Hide and Seek:
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Where are the young planets?
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26-29 June 2018, Madrid, Spain
Discussion
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Below you can find, for each session, the questions that we organisers or you participants have come up with.
You are encouraged to already look at them before the start of the meeting, to see whether you can contribute to the discussion. This could be in the form of a few slides, a sketch on a blackboard or just giving your view.
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Tuesday, June 26th
Afternoon: Session 1. General layout of the problem.
Discussion leader: René Oudmaijer
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Our general questions: Why are there so few/none planets detected around PMS stars (huge discrepancy with the number of exoplanets, (more than 3000) ? Do we have reliable direct detections of young planets, either in formation (accreting) or already formed- in disks around young stars (< 10 Myr), how many? Which candidates from the list of baby planets are the most reliable? why is it so hard to detect them?and the number of young planets that were detected?Why is there a huge discrepancy between number of exoplanets (> 3000) and the number of young planets detected? Why are there so few/none planets detected around PMS stars? Do we have reliable direct detections of young planets, either in formation (accreting) or already formed- in discs around young stars (<10 Myr), how many? Which candidates from the list of young planets are the most reliable? Why is it so hard to detect them? ...
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How are the disks around low-mass M-type stars (i.e. how are progenitors of the Trappist-1 system)? How similar or different are they from those around solar-like stars?
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Where are the planets at intermediate radii, i.e., not the close-in ones detected by RV and not the outer ones at > 100 au. Could one take advantage of the "easier" circumstellar structure during the WTTS phase to put constraints on these planets?
What are the total masses of protoplanetary disks and how do they compare with what we "see" in different ways? What is the gas vs dust ratio and how does it change with age and radius? How accurate are accretion rate estimates for low accretion rates(stellar and planetary), given all the uncertainties about stars, emission lines, photospheres, activity, and shape of veiling? Do all disks have "evolutionary signs"? Are they standard properties of disks, or signs of evolution? Is any disk "primordial" enough to be considered as a "primordial disk"?
Toy model to evaluate critically the current detection claims? Does it make sense to detect at K-band, but not at M bad and not with ALMA, and vice versa… and if so, under which condition. A pure numerical experiment… (FrMé)
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Wednesday, June 27th
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Morning: Session 2. Theoretical expectations and problems.
Discussion leader: Andrew Youdin
Our general questions: how do planets form, what accretion rates do we expect? Do planets form on a very short timescale or are current detection limits for planetary accretion rates are still above the real ones? Origin of the planet-metallicity correlation (primordial cloud, pollution,...?. Would it be possible to check that correlation in the pre-main sequence phase?
Can the chicken-and-egg problem of planet formation be solved by magnetic fields? Dust traps are needed to form a planet, but planets are needed to form a dust trap. However, vortices in the magnetic field or flare-induced density ripples in the disk create dust traps as well. So, should we consider magnetically induced planet formation?
The Galilean satellites and Titan indicate that at least some gas giant planets form with a circumplanetary disk. How is material in these disks transported? If they are shielded from the central star by circumstellar material, is there any mechanism other than a gravitational instability to drive material inward and angular momentum out? If so, it seems that circumplanetary disks should be episodically bright, and we probably haven’t looked at enough systems to detect one yet.
What is the preferred site for planet formation and the main migration mechanism?
What is the status of simulations concerning the height structure of the disks, i.e., are the planetary embryos still "vertically enclosed" by the disk or can their radiation rather freely escape in the z-direction? How "fast" and when does the runaway accretion take place, what could slow it down or speed it up? This relates to the accretion signatures that may or may not be expected from forming gas-giants.
One of the questions I would like to bring up is the interplay between planet detection statistics and binary statistics. I also thought I might mention the uncertainties in the results derived from the (many) non-detections in direct imaging surveys. For example depending on the applied priors to the planet distribution, or assumptions about atmospheric models, one can get very different inferred occurrence rates.
Distant, massive planets: how and when are they formed, as binaries or as planets? How do disks around low-metallicity stars evolve, compared to the solar- and high-metallicity ones? What is the effect of the cluster environment/stellar density/nearby massive stars on planet formation and evolution?
How do planets form, what accretion rates do we expect? Do planets form on a very short timescale or are current detection limits for planetary accretion rates still above the real ones? What is the origin of the planet-metallicity correlation (primordial cloud, pollution,...)? Would it be possible to check that correlation in the pre-main sequence phase?...
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Afternoon: Session 3. Direct observational detections.
Discussion leader: Garreth Ruane.
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Our general questions: Which is (are) the best observational approach(es) to make direct detections of young (proto)planets? What techniques would improve current detection (limit)s? What are the protoplanet accretion signatures? Protoplanet outflows? IR high-brightness contrast for already formed planets? Others?
What would the expected indirect signatures of the formation of 12 Mearth proto-jupiter core in the dust distribution? Can we detect this?
Are circumplanetary disks scaled down versions of circumstellar disks or do we expect that gas/dust ratio to be much higher? If they are colder, presumably the gas density is proportionally higher as well. In such a scenario, what kind of chemistry do we expect? If they have gravitational instabilities, should we expect scaled down versions of FUor-like spectra?
Several groups have tried to detect young protoplanets inside the gaps of disks using Halpha observations, but the detection rate is not very high (in fact, it would be very interesting to see an updated list of observed objects and detected planets).So my questions are: why is the Halpha detection rate not very high? Are the observations not sensitive/good enough? Are episodic accretion or/and circumstellar extinction an issue? Is the Halpha line the best accretion tracer to detect protoplanets?
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How reliable are images reconstructed from NIR interferometers, in particular concerning asymmetries that are interpreted as point-sources = planets. Possible contamination of these reconstructed images that introduce asymmetries like disk warps, uneven disk illumination, or jets.
On radial velocity and transits detections: here the level of activity hinders detection, what is the maximum activity level that would allow us to detect a planet? What can we do with current instrumentation? What spectroscopic/photometric accuracy do we need to make detections for active young stars? Will this accuracy be available soon? Can we currently apply these techniques to wide sub-samples of young stars? Or maybe the less active like weak T Tauris / intermediate mass Herbigs? Does it make sense to identify sub-samples of young stars and prepare a radial velocity survey?
Spectro-astrometry as an alternative approach to detect accreting planets: should we prepare a survey using this technique? (slides from IgMe)
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Thursday, June 28th
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Morning: Session 4. Indirect observational detections.
Discussion leader: Mihkel Kama
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Our general questions: What are the planetary signatures in the disc, what are the problems in these scenarios? Inner holes, spiral arms, dust traps, radial, non-keplerian streams, others...To what extent are these observables the direct result of (only) planet formation? Could it be due to photo-electric instability (Kuchner 2018)? Link:
What can we really infer about the number of planets and their properties from such indirect observations? Do transient events -red and blue absorptions in pre-main sequence stars-constitute a signpost of planetesimals in pre-main sequence stars? What is the effect of inserting a planet in a disc (descreases gas density, e.g. Tatullli 2011).
When we observe structures in discs, to what extent can we distinguish between "flat" gaps/cavities/spirals and "inclined" structures such as warps or tilts? And do we have any useful census of how common these "non-flat" structures may be?
Can a planet that carves out a gap cause the disc to spread out and become more luminous if at first some of the material is optically thick?
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Afternoon: Session 5. Planet formation vs other processes.
Discussion leader: Inga Kamp
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Our general questions: The impact that photoevaporation can have on planet formation, thus the stellar properties on the disc. Is the Mdisk-Macc plane useful to disentangle between the different scenarios driving disk evolution (i.e to identify sub-samples of stars with ongoing planet formation vs e.g. photo-evaporated disks)
Is the Macc-Mdisk correlation recently confirmed from ALMA data (Manara+, Mulders+) for Class II stars the result of *only* “disk physics” -viscous dissipation, planet formation, photoevaporation...? Are not we neglecting the influence of previous evolutionary phases? is the observed Macc-Mdisk distribution observed in Class II stars the imprint of previous Class 0-I phases? if so, can the observed Macc-Mdisk distribution in Class II stars tell us anything at all about planet formation? (slides from IgMe)
How evolved does a disk need to be for photoevaporation to overcome accretion?
Friday, June 29th
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Morning: Session 6. Planet formation and transition discs
Discussion leader: Andres Carmona
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Our general questions: Are transition discs signposts of planets? Are protoplanetary disks with large inner cavities a representative sample? Or perhaps they represent the (small) group of large, massive disks? What causes the phenomenon of a transition disc, how to distinguish between the different scenarios, what are the open problems? Can a planet that carves out a gap cause the disc to spread out and become more luminous if at first some of the material is optically thick?
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What is gas surface density inside the dust cavities of transition disks? what is the evidence for gaps and density jumps in these systems? (slides AnCa)
Do all disks go through the same transitional phase? Are all transition disks "in transition", or is this a more stable phase? Are gaps and holes a signature of imminent dispersal or a feature that last for some time?
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