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Register a new userMethanol formation in TW Hya and future prospects for detecting larger complex molecules in disks with ALMA
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Gas-phase methanol was recently detected in a protoplanetary disk for the first time with ALMA. The peak abundance and distribution of methanol observed in TW Hya differed from that predicted by chemical models. Here, the chemistry of methanol gas and ice is calculated using a physical model tailored for TW Hya with the aim to contrast the results with the recent detection in this source. New pathways for the formation of larger complex molecules (e.g., ethylene glycol) are included in an updated chemical model, as well as the fragmentation of methanol ice upon photodesorption. It is found that including fragmentation upon photodesorption improves the agreement between the peak abundance reached in the chemical models with that observed in TW Hya ($sim 10^{-11}$ with respect to ce{H2}); however, the model predicts that the peak in emission resides a factor of $2-3$ farther out in the disk than the ALMA images. Reasons for the persistent differences in the gas-phase methanol distribution between models and the observations of TW Hya are discussed. These include the location of the ice reservoir which may coincide with the compact mm-dust disk ($lesssim 60$~au) and sources of gas-phase methanol which have not yet been considered in models. The possibility of detecting larger molecules with ALMA is also explored. Calculations of the rotational spectra of complex molecules other than methanol using a parametric model constrained by the TW Hya observations suggest that the detection of individual emission lines of complex molecules with ALMA remains challenging. However, the signal-to-noise ratio can be enhanced via stacking of multiple transitions which have similar upper energy levels.
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We obtain high spatial and spectral resolution images 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 is derived with three approaches: two `direct and one modelling. The first requires a single transition and derives Tex{} directly from the line profile, yielding a vturb{}. The second assumes two different molecules are co-spatial thus their relative linewidths allow for a calculation of Tkin{} and vturb{}. Finally we fit a parametric disk model where physical properties of the disk are described by power laws, to compare our `direct methods with previous values. The two direct methods were limited to the outer $r > 40$~au disk due to beam smear. The direct method found vturb{} ranging from $approx$~vel{130} at 40~au, dropping to $approx$~vel{50} in the outer disk, qualitatively recovered with the parametric model fitting. This corresponds to roughly $0.2 - 0.4~c_s$. CN was found to exhibit strong non-LTE effects outside $r approx 140$~au, so vturb{} was limited to within this radius. The assumption that CN and CS are co-spatial is consistent with observed linewidths only within $r lesssim 100$~au, within which vturb{} was found to drop from vel{100} ($approx~0.4~c_s$) to nothing at 100~au. The parametric model yielded a near constant vel{50} 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. The magnitude of the dispersion is comparable with or below that predicted by the magneto-rotational instability theory. A more precise comparison would require to reach an absolute calibration precision of order 3%, or to find a suitable combination of light and heavy molecules which are co-located in the disk.
(Abridged) Protoplanetary disks are vital objects in star and planet formation, possessing all the material which may form a planetary system orbiting the new star. We investigate the synthesis of complex organic molecules (COMs) in disks to constrain the achievable chemical complexity and predict species and transitions which may be observable with ALMA. We have coupled a 2D model of a protoplanetary disk around a T Tauri star with a gas-grain chemical network including COMs. We compare compare synthesised line intensities and calculated column densities with observations and determine those COMs which may be observable in future. COMs are efficiently formed in the disk midplane via grain-surface chemical reactions, reaching peak grain-surface fractional abundances 1e-6 - 1e-4 that of the H nuclei number density. COMs formed on grain surfaces are returned to the gas phase via non-thermal desorption; however, gas-phase species reach lower fractional abundances than their grain-surface equivalents, 1e-12 - 1e-7. Including the irradiation of grain mantle material helps build further complexity in the ice through the replenishment of grain-surface radicals which take part in further grain-surface reactions. There is reasonable agreement with several line transitions of H2CO observed towards several T Tauri star-disk systems. The synthesised line intensities for CH3OH are consistent with upper limits determined towards all sources. Our models suggest CH3OH should be readily observable in nearby protoplanetary disks with ALMA; however, detection of more complex species may prove challenging. Our grain-surface abundances are consistent with those derived from cometary comae observations providing additional evidence for the hypothesis that comets (and other planetesimals) formed via the coagulation of icy grains in the Suns natal disk.
Connecting the composition of planet-forming disks with that of gas giant exoplanet atmospheres, in particular through C/O ratios, is one of the key goals of disk chemistry. Small hydrocarbons like $rm C_2H$ and $rm C_3H_2$ have been identified as tracers of C/O, as they form abundantly under high C/O conditions. We present resolved $rm C_3H_2$ observations from the TW Hya Rosetta Stone Project, a program designed to map the chemistry of common molecules at $15-20$ au resolution in the TW Hya disk. Augmented by archival data, these observations comprise the most extensive multi-line set for disks of both ortho and para spin isomers spanning a wide range of energies, $E_u=29-97$ K. We find the ortho-to-para ratio of $rm C_3H_2$ is consistent with 3 throughout extent of the emission, and the total abundance of both $rm C_3H_2$ isomers is $(7.5-10)times10^{-11}$ per H atom, or $1-10$% of the previously published $rm C_2H$ abundance in the same source. We find $rm C_3H_2$ comes from a layer near the surface that extends no deeper than $z/r=0.25$. Our observations are consistent with substantial radial variation in gas-phase C/O in TW Hya, with a sharp increase outside $sim30$ au. Even if we are not directly tracing the midplane, if planets accrete from the surface via, e.g., meridonial flows, then such a change should be imprinted on forming planets. Perhaps interestingly, the HR 8799 planetary system also shows an increasing gradient in its giant planets atmospheric C/O ratios. While these stars are quite different, hydrocarbon rings in disks are common, and therefore our results are consistent with the young planets of HR 8799 still bearing the imprint of their parent disks volatile chemistry.
We present Atacama Large Millimeter Array CO(3$-$2) and HCO$^+$(4$-$3) observations covering the central $1rlap{.}5$$times$$1rlap{.}5$ region of the Orion Nebula Cluster (ONC). The unprecedented level of sensitivity ($sim$0.1 mJy beam$^{-1}$) and angular resolution ($sim$$0rlap{.}09 approx 35$ AU) of these line observations enable us to search for gas-disk detections towards the known positions of submillimeter-detected dust disks in this region. We detect 23 disks in gas: 17 in CO(3$-$2), 17 in HCO$^+$(4$-$3), and 11 in both lines. Depending on where the sources are located in the ONC, we see the line detections in emission, in absorption against the warm background, or in both emission and absorption. We spectrally resolve the gas with $0.5$ km s$^{-1}$ channels, and find that the kinematics of most sources are consistent with Keplerian rotation. We measure the distribution of gas-disk sizes and find typical radii of $sim$50-200 AU. As such, gas disks in the ONC are compact in comparison with the gas disks seen in low-density star-forming regions. Gas sizes are universally larger than the dust sizes. However, the gas and dust sizes are not strongly correlated. We find a positive correlation between gas size and distance from the massive star $theta^1$ Ori C, indicating that disks in the ONC are influenced by photoionization. Finally, we use the observed kinematics of the detected gas lines to model Keplerian rotation and infer the masses of the central pre-main-sequence stars. Our dynamically-derived stellar masses are not consistent with the spectroscopically-derived masses, and we discuss possible reasons for this discrepancy.
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.
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