The physical and chemical properties of planet forming disks

VLT instruments and ALMA have revolutionized in the past five years our view and understanding of how disks turn into planetary systems. They provide exquisite insights into non-axisymmetric structures likely closely related to ongoing planet formation processes. The following cannot be a complete review of the physical and chemical properties of disks; instead I focus on a few selected aspects. I will review our current understanding of the physical properties (e.g. solid and gas mass content, snow and ice lines) and chemical composition of planet forming disks at ages of 1-few Myr, especially in the context of the planetary systems that are forming inside them. I will highlight recent advances achieved by means of consistent multi-wavelength studies of gas AND dust in protoplanetary disks.Comment: accepted for IAUS345 "Origins: From the Protosun to the First Steps of Life" proceeding

The gas temperature in the surface layers of protoplanetary disks

Models for the structure of protoplanetary disks have so far been based on the assumption that the gas and the dust temperature are equal. The gas temperature, an essential ingredient in the equations of hydrostatic equilibrium of the disk, is then determined from a continuum radiative transfer calculation, in which the continuum opacity is provided by the dust. It has been long debated whether this assumption still holds in the surface layers of the disk, where the dust infrared emission features are produced. In this paper we compute the temperature of the gas in the surface layers of the disk in a self-consistent manner. The gas temperature is determined from a heating-cooling balance equation in which processes such as photoelectric heating, dissociative heating, dust-gas thermal heat exchange and line cooling are included. The abundances of the dominant cooling species such as CO, C, C+ and O are determined from a chemical network based on the atomic species H, He, C, O, S, Mg, Si, Fe (Kamp & Bertoldi 2000). The underlying disk models to our calculations are the models of Dullemond, van Zadelhoff & Natta (2002). We find that in general the dust and gas temperature are equal to withing 10% for A_V >~ 0.1, which is above the location of the `super-heated surface layer' in which the dust emission features are produced (e.g. Chiang & Goldreich 1997). High above the disk surface the gas temperature exceeds the dust temperature and can can become -- in the presence of polycyclic aromatic hydrocarbons -- as high as 600 K at a radius of 100 AU. This is a region where CO has fully dissociated, but a significant fraction of hydrogen is still in molecular form. The densities are still high enough for non-negligible H_2 emission to be produced.....(see paper for full abstract)Comment: 28 pages, 8 figures, accepted for publication in Ap

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=2-1 rotational transition of CO with a resolution of 1.0x1.2 arcsec. 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 millimeter continuum emission is detected at a 3-sigma sensitivity of 2.1 mJy/beam. Models of disk structure and chemistry indicate that the inner disk is devoid of molecular gas, while the outer gas disk between 40 and 200 AU from the star is dominated by photochemistry from stellar and interstellar radiation. We determine parameters for a model that reproduces the basic features of the spatially resolved CO J=2-1 emission, the spectral energy distribution, and the unresolved CO J=3-2 spectrum. We investigate variations in disk chemistry and observable properties for a range of structural parameters. 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 gas-free debris disk.Comment: 11 pages, 6 figures, accepted for publication in Ap

Formation of Narrow Dust Rings in Circumstellar Debris Disks

Narrow dust rings observed around some young stars (e.g., HR 4796A) need to be confined. We present a possible explanation for the formation and confinement of such rings in optically thin circumstellar disks, without invoking shepherding planets. If an enhancement of dust grains (e.g., due to a catastrophic collision) occurs somewhere in the disk, photoelectric emission from the grains can heat the gas to temperatures well above that of the dust. The gas orbits with super(sub)-Keplerian speeds inward (outward) of the associated pressure maximum. This tends to concentrate the grains into a narrow region. The rise in dust density leads to further heating and a stronger concentration of grains. A narrow dust ring forms as a result of this instability. We show that this mechanism not only operates around early-type stars that have high UV fluxes, but also around stars with spectral types as late as K. This implies that this process is generic and may have occurred during the lifetime of each circumstellar disk. We examine the stringent upper-limit on the H2 column density in the HR 4796A disk and find it to be compatible with the presence of a significant amount of hydrogen gas in the disk. We also compute the OI and CII infrared line fluxes expected from various debris disks and show that these will be easily detectable by the upcoming Herschel mission. Herschel will be instrumental in detecting and characterizing gas in these disks.Comment: Accepted for publication in ApJ; 14 pages, 7 figure

FUV 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, 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 ProDiMo. Results. The abundance structure of neutral oxygen is 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. Ne+ and other heavy elements 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 = 1e29 erg/s) to the highest considered X-ray flux (LX = 1e32 erg/s). FUV confines the Ne+ ionized region to areas closer to the star at low X-ray luminosities (LX = 1e29 erg/s). H2O abundances are enhanced by X-rays due to higher temperatures in the inner disk and higher ionization fractions 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. (abridged)Comment: 47 pages, accepted by Astronomy and Astrophysics, a high resolution version of the paper is located at http://www.astro.rug.nl/~meijerink/disk_paperI_xrays.pd

The effects of dust evolution on disks in the mid-IR

In this paper, we couple together the dust evolution code two-pop-py with the thermochemical disk modelling code ProDiMo. We create a series of thermochemical disk models that simulate the evolution of dust over time from 0.018 Myr to 10 Myr, including the radial drift, growth, and settling of dust grains. We examine the effects of this dust evolution on the mid-infrared gas emission, focussing on the mid-infrared spectral lines of C2H2, CO2, HCN, NH3, OH, and H2O that are readily observable with Spitzer and the upcoming E-ELT and JWST. The addition of dust evolution acts to increase line fluxes by reducing the population of small dust grains. We find that the spectral lines of all species except C2H2 respond strongly to dust evolution, with line fluxes increasing by more than an order of magnitude across the model series as the density of small dust grains decreases over time. The C2H2 line fluxes are extremely low due to a lack of abundance in the infrared line-emitting regions, despite C2H2 being commonly detected with Spitzer, suggesting that warm chemistry in the inner disk may need further investigation. Finally, we find that the CO2 flux densities increase more rapidly than the other species as the dust disk evolves. This suggests that the flux ratios of CO2 to other species may be lower in disks with less-evolved dust populations.Comment: 13 pages, 9 figures, accepted in A&

Using a Logic Programming Framework to Control Database Query Dialogues in Natural Language

We present a natural language question/answering system to interface the University of Évora databases that uses clarification dialogs in order to clarify user questions. It was developed in an integrated logic programming framework, based on constraint logic programming using the GnuProlog(-cx) language [2,11] and the ISCO framework [1]. The use of this LP framework allows the integration of Prolog-like inference mechanisms with classes and inheritance, constraint solving algorithms and provides the connection with relational databases, such as PostgreSQL. This system focus on the questions’ pragmatic analysis, to handle ambiguity, and on an efficient dialogue mechanism, which is able to place relevant questions to clarify the user intentions in a straightforward manner. Proper Nouns resolution and the pp-attachment problem are also handled. This paper briefly presents this innovative system focusing on its ability to correctly determine the user intention through its dialogue capability