The Foggy Disks Surrounding Herbig Ae Stars: a Theoretical Study of the H2O Line Spectra

Water is a key species in many astrophysical environments, but it is particularly important in proto-planetary disks. So far,observations of water in these objects have been scarce, but the situation should soon change thanks to the Herschel satellite. We report here a theoretical study of the water line spectrum of a proto-planetary disk surrounding Ae stars. We show that several lines will be observable with the HIFI instrument onboard the Herschel Space Observatory. We predict that some maser lines could also be observable with ground telescopes and we discuss how the predictions depend not only on the adopted physical and chemical model but also on the set of collisional coefficients used and on the H2 ortho to para ratio through its effect on collisional excitation. This makes the water lines observations a powerful, but dangerous -if misused- diagnostic tool.Comment: Accepted for publication in ApJ Letter

The Herschel view of GAS in Protoplanetary Systems (GASPS): First comparisons with a large grid of models

The Herschel GASPS key program is a survey of the gas phase of protoplanetary discs, targeting 240 objects which cover a large range of ages, spectral types, and disc properties. To interpret this large quantity of data and initiate self-consistent analyses of the gas and dust properties of protoplanetary discs, we have combined the capabilities of the radiative transfer code MCFOST with the gas thermal balance and chemistry code ProDiMo to compute a grid of ≈300  000 disc models (DENT). We present a comparison of the first Herschel/GASPS line and continuum data with the predictions from the DENT grid of models. Our objective is to test some of the main trends already identified in the DENT grid, as well as to define better empirical diagnostics to estimate the total gas mass of protoplanetary discs. Photospheric UV radiation appears to be the dominant gas-heating mechanism for Herbig stars, whereas UV excess and/or X-rays emission dominates for T Tauri stars. The DENT grid reveals the complexity in the analysis of far-IR lines and the difficulty to invert these observations into physical quantities. The combination of Herschel line observations with continuum data and/or with rotational lines in the (sub-)millimetre regime, in particular CO lines, is required for a detailed characterisation of the physical and chemical properties of circumstellar discs

Benchmark problems for continuum radiative transfer. High optical depths, anisotropic scattering, and polarisation

Solving the continuum radiative transfer equation in high opacity media requires sophisticated numerical tools. In order to test the reliability of such tools, we present a benchmark of radiative transfer codes in a 2D disc configuration. We test the accuracy of seven independently developed radiative transfer codes by comparing the temperature structures, spectral energy distributions, scattered light images, and linear polarisation maps that each model predicts for a variety of disc opacities and viewing angles. The test cases have been chosen to be numerically challenging, with midplane optical depths up 10^6, a sharp density transition at the inner edge and complex scattering matrices. We also review recent progress in the implementation of the Monte Carlo method that allow an efficient solution to these kinds of problems and discuss the advantages and limitations of Monte Carlo codes compared to those of discrete ordinate codes. For each of the test cases, the predicted results from the radiative transfer codes are within good agreement. The results indicate that these codes can be confidently used to interpret present and future observations of protoplanetary discs.Comment: 15 pages, 10 figures, accepted for publication in A&

On the stratified dust distribution of the GG Tau circumbinary ring

Our objective is to study the vertical dust distribution in the circumbinary ring of the binary system GG Tau and to search for evidence of stratification, one of the first steps expected to occur during planet formation. We present a simultaneous analysis of four scattered light images spanning a range of wavelength from 800 nm to 3800 nm and compare them with (i) a parametric prescription for the vertical dust stratification, and (ii) with the results of SPH bi-fluid hydrodynamic calculations. The parametric prescription and hydrodynamical calculations of stratification both reproduce the observed brightness profiles well. These models also provide a correct match for the observed star/ring integrated flux ratio. Another solution with a well-mixed, but ``exotic'', dust size distribution also matches the brightness profile ratios but fails to match the star/ring flux ratio. These results give support to the presence of vertical stratification of the dust in the ring of GG Tau and further predict the presence of a radial stratification also.Comment: 9 pages, 11 figures. Accepted for publication in A&

The accumulation and trapping of grains at planet gaps: effects of grain growth and fragmentation

We model the dust evolution in protoplanetary disks with full 3D, Smoothed Particle Hydrodynamics (SPH), two-phase (gas+dust) hydrodynamical simulations. The gas+dust dynamics, where aerodynamic drag leads to the vertical settling and radial migration of grains, is consistently treated. In a previous work, we characterized the spatial distribution of non-growing dust grains of different sizes in a disk containing a gap-opening planet and investigated the gap's detectability with the Atacama Large Millimeter/submillimeter Array (ALMA). Here we take into account the effects of grain growth and fragmentation and study their impact on the distribution of solids in the disk. We show that rapid grain growth in the two accumulation zones around planet gaps is strongly affected by fragmentation. We discuss the consequences for ALMA observations.Comment: Accepted for publication in Planetary and Space Science. 13 pages, 4 figure

Where is the warm H2 ? A search for H2 emission from disks around Herbig Ae/Be stars

Mid-IR emission lines of H2 are useful probes to determine the mass of warm gas present in the surface layers of disks. Numerous observations of Herbig Ae/Be stars (HAeBes) have been performed, but only 2 detections of mid-IR H2 toward HD97048 and AB Aur have been reported. We aim at tracing the warm gas in the disks of 5 HAeBes with gas-rich environments and physical characteristics close to those of AB Aur and HD97048, to discuss whether the detections toward these 2 objects are suggestive of peculiar conditions for the gas. We search for the H2 S(1) emission line at 17.035 \mu\m with VISIR, and complemented by CH molecule observations with UVES. We gather the H2 measurements from the literature to put the new results in context and search for a correlation with some disk properties. None of the 5 VISIR targets shows evidence for H2 emission. From the 3sigma upper limits on the integrated line fluxes we constrain the amount of optically thin warm gas to be less than 1.4 M_Jup in the disk surface layers. There are now 20 HAeBes observed with VISIR and TEXES instruments to search for warm H2, but only two detections (HD97048 and AB Aur) were made so far. We find that the two stars with detected warm H2 show at the same time high 30/13 \mu\m flux ratios and large PAH line fluxes at 8.6 and 11.3 \mu\m compared to the bulk of observed HAeBes and have emission CO lines detected at 4.7 \mu\m. We detect the CH 4300.3A absorption line toward both HD97048 and AB Aur with UVES. The CH to H2 abundance ratios that this would imply if it were to arise from the same component as well as the radial velocity of the CH lines both suggest that CH arises from a surrounding envelope, while the detected H2 would reside in the disk. The two detections of the S(1) line in the disks of HD97048 and AB Aur suggest either peculiar physical conditions or a particular stage of evolution.Comment: accepted for publication in A&A : 10 pages, 6 figure

A library of near-infrared integral field spectra of young M-L dwarfs

We present a library of near-infrared (1.1-2.45 microns) medium-resolution (R~1500-2000) integral field spectra of 15 young M6-L0 dwarfs, composed of companions with known ages and of isolated objects. We use it to (re)derive the NIR spectral types, luminosities and physical parameters of the targets, and to test (BT-SETTL, DRIFT-PHOENIX) atmospheric models. We derive infrared spectral types L0+-1, L0+-1, M9.5+-0.5, M9.5+-0.5, M9.25+-0.25, M8+0.5-0.75, and M8.5+-0.5 for AB Pic b, Cha J110913-773444, USco CTIO 108B, GSC 08047-00232 B, DH Tau B, CT Cha b, and HR7329B, respectively. BT-SETTL and DRIFT-PHOENIX models yield close Teff and log g estimates for each sources. The models seem to evidence a 600-300+600 K drop of the effective temperature at the M-L transition. Assuming the former temperatures are correct, we derive new mass estimates which confirm that DH Tau B, USco CTIO 108B, AB Pic b, KPNO Tau 4, OTS 44, and Cha1109 lay inside or at the boundary of the planetary mass range. We combine the empirical luminosities of the M9.5-L0 sources to the Teff to derive semi-empirical radii estimates that do not match "hot-start" evolutionary models predictions at 1-3 Myr. We use complementary data to demonstrate that atmospheric models are able to reproduce the combined optical and infrared spectral energy distribution, together with the near-infrared spectra of these sources simultaneously. But the models still fail to represent the dominant features in the optical. This issue casts doubts on the ability of these models to predict correct effective temperatures from near-infrared spectra alone. We advocate the use of photometric and spectroscopic data covering a broad range of wavelengths to study the properties of very low mass young companions to be detected with the planet imagers (Subaru/SCExAO, LBT/LMIRCam, Gemini/GPI, VLT/SPHERE).Comment: 27 pages, 14 tables, 19 figures, accepted for publication in Astronomy & Astrophysic

An Upper Limit on the Mass of the Circumplanetary Disk for DH Tau b

Indexación: Scopus.DH Tau is a young (sim;1 Myr) classical T Tauri star. It is one of the few young PMS stars known to be associated with a planetary mass companion, DH Tau b, orbiting at large separation and detected by direct imaging. DH Tau b is thought to be accreting based on copious Ha emission and exhibits variable Paschen Beta emission. NOEMA observations at 230 GHz allow us to place constraints on the disk dust mass for both DH Tau b and the primary in a regime where the disks will appear optically thin. We estimate a disk dust mass for the primary, DH Tau A of 17.2 ± 1.7 MÅ, which gives a disk to star mass ratio of 0.014 (assuming the usual gas to dust mass ratio of 100 in the disk). We find a conservative disk dust mass upper limit of 0.42M⊕ for DH Tau b, assuming that the disk temperature is dominated by irradiation from DH Tau b itself. Given the environment of the circumplanetary disk, variable illumination from the primary or the equilibrium temperature of the surrounding cloud would lead to even lower disk mass estimates. A MCFOST radiative transfer model, including heating of the circumplanetary disk by DH Tau b and DH Tau A, suggests that a mass-averaged disk temperature of 22 K is more realistic, resulting in a dust disk mass upper limit of 0.09M⊕ for DH Tau b. We place DH Tau b in context with similar objects and discuss the consequences for planet formation models.http://iopscience.iop.org/article/10.3847/1538-3881/aa74cd/met

The Taurus Boundary of Stellar/Substellar (TBOSS) Survey I: far-IR disk emission measured with Herschel

With Herschel/PACS 134 low mass members of the Taurus star-forming region spanning the M4-L0 spectral type range and covering the transition from low mass stars to brown dwarfs were observed. Combining the new Herschel results with other programs, a total of 150 of the 154 M4-L0 Taurus members members have observations with Herschel. Among the 150 targets, 70um flux densities were measured for 7 of the 7 ClassI objects, 48 of the 67 ClassII members, and 3 of the 76 ClassIII targets. For the detected ClassII objects, the median 70um flux density level declines with spectral type, however, the distribution of excess relative to central object flux density does not change across the stellar/substellar boundary in the M4-L0 range. Connecting the 70um TBOSS values with the results from K0-M3 ClassII members results in the first comprehensive census of far-IR emission across the full mass spectrum of the stellar and substellar population of a star-forming region, and the median flux density declines with spectral type in a trend analogous to the flux density decline expected for the central objects. SEDs were constructed for all TBOSS targets covering the optical to far-IR range and extending to the submm/mm for a subset of sources. Based on an initial exploration of the impact of different physical parameters; inclination, scale height and flaring have the largest influence on the PACS flux densities. From the 24um to 70um spectral index of the SEDs, 5 new candidate transition disks were identified. The steep 24um to 70um slope for a subset of 8 TBOSS targets may be an indication of truncated disks in these systems.Two examples of mixed pair systems that include secondaries with disks were measured. Finally, comparing the TBOSS results with a Herschel study of Ophiuchus brown dwarfs reveals a lower fraction of disks around the Taurus substellar population.Comment: 64 pages, 33 figures, 12 tables, accepted for publication in A&