Oral presentations and abstracts

Exoplanetary science is now pushing to constrain the atmospheric compositions of exoplanets. This quest will be further aided by the next generation of facilities, such as the JWST and ground-based ELTs. Linking the observed composition of exoplanet atmospheres to where and how these atmospheres formed in their natal protoplanetary disks often involves comparing the observed exoplanetary atmospheric carbon-to-oxygen (C/O) ratio to a model of a disk midplane with a fixed chemical composition. In this scenario, chemical evolution in the midplane prior to and during the planet formation era is not considered. The C/O ratios of gas and ice in the disk midplane are simply defined by icelines of volatile molecules such as water and CO in the midplane. However, kinetic chemical evolution during the lifetime of the gaseous disk can change the relative abundances of volatile molecules, thus altering the C/O ratios of the planet-forming material. In my chemical evolution models, I utilize a large network of gas-phase, grain-surface and gas-grain interaction reactions, thus providing a comprehensive treatment of chemistry. In my talk, I will outline how such chemical reactions can cause the chemical composition in the disk midplane to evolve, how this affects the C/O ratios of the gas and solid material that form planets, and how such changes to the midplane chemical composition can lead to differences in exoplanet atmospheric compositions. These differences in exoplanet atmospheric compositions may be discernible with JWST observations.

How to cite: Eistrup, C.: From disk midplanes to exoplanet atmospheres. The volatile chemical link., Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-48, https://doi.org/10.5194/epsc2020-48, 2020.

Snow-lines are thought to play a vital role in the evolution of protoplanetary discs and planet formation at all scales. Snow-lines occur in regions of the protoplanetary discs where the temperature reaches the sublimation temperature and volatiles transition from the solid phase to the vapour phase (or vice-versa). However, in the outer region of protoplanetary discs (beyond a few AU), the temperature is set by the distribution of solids and their ability to absorb stellar light. Thus, the thermodynamics of the disc and the volatile phases are inextricably linked. In this talk, I will show this coupling is thermally unstable, and snow-lines continually evolve in regions of the disc that are marginally optically thick. Patches of the disc proceeding through a limit cycle, where volatiles in a region of the disc continually condense and then sublimate. Using numerical simulations of the CO snow-line I will show it can move 10s AU over 10,000 years, repeatedly. I will use these simulations to discuss how this new process may effect measured Carbon abundances, solid evolution and ultimately planet formation, making connections to high-resolution images of protoplanetary discs. 

How to cite: Owen, J.: The dance of snow-lines, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-207, https://doi.org/10.5194/epsc2020-207, 2020.

How to cite: Lichtenberg, T., Dra̧żkowska, J., Schönbächler, M., Golabek, G. J., and Hands, T. O.: Earliest isotopic and chemical bifurcation of planetary building blocks, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-243, https://doi.org/10.5194/epsc2020-243, 2020.

Protoplanets formed by core accretion can become massive enough to accrete gas from the disk they are born in. If the
planetary proto-atmosphere exceeds a critical mass, runaway gas accretion starts and the planetary atmosphere collapses into a gas
giant. In recent years, many close-in super-Earths have been observed which raises the question on how they avoided becoming hot
Jupiters. We investigate the recycling hypothesis as a possible mechanism to avoid the collapse of the atmosphere.
We use three-dimensional radiation-hydrodynamics to simulate the formation of proto-atmosphere in the local frame around
the planet. In post-processing we use tracer particles to calculate the shape of the atmosphere and determine the non-uniform recycling
timescale in a quantitative manner. Our simulations converge to a quasi-steady state where the velocity field of the gas does not change anymore. For the
parameter space explored, a = 0.1 au, m_c ∈ [1, 2, 5, 10] M_Earth, we find that recycling of the atmosphere counteracts the collapse by
preventing the gas from cooling efficiently.

How to cite: Moldenhauer, T., Kuiper, R., Kley, W., and Ormel, C.: Recycling of Planetary Proto-Atmospheres, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-390, https://doi.org/10.5194/epsc2020-390, 2020.

How to cite: Veras, D., Tremblay, P.-E., Hermes, J., McDonald, C., Kennedy, G., Meru, F., and Gänsicke, B.: Constraining planet formation around 6-8 M☉ stars, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-412, https://doi.org/10.5194/epsc2020-412, 2020.

We present a new numerical framework to model the formation and evolution of giant planets. The code is based on the further development of the stellar evolution toolkit Modules for Experiments in Stellar Astrophysics (MESA). 
The model includes the dissolution of the accreted planetesimals/pebbles in the planetary gaseous envelope, and the effect of envelope enrichment on the planetary growth and internal structure is computed self-consistently. 
We apply our simulations to Jupiter and investigate the impact of different heavy-element and gas accretion rates on its formation history. 
We show that the assumed runaway gas accretion rate significantly affect the planetary radius and luminosity.
It is confirmed that heavy-element enrichment leads to shorter formation timescales due to more efficient gas accretion. 
We find that with heavy-element enrichment Jupiter's formation timescale is compatible with typical disks' lifetimes even when assuming a low heavy-element accretion rate (oligarchic regime). 
Finally, we provide an approximation for the heavy-element profile in the innermost part of the planet, providing a link between the internal structure and the planetary growth history.

How to cite: Valletta, C. and Helled, R.: Giant planet formation models with a self-consistent treatment of heavy-element., Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-648, https://doi.org/10.5194/epsc2020-648, 2020.

1. Introduction:
In the traditional core accretion scenario, a planet grows by the subsequent accretion of a solid core and a gaseous envelope [3]. However, the accretion of these solids generates a large amount of heat, which can easily vaporize incoming pebbles and fractured planetesimals before the core has grown massive [1,4,5]. This naturally leads to the formation of planets with polluted envelopes that are characterized by different interior conditions and that follow an altered evolutionary pathway. In this series of papers [1,2+forthcoming], we develop new analytical and numerical models to describe the formation and evolution of polluted planets and link emerging trends in their formation to observations of planetary systems.

2. Formation of polluted planets:

Fig. 1. A sketch of the four potential evolutionary phases of a polluted planet.

We find that envelope pollution substantially alters the structure of proto-planets in a number of ways and we suggest that their evolution can be described by four distinct phases, sketched in Fig. 1.

(I). In the first phase of direct core growth, the envelope is still cold enough for solids to reach the central core. As the planet's internal temperatures rise, an increasing fraction of the accreting solids sublimates and is absorbed in the envelope. This slows down the growth of the core until it halts completely when all incoming solids become vaporized and remain in the envelope. We find that in the case of pebble accretion, this limits the size of the central cores to ‹ 1-2 M_⊕, depending on conditions.

(II). We refer to the second phase as that of envelope growth, as this is where all the accreted solids end up after direct core growth ends. In our analytical model, we assume that it mixes efficiently with the nebular gas but we relax this assumption in a forthcoming numerical work. Regardless, we find that polluted interiors become very hot and dense due to a higher mean molecular weight, lower adiabatic index and smaller core. Interior temperatures can already reach values in excess of 10⁴ K at only a few Earth masses. Traditional models use the critical core mass as a criterion to identify the transition to runaway gas accretion but this term becomes a meaningless in planets that do not grow their cores beyond a certain size. We therefore suggest the critical metal mass (M_z,crit) as an equivalent criterion to supercede it. It is defined as the total mass in solids (core + vapor) that a planet needs to accrete in order to reach runaway growth. We derive the first expression for this mass:

where κ_rcb is the opacity at the radiative-convective boundary, d is the planet's semi-major axis, T_vap is the vaporization temperature of the solids,  is their accretion rate and M_c is the mass of the central core. Planets that form beyond the ice-line accrete a larger fraction of volatile materials and therefore form smaller cores with material that is characterized by lower vaporization temperatures. Both these effects reduce the critical metal mass compared to the inner disk where super-Earths and mini-Neptunes are more resistant to runaway gas accretion. 

(III). If a planet stops accreting solids before it reaches runaway accretion and while the disk is still present, it enters a phase of embedded cooling. This naturally happens in pebble accretion when a planet reaches the pebble isolation mass and begins to perturb the surrounding disk. The continued inward drift of tiny dust allows the planet to maintain a high opacity, limiting the pace of cooling. Besides this, the dilution of the interior can counteract the contraction of the envelope and further limit nebular accretion, although this requires the interior to remain compositionally mixed. We suggest that a combination of these effects can help explain why Uranus and Neptune did not reach runaway gas accretion, even if their solids flux dried up while the disk was still present.

(IV). Finally, the proto-planetary disk will dissipate and the planets eventually enter phase IV of isolated cooling. In traditional models, this is mainly associated with contraction and potential mass loss. We suggest that in the case of a polluted planet, the cooling will eventually lead to the rainout of the vapor interior and generate a second phase of (indirect) core growth after several Gyr. While the process of photo-evaporation should not be altered by this, we find that it makes internal energy release an ineffective mass-loss mechanism. This is because most of the energy is only liberated late in the planet's evolution after substantial contraction, when mass loss from winds is far less efficient.

2.1 Opacity in pebble accretion

Fig. 2. Trends in the critical metal mass from the opacity of gas, dust and pebbles.

We model the opacity during pebble accretion in a forthcoming work with a combination of molecular, dust and pebble contributions. We find that pebbles can effectively reduce the dust abundance through sweep-up, but only in the early stages when nebular gas accretion is outpaced by the pebble flux. Near the onset of runaway accretion, the opacity displays a dichotomy between the hot inner disk where molecular opacity dominates and the outer disk where dust obscures the envelopes. The result is an opacity valley around 1-3 AU that translates to an equivalent minimum in the critical metal mass at the same location (see Fig. 2), which can help explain the abundance of warm Jupiters in this region.

Acknowledgements:
This work has benefited from discussions at the ISSI Ice Giants Meetings in Bern 2019 & 2020. Marc Brouwers acknowledges the support of a Royal Society Studentship, RG 160509.

References:

[1] Brouwers, M. G., Vazan, A., & Ormel, C. W. 2018, A&A, 611, A65

[2] Brouwers, M. G. & Ormel, C. W. 2020, A&A, 634, A15

[3] Pollack, J. B., Hubickyj, O., Bodenheimer, P., et al. 1996, Icarus, 124, 62

[4] Mordasini, C., Mollière, P., Dittkrist, K.-M., Jin, S., & Alibert, Y. 2015, International Journal of Astrobiology, 14, 201

[5] Valletta, C. & Helled, R. 2019, ApJ, 871, 127

How to cite: Brouwers, M., Ormel, C., Vazan, A., and Bonsor, A.: The evolutionary pathway of polluted proto-planets, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-1039, https://doi.org/10.5194/epsc2020-1039, 2020.

The study of dust agglomeration has been a prevalent subject in laboratory astrophysics since its inception. It aims to bridge the gaps in our understanding of early planet formation and helps us interpret the data we receive from telescopes like ALMA, which observe protoplanetary disks.

As a contribution to this field, the ICAPS experiment (Interactions in Cosmic and Atmospheric Particle Systems) studies the agglomeration of micrometer-sized, monomeric silica grains under microgravitational conditions. To achieve stable microgravity, the experiment flew onboard the Texus 56 sounding rocket, continuously monitoring and controlling the movement of the dust cloud inside the vacuum chamber. For this, ICAPS employed and tested a novel approach, the thermal trap. It consists of four rings that are fitted with heating elements to keep the cloud centered using thermophoresis. The data from two overview cameras with perpendicular fields of view is used as a reference for the thermal trap. An exemplary image of the cloud can be found on the left hand side of the below figure. The right hand side shows an agglomerate as seen by the on-board long distance microscope and captured by a high speed camera.

However, the main focus of this session lies on the scientific results. Within the first two minutes, the silica grains grew to agglomerates of around 10³-10⁴ constituent particles via hit-and-stick collisions. The analysis revealed insight into their collision speed, rate of growth and fractal dimension. Also, the influence of their charge could be studied because at several stages of the experiment an electric field was applied. In particular, it was possible to reconstruct some particles in three dimensions by linear displacement through the focal plane of the long distance microscope using the thermal trap. With these findings, it will be possible to classify the growth processes at play.

How to cite: Schubert, B., Blum, J., Molinski, N., Vedernikov, A., Balapanov, D., von Borstel, I., Rüffert, L., Aktas, C., Schräpler, R., and Westermeier, M.: ICAPS Sounding Rocket - Particle Growth, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-567, https://doi.org/10.5194/epsc2020-567, 2020.

The role of magnetic fields in the evolution and dispersal of protoplanetary disks remains unclear to date partially due to the uncertainty regarding the sources of ionisation present in protoplanetary disks. Magnetic fields can only influence protoplanetary disk dynamics if the disks are sufficiently ionised. Ionisation due to X-rays, FUV photons and radioactivity is well-studied and generally only leads to high levels of ionisation close to the young star and in the surface layers of protoplanetary disks due to high disk column densities. Here I will instead focus on the importance of stellar cosmic rays which may provide a source of ionisation for the outer regions, and closer to the midplane, of protoplanetary disks.

Young solar-type stars are very magnetically active and drive stronger stellar winds in comparison to the present day Sun. The increased magnetic activity of young solar-type stars suggests that they are efficient ~GeV particle accelerators producing so-called stellar cosmic rays. Thus, protoplanetary disks are likely to be bombarded by stellar cosmic rays, influencing their chemical and dynamic evolution. These incident particles are believed to trigger the formation of complex organic molecules. Thus, they are essential to advance our understanding of how organic molecules, the building blocks of life in the Universe, form.

Recent ALMA observations have provided a number of tantalising clues as to the possible importance of stellar cosmic rays in protoplanetary disks. On the one hand, chemical modelling of observations of TW Hya’s protoplanetary disk suggest that the overall ionisation rate is remarkably low. While on the other hand, ALMA observations have been used to infer the presence of significant turbulent motion in DM Tau’s protoplanetary disk. This turbulent motion is likely driven by the magneto-rotational instability which would require a much higher level of ionisation than was inferred in TW Hya’s disk for instance. I will discuss the potential influence of stellar cosmic rays in these disks. 

More generally, I will present recent results which investigated the propagation, and ionising effect, of stellar cosmic rays in protoplanetary disks around young solar-mass stars. Unlike X-rays and FUV photons, stellar cosmic rays may effectively avoid being attenuated by the high column densities in the inner regions of protoplanetary disks due to their diffusive transport. To construct our disk density profiles, we use observationally inferred values from nearby star-forming regions for the total disk mass and the radial density profile. By varying the disk mass within the observed scatter for a solar-mass star, we find for a large range of disk masses and density profiles that protoplanetary disks are “optically thin” to low energy stellar cosmic rays. I will describe how our results indicate, for a wide range of disk masses, that low energy stellar cosmic rays provide an important source of ionisation at the disk midplane at large radii (∼70 au). Finally, I will discuss the type of systems where we expect that stellar cosmic rays are likely to be most influential. 

How to cite: Rodgers-Lee, D., Taylor, A., Downes, T., and Ray, T.: The role of stellar cosmic rays in protoplanetary disk evolution, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-20, https://doi.org/10.5194/epsc2020-20, 2020.

There is growing observational evidence that giant planet formation happens early, within a million years of the coalescence of the protoplanetary disk. Ionization rate is one of the most important parameters controlling both the chemical and dynamical processes in these disks. What few observational constrains on ionization currently exists suggest overall low ionization, limiting the processes able to take place. This is seemingly in conflict with chemical models which demonstrate the importance of ionization for the chemical processing of volatile carbon and observations which suggest such processing is ubiquitous and happens quickly. I will present new NOEMA observations which, when combined with chemical modeling, are indicative of enhanced ionization rates in the envelopes of three Class I protostars.  I will then discuss the potential impact of this early enhancement on the chemical composition of the material available to forming planets.

How to cite: Schwarz, K., Maret, S., Lefevre, C., Andre, P., Belloche, A., Bergin, E., and Codella, C.: Evidence of Enhanced Ionization During Early Planet Formation, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-361, https://doi.org/10.5194/epsc2020-361, 2020.

I will present a demographic study of the gas content in protoplanetary disks of the Lupus star-forming region, based on the previous ALMA surveys of the region.

Planets form around stars during their pre-main sequence phase, when still surrounded by a circumstellar disk of dust and gas. Setting observational constraints on the gas and dust properties of protoplanetary disks is crucial in order to understand what are the ongoing physical processes in the disk. These processes shape the planet formation mechanisms, and ultimately tell us about the disk’s ability to form planets.

The advent of ALMA allowed us to characterize dust properties in large populations of disks in several star-forming regions. Nevertheless, demographic studies of the gas content in these disk populations are scarce and generally incomplete, due to the fewer detections, and other difficulties when studying gas content.

In this work, we were able to assemble a large and homogeneous sample of disks from the Lupus region, all detected in ¹²CO and dust continuum. Gas emission profiles and sizes are estimated on 43 disks of the Lupus region. The profiles are inferred from the integrated emission maps of the ¹²CO transition line in ALMA Band 6. The observed emission is modeled using empirical functions: either the Nuker profile or an elliptical Gaussian for more compact sources. The gas size, defined as a certain fraction (e.g. 68%) of the total flux, is inferred from the modeled emission profiles.

These gas properties are then compared to the dust properties of the same objects, estimated from ALMA surveys in Band 7 and using analogous methodology.

The relative size of gas and dust is a key diagnostic of dust evolution. Large dust grains are decoupled from gas and drift inwards. Thus, if dust growth is prominent in these disks, the detected dust continuum emission in sub-mm wavelengths are expected to be several times smaller than the gas extent.

The results of our extensive sample confirm the larger gas size when compared to the dust size. The gas disk size is on average 2.6 times larger than the dust disk. This size difference can be explained by effective drifting of dust, but also by the optical depth difference between ¹²CO and dust continuum. Disentangling between these two effects is in general difficult; only large size ratios (typically beyond 4) unequivocally exhibit prominent dust evolution.

Only a small fraction (~18%) of the disk population has a size ratio larger than 4. Radial drift is intimately linked to grain growth, both are crucial processes to form the cores of planets. Our results might suggest that dust evolution is less common than previously thought.

We also investigated possible trends of the size ratio with stellar and disk properties, e.g. stellar mass, disk mass, integrated CO flux; no clear correlation can be found. Interestingly, the only Brown Dwarf of the sample with characterized gas and dust disk sizes shows a relatively large ratio of 3.8. On the other stellar mass range end, disks around stars with mass > 0.8 M_sun have a tentative lower ratio of 2.1. Larger samples in the low mass regime and in the rest of stellar mass ranges are needed in order to discern possible trends between spectral types or other properties of the host stars.

How to cite: Sanchis, E.: Gas extent in protoplanetary disks of the Lupus star forming region, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-803, https://doi.org/10.5194/epsc2020-803, 2020.

What is the initial reservoir of mass available for making planets? This key question for planet formation got even more relevant when the measured masses of the Class II disks (1-3 Myr old) were found consistently smaller than what is needed for the formation of gas giant cores. One way to solve this conundrum is to investigate whether the disks at the younger stages contain enough dust. Should planets be formed earlier, in the first 0.5 Myr, the physical conditions and chemical composition assumed of the beginning of planet formation needs to be revised.

We use Atacama Large Millimeter/submillimeter Array (ALMA) observations of embedded disks in Perseus molecular cloud together with existing Very Large Array (VLA) data to provide a robust estimate of disk masses and to compare the Perseus survey of dust masses with other ALMA surveys of young and mature disks. The combination of the two interferometric facilities observing at different wavelengths enables detailed characterization of the dust emission from young disks. Our best estimate of the median dust disk masses is 158 M_⊕ (Earth masses) and 50 M_⊕, for Class 0 and Class I respectively. These values are much higher than the median found typically in Class II disks (~5 M_⊕) (Fig. 1). In Fig. 2 we show that the masses typically found in exoplanetary systems are well in a range of masses available in Class 0 and Class I.

We put the improved disk mass estimates in the context of masses of known exoplanetary systems. The masses of Class 0 and I disks in Perseus can produce the observed exoplanet systems with efficiencies acceptable by planet formation models. The most massive observed exoplanets can still be produced by the most massive Class 0 disks with an efficiency of 15%, higher efficiencies on the order of 30% are needed if the planet formation starts in Class I. Interestingly, we find that most massive exoplanetary systems require higher efficiencies. (Fig. 1, right).

Our results are most consistent with the starting point of the planet formation already in the Class 0 stage, first 0.1 Myr after the beginning of the star and disk formation process. This has major implications for the physical and chemical conditions of the formation of extrasolar planets and of our own Solar System.

Fig. 1. Cumulative distribution function of dust disk masses and solid content of exoplanets. Top: Cumulative distribution function of dust masses for Class 0 (red) and Class I (blue) disks in Perseus and Class II disks (yellow) in Lupus measured with ALMA (Ansdell et al. 2016). In black, the masses of the exoplanet systems are normalized to the fraction of the gaseous planets (Cumming et al. 2008). Perseus disk masses calculated with κ_9mm=0.28 cm²g⁻¹ from the VLA fluxes. Medians are indicated in the labels. Bottom: Zoom-in to the ranges where exoplanets are present. The color scale shows the efficiency needed for the planet formation for a given bin of the distribution.