I. Kamp

DUst around NEarby Stars. The survey observational results

Context. Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their solar system counterparts are the asteroid and Edgeworth-Kuiper belts. Aims. 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. Methods. We used 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. Results. 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 similar to 12.1% +/- 5% before Herschel to similar to 20.2% +/- 2%. A significant fraction (similar to 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.

An icy Kuiper belt around the young solar-type star HD 181327

Context. HD 181327 is a young main sequence F5/F6 V star belonging to the beta Pictoris moving group (age similar to 12 Myr). It harbors an optically thin belt of circumstellar material at radius similar to 90 AU, presumed to result from collisions in a population of unseen planetesimals. Aims. We aim to study the dust properties in the belt in details, and to constrain the gas-to-dust ratio. Methods. We obtained far-infrared photometric observations of HD 181327 with the PACS instrument onboard the Herschel Space Observatory(star), complemented by new 3.2 mm observations carried with the ATCA(star star) array. The geometry of the belt is constrained with newly reduced HST/NICMOS scattered light images that allow the degeneracy between the disk geometry and the dust properties to be broken. We then use the radiative transfer code GRATER to compute a large grid of models, and we identify the grain models that best reproduce the spectral energy distribution (SED) through a Bayesian analysis. We attempt to detect the oxygen and ionized carbon fine-structure lines with Herschel/PACS spectroscopy, providing observables to our photochemical code ProDiMo. Results. The HST observations confirm that the dust is confined in a narrow belt. The continuum is detected with Herschel/PACS completing nicely the SED in the far-infrared. The disk is marginally resolved with both PACS and ATCA. A medium integration of the gas spectral lines only provides upper limits on the [OI] and [CII] line fluxes. We show that the HD 181327 dust disk consists of micron-sized grains of porous amorphous silicates and carbonaceous material surrounded by an important layer of ice, for a total dust mass of similar to 0.05 M-circle plus. (in grains up to 1 mm). We discuss evidences that the grains consists of fluffy aggregates. The upper limits on the gas atomic lines do not provide unambiguous constraints: only if the PAH abundance is high, the gas mass must be lower than similar to 17 M-circle plus. Conclusions. Despite the weak constraints on the gas disk, the age of HD 181327 and the properties of the dust disk suggest that it has passed the stage of gaseous planets formation. The dust reveals a population of icy planetesimals, similar to the primitive Edgeworth-Kuiper belt, that may be a source for the future delivery of water and volatiles onto forming terrestrial planets.

GASPS—A Herschel survey of gas and dust in protoplanetary disks: summary and initial statistics

We describe a large-scale far-infrared line and continuum survey of protoplanetary disk through to young debris disk systems carried out using the ACS instrument on the Herschel Space Observatory. This Open Time Key program, known as GASPS (Gas Survey of Protoplanetary Systems), targeted similar to 250 young stars in narrow wavelength regions covering the [OI] fine structure line at 63 mu m the brightest far-infrared line in such objects. A subset of the brightest targets were also surveyed in [OI]145 mu m, [CII] at 157 mu m, as well as several transitions of H2O and high-excitation CO lines at selected wavelengths between 78 and 180 mu m. Additionally, GASPS included continuum photometry at 70, 100 and 160 mu m, around the peak of the dust emission. The targets were SED Class II-III T Tauri stars and debris disks from seven nearby young associations, along with a comparable sample of isolated Herbig AeBe stars. The aim was to study the global gas and dust content in a wide sample of circumstellar disks, combining the results with models in a systematic way. In this overview paper we review the scientific aims, target selection and observing strategy of the program. We summarise some of the initial results, showing line identifications, listing the detections, and giving a first statistical study of line detectability. The [OI] line at 63 mu m was the brightest line seen in almost all objects, by a factor of similar to 10. Overall [OI]63 mu m detection rates were 49%, with 100% of HAeBe stars and 43% of T Tauri stars detected. A comparison with published disk dust masses (derived mainly from sub-mm continuum, assuming standard values of the mm mass opacity) shows a dust mass threshold for [OI] 63 mu m detection of similar to 10(-5) M-circle dot. Normalising to a distance of 140 pc, 84% of objects with dust masses >= 10(-5) M-circle dot can be detected in this line in the present survey; 32% of those of mass 10(-6)-10(-5) M-circle dot, and only a very small number of unusual objects with lower masses can be detected. This is consistent with models with a moderate UV excess and disk flaring. For a given disk mass, [OI] detectability is lower for M stars compared with earlier spectral types. Both the continuum and line emission was, in most systems, spatially and spectrally unresolved and centred on the star, suggesting that emission in most cases was from the disk. Approximately 10 objects showed resolved emission, most likely from outflows. In the GASPS sample, [OI] detection rates in T Tauri associations in the 0.3-4 Myr age range were similar to 50%. For each association in the 5-20 Myr age range, similar to 2 stars remain detectable in [OI]63 mu m, and no systems were detected in associations with age >20 Myr. Comparing with the total number of young stars in each association, and assuming a ISM-like gas/dust ratio, this indicates that similar to 18% of stars retain a gas-rich disk of total mass similar to 1 M-Jupiter for 1-4 Myr, 1-7% keep such disks for 5-10 Myr, but none are detected beyond 10-20 Myr. The brightest [OI] objects from GASPS were also observed in [OI]145 mu m, [CII]157 mu m and CO J = 18 - 17, with detection rates of 20-40%. Detection of the [CII] line was not correlated with disk mass, suggesting it arises more commonly from a compact remnant envelope.

Radiation thermo-chemical models of protoplanetary discs IV. Modelling CO ro-vibrational emission from Herbig Ae discs

Context. The carbon monoxide (CO) ro-vibrational emission from discs around Herbig Ae stars and T Tauri stars with strong ultraviolet emissions suggests that fluorescence pumping from the ground X 1 Σ + to the electronic A 1 Π state of CO should be taken into account in disc models. Aims. We wish to understand the excitation mechanism of CO ro-vibrational emission seen in Herbig Ae discs, in particular in transitions involving highly excited rotational and vibrational levels. Methods. We implemented a CO model molecule that includes up to 50 rotational levels within nine vibrational levels for the ground and A-excited states in the radiative-photochemical code ProDiMo. We took CO collisions with hydrogen molecules (H2), hydrogen atoms (H), helium (He), and electrons into account. We estimated the missing collision rates using standard scaling laws and discussed their limitations. We tested the effectiveness of ultraviolet (UV) fluorescence pumping for the population of high-vibrational levels (v = 1–9, J = 1–50) for four Herbig Ae disc models (disc mass Mdisc = 10 −2 ,1 0 −4 and inner radius Rdisc = 1, 20 AU). We tested the effect of infrared (IR) pumping on the CO vibrational temperature and the rotational population in the ground vibrational level. Results. UV fluorescence and IR pumping impact on the population of ro-vibrational v> 1l evels. Thev = 1 rotational levels are populated at rotational temperatures between the radiation temperature around 4.6 μm and the gas kinetic temperature. The UV pumping efficiency increases with decreasing disc mass. The consequence is that the vibrational temperatures Tvib, which measure the relative populations between the vibrational levels, are higher than the disc gas kinetic temperatures (suprathermal population of the vibrational levels). The effect is more important for low-density gases because of lower collisional de-excitations.The UV pumping is more efficient for low-mass (Mdisc 10 −3 M� ) discs. Rotational temperatures from fundamental transitions derived using optically thick 12 CO v = 1−0 lines do not reflect the gas kinetic temperature. Uncertainties in the rate coefficients within an order of magnitude result in variations in the CO line fluxes up to 20%. CO pure rotational levels with energies lower than 1000 K are populated in local thermodynamic equilibrium but are sensitive to a number of vibrational levels included in the model. The 12 CO pure rotational lines are highly optically thick for transition from levels up to Eupper = 2000 K. The model line fluxes are comparable with the observed line fluxes from typical Herbig Ae low- and high-mass discs.

The role of OH in the chemical evolution of protoplanetary disks. II. Gas-rich environments

Context. We present a method for including gas extinction of cosmic-ray-generated UV photons in chemical models of the midplane of protoplanetary disks, focusing on its implications on ice formation and chemical evolution. Aims. Our goal is to improve on chemical models by treating cosmic rays, the main source of ionization in the midplane of the disk, in a way that is consistent with current knowledge of the gas and grain environment present in those regions. We trace the effects of cosmic rays by identifying the main chemical reaction channels and also the main contributors to the gas opacity to cosmic-ray-induced UV photons. This information is crucial in implementing gas opacities for cosmic-ray-induced reactions in full 2D protoplanetary disk models. Methods. We considered time-dependent chemical models within the range 1–10 AU in the midplane of a T Tauri disk. The extinction of cosmic-ray-induced UV photons by gaseous species was included in the calculation of photorates at each timestep. We integrated the ionization and dissociation cross sections of all atoms/molecules over the cosmic-ray-induced UV emission spectrum of H2 .B y analyzing the relative contribution of each gas phase species over time, we were able to identify the main contributors to the gas opacity in the midplane of protoplanetary disks. Results. At 1 AU the gas opacity contributes up to 28.2% of the total opacity, including the dust contribution. At 3–5 AU the gas contribution is 14.5% of the total opacity, and at 7–8 AU it reaches a value of 12.2%. As expected, at 10–15 AU freeze-out of species causes the gas contribution to the total opacity to be very low (6%). The main contributors to the gas opacity are CO, CO2 ,S , SiO, and O2. OH also contributes to the gas opacity, but only at 10–15 AU.

Far-ultraviolet and X-ray irradiated protoplanetary disks: a grid of models: I. The disk structure

Context. Planets are thought to eventually form from the mostly gaseous (~99% of the mass) disks around young stars. The density structure and chemical composition of protoplanetary disks are affected by the incident radiation field at optical, far-ultraviolet (FUV), and X-ray wavelengths, as well as by the dust properties. Aims: The effect of FUV and X-rays on the disk structure and the gas chemical composition are investigated. This work forms the basis of a second paper, which discusses the impact on diagnostic lines of, e.g., C+, O, H2O, and Ne+ observed with facilities such as Spitzer and Herschel. Methods: A grid of 240 models is computed in which the X-ray and FUV luminosity, minimum grain size, dust size distribution, and surface density distribution are varied in a systematic way. The hydrostatic structure and the thermo-chemical structure are calculated using Protoplanetary Disk Model (ProDiMo), with the recent addition of X-rays. Results: The abundance structure of neutral oxygen is very stable to changes in the X-ray and FUV luminosity, and the emission lines will thus be useful tracers of the disk mass and temperature. The C+ abundance distribution is sensitive to both X-rays and FUV. The radial column density profile shows two peaks, one at the inner rim and a second one at a radius r = 5-10 AU. The minimum is caused by shadowing from the inner rim. The fluctuations in value of the column density as a function of radius are smoothed out when FUV and X-ray luminosities increase. Ne+ and other heavy elements with an ionization potential higher than IP > 13.6 eV have a very strong response to X-rays, and the column density in the inner disk increases by two orders of magnitude from the lowest (LX = 1029 erg s-1) to the highest considered X-ray flux (LX = 1032 erg s-1). FUV confines the Ne+ ionized region to areas closer to the star at low X-ray luminosities (LX = 1029 erg s-1). This is indirectly caused by changes in the disk structure. The radial column densities of Ne+ are higher than 1012 cm-1 out to radii r > 100 AU (at LFUV ≥ 1031 erg s-1), whereas the column density already drops below this value at radii r > 20 AU at LFUV = 1032 erg s-1. H2O abundances are enhanced by X-rays due to higher temperatures in the inner disk than in the FUV only case, thus leading to a more efficient neutral-neutral formation channel. Also, the higher ionization fraction provides an ion-molecule route in the outer disk. The line fluxes and profiles are affected by the effects on these species, thus providing diagnostic value in the study of FUV and X-ray irradiated disks around T Tauri stars. Appendix B is available in electronic form at http://www.aanda.org

Modelling mid-infrared molecular emission lines from T Tauri stars

We introduce a new modelling framework called FLiTs to simulate infrared line emission spectra from protoplanetary discs. This paper focuses on the mid-IR spectral region between 9.7 um to 40 um for T Tauri stars. The generated spectra contain several tens of thousands of molecular emission lines of H2O, OH, CO, CO2, HCN, C2H2, H2 and a few other molecules, as well as the forbidden atomic emission lines of SI, SII, SIII, SiII, FeII, NeII, NeIII, ArII and ArIII. In contrast to previously published works, we do not treat the abundances of the molecules nor the temperature in the disc as free parameters, but use the complex results of detailed 2D ProDiMo disc models concerning gas and dust temperature structure, and molecular concentrations. FLiTs computes the line emission spectra by ray tracing in an efficient, fast and reliable way. The results are broadly consistent with R=600 Spitzer/IRS observational data of T Tauri stars concerning line strengths, colour, and line ratios. In order to achieve that agreement, however, we need to assume either a high gas/dust mass ratio of order 1000, or the presence of illuminated disc walls at distances of a few au. The molecules in these walls cannot be photo-dissociated easily by UV because of the large densities in the walls which favour their re-formation. Most observable molecular emission lines are found to be optically thick, rendering a standard analysis with column densities difficult. We find that the difference between gas and dust temperatures in the disc surface is important for the line formation. We briefly discuss the effects of C/O ratio and choice of chemical rate network on these results. Our analysis offers new ways to infer the chemical and temperature structure of T Tauri discs from future JWST/MIRI observations, and to possibly detect secondary illuminated disc walls based on their specific mid-IR molecular signature.

The Cosmic-Ray Dominated Region of Protoplanetary Disks

We investigate the chemical evolution in the midplane of protoplanetary disks in the region 1 AU ≤ r ≤ 10 AU, focusing on cosmic ray induced processes. These processes drive the chemical pathways of formation of gas phase molecules which later can be adsorbed onto the surface of grains. We improve on previously existing chemical models by treating the interaction of cosmic rays with the gas/grain environment in a way that is consistent with the local conditions. This means including the effects of dust aggregation in the disk and the extinction of cosmic ray induced UV photons by the gas. We conclude that the effects of cosmic ray UV flux enhancement brought about by grain growth are as relevant as their extinction by gas species. Thus we identify CO, CO2, SiO, S and O2 as the main species that contribute to the gas extinction in these regions. The implementation of this method seeks to complete other models that use steady state estimations of the chemical composition of the disk.

GASPS observations of Herbig Ae/Be stars with PACS/Herschel ? The atomic and molecular content of their protoplanetary discs

We observed a sample of 20 representative Herbig Ae/Be stars and 5 A-type debris discs with PACS onboard Herschel, as part of the GAS in Protoplanetary Systems (GASPS) project. The observations were done in spectroscopic mode, and cover the far-infrared lines of [O i], [C ii], CO, CH, H2O, and OH. We have a [O i] 63 μm detection rate of 100% for the Herbig Ae/Be and 0% for the debris discs. The [O i] 145 μm line is only detected in 25% and CO J = 18–17 in 45% (and fewer cases for higher J transitions) of the Herbig Ae/Be stars, while for [C ii] 157 μm, we often find spatially variable background contamination. We show the first detection of water in a Herbig Ae disc, HD 163296, which has a settled disc. Hydroxyl is detected as well in this disc. First seen in HD 100546, CH emission is now detected for the second time in a Herbig Ae star, HD 97048. We report fluxes for each line and use the observations as line diagnostics of the gas properties. Furthermore, we look for correlations between the strength of the emission lines and either the stellar or disc parameters, such as stellar luminosity, ultraviolet and X-ray flux, accretion rate, polycyclic aromatic hydrocarbon (PAH) band strength, and flaring. We find that the stellar ultraviolet flux is the dominant excitation mechanism of [O i] 63 μm, with the highest line fluxes being found in objects with a large amount of flaring and among the largest PAH strengths. Neither the amount of accretion nor the X-ray luminosity has an influence on the line strength. We find correlations between the line flux of [O i] 63 μm and [O i] 145 μm, CO J = 18–17 and [O i] 6300 A, and between the continuum flux at 63 μm and at 1.3 mm, while we find weak correlations between the line flux of [O i] 63 μm and the PAH luminosity, the line flux of CO J = 3–2, the continuum flux at 63 μm, the stellar effective temperature, and the Brγ luminosity. Finally, we use a combination of the [O i] 63 μm and 12CO J = 2–1 line fluxes to obtain order of magnitude estimates of the disc gas masses, in agreement with the values that we find from detailed modelling of two Herbig Ae/Be stars, HD 163296 and HD 169142.We observed a sample of 20 representative Herbig Ae/Be stars and five A-type debris discs with PACS onboard of Herschel. The observations were done in spectroscopic mode, and cover far-IR lines of [OI], [CII], CO, CH+, H2O and OH. We have a [OI]63 micron detection rate of 100% for the Herbig Ae/Be and 0% for the debris discs. [OI]145 micron is only detected in 25%, CO J=18-17 in 45% (and less for higher J transitions) of the Herbig Ae/Be stars and for [CII] 157 micron, we often found spatially variable background contamination. We show the first detection of water in a Herbig Ae disc, HD 163296, which has a settled disc. Hydroxyl is detected as well in this disc. CH+, first seen in HD 100546, is now detected for the second time in a Herbig Ae star, HD 97048. We report fluxes for each line and use the observations as line diagnostics of the gas properties. Furthermore, we look for correlations between the strength of the emission lines and stellar or disc parameters, such as stellar luminosity, UV and X-ray flux, accretion rate, PAH band strength, and flaring. We find that the stellar UV flux is the dominant excitation mechanism of [OI]63 micron, with the highest line fluxes found in those objects with a large amount of flaring and greatest PAH strength. Neither the amount of accretion nor the X-ray luminosity has an influence on the line strength. We find correlations between the line flux of [OI]63 micron and [OI]145 micron, CO J = 18-17 and [OI]6300 AA, and between the continuum flux at 63 micron and at 1.3 mm, while we find weak correlations between the line flux of [OI]63 micron and the PAH luminosity, the line flux of CO J = 3-2, the continuum flux at 63 micron, the stellar effective temperature and the Brgamma luminosity. (Abbreviated version)

Far-ultraviolet and X-ray irradiated protoplanetary disks: a grid of models: II. Gas diagnostic line emission

Context. Most of the mass in protoplanetary disks is in the form of gas. The study of the gas and its diagnostics is of fundamental importance in order to achieve a detailed description of the thermal and chemical structure of the disk. Both radiation from the central star (from optical to X-ray wavelengths) and viscous accretion are the main sources of energy, dominating disk physics and chemistry in its early stages. This is the environment in which the first phases of planet formation will proceed. Aims: We investigate how stellar and disk parameters impact the fine-structure cooling lines [Ne ii], [Ar ii], [O i], [C ii], and H2O rotational lines in the disk. These lines are potentially powerful diagnostics of the disk structure, and their modeling permits a thorough interpretation of the observations carried out with instrumental facilities such as Spitzer and Herschel. Methods: Following our earlier paper, we computed a grid of 240 disk models, in which the X-ray luminosity, UV-excess luminosity, minimum dust grain size, dust size distribution power law, and surface density distribution power law are systematically varied. We solve self-consistently for the disk vertical hydrostatic structure in every model and apply detailed line radiative transfer to calculate line fluxes and profiles for a series of well-known mid- and far-infrared cooling lines. Results: The [O i] 63 μm line flux increases with increasing LFUV when LX 30 erg s-1 and with increasing X-ray luminosity when LX > 1030 erg s-1. While [C ii] 157 μm is mainly driven by LFUV via C+ production, X-rays affect the line flux to a lesser extent. In addition, [Ne ii] 12.8 μm correlates with X-rays; the line profile emitted from the disk atmosphere shows a double-peaked component caused by emission in the static disk atmosphere, next to a high-velocity double-peaked component caused by emission in the very inner rim. Water transitions, depending on the disk region they arise from, show different slopes in correlation with the [O i] 63 μm line.