| 1. |
- Andersson, Eric P., et al.
(författare)
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Pre-supernova feedback sets the star cluster mass function to a power law and reduces the cluster formation efficiency
- 2024
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Ingår i: Astronomy and Astrophysics. - 0004-6361. ; 681
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Tidskriftsartikel (refereegranskat)abstract
- Context. The star cluster initial mass function is observed to have an inverse power law exponent around 2, yet there is no consensus on what determines this distribution, and why some variation is observed in different galaxies. Furthermore, the cluster formation efficiency (CFE) covers a range of values, particularly when considering different environments. These clusters are often used to empirically constrain star formation and as fundamental units for stellar feedback models. Detailed galaxy models must therefore accurately capture the basic properties of observed clusters to be considered predictive. Aims. We study how feedback mechanisms acting on different timescales and with different energy budgets affect the star cluster mass function and CFE. Methods. We use hydrodynamical simulations of a dwarf galaxy as a laboratory to study star cluster formation. We test different combinations of stellar feedback mechanisms, including stellar winds, ionizing radiation, and supernovae (SNe). Results. Each feedback mechanism affects the CFE and cluster mass function. Increasing the feedback budget by combining the different types of feedback decreases the CFE by reducing the number of massive clusters. Ionizing radiation is found to be especially influential. This effect depends on the timing of feedback initiation, as shown by comparing early and late feedback. Early feedback occurs from ionizing radiation and stellar winds with onset immediately after a massive star is formed. Late feedback occurs when energy injection only starts after the main-sequence lifetime of the most massive SN progenitor, a timing that is further influenced by the choice of the most massive SN progenitor. Late feedback alone results in a broad, flat mass function, approaching a log-normal shape in the complete absence of feedback. Early feedback, on the other hand, produces a power-law cluster mass function with lower CFE, albeit with a steeper slope than that usually observed.
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| 2. |
- Del Sordo, Fabio, 1979-
(författare)
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From irrotational flows to turbulent dynamos
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Many of the celestial bodies we know are found to be magnetized:the Earth, many of the planets so far discovered, the Sun and other stars,the interstellar space, the Milky Way and other galaxies.The reason for that is still to be fully understood, and this work is meant to be a step in that direction.The dynamics of the interstellar medium is dominated by events likesupernovae explosions that can be modelled as irrotational flows.The first part of this thesis is dedicated to the analysis of some characteristics of these flows, in particular how they influencethe typical turbulent magnetic diffusivity of a medium, and it is shownthat the diffusivity is generally enhanced, except for some specific casessuch as steady potential flows, where it can be lowered.Moreover, it is examined how such flows can develop vorticity when they occur in environments affected by rotation or shear,or that are not barotropic.Secondly, we examine helical flows, that are of basic importance for the phenomenon of the amplification of magnetic fields, namely the dynamo.Magnetic helicity can arise from the occurrence of an instability: here we focus on theinstability of purely toroidal magnetic fields, also known as Tayler instability.It is possible to give a topological interpretation of magnetic helicity.Using this point of view, and being aware that magnetic helicity is a conserved quantity in non-resistive flows,it is illustrated how helical systems preserve magnetic structureslonger than non-helical ones.The final part of the thesis deals directly with dynamos.It is shown how to evaluate dynamo transport coefficients with two of the most commonly used techniques, namely theimposed-field and the test-field methods.After that, it is analyzed how dynamos are affected by advectionof magnetic fields and material away from the domain in which theyoperate.It is demonstrated that the presence of an outflow, likestellar or galactic winds in real astrophysical cases,alleviates the so-calledcatastrophic quenching, that is the damping of a dynamoin highly conductive media, thus allowing the dynamo process to work better.
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| 3. |
- Johansen, Anders, et al.
(författare)
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Growth of asteroids, planetary embryos, and Kuiper belt objects by chondrule accretion.
- 2015
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Ingår i: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 1:3
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Tidskriftsartikel (refereegranskat)abstract
- Chondrules are millimeter-sized spherules that dominate primitive meteorites (chondrites) originating from the asteroid belt. The incorporation of chondrules into asteroidal bodies must be an important step in planet formation, but the mechanism is not understood. We show that the main growth of asteroids can result from gas drag-assisted accretion of chondrules. The largest planetesimals of a population with a characteristic radius of 100 km undergo runaway accretion of chondrules within ~3 My, forming planetary embryos up to Mars's size along with smaller asteroids whose size distribution matches that of main belt asteroids. The aerodynamical accretion leads to size sorting of chondrules consistent with chondrites. Accretion of millimeter-sized chondrules and ice particles drives the growth of planetesimals beyond the ice line as well, but the growth time increases above the disc lifetime outside of 25 AU. The contribution of direct planetesimal accretion to the growth of both asteroids and Kuiper belt objects is minor. In contrast, planetesimal accretion and chondrule accretion play more equal roles in the formation of Moon-sized embryos in the terrestrial planet formation region. These embryos are isolated from each other and accrete planetesimals only at a low rate. However, the continued accretion of chondrules destabilizes the oligarchic configuration and leads to the formation of Mars-sized embryos and terrestrial planets by a combination of direct chondrule accretion and giant impacts.
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| 4. |
- McNally, Colin P., et al.
(författare)
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Temperature Fluctuations Driven by Magnetorotational Instability in Protoplanetary Disks
- 2014
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Ingår i: Astrophysical Journal. - 0004-637X. ; 791:1
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Tidskriftsartikel (refereegranskat)abstract
- The magnetorotational instability (MRI) drives magnetized turbulence in sufficiently ionized regions of protoplanetary disks, leading to mass accretion. The dissipation of the potential energy associated with this accretion determines the thermal structure of accreting regions. Until recently, the heating from the turbulence has only been treated in an azimuthally averaged sense, neglecting local fluctuations. However, magnetized turbulence dissipates its energy intermittently in current sheet structures. We study this intermittent energy dissipation using high resolution numerical models including a treatment of radiative thermal diffusion in an optically thick regime. Our models predict that these turbulent current sheets drive order-unity temperature variations even where the MRI is damped strongly by Ohmic resistivity. This implies that the current sheet structures where energy dissipation occurs must be well-resolved to correctly capture the flow structure in numerical models. Higher resolutions are required to resolve energy dissipation than to resolve the magnetic field strength or accretion stresses. The temperature variations are large enough to have major consequences for mineral formation in disks, including melting chondrules, remelting calcium-aluminum-rich inclusions, and annealing silicates; and may drive hysteresis: current sheets in MRI active regions could be significantly more conductive than the remainder of the disk.
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| 5. |
- Yang, Chao Chin, et al.
(författare)
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Diffusion and Concentration of Solids in the Dead Zone of a Protoplanetary Disk
- 2018
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 868:1
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Tidskriftsartikel (refereegranskat)abstract
- The streaming instability is a promising mechanism to drive the formation of planetesimals in protoplanetary disks. To trigger this process, it has been argued that sedimentation of solids onto the mid-plane needs to be efficient, and therefore that a quiescent gaseous environment is required. It is often suggested that dead-zone or disk-wind structure created by non-ideal magnetohydrodynamical (MHD) effects meets this requirement. However, simulations have shown that the mid-plane of a dead zone is not completely quiescent. In order to examine the concentration of solids in such an environment, we use the local-shearing-box approximation to simulate a particlegas system with an Ohmic dead zone including mutual drag force between the gas and the solids. We systematically compare the evolution of the system with ideal or non-ideal MHD, with or without backreaction drag force from particles on gas, and with varying solid abundances. Similar to previous investigations of deadzone dynamics, we find that particles of dimensionless stopping time ts = 0.1 do not sediment appreciably more than those in ideal magnetorotational turbulence, resulting in a vertical scale height an order of magnitude larger than in a laminar disk. Contrary to the expectation that this should curb the formation of planetesimals, we nevertheless find that strong clumping of solids still occurs in the dead zone when solid abundances are similar to the critical value for a laminar environment. This can be explained by the weak radial diffusion of particles near the mid-plane. The results imply that the sedimentation of particles to the mid-plane is not a necessary criterion for the formation of planetesimals by the streaming instability.
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