Dmitry A. Semenov

Chemistry in Protoplanetary Disks

Protoplanetary disks (PPDs) surrounding young stars are short-lived (~0.3-10 Myr), compact (~10-1000 AU) rotating reservoirs of gas and dust. PPDs are believed to be birthplaces of planetary systems, where tiny grains are assembled into pebbles, then rocks, planetesimals, and eventually planets, asteroids, and comets. Strong variations of physical conditions (temperature, density, ionization rate, UV/X-rays intensities) make a variety of chemical processes active in disks, producing simple molecules in the gas phase and complex polyatomic (organic) species on the surfaces of dust particles. In this entry, we summarize the major modern observational methods and theoretical paradigms used to investigate disk chemical composition and evolution, and present the most important results. Future research directions that will become possible with the advent of the Atacama Large Millimeter Array (ALMA) and other forthcoming observational facilities are also discussed.

CHEMISTRY OF A PROTOPLANETARY DISK WITH GRAIN SETTLING AND Lyα RADIATION

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 Lymanradiation 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 deter- mining 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, which leads to an increase in column density for molecular species such as CO, CN and SO. The inclusion of Lyman � radiation impacts the disk chemistry through specific species that have large photodissociation cross sections at 1216 u These include HCN, NH3 and CH4, for which the column densities are decreased by an order of magnitude or more due to the presence of Lymanradiation in the UV spectrum. A few species, such as CO2 and SO, are enhanced by the presence of Lymanradiation, but rarely by more than a factor of a few. Subject headings: astrochemistry, circumstellar matter, ISM: abundances, ISM: molecules, planetary systems: protoplanetary disks, stars: pre-main-sequence

Chemistry in Protoplanetary Disks: A Sensitivity Analysis

We study how uncertainties in the rate coefficients of chemical reactions in the RATE 06 database affect abundances and column densities of key molecules in protoplanetary disks. We randomly varied the gas-phase reaction rates within their uncertainty limits and calculated the time-dependent abundances and column densities using a gas-grain chemical model and a flaring steady state disk model. We find that key species can be separated into two distinct groups according to the sensitivity of their column densities to the rate uncertainties. The first group includes CO, C+, H+3, H2O, NH3, N2H+, and HCNH+. For these species the column densities are not very sensitive to the rate uncertainties, but the abundances in specific regions are. The second group includes CS, CO2, HCO+, H2CO, C2H, CN, HCN, HNC, and other, more complex species, for which high abundances and abundance uncertainties coexist in the same disk region, leading to larger scatters in column densities. However, even for complex and heavy molecules, the dispersion in their column densities is not more than a factor of ~4. We perform a sensitivity analysis of the computed abundances to rate uncertainties and identify those reactions with the most problematic rate coefficients. We conclude that the rate coefficients of about a hundred chemical reactions need to be determined more accurately in order to greatly improve the reliability of modern astrochemical models. This improvement should be an ultimate goal of future laboratory studies and theoretical investigations.

Chemistry in disks. I. Deep search for N2H+ in the protoplanetary disks around LkCa 15, MWC 480, and DM Tauri

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. Methods: We used the IRAM PdBI array to observe the N2H+ J=1-0 line and applied a ?^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 N2H+ detections for LkCa 15 and DM Tau at the 5 ? 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 [ N2H^+/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 N2H+ 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. Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). Research partially supported by PCMI, the French national program for the Physics and Chemistry of the Interstellar Medium.

Millimeter Observations and Modeling of the AB Aurigae System

We present the results of millimeter observations and a suitable chemical and radiative transfer model of the AB Aurigae (HD 31293) circumstellar disk and surrounding envelope. The integral molecular content of this system is studied by observing CO, C18O, CS, HCO+, DCO+, H2CO, HCN, HNC, and SiO rotational lines with the IRAM 30 m antenna, while the disk is mapped in the HCO+ (1-0) transition with the Plateau de Bure Interferometer. Using a flared disk model with a vertical temperature gradient and an isothermal spherical envelope model with a shadowed midplane and two unshielded cones together with a gas-grain chemical network, time-dependent abundances of observationally important molecules are calculated. Then a two-dimensional non-LTE line radiative transfer code is applied to compute excitation temperatures of several rotational transitions of HCO+, CO, C18O, and CS molecules. We synthesize the HCO+ (1-0) interferometric map along with single-dish CO (2-1), C18O (2-1), HCO+ (1-0), HCO+ (3-2), CS (2-1), and CS (5-4) spectra and compare them with the observations. Our disk model successfully reproduces observed interferometric HCO+ (1-0) data, thereby constraining the following disk properties: (1) the inclination angle i=17+6-3 deg, (2) the position angle φ=80deg+/-30deg, (3) the size Rout=400+/-200 AU, (4) the mass Mdisk=1.3×10-2 Msolar (with a factor of ~7 uncertainty), and (5) that the disk is in Keplerian rotation. Furthermore, indirect evidence for a local inhomogeneity of the envelope at >~600 AU is found. The single-dish spectra are synthesized for three different cases, namely, for the disk model, for the envelope model, and for their combination. An overall reasonable agreement between all modeled and acquired line intensities, widths, and profiles is achieved for the latter model, with the exception of the CS (5-4) data that require the presence of high-density clumpy structures in the model. This allows us to constrain the physical structure of the AB Aur inner envelope: (1) its mass-average temperature is about 35+/-14 K; (2) the density goes inversely down with the radius, ρ~r-1.0+/-0.3, starting from an initial value n0~3.9×105 cm-3 at 400 AU; and (3) the mass of the shielded region within 2200 AU is about 4×10-3 Msolar (the latter two quantities are uncertain by a factor of ~7). In addition, evolutionary nature and lifetime for dispersal of the AB Aur system and Herbig Ae/Be systems in general are discussed.

Chemistry in disks - IX. Observations and modelling of HCO+ and DCO+ in DM Tauri

Aims. We study the deuteration and ionization structure of the DM Tau disk via interferometric observations and modeling of the key molecular ions, HCO + and DCO + . Methods. The Plateau de Bure Array is used to observe DM Tau in lines of HCO + (1-0), (3-2) and DCO + (3-2) with a 1:5 “ angular and 0:2 km s 1 spectral resolution. Using a power-law fitting approach the observed column densities profiles are derived and thus the isotopic ratio RD = DCO + / HCO + . Chemical modeling allowed an exploration of the sensitivity of HCO + and DCO + abundances to physical parameters out with temperature. A steady state approximation was employed to observationally constrain the ionization fraction x(e ). Results. Fitting of radiative transfer models suggests that there is a chemical hole in HCO + and DCO + , extending up to 50 AU from the star. More work is required to discern the cause of this. The observed column densities of HCO + and DCO + at 100 AU were

Chemical evolution in the early phases of massive star formation. I

Understanding the chemical evolution of young (high-mass) star-forming regions is a central topic in star formation research. Chemistry is employed as a unique tool 1) to investigate the underlying physical processes and 2) to characterize the evolution of the chemical composition. With these aims in mind, we observed a sample of 59 high-mass star-forming regions at different evolutionary stages varying from the early starless phase of infrared dark clouds to high-mass protostellar objects to hot molecular cores and, finally, ultra-compact Hii regions at 1 mm and 3 mm with the IRAM 30 m telescope. We determined their large-scale chemical abundances and found that the chemical composition evolves along with the evolutionary stages. On average, the molecular abundances increase with time. We modeled the chemical evolution, using a 1D physical model where density and temperature vary from stage to stage coupled with an advanced gas-grain chemical model and derived the best-fit χ 2 values of all relevant parameters. A satisfying overall agreement between observed and modeled column densities for most of the molecules was obtained. With the bestfit model we also derived a chemical age for each stage, which gives the timescales for the transformation between two consecutive stages. The best-fit chemical ages are ∼10 000 years for the IRDC stage, ∼60 000 years for the HMPO stage, ∼40 000 years for the HMC stage, and ∼10 000 years for the UCHii stage. Thus, the total chemical timescale for the entire evolutionary sequence of the high-mass star formation process is on the order of 10 5 years, which is consistent with theoretical estimates. Furthermore, based on the approach of a multiple-line survey of unresolved data, we were able to constrain an intuitive and reasonable physical and chemical model. The results of this study can be used as chemical templates for the different evolutionary stages in high-mass star formation.

Measuring turbulence in TW Hydrae with ALMA: methods and limitations

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 T ex directly from the line profile, yielding a v turb . The second method assumes that two different molecules are co-spatial, which allows using their relative line widths for calculating T kin and v turb . 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 v turb to range from ≈130 mu2009s -1 at 40 au, and to drop to ≈50 mu2009s -1 in the outer disk, which is qualitatively recovered with the parametric model fitting. This corresponds to roughly 0.2−0.4 c s . CN was found to exhibit strong non-local thermal equilibrium effects outside r ≈ 140 au, so that v turb 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 v turb was found to drop from 100 mu2009s -1 (≈0.4 c s ) to zero at 100 au. The parametric model yielded a nearly constant 50 mu2009s -1 for CS (0.2−0.4 c s ). 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.

Influence of uncertainties in the rate constants of chemical reactions on astrochemical modeling results

Abstract. With the chemical reaction rate database UMIST95 (Millar etal. 1997) we analyze how uncertainties in rate constants of gas-phase chemical reactions influence the modelling of the molecular abundances in the interstellar medium. Random variations are introduced into the rate constants to estimate the scatter in theoretical abundances. Calculations were performed for the dark and translucent molecular clouds where gas phase chemistry is adequate (Terzieva & Herbst 1998). Similar approach was used by Pineau des Forets & Roueff (2000) for the study of chemical bistability. All the species are divided into 6 sensitivity groups according to the value of the scatter in their model abundances computed with varied rate constants. It is shown that the distribution of species within these groups depends on the number of atoms in them and on the adopted physical conditions. The simple method is suggested which allows to single out reactions that are most important for the evolution of a given species.We analyze the influence of errors in the rate constants of gas-phase chemical reactions on the model abundances of molecules in the interstellar medium using the UMIST 95 chemical database. By randomly varying the rate constants within the limits of the errors given in UMIST 95, we have estimated the scatters in theoretical abundances for dark and diffuse molecular clouds. All of the species were divided into six groups by the scatter in their model equilibrium abundances when varying the rate constants of chemical reactions. The distribution of the species in groups depends on the physical conditions. The scatters in the abundances of simple species lie within 0.5–1 order of magnitude, but increase significantly as the number of atoms in the molecule increases. We suggest a simple method for identifying the reactions whose rate constants have the strongest effect on the abundance of a selected species. This method is based on an analysis of the correlations between the abundance of species and the reaction rate constants and allows the extent to which an improvement in the rate constant of a specific reaction reduces the uncertainty in the abundance of the species concerned to be directly estimated.

CHEMISTRY IN DISKS. VII. FIRST DETECTION OF HC3N IN PROTOPLANETARY DISKS

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 30?m telescope by using the new broadband correlator (fast Fourier Transform Spectrometer) to search for so far undetected molecules in the protoplanetary disks surrounding the T Tauri stars DM Tau, GO Tau, LkCa 15, and the Herbig Ae star MWC?480. We report the first detection of HC3N at 5? in the GO Tau and MWC 480 disks with the IRAM 30?m, and in the LkCa 15 disk (5?), using the IRAM array, with derived column densities of the order of 1012?cm?2. We also obtain stringent upper limits on CCS (N < 1.5 ? 1012 cm?3). We discuss the observational results by comparing them to column densities derived from existing chemical disk models (computed using the chemical code Nautilus) and based on previous nitrogen- and sulfur-bearing molecule observations. The observed column densities of HC3N are typically two orders of magnitude lower than the existing predictions and appear to be lower in the presence of strong UV flux, suggesting that the molecular chemistry is sensitive to the UV penetration through the disk. The CCS upper limits reinforce our model with low elemental abundance of sulfur derived from other sulfur-bearing molecules (CS, H2S, and SO).