The composition of a protoplanetary disk is set by a combination of interstellar inheritance and gas and grain surface chemical reactions within the disk. The survival of inherited molecules, as well as the disk in situ chemistry depends on the local temperature, density and irradiation environment, which can change over time due to stellar and disk evolution, as well as transport in the disk. We address one aspect of this coupling between the physical and chemical evolution in disks by following accretion streamlines of gas and small grains in the disk midplane, while simultaneously taking the evolving star into account. This approach is computationally efficient and enables us to take into account changing physical conditions without reducing the chemical network. We find that many species are enhanced in the inner disk midplane in the dynamic model due to inward transport of cosmic-ray driven chemical products, resulting in, e.g., orders-of magnitude hydrocarbon enhancements at 1 au, compared to a static disk. For several other chemical families, there is no difference between the static and dynamic models, indicative of a robust chemical reset, while yet others show differences between static and dynamic models that depend on complex interactions between physics and chemistry during the inward track. The importance of coupling dynamics and chemistry when modeling the chemical evolution of protoplanetary disks is thus depends on what chemistry is of interest.
Protoplanetary disks are thought to evolve viscously, where the disk mass - the reservoir available for planet formation - decreases over time as material is accreted onto the central star. Observations show a correlation between dust mass and the stellar accretion rate, as expected from viscous theory. However, the gas mass inferred from 13CO and C18O line fluxes, which should be a more direct measure, shows no such correlation. Using thermochemical DALI models, we investigate how 13CO and C18O J=3-2 line fluxes change over time in a viscously evolving disk. We also investigate if the chemica…
Context. Several sulfur-bearing molecules are observed in the interstellar medium and in comets, in strong contrast to protoplanetary disks where only CS, H$_2$CS and SO have been detected so far. Aims. We combine observations and chemical models to constrain the sulfur abundances and their sensitivity to physical and chemical conditions in the DM Tau protoplanetary disk. Methods. We obtained $0.5^{"}$ ALMA observations of DM Tau in Bands 4 and 6 in lines of CS, SO, SO$_2$, OCS, CCS, H$_2$CS and H$_2$S, achieving a $\sim 5$ mJy sensitivity. Using the non-LTE radiative transfer code RADEX and t…
Max Planck Institute for Astronomy（马克斯·普朗克天文研究所）；Institute of Astronomy, Russian Academy of Sciences（俄罗斯科学院天文研究所）；Ludwig Maximilian University（路德维希·马克西米利安大学）
Aims. With the large amount of molecular emission data from (sub)millimeter observatories and incoming James Webb Space Telescope infrared spectroscopy, access to fast forward models of the chemical composition of protoplanetary disks is of paramount importance. Methods. We used a thermo-chemical modeling code to generate a diverse population of protoplanetary disk models. We trained a K-nearest neighbors (KNN) regressor to instantly predict the chemistry of other disk models. Results. We show that it is possible to accurately reproduce chemistry using just a small subset of physical condition…
Until now, axisymmetric, alpha-disc models have been adopted for calculations of the chemical composition of protoplanetary discs. While this approach is reasonable for many discs, it is not appropriate when self-gravity is important. In this case, spiral waves and shocks cause temperature and density variations that affect the chemistry. We have adopted a dynamical model of a solar-mass star surrounded by a massive (0.39 Msun), self-gravitating disc, similar to those that may be found around Class 0 and early Class I protostars, in a study of disc chemistry. We find that for each of a number …
H$_2$CO ice on dust grains is an important precursor of complex organic molecules (COMs). H$_2$CO gas can be readily observed in protoplanetary disks and may be used to trace COM chemistry. However, its utility as a COM probe is currently limited by a lack of constraints on the relative contributions of two different formation pathways: on icy grain-surfaces and in the gas-phase. We use archival ALMA observations of the resolved distribution of H$_2$CO emission in the disk around the young low-mass star DM Tau to assess the relative importance of these formation routes. The observed H$_2$CO em…
Gas chemistry in the dust depleted inner regions of protoplanetary disks. I. Near-IR spectra and overtones
Max-Planck Institute for extraterrestrial physics（马克斯·普朗克地外物理研究所）；Kapteyn Astonomical Institute, University of Groningen（格罗宁根大学卡普坦天文研究所）；University Observatory, Faculty of Physics, Ludwig-Maximilians-University at Munchen（慕尼黑路德维希-马克西米利安大学物理学院天文台）；Max-Planck-Institut fur Astronomie（马克斯·普朗克天文学研究所）
The volatile composition of a planet is determined by the inventory of gas and ice in the parent disk. The volatile chemistry in the disk is expected to evolve over time, though this evolution is poorly constrained observationally. We present ALMA observations of C18O, C2H, and the isotopologues H13CN, HC15N, and DCN towards five Class 0/I disk candidates. Combined with a sample of fourteen Class II disks presented in Bergner et al. (2019b), this data set offers a view of volatile chemical evolution over the disk lifetime. Our estimates of C18O abundances are consistent with a rapid depletion …
We study the content in S-bearing molecules of protoplanetary disks around low-mass stars. We used the new IRAM 30-m receiver EMIR to perform simultaneous observations of the $1_{10}-1_{01}$ line of H$_2$S at 168.8 GHz and $2_{23}-1_{12}$ line of SO at 99.3 GHz. We compared the observational results with predictions coming from the astrochemical code NAUTILUS, which has been adapted to protoplanetary disks. The data were analyzed together with existing CS J=3-2 observations. We fail to detect the SO and H$_2$S lines, although CS is detected in LkCa15, DM\,Tau, and GO\,Tau but not in MWC\,480. …
Molecular line emission from protoplanetary disks is a powerful tool to constrain their physical and chemical structure. Nevertheless, only a few molecules have been detected in disks so far. We take advantage of the enhanced capabilities of the IRAM 30m telescope by using the new broad band correlator (FTS) to search for so far undetected molecules in the protoplanetary disks surrounding the TTauri stars DM Tau, GO Tau, LkCa 15 and the Herbig Ae star MWC 480. We report the first detection of HC3N at 5 sigma in the GO Tau and MWC 480 disks with the IRAM 30-m, and in the LkCa 15 disk (5 sigma),…
We present the results of models of the chemistry, including deuterium, in the inner regions of protostellar disks. We find good agreement with recent gas phase observations of several (non--deuterated) species. We also compare our results with observations of comets and find that in the absence of other processing e.g. in the accretion shock at the surface of the disk, or by mixing in the disk, the calculated D/H ratios in ices are higher than measured and reflect the D/H ratio set in the molecular cloud phase. Our models give quite different abundances and molecular distributions to other in…
My thesis work aims to study the inter-relation between various physical and chemical conditions in a wide range of astrophysical environments. Our studied regions range from the super-hot regions (i.e., nebular, photon-dominated, or photodissociation regions, diffuse area, through which the lights of the background stars can reach us) to the super-cold regions (i.e., dense molecular clouds, proto-planetary disks, etc. where interstellar dust particles absorb all background visible and ultra-violet lights). The chemical complexity of the interstellar cloud gradually evolves due to the evolutio…
We present carbon and oxygen abundances for 941 FGK stars-the largest such catalog to date. We find that planet-bearing systems are enriched in these elements. We self-consistently measure C/O, which is thought to play a key role in planet formation. We identify 46 stars with C/O \geq 1.00 as potential hosts of carbon-dominated exoplanets. We measure a downward trend in [O/Fe] versus [Fe/H] and find distinct trends in the thin and thick disks, supporting the work of Bensby et al. Finally, we measure sub-solar C/O = 0.40+0.11 - 0.07, for WASP-12, a surprising result as this star is host to a tr…
Grain surface chemistry is key to the composition of protoplanetary disks around young stars. The temperature of grains depends on their size. We evaluate the impact of this temperature dependence on the disk chemistry. We model a moderately massive disk with 16 different grain sizes. We use POLARIS to calculate the dust grain temperatures and the local UV flux. We model the chemistry using the 3-phase astrochemical code NAUTILUS. Photoprocesses are handled using frequency-dependent cross-sections, and a new method to account for self and mutual shielding. The multi-grain model outputs are com…
Molecular line surveys are among the main tools to probe the structure and physical conditions in protoplanetary disks (PPDs), the birthplace of planets. The large radial and vertical temperature as well as density gradients in these PPDs lead to a complex chemical composition, making chemistry an important step to understand the variety of planetary systems. We aimed to study the chemical content of the protoplanetary disk surrounding GG Tau A, a well-known triple T Tauri system. We used NOEMA with the new correlator PolyFix to observe rotational lines at 2.6 to 4 mm from a few dozen molecule…
We investigate the chemistry in a radiation-hydrodynamics model of star-forming core which evolves from a cold ($\sim 10$ K) prestellar core to the main accretion phase in $\sim 10^5$ yr. A rotationally-supported gravitationally unstable disk is formed around a protostar. We extract the temporal variation of physical parameters in $\sim 1.5 \times 10^3$ SPH particles which end up in the disk, and perform post-processing calculations of the gas-grain chemistry adopting a three-phase model. Inside the disk, the SPH particles migrate both inward and outward. Since a significant fraction of volati…
Most stars form in a clustered environment. Therefore, it is important to assess how this environment influences the evolution of protoplanetary discs around young stars. In turn, this affects their ability to produce planets and ultimately life. We present here for the first time 3D SPH/N-body simulations that include both the hydrodynamical evolution of the discs around their natal stars, as well as the dynamics of the stars themselves. The discs are viscously evolving, accreting mass onto the central star and spreading. We find penetrating encounters to be very destructive for the discs as …
In this paper, we model the chemical evolution of a 0.25 M$_{\odot}$ protoplanetary disc surrounding a 1 M$_{\odot}$ star that undergoes fragmentation due to self-gravity. We use Smoothed Particle Hydrodynamics including a radiative transfer scheme, along with time-dependent chemical evolution code to follow the composition of the disc and resulting fragments over approximately 4000 yrs. Initially, four quasi-stable fragments are formed, of which two are eventually disrupted by tidal torques in the disc. From the results of our chemical modelling, we identify species that are abundant in the f…
We present results from a model of the chemical evolution of protoplanetary disks. In our models we directly calculate the changing propagation and penetration of a high energy radiation field with Lyman alpha radiation included. We also explore the effect on our models of including dust grain settling. We find that, in agreement with earlier studies, the evolution of dust grains plays a large role in determining how deep the UV radiation penetrates into the disk. Significant grain settling at the midplane leads to much smaller freeze-out regions and a correspondingly larger molecular layer, w…
The origins of planets, and perhaps life itself, is intrinsically linked to the chemistry of planet formation. In this chapter we will attempt to explore the chemistry of planet-forming disks from the perspective of knowledge gained from decades of solar system study, combined with our rapidly growing knowledge of extra-solar protoplanetary disks. Our aim is not to provide a comprehensive survey of the literature, but rather to present the chemical and physical tools that are used to explore chemical evolution during the earliest stages of planet formation. We include a discussion of observati…
We develop a simple iterative scheme to include vertical turbulent mixing and diffusion in ProDiMo thermo-chemical models for protoplanetary discs. The models are carefully checked for convergence toward the time-independent solution of the reaction-diffusion equations, as e.g. used in exoplanet atmosphere models. A series of five T Tauri disc models is presented where we vary the mixing parameter α mix from 0 to 0.01 and take into account (a) the radiative transfer feedback of the opacities of icy grains that are mixed upward and (b) the feedback of the changing molecular abundances on the ga…
Disentangling CO Chemistry in a Protoplanetary Disk Using Explanatory Machine Learning Techniques
University of Virginia（弗吉尼亚大学）；Carnegie Institution for Science（卡内基科学研究所）；Space Telescope Science Institute（空间望远镜科学研究所）；National Radio Astronomy Observatory（美国国家射电天文台）
本文提出用可解释回归机器学习模型解析原行星盘中 CO 空间密度与多种物理参数的关系，发现参数组合比单一参数更关键，并验证其与高成本化学模拟结论一致。
Observations have revealed that the elemental abundances of carbon and oxygen in the warm molecular layers of some protoplanetary disks are depleted compared to those is the interstellar medium by a factor of ~10-100. Meanwhile, little is known about nitrogen. To investigate the time evolution of nitrogen, carbon, and oxygen elemental abundances in disks, we develop a one-dimensional model that incorporates dust settling, turbulent diffusion of dust and ices, as well as gas-ice chemistry including the chemistry driven by stellar UV/X-rays and the galactic cosmic rays. We find that gaseous CO i…
The gas mass of protoplanetary disks, and the gas-to-dust ratio, are two key elements driving the evolution of these disks and the formation of planetary system. We explore here to what extent CO (or its isotopologues) can be used as a tracer of gas mass. We use a detailed gas-grain chemical model and study the evolution of the disk composition, starting from a dense pre-stellar core composition. We explore a range of disk temperature profiles, cosmic rays ionization rates, and disk ages for a disk model representative of T Tauri stars. At the high densities that prevail in disks, we find that…
Context: T Tauri stars have X-ray luminosities ranging from L_X = 10^28-10^32 erg/s. These luminosities are similar to UV luminosities (L_UV 10^30-10^31 erg/s) and therefore X-rays are expected to affect the physics and chemistry of the upper layers of their surrounding protoplanetary disks. Aim: The effects and importance of X-rays on the chemical and hydrostatic structure of protoplanetary disks are investigated, species tracing X-ray irradiation (for L_X >= 10^29 erg/s) are identified and predictions for [OI], [CII] and [NII] fine structure line fluxes are provided. Methods: We have impleme…
Context: The global size and spatial distribution of dust is an important ingredient in the structure and evolution of protoplanetary disks and in the formation of larger bodies, such as planetesimals. Aims: We aim to derive simple equations that explain the global evolution of the dust surface density profile and the upper limit of the grain size distribution and which can readily be used for further modeling or for interpreting of observational data. Methods: We have developed a simple model that follows the upper end of the dust size distribution and the evolution of the dust surface densit…
The first detection of gas-phase methanol in a protoplanetary disk (TW Hya) is presented. In addition to being one of the largest molecules detected in disks to date, methanol is also the first disk organic molecule with an unambiguous ice chemistry origin. The stacked methanol emission, as observed with ALMA, is spectrally resolved and detected across six velocity channels ($>3 σ$), reaching a peak signal-to-noise of $5.5σ$, with the kinematic pattern expected for TW~Hya. Using an appropriate disk model, a fractional abundance of $3\times 10^{-12} - 4 \times 10^{-11}$ (with respect to H$_2$) …
We present an observational and theoretical study of the primary ionizing agents (cosmic rays and X-rays) in the TW Hya protoplanetary disk. We use a set of resolved and unresolved observations of molecular ions and other molecular species, encompassing eleven lines total, in concert with a grid of disk chemistry models. The molecular ion constraints comprise new data from the Submillimeter Array on HCO$^+$, acquired at unprecedented spatial resolution, and data from the literature, including ALMA observations of N$_2$H$^+$. We vary the model incident CR flux and stellar X-ray spectra and find…
Dust constitutes only about one percent of the mass of circumstellar disks, yet it is of crucial importance for the modeling of planet formation, disk chemistry, radiative transfer and observations. The initial growth of dust from sub-micron sized grains to planetesimals and also the radial transport of dust in disks around young stars is the topic of this thesis. Circumstellar dust is subject to radial drift, vertical settling, turbulent mixing, collisional growth, fragmentation and erosion. We approach this subject from three directions: analytical calculations, numerical simulations, and co…
CO is commonly used as a tracer of the total gas mass in both the interstellar medium and in protoplanetary disks. Recently there has been much debate about the utility of CO as a mass tracer in disks. Observations of CO in protoplanetary disks reveal a range of CO abundances, with measurements of low CO to dust mass ratios in numerous systems. One possibility is that carbon is removed from CO via chemistry. However, the full range of physical conditions conducive to this chemical reprocessing is not well understood. We perform a systematic survey of the time dependent chemistry in protoplanet…
The evolution of protoplanetary disks is dominated by the conservation of angular momentum, where the accretion of material onto the central star is driven by viscous expansion of the outer disk or by disk winds extracting angular momentum without changing the disk size. Studying the time evolution of disk sizes allows us therefore to distinguish between viscous stresses or disk winds as the main mechanism of disk evolution. Observationally, estimates of the disk gaseous outer radius are based on the extent of the CO rotational emission, which, during the evolution, is also affected by the cha…
We investigate deuterium chemistry coupled with the nuclear spin-state chemistry of H$_2$ and H$_3^+$ in protoplanetary disks. Multiple paths of deuterium fractionation are found; exchange reactions with D atoms, such as HCO$^+$ + D, are effective in addition to those with HD. In a disk model with grain sizes appropriate for dark clouds, the freeze-out of molecules is severe in the outer midplane, while the disk surface is shielded from UV radiation. Gaseous molecules, including DCO$^+$, thus become abundant at the disk surface, which tends to make their column density distribution relatively …
It is still unclear whether the evolution of protoplanetary disks, a key ingredient in the theory of planet formation, is driven by viscous turbulence or magnetic disk winds. As viscously evolving disks expand outward over time, the evolution of disk sizes is a discriminant test for studying disk evolution. However, it is unclear how the observed disk size changes over time if disk evolution is driven by magnetic disk winds. Combining the thermochemical code DALI with the analytical wind-driven disk evolution model presented in Tabone et al. (2021a), we study the time evolution of the observed…
In this Letter we report the CO abundance relative to H2 derived toward the circumstellar disk of the T-Tauri star TW Hya from the HD (1-0) and C18O (2-1) emission lines. The HD (1-0) line was observed by the Herschel Space Observatory Photodetector Array Camera and Spectrometer whereas C18O (2-1) observations were carried out with the Submillimeter Array at a spatial resolution of 2.8" x 1.9" (corresponding to 142 x 97 AU). In the disk's warm molecular layer (T>20 K) we measure a disk-averaged gas-phase CO abundance relative to H2 of $χ{\rm(CO)}=(0.1-3)x10^{-5}$, substantially lower than the …
(Abridged) We explore the role of X-ray photoevaporation in the evolution and dispersal of viscously evolving T-Tauri discs. We show that the X-ray photoevaporation wind rates scale linearly with X-ray luminosity, such that the observed range of X-ray luminosities for solar-type T-Tauri stars (10e28-10e31 erg\s) gives rise to vigorous disc winds with rates of order 10e-10-10e-7 M_sun/yr. We use the wind solutions from radiation-hydrodynamic models, coupled to a viscous evolution model to construct a population synthesis model so that we may study the physical properties of evolving discs and s…
[abridged] We present a 1+1D global magnetically-driven disk accretion model that captures the essence of the MRI-driven accretion, without resorting to 3D global non-ideal MHD simulations. The gas dynamics is assumed to be solely controlled by the MRI and hydrodynamic instabilities. For given stellar and disk parameters, the Shakura-Sunyaev viscosity parameter $α$ is determined self-consistently under the framework of viscously-driven accretion from detailed considerations of the MRI with non-ideal MHD effects (Ohmic resistivity and ambipolar diffusion), accounting for disk heating by stellar…
(Abridged) The birth environment of the Sun will have influenced the conditions in the pre-solar nebula, including the attainable chemical complexity, important for prebiotic chemistry. The formation and distribution of complex organic molecules (COMs) in a disk around a T Tauri star is investigated for two scenarios: (i) an isolated disk, and (ii) a disk irradiated externally by a nearby massive star. The chemistry is calculated along the accretion flow from the outer disk inwards using a comprehensive network. Two simulations are performed, one beginning with complex ices and one with simple…
The volatile contents of protoplanetary disks both set the potential for planetary chemistry and provide valuable probes of defining disk system characteristics such as stellar mass, gas mass, ionization, and temperature structure. Current disk molecular inventories are fragmented, however, giving an incomplete picture: unbiased spectral line surveys are needed to assess the volatile content. We present here an overview of such a survey of the protoplanetary disks around the Herbig Ae star MWC 480 and the T Tauri star LkCa 15 in ALMA Band 7, spanning $\sim$36 GHz from 275--317 GHz and represen…
Protoplanetary disks are the birthplaces of planetary systems. The evolution of the star-disk system and the disk chemical composition determines the initial conditions for planet formation. Therefore a comprehensive understanding of the main physical and chemical processes in disks is crucial for our understanding of planet formation. We give an overview of the early evolution of disks, discuss the importance of the stellar high-energy radiation for disk evolution and describe the general thermal and chemical structure of disks. Finally we provide an overview of observational tracers of the g…
We present a self-consistent model of a protoplanetary disk: 'ANDES' ('AccretioN disk with Dust Evolution and Sedimentation'). ANDES is based on a flexible and extendable modular structure that includes 1) a 1+1D frequency-dependent continuum radiative transfer module, 2) a module to calculate the chemical evolution using an extended gas-grain network with UV/X-ray-driven processes surface reactions, 3) a module to calculate the gas thermal energy balance, and 4) a 1+1D module that simulates dust grain evolution. For the first time, grain evolution and time-dependent molecular chemistry are in…
The distributions and abundances of small organics in protoplanetary disks are potentially powerful probes of disk physics and chemistry. HNC is a common probe of dense interstellar regions and the target of this study. We use the Submillimeter Array (SMA) to observe HNC 3--2 towards the protoplanetary disks around the T Tauri star TW Hya and the Herbig Ae star HD 163296. HNC is detected toward both disks, constituting the first spatially resolved observations of HNC in disks. We also present SMA observations of HCN 3--2, and IRAM 30m observations of HCN and HNC 1--0 toward HD 163296. The disk…
It is key to constrain the gas surface density distribution, Sigma_gas, as function of disk radius in protoplanetary disks. In this work we investigate if spatially resolved observations of rarer CO isotopologues may be good tracers of Sigma_gas. Physical-chemical models with different input Sigma_gas(R) are run. The input disk surface density profiles are compared with the simulated 13CO intensity radial profiles to check if and where the two follow each other. There is always an intermediate region in the disk where the slope of the 13CO radial emission profile and Sigma_gas(R) coincide. At …
A key problem in protoplanetary disc evolution is understanding the efficiency of dust radial drift. This process makes the observed dust disc sizes shrink on relatively short timescales, implying that discs started much larger than what we see now. In this paper we use an independent constraint, the gas radius (as probed by CO rotational emission), to test disc evolution models. In particular, we consider the ratio between the dust and gas radius, $R_{\rm CO}/R_{\rm dust}$. We model the time evolution of protoplanetary discs under the influence of viscous evolution, grain growth, and radial d…
M-stars are the most common hosts of planetary systems in the Galaxy. Protoplanetary disks around M-stars thus offer a prime opportunity to study the chemistry of planet-forming environments. We present an ALMA survey of molecular line emission toward a sample of five protoplanetary disks around M4-M5 stars (FP Tau, J0432+1827, J1100-7619, J1545-3417, and Sz 69). These observations can resolve chemical structures down to tens of AU. Molecular lines of $^{12}$CO, $^{13}$CO, C$^{18}$O, C$_2$H, and HCN are detected toward all five disks. Lines of H$_2$CO and DCN are detected toward 2/5 and 1/5 di…
