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Register a new userChemical complexity in protoplanetary disks in the era of ALMA and Rosetta
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Catherine Walsh
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Comets provide a unique insight into the molecular composition and complexity of the material in the primordial solar nebula. Recent results from the Rosetta mission, currently monitoring comet 67P/Churyumov-Gerasimenko in situ, and ALMA (the Atacama Large Millimeter/submillimeter Array), have demonstrated a tantalising link between the chemical complexity now confirmed in disks (via the detection of gas-phase CH3CN; Oberg et al. 2015) and that confirmed on the surface of 67P (Goesmann et al. 2015), raising questions concerning the chemical origin of such species (cloud or inheritance versus disk synthesis). Results from an astrochemical model of a protoplanetary disk are presented in which complex chemistry is included and in which it is assumed that simple ices only are inherited from the parent molecular cloud. The model results show good agreement with the abundances of several COMs observed on the surface of 67P with Philae/COSAC. Cosmic-ray and X-ray-induced photoprocessing of predominantly simple ices inherited by the protoplanetary disk is sufficient to generate a chemical complexity similar to that observed in comets. This indicates that the icy COMs detected on the surface of 67P may have a disk origin. The results also show that gas-phase CH3CN is abundant in the inner warm disk atmosphere where hot gas-phase chemistry dominates and potentially erases the ice chemical signature. Hence, CH3CN may not be an unambiguous tracer of the complex organic ice reservoir. However, a better understanding of the hot gas-phase chemistry of CH3CN is needed to confirm this preliminary conclusion.
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The $sigma$ Orionis cluster is important for studying protoplanetary disk evolution, as its intermediate age ($sim$3-5 Myr) is comparable to the median disk lifetime. We use ALMA to conduct a high-sensitivity survey of dust and gas in 92 protoplanetary disks around $sigma$ Orionis members with $M_{ast}gtrsim0.1 M_{odot}$. Our observations cover the 1.33 mm continuum and several CO $J=2-1$ lines: out of 92 sources, we detect 37 in the mm continuum and six in $^{12}$CO, three in $^{13}$CO, and none in C$^{18}$O. Using the continuum emission to estimate dust mass, we find only 11 disks with $M_{rm dust}gtrsim10 M_{oplus}$, indicating that after only a few Myr of evolution most disks lack sufficient dust to form giant planet cores. Stacking the individually undetected continuum sources limits their average dust mass to 5$times$ lower than that of the faintest detected disk, supporting theoretical models that indicate rapid dissipation once disk clearing begins. Comparing the protoplanetary disk population in $sigma$ Orionis to those of other star-forming regions supports the steady decline in average dust mass and the steepening of the $M_{rm dust}$-$M_{ast}$ relation with age; studying these evolutionary trends can inform the relative importance of different disk processes during key eras of planet formation. External photoevaporation from the central O9 star is influencing disk evolution throughout the region: dust masses clearly decline with decreasing separation from the photoionizing source, and the handful of CO detections exist at projected separations $>1.5$ pc. Collectively, our findings indicate that giant planet formation is inherently rare and/or well underway by a few Myr of age.
We present new Atacama Large Millimeter/submillimeter Array (ALMA) observations for three protoplanetary disks in Taurus at 2.9,mm and comparisons with previous 1.3,mm data both at an angular resolution of $sim0.1$ (15,au for the distance of Taurus). In the single-ring disk DS Tau, double-ring disk GO Tau, and multiple-ring disk DL Tau, the same rings are detected at both wavelengths, with radial locations spanning from 50 to 120,au. To quantify the dust emission morphology, the observed visibilities are modeled with a parametric prescription for the radial intensity profile. The disk outer radii, taken as 95% of the total flux encircled in the model intensity profiles, are consistent at both wavelengths for the three disks. Dust evolution models show that dust trapping in local pressure maxima in the outer disk could explain the observed patterns. Dust rings are mostly unresolved. The marginally resolved ring in DS Tau shows a tentatively narrower ring at the longer wavelength, an observational feature expected from efficient dust trapping. The spectral index ($alpha_{rm mm}$) increases outward and exhibits local minima that correspond to the peaks of dust rings, indicative of the changes in grain properties across the disks. The low optical depths ($tausim$0.1--0.2 at 2.9,mm and 0.2--0.4 at 1.3,mm) in the dust rings suggest that grains in the rings may have grown to millimeter sizes. The ubiquitous dust rings in protoplanetary disks modify the overall dynamics and evolution of dust grains, likely paving the way towards the new generation of planet formation.
Aims: To trace the radial and vertical spatial distribution of H2CS, a key species of the S-bearing chemistry, in protoplanetary disks. To analyse the observed distributions in light of the H2CS binding energy, in order to discuss the role of thermal desorption in enriching the gas disk component. Methods: In the context of the ALMA chemical survey of Disk-Outflow sources in the Taurus star forming region (ALMA-DOT), we observed five Class I or early Class II sources with the o-H2CS(7_1,6-6_1,5) line on a 40 au scale. We estimated the binding energy (BEs) of H2CS using quantum mechanical calculations, for the first time, for an extended, periodic, crystalline ice. Results: We imaged H2CS in two rotating molecular rings in the HL Tau and IRAS04302+2247 disks. The outer radii are about 140 au (HL Tau), and 115 au (IRAS 04302+2247). The edge-on geometry of IRAS 04302+2247 reveals that H2CS emission peaks, at radii of 60-115 au, at z = +- 50 au from the equatorial plane. The column densities are about 10^14 cm^-2. For HL Tau, we derive, for the first time, the [H2CS]/[H] abundance in a protoplanetary disk (about 10^-14). The BEs of H2CS computed for extended crystalline ice and amorphous ices is 4258 K and 3000-4600 K, respectively, implying a thermal evaporation where dust temperature is larger than 50-80 K. Conclusions: H2CS traces the so-called warm molecular layer, a region previously sampled using CS, and H2CO. Thioformaldehyde peaks closer to the protostar than H2CO and CS, plausibly due to the relatively high-excitation level of observed 7_1,6-6_1,5 line (60 K). The H2CS BEs implies that thermal desorption dominates in thin, au-sized, inner and/or upper disk layers, indicating that the observed H2CS emitting up to radii larger than 100 au is likely injected in the gas due to non-thermal processes.
We study details of the UV radiation transfer in a protoplanetary disk, paying attention to the influence of dust growth and sedimentation on the disk density and temperature. Also, we show how the dust evolution affects photoreaction rates of key molecules, like CN and CS.
It has been recently suggested that the multiple concentric rings and gaps discovered by ALMA in many protoplanetary disks may be produced by a single planet, as a result of the complex propagation and dissipation of the multiple spiral density waves it excites in the disk. Numerical efforts to verify this idea have largely utilized the so-called locally isothermal approximation with a prescribed disk temperature profile. However, in protoplanetary disks this approximation does not provide an accurate description of the density wave dynamics on scales of tens of au. Moreover, we show that locally isothermal simulations tend to overestimate the contrast of ring and gap features, as well as misrepresent their positions, when compared to simulations in which the energy equation is evolved explicitly. This outcome is caused by the non-conservation of the angular momentum flux of linear perturbations in locally isothermal disks. We demonstrate this effect using simulations of locally isothermal and adiabatic disks (with essentially identical temperature profiles) and show how the dust distributions, probed by mm wavelength observations, differ between the two cases. Locally isothermal simulations may thus underestimate the masses of planets responsible for the formation of multiple gaps and rings on scales of tens of au observed by ALMA. We suggest that caution should be exercised in using the locally isothermal simulations to explore planet-disk interaction, as well as in other studies of wave-like phenomena in astrophysical disks.
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