Inga Kamp

Radiation thermo-chemical models of protoplanetary disks - I. Hydrostatic disk structure and inner rim

Context. Emission lines from protoplanetary disks originate mainly in the irradiated surface layers, where the gas is generally warmer than the dust. Therefore, interpreting emission lines requires detailed thermo-chemical models, which are essential to converting line observations into understanding disk physics. Aims. We aim at hydrostatic disk models that are valid from 0.1 AU to 1000 AU to interpret gas emission lines from UV to sub-mm. In particular, our interest lies in interpreting far IR gas emission lines, such as will be observed by the Herschel observatory, related to the Gasps open time key program. This paper introduces a new disk code called ProDiMo. Methods. We combine frequency-dependent 2D dust continuum radiative transfer, kinetic gas-phase and UV photo-chemistry, ice formation, and detailed non-LTE heating & cooling with the consistent calculation of the hydrostatic disk structure. We include Fe ii and CO ro-vibrational line heating/cooling relevant to the high-density gas close to the star, and apply a modified escapeprobability treatment. The models are characterised by a high degree of consistency between the various physical, chemical, and radiative processes, where the mutual feedbacks are solved iteratively. Results. In application to a T Tauri disk extending from 0.5 AU to 500 AU, the models show that the dense, shielded and cold midplane

Radiation thermo-chemical models of protoplanetary disks. II. Line diagnostics

Aims. In this paper, we explore the diagnostic power of the far-IR fine-structure lines of [Oi] 63.2 mu m, 145.5 mu m, [Cii] 157.7 mu m, as well as the radio and sub-mm lines of CO J = 1-0, 2-1 and 3-2 in application to disks around Herbig Ae stars. We aim at understanding where the lines originate from, how the line formation process is affected by density, temperature and chemical abundance in the disk, and to what extent non-LTE effects are important. The ultimate aim is to provide a robust way to determine the gas mass of protoplanetary disks from line observations. Methods. We use the recently developed disk code ProDiMo to calculate the physico-chemical structure of protoplanetary disks and apply the Monte-Carlo line radiative transfer code Ratran to predict observable line profiles and fluxes. We consider a series of Herbig Ae type disk models ranging from 10(-6) M(circle dot) to 2.2 x 10(-2) M(circle dot) (between 0.5 and 700 AU) to discuss the dependency of the line fluxes and ratios on disk mass for otherwise fixed disk parameters. This paper prepares for a more thorough multi-parameter analysis related to the Herschel open time key program Gasps. Results. We find the [Cii] 157.7 mu m line to originate in LTE from the surface layers of the disk, where T(g) not equal T(d). The total emission is dominated by surface area and hence depends strongly on disk outer radius. The [Oi] lines can be very bright (> 10(-16) W/m(2)) and form in slightly deeper and closer regions under non-LTE conditions. For low-mass models, the [Oi] lines come preferentially from the central regions of the disk, and the peak separation widens. The high-excitation [Oi] 145.5 mu m line, which has a larger critical density, decreases more rapidly with disk mass than the 63.2 mu m line. Therefore, the [Oi] 63.2 mu m/145.5 mu m ratio is a promising disk mass indicator, especially as it is independent of disk outer radius for R(out) > 200 AU. CO is abundant only in deeper layers A(V) greater than or similar to 0.05. For too low disk masses (M(disk) less than or similar to 10(-4) M(circle dot)) the dust starts to become transparent, and CO is almost completely photo-dissociated. For masses larger than that the lines are an excellent independent tracer of disk outer radius and can break the outer radius degeneracy in the [Oi] 63.2 mu m/[C ii] 157.7 mu m line ratio. Conclusions. The far-IR fine-structure lines of [Cii] and [Oi] observable with Herschel provide a promising tool to measure the disk gas mass, although they are mainly generated in the atomic surface layers. In spatially unresolved observations, none of these lines carry much information about the inner, possibly hot regions <30 AU.

Consistent dust and gas models for protoplanetary disks I. Disk shape, dust settling, opacities, and PAHs

We propose a set of standard assumptions for the modelling of Class II and III protoplanetary disks, which includes detailed continuum radiative transfer, thermo-chemical modelling of gas and ice, and line radiative transfer from optical to cm wavelengths. The first paper of this series focuses on the assumptions about the shape of the disk, the dust opacities, dust settling, and polycyclic aromatic hydrocarbons (PAHs). In particular, we propose new standard dust opacities for disk models, we present a simplified treatment of PAHs in radiative equilibrium which is sufficient to reproduce the PAH emission features, and we suggest using a simple yet physically justified treatment of dust settling. We roughly adjust parameters to obtain a model that predicts continuum and line observations that resemble typical multi-wavelength continuum and line observations of Class II T Tauri stars. We systematically study the impact of each model parameter (disk mass, disk extension and shape, dust settling, dust size and opacity, gas/dust ratio, etc.) on all mainstream continuum and line observables, in particular on the SED, mm-slope, continuum visibilities, and emission lines including [OI] 63 μm, high-J CO lines, (sub-)mm CO isotopologue lines, and CO fundamental ro-vibrational lines. We find that evolved dust properties, i.e. large grains, often needed to fit the SED, have important consequences for disk chemistry and heating/cooling balance, leading to stronger near- to far-IR emission lines in general. Strong dust settling and missing disk flaring have similar effects on continuum observations, but opposite effects on far-IR gas emission lines. PAH molecules can efficiently shield the gas from stellar UV radiation because of their strong absorption and negligible scattering opacities in comparison to evolved dust. The observable millimetre-slope of the SED can become significantly more gentle in the case of cold disk midplanes, which we find regularly in our T Tauri models. We propose to use line observations of robust chemical tracers of the gas, such as O, CO, and H2, as additional constraints to determine a number of key properties of the disks, such as disk shape and mass, opacities, and the dust/gas ratio, by simultaneously fitting continuum and line observations.

A Resolved Molecular Gas Disk around the Nearby A Star 49 Ceti

The A star 49 Ceti, at a distance of 61 pc, is unusual in retaining a substantial quantity of molecular gas while exhibiting dust properties similar to those of a debris disk. We present resolved observations of the disk around 49 Ceti from the Submillimeter Array in the J ¼ 2Y1 rotational transition of CO with a resolution of 1:0 00 ;1:2 00 . The observed emission reveals an extended rotating structure viewed approximately edge-on and clear of detectable CO emission out to a distance of � 90 AU from the star. No 1.3 mm continuum emission is detected at a 3 � sensitivity of 2.1mJybeam � 1 .Modelsof diskstructure andchemistry indicate that theinner diskisdevoidof moleculargas,while the outer gas disk between 40 and 200 AU from the star is dominated by photochemistry from stellar and interstellar radiation.Wedetermineparametersfor amodelthatreproducesthebasicfeaturesof thespatiallyresolvedCOJ ¼ 2Y1 emission, the spectral energy distribution, and the unresolved CO J ¼ 3Y2 spectrum. We investigate variations in disk chemistry and observable properties for a range of structural parameters. The star 49 Ceti appears to be a rare example of a system in a late stage of transition between a gas-rich protoplanetary disk and a tenuous, virtually gasfree debris disk. Subject headingg astrochemistry — circumstellar matter — planetary systems: protoplanetary disks — stars: individual (49 Ceti)

The solar photospheric nitrogen abundance - Analysis of atomic transitions with 3D and 1D model atmospheres

Context. In recent years, the solar chemical abundances have been studied in considerable detail because of discrepant values of solar metallicity inferred from different indicators, i.e., on the one hand, the “sub-solar” photospheric abundances resulting from spectroscopic chemical composition analyses with the aid of 3D hydrodynamical models of the solar atmosphere, and, on the other hand, the high metallicity inferred by helioseismology. Aims. After investigating the solar oxygen abundance using a CO 5 BOLD 3D hydrodynamical solar model in previous work, we undertake a similar approach studying the solar abundance of nitrogen, since this element accounts for a significant fraction of the overall solar metallicity, Z. Methods. We used a selection of atomic spectral lines to determine the solar nitrogen abundance, relying mainly on equivalent width measurements in the literature. We investigate the influence on the abundance analysis, of both deviations from local thermodynamic equilibrium (“NLTE effects”) and photospheric inhomogeneities (“granulation effects”). Results. We recommend use of a solar nitrogen abundance of A(N) = 7.86 ± 0.12 � , whose error bar reflects the line-to-line scatter. Conclusions. The solar metallicity implied by the CO 5 BOLD-based nitrogen and oxygen abundances is in the range 0.0145 ≤ Z ≤ 0.0167. This result is a step towards reconciling photospheric abundances with helioseismic constraints on Z. Our most suitable estimates are Z = 0.0156 and Z/X = 0.0213.

Herschel-PACS observation of the 10 Myr old T Tauri disk TW Hya : Constraining the disk gas mass

Planets are formed in disks around young stars. With an age of similar to 10 Myr, TW Hya is one of the nearest T Tauri stars that is still surrounded by a relatively massive disk. In addition a large number of molecules has been found in the TW Hya disk, making TW Hya the perfect test case in a large survey of disks with Herschel-PACS to directly study their gaseous component. We aim to constrain the gas and dust mass of the circumstellar disk around TW Hya. We observed the fine-structure lines of [OI] and [CII] as part of the open-time large program GASPS. We complement this with continuum data and ground-based (12) CO 3-2 and (CO)-C-13 3-2 observations. We simultaneously model the continuum and the line fluxes with the 3D Monte-Carlo code MCFOST and the thermo-chemical code ProDiMo to derive the gas and dust masses. We detect the [OI] line at 63 mu m. The other lines that were observed, [OI] at 145 mu m and [CII] at 157 mu m, are not detected. No extended emission has been found. Preliminary modeling of the photometric and line data assuming [(CO)-C-12]/[(CO)-C-13] = 69 suggests a dust mass for grains with radius < 1 mm of similar to 1.9 x 10(-4) M-circle dot (total solid mass of 3 x 10(-3) M-circle dot) and a gas mass of (0.5-5) x 10(-3) M-circle dot. The gas-to-dust mass may be lower than the standard interstellar value of 100.

The unusual protoplanetary disk around the T Tauri star ET Chamaeleontis

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 eta Chamaeleontis cluster. We have acquired Herschel/PACS photometric fluxes at 70 mu m and 160 mu m, as well as a detection of the [OI] 63 mu m 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 ETCha 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] 6300 angstrom, [SII] 6731 angstrom and 6716 angstrom, and [NII] 6583 angstrom, 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 ETCha. 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 dust mass with confidence, M-dust approximate to (2-5) x 10(-8) M-circle dot whereas the total disk gas mass is found to be only little constrained, M-gas approximate to (5 x 10(-5)-3 x 10(-3)) M-circle dot. 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] 6300 angstrom 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 H-2 v = 1 -> 0 S(1) at 2.122 mu m and with the [OI] 63 mu m line as seen with Herschel/PACS. An additional high-velocity component of the [OI] 6300 angstrom emission line, however, points to the existence of an additional jet/outflow of low velocity 40-65 km s(-1) with mass loss rate approximate to 10(-9) M-circle dot/yr. In relation to our low estimations of the disk mass, such a mass loss rate suggests a disk lifetime of only similar to 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 similar to 3000 yrs. The evolutionary state of this unusual protoplanetary disk is discussed.

Cold DUst around NEarby Stars (DUNES). First results A resolved exo-Kuiper belt around the solar-like star zeta(2) Ret

We present the first far-IR observations of the solar-type stars delta Pav, HR 8501, 51 Peg and zeta(2) Ret, taken within the context of the DUNES Herschel open time key programme (OTKP). This project uses the PACS and SPIRE instruments with the objective of studying infrared excesses due to exo-Kuiper belts around nearby solar-type stars. The observed 100 mu m fluxes from delta Pav, HR 8501, and 51 Peg agree with the predicted photospheric fluxes, excluding debris disks brighter than L-dust/L-star similar to 5 x 10(-7) (1 sigma level) around those stars. A flattened, disk-like structure with a semi-major axis of similar to 100 AU in size is detected around zeta(2) Ret. The resolved structure suggests the presence of an eccentric dust ring, which we interpret as an exo-Kuiper belt with L-dust/L-star approximate to 10(-5).

X-ray impact on the protoplanetary disks around T Tauri stars

Context. T Tauri stars have X-ray luminosities in the range LX = 10 28 − 10 32 erg s −1 . These luminosities are similar to their UV luminosities (LUV ∼ 10 30 −10 31 erg s −1 ) and therefore X-rays are expected to affect the physics and chemistry of the upper layers of their surrounding protoplanetary disks. Aims. The effects and importance of X-rays on the chemical and hydrostatic structure of protoplanetary disks are investigated, species tracing X-ray irradiation (for LX ≥ 10 29 erg s −1 ) are identified and predictions for [O i], [C ii], and [N ii] fine structure line fluxes are provided. Methods. We implemented X-ray physics and chemistry into the chemo-physical disk code ProDiMo. We include Coulomb heating and H2 ionization as heating processes and both primary and secondary ionization due to X-rays in the chemistry. Results. X-rays heat the gas causing it to expand in the optically thin surface layers. Neutral molecular species are not significantly affected in terms of their abundance and spatial distribution, but charged species such as N + ,O H + ,H 2O + ,a nd H 3O + display enhanced abundances in the disk surface. Conclusions. Coulomb heating by X-rays changes the vertical structure of the disk, yielding temperatures of ∼8000 K out to distances of 50 AU. The chemical structure is altered by the high electron abundance of the gas at the disk surface, causing an efficient ion-molecule chemistry. The products of this, OH + ,H 2O + and H3O + , are of great interest for observations of low-mass young stellar objects with the Herschel Space Observatory. Both [O i] (at 63 and 145 μm) and [C ii] (at 158 μm) fine structure emission are affected only for LX > 10 30 erg s −1 .

The solar photospheric abundance of carbon - Analysis of atomic carbon lines with the CO5BOLD solar model

Context. The analysis of the solar spectra using hydrodynamical simulations, with a specific selection of lines, atomic data, and method for computing deviations from local thermodynamical equilibrium, has led to a downward revision of the solar metallicity, Z. We are using the latest simulations computed with the CO5BOLD code to reassess the solar chemical composition. Our previous analyses of the key elements, oxygen and nitrogen, have not confirmed any extreme downward revision of Z, as derived in other works based on hydrodynamical models. Aims. We determine the solar photospheric carbon abundance with a radiation-hydrodynamical CO5BOLD model and compute the departures from local thermodynamical equilibrium by using the Kiel code. Methods. We measured equivalent widths of atomic C i lines on high-resolution, high signal-to-noise ratio solar atlases of disccentre intensity and integrated disc flux. These equivalent widths were analysed with our latest solar 3D hydrodynamical simulation computed with CO5BOLD. Deviations from local thermodynamic equilibrium we computed in 1D with the Kiel code, using the average temperature structure of the hydrodynamical simulation as a background model. Results. Our recommended value for the solar carbon abundance relies on 98 independent measurements of observed lines and is A(C) = 8.50 ± 0.06. The quoted error is the sum of statistical and systematic errors. Combined with our recent results for the solar oxygen and nitrogen abundances, this implies a solar metallicity of Z = 0.0154 and Z/X = 0.0211. Conclusions. Our analysis implies a solar carbon abundance that is about 0.1 dex higher than what was found in previous analyses based on different 3D hydrodynamical computations. The difference is partly driven by our equivalent width measurements (we measure, on average, larger equivalent widths than the other work based on a 3D model), in part because of the different properties of the hydrodynamical simulations and the spectrum synthesis code. The solar metallicity we obtain from the CO5BOLD analyses is in slightly better agreement with the constraints of helioseismology than the previous 3D abundance results.