Context. We describe and benchmark two sophisticated chemical models developed by the Heidelberg and Bordeaux astrochemistry groups. Aims. The main goal of this study is to elaborate on a few well-described tests for state-of-the-art astrochemical codes covering a range of physical conditions and chemical processes, in particular those aimed at constraining current and future interferometric observations of protoplanetary disks. Methods. We considered three physical models: a cold molecular cloud core, a hot core, and an outer region of a T Tauri disk. Our chemical network (for both models) is based on the original gas-phase osu_03_2008 ratefile and includes gas-grain interactions and a set of surface reactions for the H-, O-, C-, S-, and N-bearing molecules. The benchmarking was performed with the increasing complexity of the considered processes: (1) the pure gas-phase chemistry, (2) the gas-phase chemistry with accretion and desorption, and (3) the full gas-grain model with surface reactions. The chemical evolution is modeled within 10 9 years using atomic initial abundances with heavily depleted metals and hydrogen in its molecular form. Results. The time-dependent abundances calculated with the two chemical models are essentially the same for all considered physical cases and for all species, including the most complex polyatomic ions and organic molecules. This result, however, required a lot of effort to make all necessary details consistent through the model runs, e.g., definition of the gas particle density, density of grain surface sites, or the strength and shape of the UV radiation field. Conclusions. The reference models and the benchmark setup, along with the two chemical codes and resulting time-dependent abundances are made publicly available on the internet. This will facilitate and ease the development of other astrochemical models and provide nonspecialists with a detailed description of the model ingredients and requirements to analyze the cosmic chemistry as studied, e.g., by (sub-) millimeter observations of molecular lines.
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 12 CO(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 χ 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. To constrain the ionization fraction in protoplanetary disks, we present new high-sensitivity interferometric observations of N 2 H + in three disks surrounding DM Tau, LkCa 15, and MWC 480. Methods. We used the IRAM PdBI array to observe the N 2 H + J = 1−0 line and applied a χ 2 -minimization technique to estimate corresponding column densities. These values are compared, together with HCO + column densities, to results of a steady-state disk model with a vertical temperature gradient coupled to gas-grain chemistry. Results. We report two N 2 H + detections for LkCa 15 and DM Tau at the 5σ level and an upper limit for MWC 480. The column density derived from the data for LkCa 15 is much lower than previously reported. The [N 2 H + /HCO + ] ratio is on the order of 0.02−0.03. So far, HCO + remains the most abundant observed molecular ion in disks. Conclusions. All the observed values generally agree with the modelled column densities of disks at an evolutionary stage of a few million years (within the uncertainty limits), but the radial distribution of the molecules is not reproduced well. The low inferred concentration of N 2 H + in three disks around low-mass and intermediate-mass young stars implies that this ion is not a sensitive tracer of the overall disk ionization fraction.
We studied several representative circumstellar disks surrounding the Herbig Ae star MWC 480 and the T Tauri stars LkCa 15 and DM Tau at (sub-)millimeter wavelengths in lines of CCH 1 . Our aim is to characterize photochemistry in the heavily UV-irradiated 1 Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).
We present submillimeter (CSO 350µm) and millimeter (SEST 1.2 mm, OVRO 3 mm) photometry for 125 solar-type stars from the FEPS Spitzer Legacy program that have masses between ∼ 0.5 and 2.0 M ⊙ and ages from ∼ 3 Myr to 3 Gyr. Continuum emission was detected toward four stars with a signal to noise ratio ≥ 3: the classical T Tauri stars RX J1842.9−3532, RX J1852.3−3700, and PDS 66 with SEST, and the debris disk system HD 107146 with OVRO. RX J1842.9−3532 and RX J1852.3−3700 are located in projection nearby the R CrA molecular cloud with estimated ages of ∼ 10 Myr (Neuhäuser et al. 2000), while PDS 66 is a probable member of the ∼ 20 Myr old Lower Centaurus-Crux subgroup of the Scorpius-Centaurus OB association (Mamajek et al. 2004). The continuum emission toward these three sources is unresolved at the 24 ′′ SEST resolution and likely originates from circumstellar accretion disks, each with estimated dust masses of ∼ 5×10 −5 M ⊙ . Analysis of the visibility data toward HD 107146 (age ∼ 80-200 Myr) indicates that the 3 mm continuum emission is centered on the star -2within the astrometric uncertainties and resolved with a gaussian-fit FWHM size of (6.5 ′′ ± 1.4 ′′ ) × (4.2 ′′ ± 1.3 ′′ ), or 185 AU×120 AU. The results from our continuum survey are combined with published observations to quantify the evolution of dust mass with time by comparing the mass distributions for samples with different stellar ages. The frequency distribution of circumstellar dust masses around solar-type stars in the Taurus molecular cloud (age ∼ 2 Myr) is distinguished from that around 3-10 Myr and 10-30 Myr old stars at a significance level of ∼ 1.5σ and ∼ 3σ respectively. These results suggest a decrease in the mass of dust contained in small dust grains and/or changes in the grain properties by stellar ages of 10-30 Myr, consistent with previous conclusions. Further observations are needed to determine if the evolution in the amount of cold dust occurs on even shorter time scales.
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