No Arabic abstract
Probing the gas and dust in proto-planetary disks is central for understanding the process of planet formation. In disks surrounding solar type protostars, the bulk of the disk mass resides in the outer midplane, which is cold ($leq$20 K), dense ($geq 10^7$ cm$^{-3}$) and depleted of CO. Observing the disk midplane has proved, therefore, to be a formidable challenge. Ceccarelli et al. (2004) detected H$_2$D$^+$ emission in a proto-planetary disk and claimed that it probes the midplane gas. Indeed, since all heavy-elements bearing molecules condense out onto the grain mantles, the most abundant ions in the disk midplane are predicted to be H$_3^+$ and its isotopomers. In this article, we carry out a theoretical study of the chemical structure of the outer midplane of proto-planetary disks. Using a self-consistent physical model for the flaring disk structure, we compute the abundances of H$_3^+$ and its deuterated forms across the disk midplane. We also provide the average column densities across the disk of H$_3^+$, H$_2$D$^+$, HD$_2^+$ and D$_3^+$, and line intensities of the ground transitions of the ortho and para forms of H$_2$D$^+$ and HD$_2^+$ respectively. We discuss how the results depend on the cosmic ray ionization rate, dust-to-gas ratio and average grain radius, and general stellar/disk parameters. An important factor is the poorly understood freeze-out of N$_2$ molecules onto grains, which we investigate in depth. We finally summarize the diagnostic values of observations of the H$_3^+$ isotopomers.
We report the first detection of the ground transition of the deuterated water at 464 GHz in the young proto-planetary disk surrounding the solar type protostar DM Tau. The line is observed in absorption against the continuum from the cold dust in the disk midplane, with a line to continuum ratio close to unity. The observation implies that deuterated gaseous water is present, with a relatively large abundance ($sim 3times10^{-9}$), in the outer disk above the midplane, where the density is, within a factor ten, $sim 10^6$ cm$^{-3}$ and the temperature is lower than about 25 K. In these conditions, the H$_2$O condensation timescale is much smaller than the DM Tau disk age, and, therefore, water should be fully frozen onto the grain mantles. We suggest that UV photons and/or X-rays sublimate part of the mantles re-injecting the ices into the gas phase. Even though there is currently no measurement of H$_2$O, we provide arguments that the HDO/H$_2$O ratio should be about 0.01 or larger, which would be hundreds of times larger than the values measured in Solar System objects. This suggests the need of strong caution in comparing and linking the HDO/H$_2$O in Solar System and star forming environments.
We review the properties of dust in protoplanetary disks around optically visible pre-main sequence stars obtained with a variety of observational techniques, from measurements of scattered light at visual and infrared wavelengths to mid-infrared spectroscopy and millimeter interferometry. A general result is that grains in disks are on average much larger than in the diffuse interstellar medium (ISM). In many disks, there is evidence that a large mass of dust is in grains with millimeter and centimeter sizes, more similar to sand and pebbles than to grains. Smaller grains (with micron-sizes) exist closer to the disk surface, which also contains much smaller particles, e.g., polycyclic aromatic hydrocarbons. There is some evidence of a vertical stratification, with smaller grains closer to the surface. Another difference with ISM is the higher fraction of crystalline relative to amorphous silicates found in disk surfaces. There is a large scatter in dust properties among different sources, but no evidence of correlation with the stellar properties, for samples that include objects from intermediate to solar mass stars and brown dwarfs. There is also no apparent correlation with the age of the central object, over a range roughly between 1 and 10 Myr. This suggests a scenario where significant grain processing may occur very early in the disk evolution, possibly when it is accreting matter from the parental molecular core. Further evolution may occur, but not necessarily rapidly, since we have evidence that large amounts of grains, from micron to centimeter size, can survive for periods as long as 10 Myr.
At the low temperatures ($sim$10 K) and high densities ($sim$100,000 H$_2$ molecules per cc) of molecular cloud cores and protostellar envelopes, a large amount of molecular species (in particular those containing C and O) freeze-out onto dust grain surfaces. It is in these regions that the deuteration of H$_3^+$ becomes very efficient, with a sharp abundance increase of H$_2$D$^+$ and D$_2$H$^+$. The multi-deuterated forms of H$_3^+$ participate in an active chemistry: (i) their collision with neutral species produces deuterated molecules such as the commonly observed N$_2$D$^+$, DCO$^+$ and multi-deuterated NH$_3$; (ii) their dissociative electronic recombination increases the D/H atomic ratio by several orders of magnitude above the D cosmic abundance, thus allowing deuteration of molecules (e.g. CH$_3$OH and H$_2$O) on the surface of dust grains. Deuterated molecules are the main diagnostic tools of dense and cold interstellar clouds, where the first steps toward star and protoplanetary disk formation take place. Recent observations of deuterated molecules are reviewed and discussed in view of astrochemical models inclusive of spin-state chemistry. We present a new comparison between models based on complete scrambling (to calculate branching ratio tables for reactions between chemical species that include protons and/or deuterons) and models based on non-scrambling (proton hop) methods, showing that the latter best agree with observations of NH$_3$ deuterated isotopologues and their different nuclear spin symmetry states.
(abridged) We used the IRAM 30-m to perform a sensitive wideband survey of 30 protoplanetary disks in the Taurus Auriga region. We simultaneously observed HCO$^+$(3-2), HCN(3-2), C$_2$H(3-2), CS(5-4), and two transitions of SO. We combine the results with a previous survey which observed $^{13}$CO (2-1), CN(2-1), two o-H$_2$CO lines and one of SO. We use available interferometric data to derive excitation temperatures of CN and C$_2$H in several sources. We determine characteristic sizes of the gas disks and column densities of all molecules using a parametric power-law disk model. Our study is mostly sensitive to molecules at 200-400 au from the stars. We compare the derived column densities to the predictions of an extensive gas-grain chemical disk model, under conditions representative of T Tauri disks. This survey provides 20 new detections of HCO$^+$ in disks, 18 in HCN, 11 in C$_2$H, 8 in CS and 4 in SO. HCO$^+$ is detected in almost all sources, and its J=3-2 line is essentially optically thick, providing good estimates of the disk radii. The other transitions are (at least partially) optically thin. Variations of the column density ratios do not correlate with any specific property of the star or disk. Disks around Herbig Ae stars appear less rich in molecules than those around T Tauri stars, although the sample remains small. SO is only found in the (presumably younger) embedded objects, perhaps reflecting an evolution of the S chemistry due to increasing depletion with time. Overall, the molecular column densities, and in particular the CN/HCN and CN/C$_2$H ratios, are well reproduced by gas-grain chemistry in cold disks. This study provides a census of simple molecules in disks of radii $> 200-300$ au. Extending that to smaller disks, or searching for less abundant or more complex molecules requires a much more sensitive facility, i.e. NOEMA and ALMA.
By performing local three-dimensional MHD simulations of stratified accretion disks, we investigate disk winds driven by MHD turbulence. Initially given weak vertical magnetic fields are effectively amplified by magnetorotational instability and winding due to differential rotation. Large scale channel flows develop most effectively at 1.5 - 2 times the scale heights where the magnetic pressure is comparable to but slightly smaller than the gas pressure. The breakup of these channel flows drives structured disk winds by transporting the Poynting flux to the gas. These features are universally observed in the simulations of various initial fields. This disk wind process should play an essential role in the dynamical evaporation of proto-planetary disks. The breakup of channel flows also excites the momentum fluxes associated with Alfvenic and (magneto-)sonic waves toward the mid-plane, which possibly contribute to the sedimentation of small dust grains in protoplanetary disks.