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  • 1.
    Ejdetjärn, Timmy
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    Exploring the nature of ISM turbulencein disc galaxies2024Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [sv]

    Galaxbildning är en kontinuerlig process som började bara några hundra miljoner år efterBig Bang. De första galaxerna var mycket volatila, med utbrott av stjärnbildning ochoorganiserade gasrörelser. Men även efter att dessa galaxer utvecklade ordnade roterandegasskivor, förblev gasen inom skivan, kallat det interstellära mediet (ISM), fortfarandehögt turbulent. Faktum är att ISM är supersoniskt turbulent, vilket innebär att de oorgan-iserade gasrörelserna överstiger ljudets hastighet i mediet. Denna supersoniska turbulenshar kopplats till flera avgörande egenskaper relaterade till galaxutveckling; till exempel,öka (och i vissa regioner minska) ISM:ets gas densitet, stjärnbildning och gasblandning.

    Många observationer har visat att alla gasfaser i ISM upplever supersoniska nivåer avturbulens, med linjebredder (en observationsmetod för att kvantifiera mängden turbulens)så höga som σg ≲ 100 km s−1 i hög-rödförskjutnings (dvs. yngre) skivgalaxer, medanlokala lugna skivor har σg ≲ 40 km s−1. Emellertid innehåller ISM olika gasfaser somtäcker ett brett spektrum av temperaturer och densiteter, vilka uppvisar olika nivåer avturbulens. Till exempel representerar den varma joniserade gasfasen de övre gränsernasom nämns ovan, medan kallare, tätare gas endast når σg ≲ 40 km s−1 och σg ≲ 15 km s−1i hög-rödförskjutnings och lokala galaxer, respektive.

    De fysikaliska processer som driver denna turbulens är inte fullt förstådda, men enkombination av stellär feedback (t.ex. supernova) och gravitationsinstabilitet (t.ex. undermolnkollaps) har föreslagits ge en majoritet av den turbulenta energin. I synnerhet ärstellär feedback avgörande för bildandet av varm joniserad gas och kan därför ha ettbetydande bidrag till turbulensen inom joniserad gas. Dessutom komplicerar heterogenadata från mycket olika galaxer (i termer av t.ex. massa och storlek) vid olika upplösningar(vilket orsakar konstgjord linjebreddning) förståelsen av den underliggande orsaken.

    En vanligt använd spårare av joniserad gas är Hα-emissionslinjen och har använts om-fattande i undersökningar vid hög rödförskjutning. Emellertid kommer bidraget från Hα-signalen från två primära källor: de strålningsjoniserade regionerna runt massiva nyföddastjärnor inbäddade i molekylär gas (kallade H II -regioner) och diffus joniserad gas (DIG) som fyller hela den galaktiska skivan. Observationer har funnit att dessa två källor bidrar,i genomsnitt, ungefär lika mycket till Hα-signalen (dock med en stor spridning), mennivåerna av turbulens är markant olika; med DIG ungefär 2-3 gånger mer turbulent ängasen i H II-regioner.

    Numeriska simuleringar har kommit långt och kan nu simulera hela skivgalaxer medparsec-skala upplösning (i områden av intresse). Dessutom har galaxsimuleringar kunnatåterskapa den nivå av turbulens som observerats i lokala och hög-rödförskjutningsgalaxer. Men numeriska och observationsbaserade arbeten har olika tillgängliga data och reduk-tion/analys varierar mellan författare, och därför krävs noggrannhet för att göra kvalita-tiva jämförelser.

    I detta arbete utför jag numeriska simuleringar för att undersöka ISM-turbulens i olikagasfaser. Mina simuleringar modellerar jag en Vintergatan-liknande galax vid två olikarödförskutningar (användande gasfraktion som en proxy för rödförskutning) och med/utanfysik för stellär feedback, för att utvärdera dess påverkan. Jag utforskar förhållandetmellan stjärnbildningshastigheten och turbulensen, och undersöker vad som driver dettaförhållande. Dessutom analyserar jag Hα-emissionslinjen och jämför bidraget i intensitetoch linjebreddning (turbulens) från H II-regioner och DIG.

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  • 2.
    Lagae, Cis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Tracing the first generation of stars with 3D model atmospheres2023Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
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  • 3.
    Georgiev, Ivelin
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Oskar Klein Centre.
    Understanding the large-scale structure of the the21-cm signal originating from the Epoch of Reionisation2022Licentiatavhandling, monografi (Övrigt vetenskapligt)
    Abstract [en]

    The first billion years from the beginning of the Universe is the focus of multiple astronomical facilities in the upcoming decade. This unique era is marked by the formation of the first stars and galaxies, which release ionising radiation into the intergalactic medium(IGM). As a result, these sources initiate a period during which the cold and dense IGM, primarily consisting of neutral hydrogen (HI ), is heated and ionised. We refer to this era as the Epoch of Reionisation (EoR). How the EoR transpired hence depends on the properties of these ionising sources, and this forms a vital piece to the puzzle of understanding the early Universe. This licentiate thesis aims to educate the reader on the power spectrum (PS) statistic of the 21-cm signal from HI during the EoR. The PS is a prospective observable by radio interferometers, such as the Low-Frequency Array (LOFAR) and the future Square Kilometre Array (SKA). It is an essential stepping stone in comprehending the dominant physical processes affecting the IGM at a given length scale during the EoR.

    In Paper I, we analyse the decomposition of the 21-cm PS from Lidz et al. (2007) (eq. 2)and study the evolution of its terms. We conduct our investigation for a set of C 2 -Ray and 21cmFAST simulations with volumes of (714 cMpc)3, concentrating on large-scales (k < 0.3 Mpc−1 ) as the signal-to-noise ratio of observing these scales will be high. We find that the 21-cm PS tracks the PS of neutral hydrogen fluctuations, which itself past a certain scale tracks the matter PS after a global ionisation fraction of x̄HII∼ 0.1. Hence, the 21-cm PS possesses a two-regime form for which the large-scale PS is a biased version of the cosmological density field and the small-scale PS depends on the astrophysics of the EoR. We construct a bias parameter to explore whether the 21-cm PS can be used as a probe of cosmology on large k-scales. We discover a transition feature for both simulations, following the ktrans ≈ 2/λMFP empirical formula. The transition scale between the scale-independent and scale-dependent bias regimes is directly related to the value of the mean free path of ionising photons (λMFP ).

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  • 4.
    Viswanath, Gayathri
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    High-Contrast Investigation of the ε Ind system2021Licentiatavhandling, monografi (Övrigt vetenskapligt)
    Abstract [en]

    This licentiate thesis provides a broad introduction into the methodology of detecting and characterising exoplanets, with the main focus on the method of high-contrast imaging (HCI). Developments in theoretical knowledge as well as instrumentation have, in the past decade, pushed the boundaries of what HCI can achieve, both in terms of detection sensitivity and constraining planet properties. Direct imaging surveys in the near infrared (NIR) and longward wavelengths have proven particularly useful in detecting younger giant planets at wide orbital separations.

    The scientific work presented as part of this thesis is one such result of an imaging pursuit of the young giant planet, ϵ Ind Ab, which has long eluded NIR imaging surveys in the past, yet revealing its existence via radial velocity trends and astrometry of the parent star. It resides in the very interesting ϵ Ind stellar system, revolving around the primary star ϵ Ind A which is a Sun-like star only ∼12 light years away and visible in the night sky to the naked eye. With the combination of imaging data from two mid infrared (MIR) instruments, advanced post-processing techniques as well as comparative analysis using different planet atmospheric models, this work was able to place tight constraints on the age of the system and mass of the planet, although no detection was achieved. The new constraints set a firm foundation for MIR imaging surveys for the planet in future, especially with the upcoming more sensitive, advanced instruments in the later half of the decade.

    MIR imaging surveys have gained increasing significance in the recent years, due to their ability to detect colder/ smaller planets. It plays an important role in covering the missing gaps in the planet parameter space, ultimately aiding in improving our knowledge on planet formation and evolution.

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    Licentiate Thesis - Gayathri Viswanath
  • 5.
    Cavallius, Maria
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Gas in Debris Disks2020Licentiatavhandling, monografi (Övrigt vetenskapligt)
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  • 6.
    Pietrow, Alexander
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Investigating magnetic fields in the solar chromosphere2020Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Solar plage has been the topic of many studies since its initial description in the mid 19th century, but as of yet it has not been understood to the point where we can reproduce all aspects of these active regions in quasi-realistic numerical models. To a large extent, this is caused by an incomplete understanding of the magnetic structure that drives the activity in these areas. Detailed measurements have been done of the magnetic field configuration of plage in the photosphere since the late 20th century, but only a handful of papers have managed to make any measurements at all in the higher situated chromosphere, despite the fact that the magnetic field vector of plage is important in understanding chromospheric magnetic fields in general, as well as the heating processes of the higher atmosphere. In Pietrow et al. (2020) we add to these measurements by introducing what is to our knowledge the first full Stokes inversion of chromospheric plage, which allowed us to estimate the magnetic field vector at an optical depth of logτ = -3.5. The obtained value is |B| = 440 ± 90 G in the plage with an inclination of 10° ± 16° with respect to the local vertical. Our reported magnetic field strength matches with a recent study by Morosin et al. (2020), but is higher by a factor of two or more compared to previous studies that measured the field using other methods. Additionally we measure an average magnetic field strength of |B| = 300 ± 50 G in a fibrillar region close to the plage.

    In this thesis we explore the difficulties of measuring this magnetic field vector. Since plage exists in a complex environment, we will begin with a general description of the structure and properties of the solar atmosphere and the layers from which it is composed, as well as review the types of active regions that can be found in the solar atmosphere. Our focus then narrows to the chromosphere, the diagnostic properties of spectral lines that are sensitive to this layer (mainly the \cair line), plage regions, and plage chromospheric magnetic fields. Additionally, we touch upon the theory of radiative transfer and how physical characteristics of the atmosphere can be inferred from polarised light. We also give attention to the observing process with the Swedish 1-m Solar Telescope (SST) and the workings of the reduction pipeline and post-reduction methods as well as the process spectropolarimetric inversions.

    Finally, once we have understood why and how this project has been done, we summarize our findings and compare them to current literature.

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  • 7.
    Nativi, Lorenzo
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Jet-wind interaction in neutron star mergers2020Licentiatavhandling, monografi (Övrigt vetenskapligt)
    Abstract [en]

    Besides being sources of gravitational waves, there has been evidence that neutron starmergers release neutron-rich material suitable for the production of heavy r-process nuclei.The radioactive decay of these freshly synthesised elements powers a rapidly evolvingthermal transient, the “macronova” (also known as “kilonova”). Its spectral propertiesstrongly depend on the ejecta composition, since neutron rich material synthesises heavyr-process elements that can efficiently trap radiation inside the ejecta producing a longlasting signal peaking in the red part of the spectrum. The first detection of a binaryneutron star merger was also accompanied by the evidence of a relativistic jet. Despitebeing ascertained the presence of these two dynamical components, neutron-rich ejectaand ultra-relativistic jet, the observational consequences of the interplay between the twois still unclear. In the paper we investigate such interaction through dedicated specialrelativistic hydrodynamic simulations, starting from a realistic environment obtained byprevious works. Light curves are then constructed up to the time scale of days by postprocessing the hydrodynamic results adopting proper radiative transfer. I show thatjet propagation within such environment can significantly affect the observation of theradioactive transient. A relativistic outflow can in fact “punch-away” a fraction of highopacity material before the brightening of the macronova, resulting in the transient beingbrighter and bluer for on-axis observers in the first few days. In this way the jet impactsboth time scale and luminosity of the macronova peak, that are the two main observablesallowing the estimate of the ejecta properties.

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  • 8.
    Della Bruna, Lorenza
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    The emerging phase of young star clusters, constraints for UV photons leakage in local galaxies2020Licentiatavhandling, monografi (Övrigt vetenskapligt)
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  • 9.
    Calissendorff, Per
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Characterising Young Stellar Binaries2018Licentiatavhandling, monografi (Övrigt vetenskapligt)
    Abstract [sv]

    Stjärnor är utspridda över hela himlen inom vår galax, och förekommer i flera variationer av åldrar och storlekar. Att uppskatta dessa egenskaper visar sig emellertid inte alltid vara trivialt. Det finns dock omständigheter som tillåter oss att undersöka karaktärsdragen hos binära stjärnor. Lyckligtvis så tillhör de flesta stjärnor binär- eller multipelsystem, och genom deras ljusstyrka kan vi studera deras storlekar, medan deras dynamiska interaktioner tillåter oss härleda massor. Även om absoluta åldrar är näst intill omöjliga att bestämma så har vi flera metoder för att sätta begränsningar på åldern hos stjärnor vid olika evolutionssteg. Teoretiska modeller kan ge oss en uppfattning om en del av attributen hos stjärnor, men kräver ibland osäkra antaganden. När fundamentala fysiska lagar kan användas för att bestämma individuella massor hos stjärnsystem kan vi använda dessa för att kalibrera de teoretiska modellerna. Därför är mångfalden hos stjärnor av stor betydelse, där även fördelningarna av separation och massor hos stjärnsystemen kan hjälpa oss förstå och reglera bildningsscenarion inom de olika massgränserna. I den här avhandlingen samlar vi information om hur binära- och multipel-stjärnsystem kan upptäckas, men visar även hur en kan gå tillväga för att bestämma deras mest fundamentala egenskaper såsom massa och ålder. Vi jämför också de olika metoder och tekniker som används i litteraturen, påpekar de olika problem som uppstår, samt ger exempel på användning och de framtidsutsikter som förväntas med uppkommande förbättringar. I den sista delen av avhandlingen presenterar vi resultat från en nyligen publicerad artikel angående omloppsbanorna i ett trippel-stjärnsystem som ger en dynamisk massa för systemet som motsäger den teoretiska massan uppskattad från rådande teoretiska modeller för lågmassiva stjärnor.

  • 10.
    da Silva Santos, João Manuel
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Constraints on the gas temperature in the solaratmosphere from multiwavelength inversions2018Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    In this Licentiate thesis I review the properties of the solar atmosphere and the diagnostic value ofdifferent spectral lines in the visible and ultraviolet (UV) along with the millimeter (mm) continua in theelectromagnetic spectrum of the Sun.While the solar atmosphere has been routinely observed in high-resolution from ground-based opti-cal telescopes such as the Swedish Solar Telescope (SST), and more recently in the UV from space tele-scopes such as the Interface Region Imaging Spectrograph (IRIS), radio observations lag behind despitetheir great usefulness. This is likely to change thanks to the Atacama Large Millimeter Array (ALMA)that only started observing the Sun in 2016 with a few limitations, but the first results are promising.ALMA observations probe the solar chromosphere at different heights by tuning into slightly differentfrequencies at potentially milliarcsecond scales if the full array is able to operate with the longest base-lines. This new spectral window onto the Sun is expected to advance various fields of research suchas wave propagation and oscillations in the chromosphere, thermal structure of filaments/prominences,triggering of flares and microflares, and more generally chromospheric and coronal heating, because themm-intensities can be modelled by simply assuming local-thermodynamic equilibrium.In da Silva Santos et al. (2018) we find that coordinated observations from SST, IRIS and ALMA willpermit us to estimate with greater accuracy the full thermodynamical state of the plasma as a functionof optical depth based on experiments with a snapshot of a three-dimensional magnetohydrodynamicsimulation of the Sun’s atmosphere. Particularly, the mm-continuum improves the accuracy of inferredtemperatures in the chromosphere. Here we expand on the Why and How this can be done. The goal isto better constrain the temperature stratification in the solar atmosphere in order to understand chromo-spheric heating.

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  • 11.
    Karamehmetoglu, Emir
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    Looking for the high-mass progenitors of stripped-envelope supernovae2018Licentiatavhandling, monografi (Övrigt vetenskapligt)
    Abstract [en]

    Stripped-envelope supernovae were thought to be the explosions of very massive stars (& 20 M) that lost their outer layers of hydrogen and/or helium in strong stellar winds. However, recent studies have highlighted that most stripped-envelope supernovae seem to be arising from rela- tively lower-mass progenitor stars in the 12 20 M(sun) range, creating a mystery about the fate of the higher-mass stars. In this licentiate thesis, we review our knowledge of stripped-envelope supernovae, and present the astrophysical problem of their missing high-mass progenitors. The thesis focuses on observations of unique and rare stripped-envelope supernovae classified with modern optical surveys such as the intermediate Palomar Transient Factory (iPTF) and the Public European Southern Observatory Spectroscopic Survey of Transient Objects (PESSTO). In these surveys we have discovered stripped-envelope supernovae with long-lasting broad lightcurves, which are thought to be a marker for highly massive (& 20 M[sun]) progenitor stars. Despite this exciting association, there are only a handful of existing examples of stripped- envelope supernovae with broad lightcurves published in the literature, not numerous enough to account for the missing high-mass stars.

    During our efforts, the first object we focused on was OGLE-2014-SN-131, a long-lasting supernova in the southern sky initially classified by PESSTO. We re-classified it as a supernova Type Ibn interacting with a helium-rich circumstellar environment. Unlike all other Type Ibn’s in the literature, OGLE-2014-SN-131 was found to have a long rise-time and large lightcurve broadness. By modeling its bolometric lightcurve, we concluded that OGLE-2014-SN-131 must have had an unusually massive progenitor star. Furthermore, since an ordinary radioactive- decay model could not reproduce the lightcurve, we investigated both a magnetar and circum- stellar interaction as potential powering scenarios and favored the latter due to the signatures of interaction present in the spectra. Next, we looked for similar objects in the supernova dataset of the iPTF, which contains over 200 stripped-envelope supernovae. Searching in a sub-sample of 100 well-observed supernovae, we identified 11 to have unusually broad lightcurves. We also constrained the distribution of lightcurve broadness for iPTF stripped-envelope supernovae. The 11 with broad lightcurves will be studied carefully in a forthcoming paper. The first part of this forthcoming paper, which describes the careful statistical identification of these super-novae, is included in this thesis. In it we identify that 10% of the iPTF stripped-envelope supernova sample have broad lightcurves, which a surprisingly high fraction given their rarity in the published literature. Finally, we evaluate whether our estimate of the fraction of broad stripped-envelope supernovae could help explain the missing high-mass progenitors, and con- clude that they can only be a small fraction of the missing high-mass progenitors.

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    Karamehmetoglu_E_Licentiate_SESNe
  • 12.
    Menacho, Veronica
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    The violent ISM in Haro 112018Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    This thesis introduces briefly physical processes operating in the ISM around massive starsand focuses on the impact strong stellar feedback has in creating large-scale structures in a galaxy.Stellar feedback is ubiquitous in star forming galaxies and its effect on the ISM depends strongly on the energy output from the most massive stars (M$\geq$7 \Mo ) and the properties of the surrounding gas. Starburst galaxies are among the most active in producing %galaxies are among the most active galaxies and produce a large amount of massive star clusters %or even super star clusters (SSC; M$_{cl} \geq 10^5$ \Mo ), with stellar populations up to thousands of massive stars. %The most massive stars (M$\geq$30 \Mo ) are lives short, but they In the first 4 Myr of the star clusters evolution, radiative feedback of the most massive stars (M$\geq$30 \Mo ) are at work. Large amount of ionizing photons are released to the ambient medium while radiative pressure compress the surrounding gas. At the same time their stellar winds inject continuously mechanical energy and momentum in their surrounding. This mechanical feedback is then at later ages, until $\sim$ 40 Myrs, maintained by supernova explosions from the less massive stars.

    Strong stellar feedback tends to develop large-scale structures such as bubbles, loops, filaments and outflows. These are transient structures and can be seen as imprints of how the released energy is clearing or has cleared paths in the ISM. Strong stellar feedback can have devastating consequences in dwarf galaxies due to their shallow gravitational potential. It can accelerate outflows with velocities larger than their escape velocities. In this way, dwarf galaxies can lose a large fraction of their gas mass, which will be crucial in their subsequent evolution.On the other hand, galactic winds might be responsible to create holes in the ISM, allowing the easily-absorbed ionizing photons (Lyman continuum photons, LyC) to escape the galaxy. Studies on a few LyC leaking galaxies have shown that this mechanisms might have preference from a density-bound scenario, which takes place in galaxies with a highly ionized halo.

    In my paper I used deep MUSE observations to analyse the impact strong stellar feedback has in the starburst and Lyman continuum emitting galaxy: Haro 11. The paper presents three emission line diagnostics aiming to analyse the condition of the warm ionized gas in this galaxy, which are the \Ha\ emission, the level of ionization in gas and the presence of fast shocks. These diagnostics are presented in maps of 50 \kms\ bins in a velocity range from -400 to 350 \kms . Haro 11 shows a violent ISM whose warm ionized gas is almost completely shaped by effect of stellar feedback from the most massive star forming regions in the centre. Arcs, shells, outflows paths and galactic scale ionizing cones are imprinted in ISM of Haro 11. Our analysis suggests the presence of a kpc-scale superbubble which might have created galactic holes in the ISM. Beside of that, Haro 11 shows a highly ionized halo. Both mechanisms appear to facilitate the escape of LyC in this galaxy. %Finally, we observe emission at velocities up to thousand \kms\ which could hints to gas escaping the galaxy. Finally the paper presents estimates of the gas mass fraction that could escape the gravitational potential of the galaxy.

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  • 13.
    Giri, Sambit Kumar
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Tomographic study of the 21-cm signal2018Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    The next generation radio telescopes such as the Square Kilometre Array (SKA) will be able to produce images of the redshifted Hi (neutral hydrogen) 21-cm signal from the Epoch of reionization (EoR). In this thesis, we use the simulated tomographic images of the signal and investigate methods to extract and interpret information from those images. We implement the image segmentation technique to identify ionized regions in the images and study the statistical information of the EoR in terms of the probability distribution function (PDF), power spectra, bubble size distribution (BSD) etc. We investigate various BSD techniques such as the mean-free-path (MFP), spherical-average (SPA) and friends-of-friends (FOF) to determine the sizes of the ionized regions. We find that the newly implemented method performs better compared to the previously used segmentation methods.

    We find that the recovered sizes of the ionized regions are affected by the resolution of the images. As we degrade the resolution to SKA1-Low, the smaller regions are no longer visible and the sharp structures at the edges of the larger regions disappear. Therefore, the observed BSDs will be a modified form of the intrinsic ones. We investigate different line of sight effects on the estimated BSDs. As the ionized regions evolve along the frequency direction in the tomographic images due to the light-cone (LC) effect, the BSDs calculated from the tomographic images are shifted to larger sizes compared to the BSDs from the comoving simulation volume. We find that the error from the comoving boxes in estimating the BSDs is minor if the frequency width of the dataset is less than 10 MHz. On the other hand, another line of sight effect namely the redshift-space distortions (RSDs), induced by velocity gradients along the line of sight, has less than 10 per cent effect at simulation resolution and negligible effect at SKA1-Low resolution on the BSDs.

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    Omslagsframsida
  • 14.
    Nyholm, Anders
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    Bumpy light curves of interacting supernovae2017Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    A supernova (SN) is the explosive destruction of a star. Via a luminous outpouring of radiation, the SN can rival the brightness of its SN host galaxy for months or years. In the past decade, astronomical surveys regularly observing the sky to deep limiting magnitudes have revealed that core collapse SNe (the demises of massive stars) are sometimes preceded by eruptive episodes by the progenitor stars during the years before the eventual SN explosion. Such SNe tend to show strong signatures of interaction between the SN ejecta and the circumstellar medium (CSM) deposited by the star before the SN explosion, likely by mass-loss episodes like the ones we have started to observe regularly. The complex CSM resolved around certain giant stars in our own galaxy and the eruptions of giant stars like η Car in the 19th century can be seen in this context. As the SN ejecta of an interacting SN sweep up the CSM of the progenitor, radiation from this process offers observers opportunity to scan the late mass loss history of the progenitor. In this thesis, interacting SNe and eruptive mass loss of their progenitors is discussed. The SN iPTF13z (discovered by the intermediate Palomar Transient Factory, iPTF) is presented. This transient was followed with optical photometry and spectroscopy during 1000 days and displayed a light curve with several conspicuous re-brigthenings ("bumps"), likely arising from SN ejecta interacting with denser regions in the CSM. Around 200 days before discovery, in archival data we found a clear precursor outburst lasting >~ 50 days. A well-observed (but not necessarily well understood) event like SN 2009ip, which showed both precursor outbursts and a light curve bump, makes an interesting comparison object. The embedding of the (possible) SN in a CSM makes it hard to tell if a destructive SN explosion actually happened. In this respect, iPTF13z is compared to e.g. SN 2009ip but also to long-lived interacting SNe like SN 1988Z. Some suggestions for future investigations are offered, to tie light curve bumps to precursor events and to clarify the question of core collapse in the ambiguous cases of some interacting SNe.

    Ladda ner fulltext (pdf)
    fulltext
  • 15.
    Asensio Torres, Rubén
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    High-contrast imaging of faint substellar companions and debris disks2017Licentiatavhandling, monografi (Övrigt vetenskapligt)
    Abstract [en]

    Star formation results in the accumulation of circumstellar material orbiting around their host stars, such as debris disks and planetary-mass objects. A relatively modern technique that has shown to be very valu- able for studying these faint companions is high-contrast imaging, which allows to distinguish the light emitted or scattered off these objects, otherwise hidden behind the brighter stellar halo. In this Licentiate thesis we review the high-contrast imaging technique and its capabilities, giving examples of the latest achievements reached by this method. The star and planet formation scenario within young and gas-rich protoplanetary disks is also briefly discussed. We show that direct imaging observations constrain the protoplanetary disk evolution and the different planet formation processes. We also discuss the debris disk formation scenario and how direct imaging observations can help to understand their morphology and composition. The presence of planetary-mass companions within the disk can be revealed directly via high-contrast imaging or otherwise inferred from their interactions with the debris disk. Finally, we present a recent result on the polarimetry and the flux distribution in the debris disk around the HD 32297 star, which appears to be dominated by micron-sized dust particles and might have indications of a double ring structure (Paper I). 

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    fulltext
  • 16.
    Kundu, Esha
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Modeling of Radio Emission from Supernovae: Application to Type Ia2017Licentiatavhandling, monografi (Övrigt vetenskapligt)
    Abstract [en]

    The interaction of supernova (SN) ejecta with the circumstellar medium (CSM) drives a strong shock wave into the CSM. These shocks are ideal places where effective particle acceleration and magnetic field amplification can take place. The accelerated relativistic particles, in the presence of magnetic field, could emit a part of their energy via synchrotron radiation in radio wavelengths. The flux of this radiation, when compared with observations, gives an estimate of the CSM density. This could either be the particle density ($\rm n_{ISM}$) in case of the SN exploding in a constant density medium, characteristic of interstellar medium, or pre-SN mass loss rate ($\mdot$) of the progenitor system for a wind medium. In this work we have modeled the synchrotron luminosities and compared that with the radio upper limits measured for the Type Ia SNe 2011fe and 2014J. Assuming equipartition of energy between electric and magnetic fields, with 10$\%$ of the thermal shock energy in each field, we found a very low density medium, having $\rm n_{ISM} < \sim $ 0.35 $\ccc$,around both the SNe. In terms of $\mdot$ this implies an upper limit of $10^{-9}$ \msunyr for a wind velocity of 100 \kms. From the measurements of H I column density it could be expected that $\rm n_{ISM} \sim $ 1 $\ccc$ around both the SNe. If this is the true value close to the SNe, this would indicate that the energy density in magnetic field is less than that presumed for energy equipartition. The progenitors of both SNe 2011fe and 2014J are not clear. However previous studies have pointed toward a few potential  channels. Here, we have compared the CSM densities estimated by our models with that predicted by those different plausible formation channels and have tried to constrain the amplification of magnetic fields in SN shocks.  

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    fulltext
  • 17.
    Messa, Matteo
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Young Stellar Clumps and Clusters in the Local Universe2017Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    The thesis introduces the process of star formation at the scale of galaxies, where a turbulent--dominated interstellar medium (ISM) can locally collapse under self-gravity. This collapse results in a star formation process which is hierarchical both in space and time. At the high-density peaks of this hierarchical ISM is possible to form star clusters, collections of stars which can stay gravitationally bound for several Myr. Young star clusters (YSCs) are a common product of star formation in the local universe and, in fact, all star forming galaxies have been shown to host a YSC population. They have masses ranging from few hundreds \msun\ to ~107 Msun, and radii of few parsecs (up to ~10 pc). It has been proposed that the most massive of these systems could survive for several Gyr, forming populations of globular clusters.

    The majority of star formation happens in a clustered fashion and studying the properties of YSC is therefore of fundamental importance to understand the star formation process across galaxies. YSCs are much more luminous than single stars and can therefore be studied to larger distances. They are usually host a stellar population with uniform properties, in particular age and metallicity, which make them a useful probe of the host galaxy properties.In addition, due to the hierarchy of star formation, YSCs are themselves clustered into cluster complexes, or clumps. Such huge clumps (R ~1-3 kpc, M ~108-109 Msun) are major blocks of the structure of high-z galaxies and their study can shed light on star formation and galaxy evolution over cosmological times.

    In my papers, I used the new HST UV observations obtained from the Legacy Extragalactic UV Survey (LEGUS), combined with archival HST BVI observations to study the YSC population of the nearby interacting galaxy M51. In the first paper I built a catalog of the YSC population and studied its properties in the entire galaxy. A truncated mass function was found for the population, meaning that there is a physical mechanism that inhibits the formation of clusters above a specific mass. The disruption of YSC was found to be very moderate, in line with what observed in similar galaxies. The measured fraction of star formation happening in bound clusters was ~20%. These results were compared to studies of YSC populations in local galaxies and to models.In the second paper, instead, the YSC properties were studied in function of the environment inside M51. The shape of the mass function and its truncation were found uniform at different galactocentric radii, while the age function showed more variability. The biggest differences in YSC properties were recovered when the spiral arm and inter-arm environments are compared, suggesting a difference which can already set in when the giant molecular clouds, progenitor of YSCs, start forming.

  • 18.
    Libbrecht, Tine
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Exploring the diagnostic value of He I D3 in the solar chromosphere2016Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
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    fulltext
  • 19.
    Puschnig, Johannes
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    How ISM properties drive Lyman Continuum Escape2016Licentiatavhandling, monografi (Övrigt vetenskapligt)
    Abstract [en]

    The thesis introduces physical processes that are at work in astrophysical plasmas and reviews the current state of research related to the emission of ionizing photons, i.e. Lyman continuum (LyC). Star forming galaxies and active galactic nuclei are discussed as sources of LyC. Observations of LyC leakage at all redshifts are summarized and escape fractions are brought into a cosmological context, i.e. its implications for the reionization of the Universe, one of the major gas phase changes that was completed already after ∼1Gyr after the Big Bang at redshift z∼6.The main work focuses on observations of the local LyC leaking galaxy Tololo 1247-232. Physical properties of the interstellar medium, its porosity and neutral medium column density, could be derived using newly obtained Hubble Space Telescope (HST) data. The work is based on spectroscopy obtained with the Cosmic Origins Spectrograph (COS), as well as optical and ultraviolet multi-band imaging with the Wide Field Camera 3 (WFC3). An improved COS data reduction procedure was adopted. The recent detection of ionizing radiation emerging from Tololo 1247-232 could be confirmed. A LyC escape fraction of 6.6% was derived, in agreement with previous results. We used FUV absorption lines of Si II and Si IV as a probe of the neutral and ionized interstellar medium and find that most of the ISM gas is ionized, likely facilitating LyC escape from density bounded regions. Neutral gas covering as a function of line-of-sight velocity is derived using the apparent optical depth method. The ISM is found to be sufficiently clumpy, supporting the direct escape of LyC photons. We further report on broadband UV and optical continuum imaging as well as narrowband imaging of Lyα, Hα and Hβ. We also performed VLA 21cm imaging. The hydrogen hyperfine transition was not detected, but a deep upper limit atomic gas mass of 10^9 Mo could be derived. The upper limit gas fraction is only 20 percent. Evidence is found that the H I gas halo is relatively small compared to other Lyman Alpha emitters.

    Ladda ner fulltext (pdf)
    ISM_Lyman_Continuum_Tololo_1247-232_Johannes_Puschnig_2016
  • 20.
    Lee, Kai Yan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Heating the early Universe2015Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Ladda ner fulltext (pdf)
    fulltext
  • 21.
    Rivero Losada, Illa
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).
    Effects of rotation and stratification on magnetic flux concentrations2014Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    The formation of magnetic flux concentrations in the Sun is still a matter of debate. One observable manifestations of such concentrations is sunspots. A mechanism able to spontaneously form magnetic flux concentrations in strongly stratified hydromagnetic turbulence and in the presence of a weak magnetic field is the negative effective magnetic pressure instability (NEMPI). This instability is caused by the local suppression of the turbulence by the magnetic field. Due to the complexity of the system, and in order to understand the fundamental physics behind the instability, the study started by considering simplified conditions. In this thesis we aim to move towards the complexity of the Sun. Here we want to know whether the instability can develop under rotation and in the case of a polytropic stratification instead of the simpler isothermal stratification. We perform different kinds of simulations, namely direct numerical simulations (DNS)and mean field simulations (MFS) of strongly stratified turbulence in the presence of weak magnetic fields. We then study separately the effects of rotation and the change in stratification.

    It is found that slow rotation can suppress the instability. For Coriolis numbers larger than $0.1$ the MFS no longer result in growth, whereas the DNS start first with adecreaseof the growth rate of the instability % with the speed-up of rotation is alleviated and then, for $\Co > 0.06$, an increase owing to the fact that rotation leads to  the onset of the dynamo instability, which couples with NEMPI in a combined system. In fact, the suppression implies a constraint on the depth where the instability can operate in the Sun. Since rotation is very weak in the uppermost layers of the Sun, the formation of the flux concentration through this instability might be a shallow phenomenon. The same constraint is found when we study the effects of polytropic stratificationon NEMPI. In this case, the instability also develops, but it is much more concentrated in the upper parts of the simulation domain than in the isothermal case. In contrast to the isothermal case, where the density scale height is constant inthe computational domain, polytropic layers decrease their stratification deeper down, so it becomes harder for NEMPI to operate.

    With these studies we confirm that NEMPI can form magnetic flux concentrations even in the presence of weak rotation and for polytropic stratification. When applied to the Sun, the effects of rotation and the change of stratification constrain the depth where NEMPI can develop to the uppermost layers, where the rotational influence is weak and the stratification is strong enough.

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    fulltext
  • 22.
    Petrushevska, Tanja
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum.
    Search for lensed supernovae behind the galaxy cluster Abell 16892014Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Distant supernovae probe the history of cosmic expansion and star formation, and provide important information on their progenitor models. Investigation of supernovae type Ia for  are needed to reveal any evolution in their intrinsic properties, and thus their utility as existing distance indicators.  One approach to extend the redshift range beyond the reach of our telescopes is to use galaxy clusters as gravitational telescopes. Conversely, lensed supernovae can be used as probes for testing cluster lens model predictions. In this licentiate thesis, I present a 5-year ground based near-infrared search for gravitationally magnified supernovae behind the galaxy cluster A1689. Abell 1689, at z=0.187, is a one of the best-studied clusters with an extended Einstein radius, making it particularly suited as a gravitational telescope. Our search resulted in the discovery of one highly magnified supernova type II at z=1.703 and two transient objects (probably type IIP and Ia, in host galaxies at redshifts  and ), which is  in agreement with the survey expectations.

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    spikblad
  • 23.
    Cataldi, Gianni
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Debris disks from an astronomical and an astrobiological viewpoint2013Licentiatavhandling, monografi (Övrigt vetenskapligt)
    Abstract [en]

    In this licentiate thesis, I consider debris disks from an observational, astronomical viewpoint, but also discuss a potential astrobiological application. Debris disks are essentially disks of dust and rocks around main-sequence stars, analogue to the Kuiper- or the asteroid belt in our solar system. Their observation and theoretical modeling can help to constrain planet formation models and help in the understanding of the history of the solar system. After a general introduction into the field of debris disks and some basic debris disk physics, the thesis concentrates on the observation of gas in debris disks. The possible origins of this gas and its dynamics are discussed and it is considered what it can tell us about the physical conditions in the disk and possibly about the dust composition. In this way, the paper associated with this thesis (dealing with the gas in the β Pic debris disk) is set into context. More in detail, we observed the CII emission originating from the carbon-rich β Pic disk with Herschel HIFI and attempted to constrain the spatial distribution of the gas from the shape of the emission line. This is necessary since the gas production mechanism is currently unknown, but can be constraint by obtaining information about the spatial profile of the gas. The last part of the thesis describes our preliminary studies of the possibility of a debris disk containing biomarkers, created by a giant impact on a life-bearing exoplanet.

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    Licentiate_GianniCataldi
  • 24.
    Taddia, Francesco
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Observations of unusual Type II supernovae2012Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
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    Taddia_licentiate.pdf
  • 25.
    Warnecke, Jörn
    Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Flux emergence: flares and coronal mass ejections driven by dynamo action underneath the solar surface2011Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Helically shaped magnetic field structuresknown as coronal mass ejections (CMEs) are closely related to so-called eruptive flares. On the one hand, these events are broadly believed to be due tothe buoyant rise of magnetic flux tubes from the bottom of the convection zone to the photosphere where they form structures such as sunspots. On the other hand, models of eruptive flares and CMEs have no connection to the convection zone and the magnetic field generated bydynamo action. It is well known that a dynamo can produce helical structures and twisted magnetic fields as observed in the Sun. In this work we ask, how a dynamo-generated magnetic field appears above the surface without buoyancy force and how this field evolves inthe outer atmosphere of the Sun.

    We apply a new approach of a two layer model, where the lower one represents the convection zone and the upper one the solar corona. The two layers are included in one single simulation domain. In the lower layer, we use a helical forcing function added to the momentum equation to create a turbulent dynamo. Due to dynamo action, a large-scale field is formed. As a first step we use a Cartesian cube. We solve the equations of the so-called force-free model in the upper layer to create nearly force-free coronal magnetic fields. In a second step we use a spherical wedge, which extends radially from 0.7 to 2 solar radii. We include density stratification due to gravity in anisothermal domain. The wedge includes both hemispheres of the Sun and we apply a helicalforcing with different signs in each hemisphere.

    As a result, a large-scale field is generated by a turbulent dynamo acting underneath the surface. Due to the latitudinal variation of the helicity produced by the helical forcing, the dynamo is oscillating in the spherical wedge. Twisted magnetic fields emerge above the surface and form arch-like structures with strong current sheets. Plasmoids and CME-like structures are ejected recurrently into the outerlayers. In the spherical simulations we find that the magnetic helicity changes sign in the exterior, which is in agreement with recent analysis of the solar wind data.

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    Thesis
  • 26.
    Del Sordo, Fabio
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Generation of magnetic fields on galactic scale2011Licentiatavhandling, monografi (Övrigt vetenskapligt)
    Abstract [en]

    In these pages we will go through the topic of astrophysical magnetic fields, focusing on galactic fields, their observation and the theories that have been developed for a proper understanding of the these physical phenomena.We review the main work in the study of galactic magnetic fields, often seeing how it is important to deal with problems of general validity in order to be able to point out the right elements needed for a correct interpretation of specific situations. We also aim to summarize some of the conflicts that arise using different theoretical approaches to be proficient in future choices of our research guidelines.This thesis consists in an introductory text and three papers dealing with some specific topics that are introduced in the first three chapters.In the first chapter we will talk about the state of the art of the observations of galactic fields. We review current techniques and observations.In the second chapter we describe the current theories that best describe the generation of magnetic fields. We also mention here two of the three works presented in this licentiate thesis. We will then deal with the possibility to have a proper measure of the $\alpha$ effect in numerical simulationsof dynamo action.Then we consider a particular aspect of magnetic helicity, that is, its connection with the topology of the magnetic field in a given system.In the third chapter we focus on theories related to galactic fields and their validity.We also present our work on the generation of vorticity in the interstellar medium as well as a study ofturbulent diffusivity in a system presenting spherical expansion waves.

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    licentiate_thesis_fabiodelsordo
  • 27.
    Karlsson, Roland
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    18-cm VLA observations of OH towards the Galactic Centre2003Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
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    fulltext
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