(Abridged*) Models of the young solar nebula assume a hot initial disk with most volatiles are in the gas phase. The question remains whether an actively accreting disk is warm enough to have gas-phase water up to 50 AU radius. No detailed studies ha
ve yet been performed on the extent of snowlines in an embedded accreting disk (Stage 0). Quantify the location of gas-phase volatiles in embedded actively accreting disk system. Two-dimensional physical and radiative transfer models have been used to calculate the temperature structure of embedded protostellar systems. Gas and ice abundances of H$_2$O, CO$_2$, and CO are calculated using the density-dependent thermal desorption formulation. The midplane water snowline increases from 3 to 55 AU for accretion rates through the disk onto the star between $10^{-9}$-$10^{-4} M_{odot} {rm yr^{-1}}$. CO$_2$ can remain in the solid phase within the disk for $dot{M} leq 10^{-5} M_{odot} {rm yr^{-1}}$ down to $sim 20$ AU. Most of the CO is in the gas phase within an actively accreting disk independent of disk properties and accretion rate. The predicted optically thin water isotopolog emission is consistent with the detected H$_2^{18}$O emission toward the Stage 0 embedded young stellar objects, originating from both the disk and the warm inner envelope (hot core). An accreting embedded disk can only account for water emission arising from $R < 50$ AU, however, and the extent rapidly decreases for low accretion rates. Thus, the radial extent of the emission can be measured with ALMA observations and compared to this limit. Volatiles sublimate out to 50 AU in young disks and can reset the chemical content inherited from the envelope in periods of high accretion rates. A hot young solar nebula out to 30 AU can only have occurred during the deeply embedded Stage 0, not during the T-Tauri phase of our early solar system.
Abridged: Recent simulations have explored different ways to form accretion disks around low-mass stars. We aim to present observables to differentiate a rotationally supported disk from an infalling rotating envelope toward deeply embedded young ste
llar objects and infer their masses and sizes. Two 3D magnetohydrodynamics (MHD) formation simulations and 2D semi-analytical model are studied. The dust temperature structure is determined through continuum radiative transfer RADMC3D modelling. A simple temperature dependent CO abundance structure is adopted and synthetic spectrally resolved submm rotational molecular lines up to $J_{rm u} = 10$ are simulated. All models predict similar compact components in continuum if observed at the spatial resolutions of 0.5-1$$ (70-140 AU) typical of the observations to date. A spatial resolution of $sim$14 AU and high dynamic range ($> 1000$) are required to differentiate between RSD and pseudo-disk in the continuum. The peak-position velocity diagrams indicate that the pseudo-disk shows a flatter velocity profile with radius than an RSD. On larger-scales, the CO isotopolog single-dish line profiles are similar and are narrower than the observed line widths of low-$J$ lines, indicating significant turbulence in the large-scale envelopes. However a forming RSD can provide the observed line widths of high-$J$ lines. Thus, either RSDs are common or a higher level of turbulence ($b sim 0.8 {rm km s^{-1}}$ ) is required in the inner envelope compared with the outer part. Multiple spatially and spectrally resolved molecular line observations are needed. The continuum data give a better estimate on disk masses whereas the disk sizes can be estimated from the spatially resolved molecular lines observations. The general observable trends are similar between the 2D semi-analytical models and 3D MHD RSD simulations.
(Abridged) Organic molecules are important constituents of protoplanetary disks. Their ro-vibrational lines observed in the near- and mid-infrared are commonly detected toward T Tauri disks. These lines are the only way to probe the chemistry in the
inner few au where terrestrial planets form. To understand this chemistry, accurate molecular abundances have to be determined. This is complicated by excitation effects. Most analyses so far have made the assumption of local thermal equilibrium (LTE). Starting from estimates for the collisional rate coefficients of HCN, non-LTE slab models of the HCN emission were calculated to study the importance of different excitation mechanisms. Using a new radiative transfer model, the HCN emission from a full two-dimensional disk was then modeled to study the effect of the non-LTE excitation, together with the line formation. We ran models tailored to the T Tauri disk AS 205 (N) where HCN lines in both the 3 {mu}m and 14 {mu}m bands have been observed by VLT-CRIRES and the Spitzer Space Telescope. Reproducing the observed 3 {mu}m / 14 {mu}m flux ratios requires very high densities and kinetic temperatures ($n > 10^{14}$ cm$^{-3}$ and $T > 750$ K), if only collisional excitation is accounted for. Radiative pumping can, however, excite the lines easily out to considerable radii $sim$ 10 au. Consequently, abundances derived from LTE and non-LTE models do not differ by more than a factor of about 3. Models with both a strongly enhanced abundance within $sim$ 1 au (jump abundance) and constant abundance can reproduce the current observations, but future observations with the MIRI instrument on JWST and METIS on the E-ELT can easily distinguish between the scenarios and test chemical models. Depending on the scenario, ALMA can detect rotational lines within vibrationally excited levels.
(Abridged) Disks are observed around pre-main sequence stars, but how and when they form is still heavily debated. While disks around young stellar objects have been identified through thermal dust emission, spatially and spectrally resolved molecula
r line observations are needed to determine their nature. We present subarcsecond observations of dust and gas toward four Class I low-mass young stellar objects in Taurus. The 13CO and C18O J=2-1 transitions at 220 GHz were observed with the Plateau de Bure Interferometer at a spatial resolution of ~0.8 and analyzed using uv-space position velocity diagrams to determine the nature of their observed velocity radient. Rotationally supported disks (RSDs) are detected around 3 of the 4 Class I sources studied. The derived masses identify them as Stage I objects; i.e., their stellar mass is higher than their envelope and disk masses. The outer radii of the Keplerian disks toward our sample of Class I sources are <~ 100 AU. The lack of on-source C18O emission for TMR1 puts an upper limit of 50 AU on its size. Flattened structures at radii > 100 AU around these sources are dominated by infalling motion (v propto r^-1). A large-scale envelope model is required to estimate the basic parameters of the flattened structure from spatially resolved continuum data. Similarities and differences between the gas and dust disk are discussed. Combined with literature data, the sizes of the RSDs around Class I objects are best described with evolutionary models with an initial rotation of 10^-14 Hz and slow sound speeds. Based on the comparison of gas and dust disk masses, little CO is frozen out within 100 AU in these disks. RSDs with radii up to 100 AU are present around Class I embedded objects. Larger surveys of both Class 0 and I objects are needed to determine whether most disks form late or early in the embedded phase.
(Abridged) Star and planet formation theories predict an evolution in the density, temperature, and velocity structure as the envelope collapses and forms an accretion disk. The aim of this work is to model the evolution of the molecular excitation,
line profiles, and related observables during low-mass star formation. Specifically, the signatures of disks during the deeply embedded stage are investigated. Semi-analytic 2D axisymmetric models have been used to describe the evolution of the density, stellar mass, and luminosity from the pre-stellar to the T-Tauri phase. A full radiative transfer calculation is carried out to accurately determine the time-dependent dust temperatures and CO abundance structure. We present non-LTE near-IR, FIR, and submm lines of CO have been simulated at a number of time steps. In contrast to the dust temperature, the CO excitation temperature derived from submm/FIR lines does not vary during the protostellar evolution, consistent with C18O observations obtained with Herschel and from ground-based telescopes. The near-IR spectra provide complementary information to the submm lines by probing not only the cold outer envelope but also the warm inner region. The near-IR high-J (>8) absorption lines are particularly sensitive to the physical structure of the inner few AU, which does show evolution. High signal-to-noise ratio subarcsec resolution data with ALMA are needed to detect the presence of small rotationally supported disks during the Stage 0 phase and various diagnostics are discussed.
