1,252 research outputs found
Tidal stripping as a mechanism for placing globular clusters on wide orbits: the case of MGC1 in M31
The globular clusters of large spiral galaxies can be divided into two
populations: one which formed in-situ and one which comprises clusters tidally
stripped away from other galaxies. In this paper we investigate the
contribution to the outer globular cluster population in the M31 galaxy through
donation of clusters from dwarf galaxies. We test this numerically by comparing
the contribution of globular clusters from simulated encounters to the observed
M31 globular cluster population. To constrain our simulations, we specifically
investigate the outermost globular cluster in the M31 system, MGC1. The remote
location of MGC1 favours the idea of it being captured, however, the cluster is
devoid of features associated with tidal interactions. Hence we separate
simulations where tidal features are present and where they are hidden. We find
that our simulated encounters can place clusters on MGC1-like orbits. In
addition, we find that tidal stripping of clusters from dwarf galaxies leaves
them on orbits having a range of separations, broadly matching those observed
in M31. We find that the specific energies of globular clusters captured by M31
closely matches those of the incoming host dwarf galaxies. Furthermore, in our
simulations we find an equal number of accreted clusters on co-rotating and
counter-rotating orbits within M31 and use this to infer the fraction of
clusters that has been accreted. We find that even close in roughly 50% of the
clusters are accreted, whilst this figure increases to over 80% further out.Comment: 17 pages, 12 figures. Accepted for publication in MNRA
The galactic scale impact of feedback from individual stars
Feedback from stars is essential for the formation and evolution of galaxies. It is an energy source that drives gas motions and chemically enriches the galaxy by supplying metals. Without this stellar feedback, numerical galaxy simulations result in galaxies with little resemblance to those observed in our Universe. Modern galaxy simulations frequently reach a mass resolution of a few tens of solar masses. Such high-resolution warrants models incorporating individual stars. These models enable a detailed treatment of when and where stars inject feedback. In this thesis, I present such a model and provide a series of papers exploring physical mechanisms unlocked by this model.In paper I, we investigate how runaway stars affect the galactic winds driven by stellar feedback in Milky Way-like galaxies. Massive runaway stars can venture to places where these short-lived stars are otherwise not found (e.g., between spiral arms). In these regions of diffuse gas, supernovae can efficiently incorporate energy into large volumes of gas, thereby boosting the gas outflow rate of the galaxy. Furthermore, the star formation rate is not significantly affected since parts of the feedback budget move away from star-forming gas. The result is a ten-fold boost in the mass loading factor.Paper II is a follow-up investigation of a surprising signal of star formation in spiral galaxies with runaway stars, found in Paper I. The signal is produced by the rapid migration of runaway stars to the galaxy's outskirts. Via direct comparison to observational data, we find that this explains faint far-ultraviolet radiation detected outside the optical radius of nearby spiral galaxies. This radiation manifests as a trend in the star formation relation with a slope similar to one produced by runaway stars escaping to these regions.In paper III, the star-by-star model is upgraded with a more advanced model for feedback and runaway stars. We showcase this model with a suit of simulations of isolated dwarf galaxies, testing a range of parameters for the natal velocity model of individual stars responsible for incorporating runaway stars. In stark contrast to the Milky Way-like galaxy, we find runaway stars play little to no role in determining outflows in dwarf galaxies. We discuss several possible reasons for the different effects in small and large galaxies
Probing the phase diagram of cuprates with YBaCuO thin films and nanowires
We have grown and characterized 30 nm thick YBaCuO
(YBCO) films, deposited by pulsed laser deposition on both MgO (110) and
SrTiO (001) substrates, which induce opposite strain to the superconducting
layer. By carefully tuning the in-situ post-annealing oxygen pressure, we
achieved, in a reproducible way, films at different oxygen doping, spanning
from the slightly overdoped down to the strongly underdoped region of the phase
diagram. The transport properties of the films, investigated through resistance
versus temperature measurements, are in perfect qualitative agreement with
single crystals. Starting from these films, we have also successfully
fabricated nanowires with widths down to 65 nm, at different oxygen doping. The
nanostructures exhibit characteristic temperatures (as the critical temperature
and the pseudogap temperature ) similar to those of the
as-grown films and carry critical current densities close to
the critical depairing value, limited by vortex entry. This implies that the
superconducting and the normal state properties of underdoped YBCO are
preserved in our films, and they can be studied as a function of the
dimensionality of the system, down to the nanoscale.Comment: 11 pages, 9 figures, submitted to Phys. Rev. Material
True equivalent chip thickness for tools with a nose radius
A majority of the established systems for choice and optimization of cutting data are based on WoxĂ©nâs equivalent chip thickness, heW. In metal cutting theory and models, the equivalent chip thickness is of vital importance when the depth-of-cut ap is in the same order or smaller than the nose radius r. WoxĂ©n made considerable simplifications in his chip area model, that form the basis for calculations of the equivalent chip thickness. Basic mathematical solutions, e.g. describing the chip area on circular inserts, are lacking. This article describes the geometrical implications when machining with round inserts. The error in WoxĂ©nâs equivalent chip thickness is largest when the depth-of-cut is less than ÂŒ of the nose radius. The calculations of the equivalent chip thickness based on the WoxĂ©n model are up to 50 % wrong, for some combinations of cutting data in the finishing range. The presented results explain the difficulties in getting a good validity in the models used to calculate tool life in finishing machining. The error leads to an underrating of the tool load in many machining situations
Manufacturing costs and Degree of Occupancy Based on the Principle of Characteristic Parts
In making capacity estimates and cost calculations in the manufacturing industry, many products and production systems are often involved, making the data in their totality difficult to grasp. Introducing the concept of the characteristic part, which is a fabricated part seen as representative of all parts produced in terms of demand, setup time, cycle time, average batch size and total number of batches involved, makes the calculations required much more manageable and much less time-consuming. The article takes up how the characteristic part is defined and how it can be used in calculating production capacity, system utilization and manufacturing costs
Runaway stars masquerading as star formation in galactic outskirts
In the outskirts of nearby spiral galaxies, star formation is observed in
extremely low gas surface densities. Star formation in these regions, where the
interstellar medium is dominated by diffuse atomic hydrogen, is difficult to
explain with classic star formation theories. In this work, we introduce
runaway stars as an explanation to this observation. Runaway stars, produced by
collisional dynamics in young stellar clusters, can travel kilo-parsecs during
their main sequence life time. Using galactic-scale hydrodynamic simulations
including a treatment of individual stars, we demonstrate that this mechanism
enables the ejection of young massive stars into environments where the gas is
not dense enough to trigger star formation. This results in the appearance of
star formation in regions where it ought to be impossible. We conclude that
runaway stars are a contributing, if not dominant, factor to the observations
of star formation in the outskirts of spiral galaxies.Comment: Submitted to MNRAS Letters, comments welcom
Plant strategies for ornamental shrubs and their suitability for urban planting
Det utbredda slentriantÀnkandet i vÄr bransch ligger till grund för mÄnga tveksamma, och dyra lösningar.
Den generella mentaliteten verkar vara att, âman gör de man blir tillsagdâ och slutar helt fundera pĂ„ varför.
Att tÀnka sjÀlv, ifrÄgasÀtta, och utveckla sin kunskap inom ett givet omrÄde Àr den enskilt viktigaste förmÄgan
vi har. NÀr det kritiska tÀnkandet försvinner och ersÀtts med jakt pÄ större ekonomiska vinster och ointresse
Àr branschen ute pÄ hal is. DÀrför anser jag att det extra viktigt att sjÀlv försöka ta reda pÄ varför vÀxter
fungerar, utvecklar och etablerar sig sÄ olika beroende pÄ vÀxtplats och miljö. Grimes C-S-R klassificering,
och ekologiska bakgrund, förklarar pÄ ett bra sÀtt sambandet mellan vÀxtens konkurrenssförmÄga och dess
vÀxtplats.
Litterturstudien av Plant strategies, Vegetation process, and Ecosystem properties (2001) ligger till grund för
resultatdelen och diskussionen. De olika strategierna avhandlades och deras anvÀndningspotential i urban
miljö diskuterades. Olika strategier visade sig vara lÀmpliga beroende pÄ platsen och funktionskraven. Den
huvudsakliga lÀrdomen frÄn litteraturstudien var att strategierna skall matchas mot vÀxtplatsen, om en funktionell
och skötselextensiv plantering skall kunna uppnÄs. För att vidare kunna förstÄ, och applicera, den ganska
teoretiska litteraturen, valdes tre exempelplanteringar i urban miljö.
Den första exempelplanteringen Äterfinns pÄ RÄdhusrÀttens bostadsomrÄde inne i Lund. Detta omrÄde Àgs
och förvaltats av LKFŽs bostadsförening. Planteringarna Àr relativt nyligen omgjorda, och ligger pÄ bjÀlklag.
Eftersom jordvolymen Àr begrÀnsad, stÀlls högre krav pÄ vÀxtvalet. HÀr har man anvÀnt Syringa vulgaris, Budleja
davidii, Taxus x intermedia, med Vinca minor och Sedum acre som underplantering. VÀxternas strategi Àr
felmatchad till vÀxtplatsen. Andra arter med mer konkurrensstarka strategier har kunnat etablera sig. Detta
hade kunnat undvikas genom ett vÀxtval dÀr strategierna varit bÀttre anpassade till stÄndorten.
Den andra exempelplanteringen representerar hÄrdgjord stadsmiljö, och Äterfinns utanför PilÀngshallen i
Lomma. Den skiljer sig dÀrmed frÄn de bÄda andra planteringarna genom att vara helt i hÄrdgjord miljö. Den
ligger inte heller pÄ bjÀlklag, utan Àr traditionellt planterad. VÀxtvalet och vÀxternas strategi Àr bÀttre lÀmpat
för platsen. HĂ€r Ă„terfinns Cotoneaster divaricatus, Juiperus communis, troligen sorten âMeyerâ med inslag av
Forsythia x intermedia âSpectabilisâ och Amelanchier spicata, Malus sargentii, Weigela-hybrida, Syringa vulgaris
samt Prunus sargentii. StÄndorten Àr varm och nÄgot torr. HÀr Àr majoriteten av buskarnas strategier i bÀttre
samklang med vÀxtplatsen. En del invandrare skogsvÀxter i form av Ligustrum atrovirens och Corylus avellana
har dock etablerat sig. Vilket tyder pÄ att stÄndorten inte Àr extremt torr.
Den tredje exempelplanteringen Àr likt den första byggd pÄ bjÀlklag, men Àr Àldre och ligger pÄ en norrsida.
Den bestĂ„r till stora delar av Spiraea betulifolia troligen sorten âTorâ, Prunus laurocerasus âOtto Luykenâ och
Potentilla fruticosa. Förutom dessa arter vĂ€xer hĂ€r Cornus sp., Budleja sp., Hedera helix âHibernicaâ
och Hydrange petiolaris ssp. anomala. Planteringen innehÄller Àven perennerna Salvia nemorosa och
Alchemilla mollis. StÄndorten Àr ganska kall och mager. LjusinslÀppet Àr dÄligt och förutsÀttningarna Àr generellt
tuffa. Den tuffa miljön och oproduktiva stÄndorten till trots, hade ett flertal stresstrateger kunnat anvÀndas
istÀllet för det mycket brokiga vÀxtmaterialet som idag finns pÄ platsen. VÀxtvalet i de tre exempelplanteringarna
granskades och sattes i relation till stÄndort, och strategi. DÀrefter diskuterades förutsÀttningarna och
förslag gavs pÄ vÀxter vars strategi passade vÀxtplatsen.
De olika strategierna, innebÀr en evolutionÀr specialisering till en ekologisk nisch, som kan tyckas enkel att,
applicera rakt av i urbana planteringar. Men verkligheten Àr mer komplex. Det finns inga enkla svar. Slutsatsen
blev alltsÄ att mer kunskap behövs om prydnadsbuskars naturstÄndort, och odlingsförsök i urban miljö, men
inriktningen pĂ„ biologiska strategier bör utföras för att gĂ„ till grunden med frĂ„gestĂ€llningen: âVilka biologiska
strategier hos prydnadsbuskar Àr av största betydelse utifrÄn tre exempelplanteringar i urban miljö?
- âŠ