| 1. |
- Arnadottir, Anna, et al.
(författare)
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Från Jorden mot Universum
- 2016
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Konstnärligt arbete (övrigt vetenskapligt/konstnärligt)abstract
- The night sky, both beautiful and mysterious, has been the subject of campfire stories, ancient myths and awe for as long as there have been people. A desire to comprehend the Universe may well be humanity’s oldest shared intellectual experience. Yet only recently have we truly begun to grasp our place in the vast cosmos. To learn about this journey of celestial discovery, from the theories of the ancient Greek astronomers to today’s grandest telescopes.
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| 2. |
- Jansson, Karl Wahlberg, et al.
(författare)
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The role of pebble fragmentation in planetesimal formation II. Numerical simulations
- 2017
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 835:1
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Tidskriftsartikel (refereegranskat)abstract
- Some scenarios for planetesimal formation go through a phase of collapse of gravitationally bound clouds of millimeter- to centimeter-size pebbles. Such clouds can form, for example, through the streaming instability in protoplanetary disks. We model the collapse process with a statistical model to obtain the internal structure of planetesimals with solid radii between 10 and 1000 km. During the collapse, pebbles collide, and depending on their relative speeds, collisions have different outcomes. A mixture of particle sizes inside a planetesimal leads to better packing capabilities and higher densities. In this paper we apply results from new laboratory experiments of dust aggregate collisions (presented in a companion paper) to model collision outcomes. We find that the internal structure of a planetesimal is strongly dependent on both its mass and the applied fragmentation model. Low-mass planetesimals have no/few fragmenting pebble collisions in the collapse phase and end up as porous pebble piles. The number of fragmenting collisions increases with increasing cloud mass, resulting in wider particle size distributions and higher density. The collapse is nevertheless "cold" in the sense that collision speeds are damped by the high collision frequency. This ensures that a significant fraction of large pebbles survive the collapse in all but the most massive clouds. Our results are in broad agreement with the observed increase in density of Kuiper Belt objects with increasing size, as exemplified by the recent characterization of the highly porous comet 67P/Churyumov-Gerasimenko.
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| 3. |
- Wahlberg Jansson, Karl, et al.
(författare)
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Formation of pebble-pile planetesimals
- 2014
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 570
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Tidskriftsartikel (refereegranskat)abstract
- Asteroids and Kuiper belt objects are remnant planetesimals from the epoch of planet formation. The first stage of planet formation is the accumulation of dust and ice grains into mm-cm-sized pebbles. These pebbles can clump together through the streaming instability and form gravitationally bound pebble `clouds'. Pebbles inside such a cloud will undergo mutual collisions, dissipating energy into heat. As the cloud loses energy, it gradually contracts towards solid density. We model this process and investigate two important properties of the collapse: (i) the timescale of the collapse and (ii) the temporal evolution of the pebble size distribution. Our numerical model of the pebble cloud is zero-dimensional and treats collisions with a statistical method. We find that planetesimals with radii larger than ~100 km collapse on the free-fall timescale of about 25 years. Lower-mass clouds have longer pebble collision timescales and collapse much more slowly, with collapse times of a few hundred years for 10-km-scale planetesimals and a few thousand years for 1-km-scale planetesimals. The mass of the pebble cloud also determines the interior structure of the resulting planetesimal. The pebble collision speeds in low-mass clouds are below the threshold for fragmentation, forming pebble-pile planetesimals consisting of the primordial pebbles from the protoplanetary disk. Planetesimals above 100 km in radius, on the other hand, consist of mixtures of dust (pebble fragments) and pebbles which have undergone substantial collisions with dust and other pebbles. The Rosetta mission to the comet 67P/Churyumov-Gerasimenko and the New Horizons mission to Pluto will both provide valuable information about the structure of planetesimals in the Solar System. Our model predicts that 67P is a pebble-pile planetesimal consisting of primordial pebbles from the Solar Nebula, while the pebbles in the cloud which contracted to form Pluto must have been ground down substantially during the collapse.
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| 4. |
- Wahlberg Jansson, Karl
(författare)
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Formation of Pebble-Pile Planetesimals and the Interior Structure of Comets
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Planets form in protoplanetary discs of gas, dust and ice around newborn stars. In the Solar System, not only planets are found as a result from the time of planet formation, but also remnant planetesimals in the form of asteroids, Kuiper belt objects and comets. Recent observations, e.g. by the space mission Rosetta, have found that comets are porous objects barely able to hold themselves together by gravity.Gravitationally bound clouds of mm- to dm-sized pebbles can form in the protoplanetary disc by interactions between solids and the gas, e.g. through the streaming instability. Such clouds will have collisions between pebbles resulting in energy dissipation and inevitably a collapse into a solid planetesimal.In paper I we develop a statistical model to investigate, with numerical simulations, the collapse of pebble clouds into planetesimals. We find that low-mass planetesimals, e.g. comets, are porous pebble-piles with this formation mechanism. Paper II and III investigate the role of fragmenting collisions in planetesimal formation, both with laboratory experiments and numerical simulations.The above model assumes an isolated, homogeneous planetesimal. In paper IV we find that the interior structure of comets can vary with depth ('onion'-like shells) and that gas in the protoplanetary disc can affect the collapse significantly.
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| 5. |
- Wahlberg Jansson, Karl
(författare)
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Nu når vi äntligen Pluto
- 2014
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Ingår i: Populär Astronomi. - 1650-7177. ; 14:4, s. 14-19
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- I juli 2015 når NASA-rymdsonden New Horizons till solsystemets mest kända dvärgplanet Pluto. Denna artikel diskuterar vad Pluto egentligen är och vad rymdsonden kommer att hitta.
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| 6. |
- Wahlberg Jansson, Karl, et al.
(författare)
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Radially resolved simulations of collapsing pebble clouds in protoplanetary discs
- 2017
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Ingår i: Monthly Notices of the Royal Astronomical Society. - 1365-2966. ; 469
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Tidskriftsartikel (refereegranskat)abstract
- In the Solar System, asteroids and Kuiper belt objects as well as comets are remnant planetesimals from the time of planet formation. Interactions between solids and gas inside a protoplanetary disc can, e.g. through the streaming instability, form gravitationally bound planetesimal-mass clouds of pebbles. Such clouds will inevitably have inelastic collisions between pebbles, lose energy and experience a runaway collapse into planetesimals. We study the collapse process with a statistical model to find the internal structure of comet-sized planetesimals. In this paper we develop a numerical model that keep track of at what depth particles inside the pebble cloud are to get the radial structure of the resulting planetesimal. We find that the interiors of a planetesimal is heavily dependent on initial pebble sizes and depth inside the planetesimal. We also look at what effect disc gas has on the collapse by adding gas drag onto particles. This both speeds up the collapse and cause lower collision speeds which results in primordial pebbles surviving the collapse. The dependence on particle sizes result in planetesimals with an interior of “onion”-like shells. Our results are in agreement with Rosetta observations of 67P/Churyumov–Gerasimenko being a porous, homogeneous pebble-pile.
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