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Aims. We study the molecular content and chemistry of a circumstellar disk surrounding the Herbig Ae star AB Aur at (sub-)millimeter wavelengths. Our aim is to reconstruct the chemical history and composition of the AB Aur disk and to compare it with disks around low-mass, cooler T Tauri stars.

Context. The overall properties of disks surrounding intermediate PMS stars (HAe) are not yet well constrained by current observations. The disk inclination, which significantly affects spectral energy distribution modeling, is often unknown.

Context.The jet-disk connection is an important part of the star formation process. HH 30 is a rare and beautiful example of a system exhibiting a flared edge-on disk, an optical jet and a CO molecular outflow. A recent analysis of the jet wiggling has revealed that the central star is in reality a binary object. Therefore, the dust and gas disk observed around HH 30 is circumbinary.

Context.Gravitational lenses offer unique opportunities to explore the molecular content around active galactic nuclei at high redshift, through the magnification and gain in angular resolution.

Context.The physical structure of proto-planetary disks is not yet well constrained by current observations. Millimeter interferometry is an essential tool to investigate young disks.

Aims.To constrain the ionization fraction in protoplanetary disks, we present new high-sensitivity interferometric observations of N2H+in three disks surrounding DM Tau, LkCa 15, and MWC 480.

Aims.We constrain the dust distribution and its properties (temperature, emissivity) in inner proto-planetary disks

Context.HH 30 is a well-known Pre-Main-Sequence star in Taurus. HST observations have revealed a flared, edge-on disk driving a highly-collimated optical jet, making this object a case study for the disk-jet-outflow paradigm.

We report the detection of the $J=2{-}1$line of DCO+in the proto-planetary disk of DM Tau and re-analyze the spectrum covering the 465 GHz transition of HDO in this source, recently published by Ceccarelli et al. (2005, ApJ, 631, L81). A modelling of the DCO+line profile with the source parameters derived from high resolution HCO+observations yields a DCO+/HCO+abundance ratio of ${\simeq} 4\times 10^{-3}$, an order of magnitude smaller than that derived in the low mass cores. The re-analysis of the 465 GHz spectrum, using the proper continuum flux (0.5 Jy) and source systemic velocity ($6.05 \,{\rm km\,s}^{-1}$), makes it clear that the absorption features attributed to HDO and C6H are almost certainly unrelated to these species. We show that the line-to-continuum ratio of an absorption line in front of a Keplerian disk can hardly exceed the ratio of the turbulent velocity to the projected rotation velocity at the disk edge, unless the line is optically very thick ($\tau >10^4$). This ratio is typically 0.1–0.3 in proto-planetary disks and is ${\simeq} 0.15$in DM Tau, much smaller than that for the alleged absorption features. We also show that the detection of H2D+in DM Tau, previously reported by these authors, is only a 2-sigma detection when the proper velocity is adopted. So far, DCO+is thus the only deuterated molecule clearly detected in proto-planetary disks.

We present sub-arcsecond images of AB Auriga obtained with the IRAM Plateau de Bure interferometer in the isotopologues of CO, and in continuum at 3 and 1.3 mm. These observations allow us to trace the structure of the circumstellar material of AB Aurin regions where optical and IR imaging is impossible because of the emission from the star. These images reveal that the environment of AB Auris widely different from the proto-planetary disks that surround T Tauri stars like DM Tau and LkCa15 or HAeBe stars like MWC 480 in several aspects. Instead of being centrally peaked, the continuum emission is dominated by a bright, asymmetric (spiral-like) feature at about 140 AU from the central star. Little emission is associated with the star itself. The molecular emission shows that AB Auris surrounded by a very extended flattened structure (“disk”), which is rotating around the star. Bright molecular emission is also found towards the continuum “spiral”. The large-scale molecular structure suggests the AB Aurdisk is inclined between 23 and 43 degrees, but the strong asymmetry of the continuum and molecular emission prevents an accurate determination of the inclination of the inner parts. Analysis of the emission in terms of a Keplerian disk provides a reasonable fit to the data, but fails to give a consistent picture because the inclinations determined from $^{12}{\rm CO}$${J}~= 2~\rightarrow\ 1$, $^{13}{\rm CO}$${J}~= 2~\rightarrow\ 1$, $^{13}{\rm CO}$${J}~= 1~\rightarrow\ 0$and ${\rm C}^{18}{\rm O}$${J}~= 1~\rightarrow\ 0$do not agree. The mass predicted for the central star in such Keplerian models is in the range 0.9 to 1.2 ${M}_{\odot}$, much smaller than the expected 2.2 ${M}_{\odot}$from the spectral type of AB Aur. Better and more consistent fits to the $^{13}{\rm CO}$${J}~= 2~\rightarrow\ 1$, $^{13}{\rm CO}$${J}~= 1~\rightarrow\ 0$data are obtained by relaxing the Keplerian hypothesis. We find significant non-Keplerian motion, with a best fit exponent for the rotation velocity law of 0.41 ±0.01, but no evidence for radial motion. The disk has an inner hole about 70 AU in radius. The disk is warm and shows no evidence of depletion of CO. The dust properties suggest that the dust is less evolved than in typical T Tauri disks. Both the spiral-like feature and the departure from purely Keplerian motion indicates the AB Aurdisk is not in quasi-equilibrium. Disk self-gravity is insufficient to create the perturbation. This behavior may be related either to an early phase of star formation in which the Keplerian regime is not yet fully established and/or to a disturbance of yet unknown origin. An alternate, but unproven, possibility is that of a low mass companion located about 40 AU from AB Aur.

We present high angular resolution interferometric observations of the dust continuum at 2.7 and 1.3 mm, and of the HC3N ($J=12\to11$) and C18O ($J=2\to1$) emission around L1157-mm, a Class 0 object that drives a spectacular molecular outflow. The millimeter dust emission is clearly resolved into two components, a flattened compact source of ~$450\times250$AU at 1.3 mm, and mass ~0.1 $M_\odot$, plus an extended envelope of ~3000 AU at 1.3 mm, and mass ~1.1 $M_\odot$. The millimeter spectral index varies throughout the region, with the lower value found toward the compact protostar, possibly indicating grain growth in the denser regions. A strong interaction between the molecular outflow and the close protostellar environment is taking place and affects the structure of the innermost parts of the envelope. This is shown by the spatial coincidence between the molecular outflow and the dust (1.3 mm continuum) and HC3N emission: both tracers show structures associated to the edges of the outflow lobes. Basically, the global picture sketched for the Class 0 object L1157-mm by Gueth et al. ([CITE]) is supported. We find possible evidence of infall, but we do not detect any velocity gradient indicative of a rotating circumstellar disk.

We present new single-dish (30-m) observations and an improved analysis of our earlier interferometric observations (Simon et al. 2000) of BP Tau. Our analysis yields a detailed description of the properties of the circumstellar disk. The disk is small (outer radius 120 AU) and marginally optically thick in the $\ifmmode {^{12}{\rm CO}} \else {$^{12}{\rm CO}$}\fi$,$\ifmmode {{J}=2\!\!\rightarrow\!\!1} \else {$J=2\!\!\rightarrow\!\! 1$}\fi$line. It is also relatively hot, about 50 K at 100 AU. The mm continuum emission is consistent with a thermal emission from circumstellar dust with an emissivity index $\beta \simeq 0.7$, and a disk mass about $1.2\times 10^{-3} ~{M}_{\odot}$. The anomalously low $\ifmmode {^{12}{\rm CO}} \else {$^{12}{\rm CO}$}\fi$to dust emission ratio implies that either CO is depleted by a factor of the order of 150 with respect to H2, or has a very low gas to dust ratio, or highly anomalous dust properties. The disk exhibits direct evidence of Keplerian rotation, but because of a combination of insufficient resolution and optically thin CO emission, the mass derived for the star remains inaccurate (between 0.6 and $1.3 ~{M}_{\odot}$for 140 pc). The unusual properties of the circumstellar disk suggest that BP Tau may be a transient object in the process of clearing its disk.

We use high angular and spectral resolution (${\sim} 1''$& 0.1 km s-1) images in $^{12}{\rm CO}$, $^{13}{\rm CO}$$J=2\!\rightarrow\! 1$, $J=1\!\rightarrow\! 0$and ${\rm C}^{18}{\rm O}$$J=2\!\rightarrow\! 1$coming from the IRAM interferometer to probe the vertical temperature distribution in the disk of DM Tau. We investigate here a new method based on the different opacities of the CO isotopomers to sample the temperature disk structure at various vertical scales inside the disk. Typically, the $^{12}{\rm CO}$transitions are sampling at 2–4 scale heights, while the $^{13}{\rm CO}$data are more likely tracing 1 scale height for the $J=2$–1, and the disk mid-plane for the $J=1$–0 line. At the disk scale at which the IRAM observations are sensitive ($R\geq 50$–60 AU), the analysis reveals a vertical temperature gradient. The outer layers where τ($^{12}{\rm CO}$$J=2\!\rightarrow\! 1$$) \simeq 1$are warmer (${\sim} 30$K) than the inner layers probed by $^{13}{\rm CO}$and ${\rm C}^{18}{\rm O}$data (${\sim} 13$–20 K). These findings are consistent with the structure expected for flared irradiated disks around TTauri stars. We also observe that the outer radius of the disk is smaller in $^{13}{\rm CO}$and ${\rm C}^{18}{\rm O}$than in $^{12}{\rm CO}$. These differences in truncation radius are in agreement with photodissociation effects. We also note that the dynamical mass determination from CO is weakly affected by the temperature gradient.

Aims. We attempt to understand the presence of gas phase CO below its sublimation temperature in circumstellar disks. We study two promising mechanisms to explain this phenomenon: turbulent mixing and photodesorption.Methods. We compute the chemical evolution of circumstellar disks including grain surface reactions with and without turbulent mixing and CO?photodesorption.Results. We show that photodesorption significantly enhances the gas phase CO?abundance, by extracting CO from the grains when the visual extinction remains below about 5?mag. However, the resulting dependence of column density on radial distance is inconsistent with observations so far. We propose that this inconsistency could be the result of grain growth. On the other hand, the influence of turbulent mixing is not found to be straightforward. The efficiency of turbulent mixing depends upon a variety of parameters, including the disk structure. For the set of parameters we chose, turbulent mixing is not found to have any significant influence on the CO?column density.

Context. The disk-outflow connection is thought to play a key role in extracting excess angular momentum from a forming proto-star. Although jet rotation has been observed in a few objects, no rotation of molecular outflows has been unambiguously reported so far.Aims. We report new millimeter-interferometric observations of the edge-on T?Tauri?star ? disk system in the isolated Bok globule CB?26. The aim of these observations was to study the disk-outflow relation in this 1?Myr old low-mass young stellar object.Methods. The IRAM PdBI array was used to observe 12CO(2?1) at 1.3?mm in two configurations, resulting in spectral line maps with 1.5???resolution. We use an empirical parameterized steady-state outflow model combined with 2-D line radiative transfer calculations and ?2-minimization in parameter space to derive a best-fit model and constrain parameters of the outflow.Results. The data reveal a previously undiscovered collimated bipolar molecular outflow of total length ?2000?AU, escaping perpendicular to the plane of the disk. We find peculiar kinematic signatures that suggest that the outflow is rotating with the same orientation as the disk. However, we could not ultimately exclude jet precession or two misaligned flows as possible origins of the observed peculiar velocity field. There is indirect indication that the embedded driving source is a binary system, which, together with the youth of the source, could provide a clue to the observed kinematic features of the outflow.Conclusions. CB?26 is so far the most promising source in which to study the rotation of a molecular outflow. Assuming that the outflow is rotating, we compute and compare masses, mass flux, angular momenta, and angular momentum flux of the disk and outflow and derive disk dispersal timescales of 0.5 ...1?Myr, comparable to the age of the system.

Aims. We study the molecular content and chemistry of a circumstellar disk surrounding the Herbig Ae star AB Aur at (sub-)millimeter wavelengths. Our aim is to reconstruct the chemical history and composition of the AB Aur disk and to compare it with disks around low-mass, cooler T Tauri stars. Methods. We observe the AB Aur disk with the IRAM Plateau de Bure Interferometer in the C- and D-configurations in rotational lines of CS, HCN, C2H, CH3OH, HCO+, and CO isotopes. Using an iterative minimization technique, observed columns densities and abundances are derived. These values are further compared with results of an advanced chemical model that is based on a steady-state flared disk structure with a vertical temperature gradient, and gas-grain chemical network with surface reactions. Results. We firmly detect HCO+in the 1?0 transition, tentatively detect HCN, and do not detect CS, C2H, and CH3OH. The observed HCO+and 13CO column densities as well as the upper limits to the column densities of HCN, CS, C2H, and CH3OH are in good agreement with modeling results and those from previous studies.?Conclusions. The AB Aur disk possesses more CO, but is less abundant in other molecular species compared to the DM Tau disk. This is primarily caused by intense UV irradiation from the central Herbig A0 star, which results in a hotter disk where CO freeze out does not occur and thus surface formation of complex CO-bearing molecules might be inhibited.

Context. Recent modeling based on unresolved infrared observations of the spectral energy distribution (SED) of GM?Aurigae suggests that the inner disk of this single TTauri star is truncated at an inner radius of 25?AU.Aims. We attempt to find evidence of this inner hole in the gas distribution, using spectroscopy with high angular resolution.Methods. Using the IRAM array, we obtained high angular resolution (~1.5?) observations with a high S/N per channel of the 13CO?J=2-1?and C18O?J=2-1?and of the 13CO?J=1-0?lines. A standard parametric disk model is used to fit the line data in the Fourier-plane and to derive the CO?disk properties. Our measurement is based on a detailed analysis of the spectroscopic profile from the CO?disk rotating in Keplerian velocity. The millimeter continuum, tracing the dust, is also analyzed.Results. We detect an inner cavity of radius 19?? 4?AU at the 4.5??level. The hole manifests itself by a lack of emission beyond the (projected) Keplerian speed at the inner radius. We also constrain the temperature gradient in the disk.Conclusions. Our data reveal the existence of an inner hole in GM?Aur?gas disk. Its origin remains unclear, but can be linked to planet formation or to a low mass stellar companion orbiting close to the central star (~5-15?AU). The frequent finding of inner cavities suggests that either binarity is the most common scenario of star formation in Taurus or that giant planet formation starts early.

Context. The overall properties of disks surrounding intermediate PMS stars (HAe) are not yet well constrained by current observations. The disk inclination, which significantly affects spectral energy distribution modeling, is often unknown.Aims. We attempted to resolve the disks around CQ?Tau and MWC?758 to provide accurate constraints on the disk parameters, in particular the temperature and surface density distribution.Methods. We report arcsecond resolution observations of dust and CO?line emissions with the IRAM array. We also searched for the HCO+?J = 1 ? 0 transition. The disk properties are derived using a standard disk model. We use the Meudon PDR code to study the chemistry. Results. The two disks share some common properties. The mean CO?abundance is low despite disk temperatures above the CO?condensation temperature. Furthermore, the CO?surface density and dust opacity have different radial dependence. The CQ?Tau disk appears warmer and perhaps less dense than that of MWC?758. Modeling the chemistry, we find that photodissociation of CO?is a viable mechanism to explain its low abundance. The photospheric flux is not sufficient for this: a strong UV excess is required. In CQ?Tau, the high temperature is consistent with the expectation for a PDR. The PDR model has difficulty explaining the mild temperatures obtained in MWC?758, for which a low gas-to-dust ratio is preferred. A yet, unexplored alternative could be that, despite currently high gas temperatures CO?remains trapped in grains, as the models suggest that large grains can be cold enough to prevent thermal desorption of CO. The low inclination of the CQ?Tau disk, ~30?, challenges previous interpretations given for UX Ori ? like luminosity variations of this star.Conclusions. We conclude that CO?cannot be used as a simple tracer of gas-to-dust ratio, the CO?abundance being affected by photodissociation and grain growth.

Aims.We study the content in S-bearing molecules of protoplanetary disks around low-mass stars.

Context.The structure in density and temperature of protoplanetary disks surrounding low-mass stars is not well known yet. The protoplanetary disks’ midplane are expected to be very cold and thus depleted in molecules in gas phase, especially CO. Recent observations of molecules at very low apparent temperatures (~6 K) challenge this current picture of the protoplanetary disk structures.

Context.Previous studies have found that Vega is surrounded by an extended debris disc that is very smooth in the far infrared, but displays possible clumpiness at 850   μm and dust emission peaks at 1.3 mm.

Context.Dark cloud chemical models usually predict large amounts of O2, often above observational limits. Aims.We investigate the reason for this discrepancy from a theoretical point of view, inspired by the studies of Jenkins and Whittet on oxygen depletion.Methods.We use the gas-grain code Nautilus with an up-to-date gas-phase network to study the sensitivity of the molecular oxygen abundance to the oxygen elemental abundance. We use the rate coefficient for the reaction O + OH at 10 K recommended by the KIDA (KInetic Database for Astrochemistry) experts. Results.The updates of rate coefficients and branching ratios of the reactions of our gas-phase chemical network, especially N + CN and H\hbox{$_3^+$}+ O, have changed the model sensitivity to the oxygen elemental abundance. In addition, the gas-phase abundances calculated with our gas-grain model are less sensitive to the elemental C/O ratio than those computed with a pure gas-phase model. The grain surface chemistry plays the role of a buffer absorbing most of the extra carbon. Finally, to reproduce the low abundance of molecular oxygen observed in dark clouds at all times, we need an oxygen elemental abundance smaller than 1.6 × 10-4. Conclusions.The chemistry of molecular oxygen in dense clouds is quite sensitive to model parameters that are not necessarily well constrained. That O2abundance may be sensitive to nitrogen chemistry is an indication of the complexity of interstellar chemistry.

Context.Proto-planetary disks are thought to provide the initial environment for planetary system formation. The dust and gas distribution and its evolution with time is one of the key elements in the process.

Context.Recent sub-millimetric observations at the Plateau de Bure interferometer showed a cavity of  ~46 AU in radius in the proto-planetary disk of the T Tauri star LkCa15 (V1079 Tau), located in the Taurus molecular cloud. Additional Spitzer observations have corroborated this result which can possibly be explained by a massive (≥5 MJup) planetary companion, a brown dwarf or a low-mass star companion at about 30 AU from the star.

Context. The gas and dust dissipation processes of proto-planetary disks are hardly known. Transition disks between Class II (proto-planetary disks) and Class III (debris disks) remain difficult to detect.Aims. We investigate the carbon chemistry of the peculiar CQ Tau gas disk. It is likely to be a transition disk because it exhibits weak CO emission with a relatively strong millimeter continuum, indicating that the disk may currently be dissipating its gas content.Methods. We used APEX to observe the two C itransitions 3P1$\rightarrow$3P0at 492 GHz and 3P2$\rightarrow$3P1at 809 GHz in the disk orbiting CQ Tau. We compare the observations to several chemical model predictions. We focus our study on the influence of the stellar UV radiation shape and gas-to-dust ratio.Results. We did not detect the C ilines. However, our upper limits are deep enough to exclude high-C imodels. The only available models compatible with our limits imply very low gas-to-dust ratios, of the order of only a few.Conclusions. These observations strengthen the hypothesis that CQ Tau is likely to be a transition disk and suggest that gas disappears before dust.

Context. The gas and dust dissipation processes of proto-planetary disks are hardly known. Transition disks between Class II (proto-planetary disks) and Class III (debris disks) remain difficult to detect.

Context. Because of new telescopes that will dramatically improve our knowledge of the interstellar medium, chemical models will have to be used to simulate the chemistry of many regions with diverse properties. To make these models more robust, it is important to understand their sensitivity to a variety of parameters.Aims. In this article, we report a study of the sensitivity of a chemical model of a cold dense core, with homogeneous and time-independent physical conditions, to variations in the following parameters: initial chemical inventory, gas temperature and density, cosmic-ray ionization rate, chemical reaction rate coefficients, and elemental abundances.Methods. We used a Monte Carlo method to randomly vary individual parameters and groups of parameters within realistic ranges. From the results of the parameter variations, we can quantify the sensitivity of the model to each parameter as a function of time. Our results can be used in principle with observations to constrain some parameters for different cold clouds. We also attempted to use the Monte Carlo approach with all parameters varied collectively.Results. Within the parameter ranges studied, the most critical parameters turn out to be the reaction rate coefficients at times up to 4 × 105yr and elemental abundances at later times. At typical times of best agreement with observation, models are sensitive to both of these parameters. The models are less sensitive to other parameters such as the gas density and temperature.Conclusions. The improvement of models will require that the uncertainties in rate coefficients of important reactions be reduced. As the chemistry becomes better understood and more robust, it should be possible to use model sensitivities concerning other parameters, such as the elemental abundances and the cosmic ray ionization rate, to yield detailed information on cloud properties and history. Nevertheless, at the current stage, we cannot determine the best values of all the parameters simultaneously based on purely observational constraints.

Context. Because of new telescopes that will dramatically improve our knowledge of the interstellar medium, chemical models will have to be used to simulate the chemistry of many regions with diverse properties. To make these models more robust, it is important to understand their sensitivity to a variety of parameters.