Protoplanetary disk chemistry

Chemical gap theory applied to AS 209: Revealing the Chemistry of Planet Formation

Felipe Alarcón (University of Michigan)

Emission substructures in both gas and dust are a common feature in protoplanetary disks. Such substructures can be linked to planet formation or planets themselves. We explore the observed gas substructures in AS 209 from high spatial resolution data of CO isotopologues and C2H, its possible causes, and links to the dust substructures present in the disk. In this disk, observations of C18O emission exhibits a strong  depression coincident with gaps seen in dust emission, but the gap is much wider (tens of au compared to few au) in CO surface density, overlapping the two biggest gaps observed in dust continuum emission. We investigate the origin of the CO surface density depression as directly associated with a planet carving a gap in the overall surface density of Hydrogen or as the result of CO chemical processing in gas with reduced surface density perturbations in H2. We compute thermochemical models of AS209 using the rac2d code in order to discriminate these two scenarios.  We find that the CO surface is degenerate with either solution, gas surface density perturbations or chemical processing. However, the presence of a massive planet (>0.2 Jupiter Masses) would be required to account for the CO gas depression, but this is in conflict with constraints set by the dust emission and by the gas pressure profile measured by gas kinematics. Thus, we infer, via mass independent means, that a CO chemical abundance variation is required to account for the observed structure in CO emission. This is further supported by bright C2H emission anti-correlating with the drop in CO. Based in our models, we suggest that CO chemical processing explains the CO gap  and enables C2H formation in gas with C/O>2.

Fifty AU Study of the chemistry in the disk/envelope system of Solar-like protostars (FAUST) Large program first results: the hot corino in L1551 IRS5

Eleonora Bianchi (Univ. Grenoble Alpes)

Recent evidence suggests that planet formation probably starts earlier than what was previously though. In particular, Class I protostellar disks (~105 yr) represent a key step in our understanding of the gas chemical composition at the planet formation scale. In this context, we report the discovery of a hot corino at the heart of the prototypical Class I source L1551 IRS5, obtained via ALMA observations as part of the Large Program FAUST (Fifty AU Study of the chemistry in the disk/envelope system of Solar-like protostars). More specifically, FAUST is the first ALMA Large Program based on astrochemistry and is designed to survey the chemical composition of a sample of 13 Class 0 and I protostars at the planet-formation scale. We detected in L1551 IRS 5 several emission lines from interstellar complex organic molecules (iCOMs) such as methanol and its most abundant isotopologues, as well as methyl formate and ethanol. The line emission is bright toward the north component (N), although a hot corino in the south component, cannot be excluded. The non-LTE analysis of the methanol lines towards N provides constraints on the gas temperature (~ 100 K), density (≥ 1.5 x 108 cm-3) and emitting size (~0.15”, i.e. ~ 10 au in radius). The lines are predicted to be optically thick, the 13CH3OH line having an opacity ≥ 2. The methyl formate and ethanol column densities relative to methanol are ≤ 0.03 and ≤ 0.015, respectively, compatible with those measured in Class 0 sources. Thus, the present observations towards L1551 IRS5 agree with little chemical evolution in hot corinos from Class 0 to I.

Introducing STAR-MELT: STellar AccRetion-Mapping with Emission Line Tomography - a Python package for extraction, identification, fitting and analysis of spectral lines

Justyn Campbell-White (University of Dundee)

Low- and intermediate-mass stars acquire most of their mass in the protostellar phase, but accretion continues into the pre-main-sequence phase via a disk for a few million years. Accretion governs the transport of matter and angular momentum from the accretion disk to the star. This affects disk stability and evolution, stellar rotation and activity, and planet formation and migration. The main observational challenge is probing the sub-au scales of the innermost disk, which is not yet possible via interferometry. Such stars, however, possess a wealth of metallic emission lines that can reveal the nature of these accretion-related processes. Our analysis involves emission line tomography of time-resolved high-resolution spectra of young stars. This technique uses the time domain to look for distortions in the stellar emission line profiles and radial velocity signatures. Temperatures and densities can be determined for the various emission line species. With both temporal and spatial information, we can then infer a tomographic map of the accretion structures, activity spots and the innermost hot atomic gas; down to smaller scales than those achievable with direct imaging. Our analysis allows for new science results to be obtained from archival data. We have developed a Python package to extract and analyse the accretion and/or activity spectrum from the high-resolution data. Directly from the fits files, the emission lines are automatically extracted and identified, via matching to a compiled reference database of lines. Line profiles are fitted and quantified, allowing for calculations of physical properties across each individual observation. Our STAR-MELT python package would also be useful for different applications of spectral analysis, where emission line identification is required. Standard data formats for spectra are automatically compatible, with user-defined custom formats also available. Any reference database (atomic or molecular) can also be used for line identification. Temporal variations in lines can readily be displayed and quantified. In this talk, we will present a brief introduction and demonstration of the STAR-MELT package, along with early results from our emission line tomography analysis. We welcome feedback and suggestions from the community regarding implementation of features that may be useful in other areas of spectral analysis.

Resolved DCN/HCN profiles towards five protoplanetary disks from the MAPS data

Gianni Cataldi (National Astronomical Observatory of Japan)

The deuteration fraction of molecules is sensitive to the physical and chemical conditions of an environment. Therefore, the D/H ratio is often used to infer the thermal history and formation location of comets or asteroids. Eventually, we would like to connect the D/H ratio measured in Solar System bodies to the deuteration chemistry in protoplanetary disks. To further constrain that chemistry, we derived radially resolved profiles of DCN/HCN towards five protoplanetary disks observed by the ALMA large program MAPS. I will discuss what these data can tell us about the HCN deuteration chemistry operating in protoplanetary disks.

Thioformaldehyde (H2CS) in protoplanetary disks: radial and vertical spatial distributions as observed towards HL Tau and IRAS04302+2247

Claudio Codella (INAF - Osservatorio Astrofisico di rcetri)

A breakthrough results provided by ALMA is that the process of planet formation already starts around Sun-like protostars with ages less than 1 Myr. This calls for further key question: what is the chemical compositions in the early protoplanetary disks? This is the goal of the ALMA-DOT project (ALMA chemical survey of Disk-Outflow sources in Taurus), which targets Class I or early Class II disks to obtain their chemical characterization. One of the topics is to understand the chemistry of S-bearing species: here we present H2CS and CS ALMA-Band 6 observations on a Solar System scale of five Class I or early Class II sources. More specifically, we imaged H2CS and CS emission in two rotating molecular rings in the HL Tau and IRAS04302+2247 disks, up to radii larger than 150 au. The edge-on geometry of IRAS 04302+2247 allows us to reveal that H2CS emission peaks, at radii of 60-115 au, at z = +- 50 au from the equatorial plane. Assuming LTE conditions, the column densities are around 1014 cm-2. For HL Tau, we derive, for the first time, the [H2CS]/[H] abundance in a protoplanetary disk: about 10-14. To analyse the observed distributions, we estimated the binding energy of H2CS and CS using quantum mechanical calculations, for the first time, for an extended, periodic, crystalline ice. The binding energy (BE) for an extended crystalline ice are 4258 K (H2CS) and 3861 K (CS). On the other end, for amorphous ice models the estimated BEs are 3000-4600 K (H2CS), and 2700-4000 K (CS). These findings imply a thermal evaporation where dust temperature is larger than 50-80 K. Both H2CS and CS trace the same region, i.e. the so-called warm molecular layer. Thioformaldehyde peaks closer to the protostar than and CS, plausibly due to the relatively high-excitation level of observed H2CS line (Eu = 60 K). The binding energies show that the molecular emission observed at radii larger than 100 au are injected into the gas due to non-thermal processes (photo-/CR- and/or reactive-desorption).

Molecules with ALMA at Planet-forming Scales (MAPS): An overview of radial and vertical disk chemical substructures

Charles Law (Harvard-Smithsonian Center for Astrophysics)

Planets form and obtain their compositions in dust- and gas-rich disks around young stars. This process is intimately connected to the spatial arrangement of disk material, but only recently has it become clear that disks are not always smooth in either their dust or gas distributions. Dust substructure at the au-to-10 au scale is ubiquitous, as revealed by the DSHARP program, but far fewer observations have probed gas substructure at similar scales. As a result, it is not known whether dust and chemical substructures are linked on such scales, and the causal relationship between dust and molecular distributions remains largely unexplored. To address this, I will present initial results from the The Molecules with ALMA at Planet-forming Scales (MAPS) Large Program, which explores radial and vertical disk chemical structures at 10 au scales in five disks where dust substructure is detected and planet formation appears to be ongoing. A wide diversity of line morphologies, including rings, gaps, and emission plateaus, is observed with substructures occurring at almost any radius in which line emission is detected. I will discuss how these chemical substructures correlate with one another and with dust gaps, rings, and outer edges. I will pay special attention to the inner 50 au of disks, and other regions where there are signs of ongoing planet formation, and provide some first conclusions about what MAPS is teaching us about the chemistry of planet formation.

Icy conditions in protoplanetary disks: how and where to find the water snowline

Margot Leemker (Leiden Observatory, Leiden University)

Snowlines, in particular the water snowline, are important for the formation of planets in protoplanetary disks. However, locating the water snowline directly is challenging. First, due to the proximity of the water snowline to the host star, which is generally at <10 au even for Herbig disks. Second, due to the absorption of water in the Earth's atmosphere. A chemical tracer, HCO+, observed with ALMA at high resolution potentially provides a solution to both problems. HCO+ is destroyed by gas-phase water, therefore the HCO+ abundance drops by orders of magnitude when water desorbs from the grains. Here we present new ALMA band 6 observations of HCO+ in the disk around the outbursting source V883 Ori. These observations strongly suggest that HCO+ is tracing the water snowline and that this snowline is located well outside the radius of ~50 AU previously suggested by an intensity break in continuum observations. Thus, there appears no link between dust substructure and the water snowline in this case. We also provide generic models for HCO+ and H13CO+ emission from disks around non-outbursting Herbig stars, highlighting the complications due to continuum optical depth and excitation in tracing snowlines when they are located close to the star.

The impact of pre-main sequence stellar evolution on the initial conditions for planet formation

James Miley (ALMA JAO/NAOJ)

Midplane temperature structure within protoplanetary discs determines the position of key snowlines, which in turn dictates the phase of key volatiles and plays a key role in setting the carbon-to-oxygen ratio of planet building material as a function of radius. However, the evolution of the host star's luminosity alters the disc midplane temperature, suggesting temporal variations of disc composition for any given orbital radius in the disc. Through modelling of protoplanetary disc midplanes a dichotomy is identified between the temperature, and subsequently the composition, of disc midplanes around low and intermediate mass stars. Applying this modelling to the HR8799 planetary system, constraints are placed on the radial location and time at which the carbon-rich planet HR8799b was formed.

The dynamic model of volatiles in gas-dust protoplanetary disks

Tamara Molyarova(Institute of Astronomy, Russian Academy of Sciences)

Volatile species are an important component of protoplanetary disks. They can either be in the gas phase or settle as ice on dust grain surface and form icy mantles capable of influencing dust evolution. In particular, according to some laboratory studies, dust particles covered with ice are more resistant to fragmentation, as they have higher fragmentation velocity. We model the dynamics of volatile species in a protoplanetary disk that includes evolving dust. We use the hydrodynamic code FEOSAD in the thin-disk limit, which allows us to model the long-term disk evolution starting from the cloud collapse and includes a two-component model of dust dynamics and evolution. We added to the model the transport, evaporation, and condensation of four volatiles (H2O, CO2, CH4, CO), as well as the feedback of ice mantles on dust fragmentation. The simulation results show a complex distribution of ices in the disk. There are multiple snowlines for each molecule and the volatiles accumulate around snowlines. We highlight the effects of dust and ice co-evolution and analyze the composition and thickness of evolving icy mantles on the considered dust populations.

ALMA-DOT: the ALMA chemical survey of Disk-Outflow sources in Taurus

Linda Podio (INAF - Osservatorio Astrofisico di Arcetri)

In the last five years ALMA revolutionised our comprehension of planet formation. The first breakthrough was delivered by the impacting image of the rings in the disk of HL Tau, suggesting that planet formation occurs early, in disks of less than 1 Myr. ALMA is revolutionising also our comprehension of the disk chemistry, which is crucial to answer another key question about planet formation: what chemical composition planets inherit from their natal environment? Answering this question is the goal of the ALMA chemical survey of Disk-Outflow sources in Taurus (ALMA-DOT). ALMA-DOT targets six Class I, early Class II disks (0.1-1 Myr) in a number of molecular tracers: CO, CN, S-bearing, CS, H2CS, SO2, deuterated water, HDO, and simple organics, H2CO and CH3OH. The survey allowed us to obtain a comprehensive view of the radial distribution of molecules in young disks, to reveal the disk vertical structure with the first image of the “molecular” and “freeze-out” layer, and to derive the molecules gas-phase abundance ratios. These are compared with the abundance ratios in Class 0 and II sources, and in comets, to reconstruct the chemical evolution from protostars to planets. Moreover, we report the first detection of methanol in a Class I disk, a key organic molecule for the build-up of chemical complexity. The results obtained by ALMA-DOT are the first step towards the characterisation of the disk chemical evolution and of the molecular heritage delivered to the assembling planets.

AB Aur, a Rosetta stone for astrochemical studies

Pablo Rivière (Observatorio Astronómico Nacional (OAN), Spain)

The Herbig  star  AB  Aur  is  a  widely  studied  system hosting a transitional disk. Located at 163 pc from the  Sun,  it  is perfectly  suited to studying the  spatial  distribution  of  gas  and  dust  in the  circumstellar environment in detail. The disk shows prominent emission from spiral arms at the near-infrared (NIR) and radio wavelength ranges, which could be explained by the presence of one or several forming giant planets. The system also shows a cavity in continuum emission that extends from∼70 to∼100 au.  Inside  the  cavity  a  compact source has been detected by. CO J=2-1 observations with ALMA highlighted the presence of prominent spiral arms at a radius of 0.""33, within the dust cavity. On the basis of the comparison of NOEMA continuum images at 1 and 2 mm with two-fluid hydrodynamical simulations, it was suggested that the dust trap observed in AB Aur was associated with a decaying vortex. So far, a few molecular species have been detected in ABAur, including CO, SO, HCO+, HCN, and H2CO. It is remarkable that the first detection of SO in a protoplanetary disk was reported towards AB Aur.  Most  species depicted  ring-like  emission  co-spatial with the  dust  ring  with a minimum towards the central dust gap, except 12CO and HCO+.NOEMA images of the HCO+3-2 line revealed the presence of the ring, a compact emission towards the center, and a filament connecting the center with the ring. The filamentary structure could be tracing an accretion flow from the outer ring into the inner disk. From a very complete spectroscopic study, we derived, for the first time, the gas temperature and the gas-to-dust ratio along the disk, providing information that is essential to constraining hydrodynamical simulations. Moreover, we explored the gas chemistry and, in particular, the sulfur depletion. The derived sulfur depletion is dependent on the assumed C/O ratio. Our data are better explained with C/O ~ 0.7 and S/H = 8 × 10-8.

Reading between the lines: Probing magnetospheric accretion, winds, and the innermost disk with emission line tomography

Aurora Sicilia-Aguilar (University of Dundee)

What happens in the planet-forming region at the time of planet formation? How are stars like our Sun formed? Observationally-based answers have to deal with one fact: the highest spatial resolution available nowadays barely traces the location of Jupiter in nearby star-forming regions. Thus to gain information on the tiny scales of stellar radii and the innermost planet-forming regions of disks, indirect methods are required. Young stars are rich in emission (and absorption) lines, related to their winds, accretion, spots, and innermost disk gas. Optical lines contain a large number of species with various excitation potentials and critical densities, providing information on the temperature, density, and velocity of hot and tiny structures. Combining the velocity information with repeated, time-resolved data, we can reconstruct scales far beyond the spatial resolution of the most powerful interferometers. Using time-resolved spectroscopy covering several rotational and disk orbital periods, we can obtain a very detailed view of the structure and variability of accretion columns and spots and information on the presence and launching points of stellar/disk winds in young stars. Understanding these processes and how they affect the observed spectra can also help us to identify (or rule out) the presence of young and newly-formed planets and stellar companions that may be perturbing the disk. Highly variable sources provide a further point: with the temperature varying during outbursts, we can spectroscopically access an even larger region of the disk and surroundings. I will present the results on young stars with very different spectral types and behaviours, discussing the power and limitations of emission line tomography and exploring what we can learn from "reading between the (spectral) lines" in these and other objects... and where the future can lead us to!

Warm initial conditions for planet formation

Merel van 't Hoff (University of Michigan)

Temperature is an important parameter in planet formation as it governs the chemical composition of both the ice and the gas through the freeze out of molecules. As evidence is piling up that planets start already forming during the embedded stage, studying the temperature structure of these young disks is key to understanding the initial conditions for planet formation. I will present ALMA observations of five young disks in Taurus targeting C17O, H2CO, HDO and CH3OH. The different freeze-out temperatures of these molecules allow to derive a global temperature structure, in particular for the edge-on disks L1527 and IRAS 04302 where the vertical structure is beautifully visible. These results show that young disks are warmer than more evolved protoplanetary disks around solar analogues, with no CO freeze-out or CO processing.

ALMA Observations of N2H+ and N2D+ in Protoplanetary Disks

Yoshihide Yamato (The University of Tokyo)

Recent observations with the ALMA telescope have revealed that a variety of structures, such as rings, gaps, and spiral arms, are found in many protoplanetary disks. If these structures are caused by gravitational interaction with planets, similar structures are expected to be found in molecular gas. In addition, molecular emission lines contain a wealth of information about the disk's radial and vertical temperature distribution and chemical composition. However, the spatially resolved observation of molecular emission lines has not been carried out very often despite its importance because of the long integration time required. In MAPS project, ALMA Cycle 6 Large Program, we observe more than 20 molecular emission lines from five protoplanetary disks with a spatial resolution close to 0′′.1, in order to elucidate the distribution of gas in the disks. In this talk, we will present the results of the N2D+ (J= 3-2) emission line analysis. N2D+ is an important ion for ionization and deuteration, and is expected to exist at low temperatures below 30 K. Therefore, it may be the best and only tracer of the ionization and deuteration near the midplane of the disk at low temperatures. The amount of N2D+ decreases in the region where CO sublimates from the dust surface and exists as a gas phase because it is destroyed by ion-molecule reactions with CO. Therefore, it can also be an indicator of the CO snowline near the midplane. In this study, we have reliably detected N2D+ (J= 3-2) emission lines in four out of five objects. The N2D+ molecules are found to be distributed in a ring shape with peaks beyond 50 au. The inner edge of the ring is in good agreement with the position of the CO snowline inferred from the theoretical model of the disk constructed by the project members and previous observations, indicating for the first time the usefulness of N2D+ molecules as an indicator of CO snowlines. The column densities of the obtained N2D+ molecules are higher than 10^11 cm-2 at the peak position, suggesting a high ionization rate exceeding 10^-18 s-1 according to a typical disk model. The N2D+/N2H+ ratio is found to be about 0.1 - 1. this is much higher than the elemental abundance ratios of interstellar medium (D /H = 10-5). In particular, the N2D+/N2H+ ratio tends to be higher at the outer edge of the disk, which is comparable to or larger than the N2D+/N2H+ ratio in the starless core and the Class 0 proto-stellar envelope. This indicates that deuteration of molecules by ion-molecule reactions in the disk is effective.