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. Aims. We attempt to characterize the radial distribution of dust in disks around a sample of young stars from an observational point of view, and, when possible, in a model-independent way, by using parametric laws. Methods. We used the IRAM PdBI interferometer to provide very high angular resolution (down to 0.4′′ in some sources) observations of the continuum at 1.3 mm and 3 mm around a sample of T Tauri stars in the Taurus-Auriga region. The sample includes single and multiple systems, with a total of 23 individual disks. We used track-sharing observing mode to minimize the biases. We fitted these data with two kinds of models: a “truncated power law” model and a model presenting an exponential decay at the disk edge (“viscous” model). Results. Direct evidence for tidal truncation is found in the multiple systems. The temperature of the mm-emitting dust is constrained in a few systems. Unambiguous evidence for large grains is obtained by resolving out disks with very low values of the dust emissivity index β. In most disks that are sufficiently resolved at two different wavelengths, we find a radial dependence of β, which appears to increase from low values (as low as 0) at the center to about 1.7−2 at the disk edge. The same behavior could apply to all studied disks. It introduces further ambiguities in interpreting the brightness profile, because the regions with apparent β ≈ 0 can also be interpreted as being optically thick when their brightness temperature is high enough. Despite the added uncertainty on the dust absorption coefficient, the characteristic size of the disk appears to increase with a higher estimated star age. Conclusions. These results provide the first direct evidence of the radial dependence of the grain size in proto-planetary disks. Constraints of the surface density distributions and their evolution remain ambiguous because of a degeneracy with the β(r) law.
Aims. We aim to obtain a spatially resolved measurement of velocity dispersions in the disk of TW Hya. Methods. We obtained images with high spatial and spectral resolution of the CO J = 2–1, CN N = ...2–1 and CS J = 5–4 emission with ALMA in Cycle 2. The radial distribution of the turbulent broadening was derived with two direct methods and one modelling approach. The first method requires a single transition and derives Tex directly from the line profile, yielding a vturb. The second method assumes that two different molecules are co-spatial, which allows using their relative line widths for calculating Tkin and vturb. Finally we fitted a parametric disk model in which the physical properties of the disk are described by power laws, to compare our direct methods with previous values. Results. The two direct methods were limited to the outer r > 40 au disk because of beam smear. The direct method found vturb to range from ≈130 m s-1 at 40 au, and to drop to ≈50 m s-1 in the outer disk, which is qualitatively recovered with the parametric model fitting. This corresponds to roughly 0.2−0.4 cs. CN was found to exhibit strong non-local thermal equilibrium effects outside r ≈ 140 au, so that vturb was limited to within this radius. The assumption that CN and CS are co-spatial is consistent with observed line widths only within r ≲ 100 au, within which vturb was found to drop from 100 m s-1 (≈0.4 cs) to zero at 100 au. The parametric model yielded a nearly constant 50 m s-1 for CS (0.2−0.4 cs). We demonstrate that absolute flux calibration is and will be the limiting factor in all studies of turbulence using a single molecule. Conclusions. The magnitude of the dispersion is comparable with or below that predicted by the magneto-rotational instability theory. A more precise comparison would require reaching an absolute calibration precision of about 3%, or finding a suitable combination of light and heavy molecules that are co-located in the disk.
We report new dynamical masses for five pre-main sequence (PMS) stars in the L1495 region of the Taurus star-forming region (SFR) and six in the L1688 region of the Ophiuchus SFR. Since these regions ...have VLBA parallaxes, these are absolute measurements of the stars' masses and are independent of their effective temperatures and luminosities. Seven of the stars have masses , thus providing data in a mass range with little data, and of these, six are measured to precision . We find eight stars with masses in the range 0.09-1.1 that agree well with the current generation of PMS evolutionary models. The ages of the stars we measured in the Taurus SFR are in the range 1-3 Myr, and Myr for those in L1688. We also measured the dynamical masses of 14 stars in the ALMA archival data for Akeson & Jensen's Cycle 0 project on binaries in the Taurus SFR. We find that the masses of seven of the targets are so large that they cannot be reconciled with reported values of their luminosity and effective temperature. We suggest that these targets are themselves binaries or triples.
Context. The increased sensitivity of millimeter-wave facilities now makes possible the detection of low amounts of gas in debris disks. Some of the gas-rich debris disks harbor peculiar properties, ...with possible pristine gas and secondary generated dust. The origin of the gas in these hybrid disks is strongly debated and the current sample is too sparse to understand this phenomenon. Aims. More detections are necessary to increase the statistics on this population. Lying at the final stages of evolution of proto-planetary disks and at the beginning of the debris disk phase, these objects could provide new insight into the processes involved in the making of planetary systems. Methods. We carried out a deep survey of the CO J = 2 → 1 and CO J = 3 → 2 lines with the APEX and IRAM radiotelescopes in young debris disks selected according to hybrid disk properties. The survey is complemented with a bibliographic study of the ratio between the emission of the gas and the continuum (SCO/Fcont) in CTTS, Herbig Ae, WTTS, hybrid, and debris disks. Results. Our sub-mm survey comprises 25 stars, including 17 new targets, and we increase the sensitivity limit by a factor 2 on eight sources compared to similar published studies. We report a 4σ tentative detection of a double-peaked CO J = 2 → 1 line around HD 23642; an eclipsing binary located in the Pleiades. We also reveal a correlation between the emission of the CO gas and the dust continuum from CTTS, Herbig Ae and few debris disks. The observed trend of the gas to dust flux ratio suggests a concurrent dissipation of the dust and gas components. Hybrid disks systematically lie above this trend, suggesting that these systems may witness a transient phase, when the dust has evolved more rapidly than the gas, with a flux ratio SCO/Fcont enhanced by a factor of between 10 and 100 compared to standard (proto-)planetary disks.
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.We study ...the vertical and radial temperature distribution in a few well-known disks from an observational perspective. The surface density distribution of CO and HCO+ and the scale-height are also investigated. Methods.We report CO observations at sub-arcsecond resolution with the IRAM array of the disks surrounding MWC 480, LkCa 15, and DM Tau, and simultaneous measurements of HCO+ J = 1$\rightarrow$ 0. To derive the disk properties, we fit a standard disk model in which all parameters are power laws of the distance to the star to the data. Possible biases associated with the method are detailed and explained. We compare the properties of the observed disks with similar objects. Results.We find evidence for a vertical temperature gradient in the disks of MWC 480 and DM Tau, as in AB Aur, but not in LkCa 15. The disk temperatures increase with stellar effective temperature. Except for AB Aur, the bulk of the CO gas is at temperatures smaller than 17 K, below the condensation temperature on grains. We find the scale height of the CO distribution to be larger (by 50%) than the expected hydrostatic scale height. The total amount of CO and the isotopologue ratio depends globally on the star. The more UV luminous objects appear to have more CO, but there is no simple dependency. The 13CO / HCO+ ratio is ~600, with substantial variations between sources, and with radius. The temperature behavior is consistent with expectations, but published chemical models have difficulty reproducing the observed CO quantities. Changes in the slope of the surface density distribution of CO, compared to the continuum emission, suggest a more complex surface density distribution than is usually assumed in models. Vertical mixing seems an important chemical agent, as does photo-dissociation by the ambient UV radiation at the disk's outer edge.
We present ALMA Cycle 2 observations at 0 5 resolution of TW Hya of CS emission. The radial profile of the integrated line emission displays oscillatory features outward of 1 5 ( au). A dip-like ...feature at 1 6 is coincident in location, depth, and width with features observed in dust scattered light at near-infrared wavelengths. Using a thermochemical model indicative of TW Hya, gas-grain chemical modeling, and non-LTE radiative transfer, we demonstrate that such a feature can be reproduced with a surface density depression, consistent with the modeling performed for scattered-light observations of TW Hya. We further demonstrate that a gap in the dust distribution and dust opacity only cannot reproduce the observed CS feature. The outer enhancement at 3 1 is identified as a region of intensified desorption due to enhanced penetration of the interstellar far-UV radiation at the exponential edge of the disk surface density, which intensifies the photochemical processing of gas and ices.
Context. Determining the gas density and temperature structures of protoplanetary disks is a fundamental task in order to constrain planet formation theories. This is a challenging procedure and most ...determinations are based on model-dependent assumptions. Aims. We attempt a direct determination of the radial and vertical temperature structure of the Flying Saucer disk, thanks to its favorable inclination of 90 degrees. Methods. We present a method based on the tomographic study of an edge-on disk. Using ALMA, we observe at 0.5″ resolution the Flying Saucer in CO J = 2–1 and CS J = 5–4. This edge-on disk appears in silhouette against the CO J = 2–1 emission from background molecular clouds in ρ Oph. The combination of velocity gradients due to the Keplerian rotation of the disk and intensity variations in the CO background as a function of velocity provide a direct measure of the gas temperature as a function of radius and height above the disk mid-plane. Results. The overall thermal structure is consistent with model predictions, with a cold (<12−15 K) CO-depleted mid-plane and a warmer disk atmosphere. However, we find evidence for CO gas along the mid-plane beyond a radius of about 200 au, coincident with a change of grain properties. Such behavior is expected in the case of efficient rise of UV penetration re-heating the disk and thus allowing CO thermal desorption or favoring direct CO photo-desorption. CO is also detected at up to 3–4 scale heights, while CS is confined to around 1 scale height above the mid-plane. The limits of the method due to finite spatial and spectral resolutions are also discussed. Conclusions. This method appears to be a very promising way to determine the gas structure of planet-forming disks, provided that the molecular data have an angular resolution which is high enough, on the order of 0.3−0.1″ at the distance of the nearest star-forming regions.
Context.
Grain surface chemistry is fundamental to the composition of protoplanetary disks around young stars.
Aims.
The temperature of grains depends on their size. We evaluate the impact of this ...temperature dependence on the disk chemistry.
Methods.
We modeled a moderately massive disk with 16 different grain sizes. We used the 3D Monte Carlo POLARIS code to calculate the dust grain temperatures and the local
uv
flux. We modeled the chemistry using the three-phase astrochemical code NAUTILUS. Photo processes were handled using frequency-dependent cross sections and a new method to account for self and mutual shielding. The multi-grain model outputs are compared to those of single-grain size models (0.1 μm); there are two different assumptions for their equivalent temperature.
Results.
We find that the Langmuir-Hinshelwood mechanism at equilibrium temperature is not efficient to form H
2
at 3–4 scale heights (
H
), and we adopt a parametric fit to a stochastic method to model H
2
formation instead. We find the molecular layer composition (1–3
H
) to depend on the amount of remaining H atoms. Differences in molecular surface densities between single and multi-grain models are mostly due to what occurs above 1.5
H
. At 100 au, models with colder grains produce H
2
O and CH
4
ices in the midplane, and those with warmer grains produce more CO
2
ices; both of these allow for an efficient depletion of C and O as soon as CO sticks on grain surfaces. Complex organic molecules production is enhanced by the presence of warmer grains in the multi-grain models. Using a single-grain model mimicking grain growth and dust settling fails to reproduce the complexity of gas-grain chemistry.
Conclusions.
Chemical models with a single-grain size are sensitive to the adopted grain temperature and cannot account for all expected effects. A spatial spread of the snowlines is expected to result from the ranges in grain temperature. The amplitude of the effects depends on the dust disk mass.
Context. With its high complexity, large size, and close distance, the ringworld around GG Tau A is an appealing case to study the formation and evolution of protoplanetary disks around multiple star ...systems. However, investigations with radiative transfer models usually neglect the influence of the circumstellar dust around the individual stars. Aims. We investigate how circumstellar disks around the stars of GG Tau A influence the emission that is scattered at the circumbinary disk and if constraints on these circumstellar disks can be derived. Methods. We performed radiative transfer simulations with the POLArized RadIation Simulator (POLARIS) to obtain spectral energy distributions and emission maps in the H-Band (near-infrared). Subsequently, we compared them with observations to achieve our aims. Results. We studied the ratio of polarized intensity at different locations in the circumbinary disk. We conclude that the observed scattered-light near-infrared emission is best reproduced if the circumbinary disk lies in the shadow of at least two coplanar circumstellar disks surrounding the central stars. This implies that the inner wall of the circumbinary disk is strongly obscured around the midplane, while the observed emission is actually dominated by the upper-most disk layers. In addition, the inclined dark lane (“gap”) on the western side of the circumbinary disk, which has been a stable, nonrotating, feature for approximately 20 yr, can only be explained by the self-shadowing of a misaligned circumstellar disk surrounding one of the two components of the secondary close-binary star GG Tau Ab.
ABSTRACT The first hydrostatic core, also called the first Larson core, is one of the first steps in low-mass star formation as predicted by theory. With recent and future high-performance ...telescopes, the details of these first phases are becoming accessible, and observations may confirm theory and even present new challenges for theoreticians. In this context, from a theoretical point of view, we study the chemical and physical evolution of the collapse of prestellar cores until the formation of the first Larson core, in order to better characterize this early phase in the star formation process. We couple a state-of-the-art hydrodynamical model with full gas-grain chemistry, using different assumptions for the magnetic field strength and orientation. We extract the different components of each collapsing core (i.e., the central core, the outflow, the disk, the pseudodisk, and the envelope) to highlight their specific physical and chemical characteristics. Each component often presents a specific physical history, as well as a specific chemical evolution. From some species, the components can clearly be differentiated. The different core models can also be chemically differentiated. Our simulation suggests that some chemical species act as tracers of the different components of a collapsing prestellar dense core, and as tracers of the magnetic field characteristics of the core. From this result, we pinpoint promising key chemical species to be observed.
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