key program GASPS (GAS in Protoplanetary Systems), consisting of a detection of the [Oi] 63 μm line and upper limits for several other far infrared lines. We complement this with continuum data and ground-based observations of the 12CO 3-2, 2-1 and 13CO J = 1-0 line transitions, as well as an upper limit for the H 2 0-0 S(1) transition. We explore the effects of stellar ultraviolet variability and dust settling on the line emission, and on the derived disc properties. Our fitting efforts lead to derived gas/dust ratios in the range 9-100, depending on the assumptions made. We note that the line fluxes are sensitive in general to the degree of dust settling in the disc, with an increase in line flux for settled models. This is most pronounced in lines which are formed in the warm gas in the inner disc, but the low excitation molecular lines are also affected. This has serious implications for attempts to derive the disc gas mass from line observations. We derive fractional PAH abundances between 0.007 and 0.04 relative to ISM levels. Using a stellar and UV excess input spectrum based on a detailed analysis of observations, we find that the all observations are consistent with the previously assumed disc geometry. © 2012 ESO.

Woitke P.,University of Vienna | Woitke P.,Astronomy Technology Center | Woitke P.,University of St. Andrews | Riaz B.,US Space Telescope Science Institute | And 17 more authors.
Astronomy and Astrophysics | Year: 2011

We present new continuum and line observations, along with modelling, of the faint (6-8) Myr old T Tauri star ET Cha belonging to the η Chamaeleontis cluster. We have acquired Herschel/PACS photometric fluxes at 70 μm and 160 μm, as well as a detection of the [OI] 63 μm fine-structure line in emission, and derived upper limits for some other far-IR OI, CII, CO and o-H2O lines. These observations were carried out in the frame of the open time key programme GASPS, where ET Cha was selected as one of the science demonstration phase targets. The Herschel data is complemented by new simultaneous ANDICAM B-K photometry, new HST/COS and HST/STIS UV-observations, a non-detection of CO J = 3 → 2 with APEX, re-analysis of a UCLES high-resolution optical spectrum showing forbidden emission lines like [OI] 6300Å, [SII] 6731Å and 6716Å, and [NII] 6583Å, and a compilation of existing broad-band photometric data. We used the thermo-chemical disk code ProDiMo and the Monte-Carlo radiative transfer code MCFOST to model the protoplanetary disk around ET Cha. The paper also introduces a number of physical improvements to the ProDiMo disk modelling code concerning the treatment of PAH ionisation balance and heating, the heating by exothermic chemical reactions, and several non-thermal pumping mechanisms for selected gas emission lines. By applying an evolutionary strategy to minimise the deviations between model predictions and observations, we find a variety of united gas and dust models that simultaneously fit all observed line and continuum fluxes about equally well. Based on these models we can determine the disk dustmass with confidence, Mdust â‰̂ (2-5) × 10-8 MâŠ(tm) whereas the total disk gas mass is found to be only little constrained, Mgas â‰̂ (5 × 10-5-3 × 10-3) MâŠ(tm). Both mass estimates are substantially lower than previously reported. In the models, the disk extends from 0.022 AU (just outside of the co-rotation radius) to only about 10 AU, remarkably small for single stars, whereas larger disks are found to be inconsistent with the CO J = 3 → 2 non-detection. The low velocity component of the [OI] 6300Å emission line is centred on the stellar systematic velocity, and is consistent with being emitted from the inner disk. The model is also consistent with the line flux of H2 v = 1 → 0 S(1) at 2.122 μm and with the [OI] 63 μm line as seen with Herschel/PACS. An additional high-velocity component of the [OI] 6300Å emission line, however, points to the existence of an additional jet/outflow of low velocity 40-65 km s-1 with mass loss rate â‰̂ 10 -9 M·/yr. In relation to our low estimations of the disk mass, such a mass loss rate suggests a disk lifetime of only ∼0.05-3 Myr, substantially shorter than the cluster age. If a generic gas/dust ratio of 100 was assumed, the disk lifetime would be even shorter, only ∼3000 yrs. The evolutionary state of this unusual protoplanetary disk is discussed. © 2011 ESO.

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