<p dir="ltr">Advancements in astronomical imaging all-sky surveys are revolutionising the field of time domain astronomy. However, the immense volume of alert data presents a critical bottleneck in maximizing scientific returns from these surveys. Effectively analyzing alert streams to discover transients in unexplored physical parameter spaces is crucial for enhancing scientific output. Building robust infrastructure to identify, prioritize, and execute efficient follow-up strategies on alert streams from all-sky surveys becomes critical. My thesis confronts this challenge through a multidisciplinary approach, by integrating statistical methods, machine learning algorithms, and hydrodynamical simulations to constrain transient explosion properties and motivate effective follow-up initiatives. </p><p dir="ltr">I analyze a sample of 45 Type II supernovae from the Zwicky Transient Facility (ZTF) public survey using a grid of hydrodynamical models in order to assess whether theoretically driven forecasts can intelligently guide follow-up observations supporting all-sky survey alert streams. I estimate several progenitor properties and explosion physics parameters, including zero-age main-sequence (ZAMS) mass, mass-loss rate, kinetic energy, ⁵⁶Ni mass synthesized, host extinction, and the time of the explosion. This work involves simulations of real-time of evolving incomplete light curves of the sample (∆t < 25 days, ∆t < 50 days, all data) leading to the conclusion that certain physical parameters exhibit greater reliability as indicators of true values during early epochs. This study emphasises the vital role of real-time modeling of transients, supported by multi-band synthetic light curves tailored to survey passbands, for identifying interesting transients based on their progenitor and explosion properties and determining critical epochs for follow-up observations.</p><p dir="ltr">In my thesis, I report multi wavelength observations and characterization of the ultraluminous transient AT 2021lwx (ZTF20abrbeie; aka“Barbie”) identified in the ZTF alert stream, that was flagged as an anomaly by the Recommender Engine For Intelligent Transient Track-ing (REFITT). From a spectroscopically measured redshift of 0.9945, and slowly fading g and r light curves spanning over 1000 observer-frame days that peak with an absolute magnitude of Mr = −25.7 mag, AT 2021lwx has an extraordinary peak pseudo-bolometric luminosity of log (Lmax/[erg/s]) = 45.7. The total radiative energy is over 10⁵³ erg, and as of today, the transient continues to decline slowly following a t^−5/3 power-law. Modeling available photometry with MOSFiT suggests that AT 2021lwx is a tidal disruption event (TDE) candidate involving a ≈ 14 or 15 solar mass star accreting onto a supermassive black hole (SMBH) with mass M_BH ≈ 10⁸ solar mass. But, intriguingly, no host galaxy associated with the theorized SMBH is detected yet. The Pan-STARRS non-detections do not definitively exclude the existence of a galaxy hosting AT 2021lwx. Utilizing EzGal, upper limits on the stellar mass for different stellar population models were determined. These upper limits suggest that a 10¹⁰ solar mass host galaxy cannot be ruled out.</p><p dir="ltr">The enhanced sensitivity of upcoming large-scale all-sky surveys enables the early detection of transients, providing unique insights into their progenitor systems, an example being detection of shock cooling emission (SCE) in light curves of stripped-envelope supernovae(SESNe). Leveraging a statistically significant sample of these events with early detections from all-sky surveys presents an invaluable opportunity to constrain their environments, pro-genitors, and explosion properties. In my final study contributing to this thesis, I analyze a sample of 16 SESNe identified from the ZTF survey, characterised by prominent shock cooling emission features in their light curves. By modeling the SCE and the radioactive peak in these transients, I derive estimates of progenitor radius, mass of the extended envelope and explosion properties of SESNe. This analysis yields upper limits and ranges for the compactness and envelope structures of the SESN progenitors that exhibit SCE in their light curves. I conclude my thesis with a summary of the findings and their future applications.</p>

Populärwissenschaftlicher Abstract: <br /> <br /> Bislang gibt es in der beobachtenden optischen Astronomie zwei verschiedene Herangehensweisen: Einerseits werden Objekte durch Kameras abbildend erfaßt, andererseits werden durch die wellenlängenabhängige Zerlegung ihres Lichtes Spektren gewonnen. Das Integral - Field - Verfahren ist eine relativ neue Technik, welche die genannten Beobachtungsmethoden vereint. Das Objektbild im Teleskopfokus wird in räumlich zerlegt und jedes Ortselement einem gemeinsamen Spektrografen zugeführt. Hierdurch wird das Objekt nicht nur zweidimensional räumlich erfaßt, sondern zusätzlich die spektrale Kompenente als dritte Dimension erhalten, weswegen das Verfahren auch als 3D-Methode bezeichnet wird. Anschaulich kann man sich das Datenresultat als eine Abbildung vorstellen, in der jeder einzelne Bildpunkt nicht mehr nur einen Intensitätswert enthält, sondern gleich ein ganzes Spektrum. Diese Technik ermöglicht es, ausgedehnte Objekte im Unterschied zu gängigen Spaltspektrografen komplett zu erfassen. Die besondere Stärke der Methode ist die Möglichkeit, die Hintergrundkontamination der unmittelbaren Umgebung des Objektes zu erfassen und in der Auswertung zu berücksichtigen. Durch diese Fähigkeit erscheint die 3D-Methode prädestiniert für den durch moderne Großteleskope erschlossenen Bereich der extragalaktischen Stellarastronomie. Die detaillierte Untersuchung aufgelöster stellare Populationen in nahegelegenen Galaxien ist erst seit kurzer Zeit dank der Fortschritte mit modernen Grossteleskopen und fortschrittlicher Instrumentierung möglich geworden. Wegen der Bedeutung für die Entstehung und Evolution von Galaxien werden diese Arbeiten zukünftig weiter an Bedeutung gewinnen.<br /> <br /> In der vorliegenden Arbeit wurde die Integral-Field-Spektroskopie an zwei planetarischen Nebeln in der nächstgelegenen großen Spiralgalaxie M31 (NGC 224) getestet, deren Helligkeiten und Koordinaten aus einer Durchmusterung vorlagen. Hierzu wurden Beobachtungen mit dem MPFS-Instrument am russischen 6m - Teleskop in Selentschuk/Kaukasus sowie mit INTEGRAL/WYFFOS am englischen William-Herschel-Teleskop auf La Palma gewonnen. Ein überraschendes Ergebnis war, daß eins der beiden Objekte falsch klassifiziert wurde. Sowohl die meßbare räumliche Ausdehnung des Objektes als auch das spektrale Erscheinungsbild schlossen die Identität mit einem planetarischen Nebel aus. Mit hoher Wahrscheinlichkeit handelt es sich um einen Supernovaüberrest, zumal im Rahmen der Fehler an gleicher Stelle eine vom Röntgensatelliten ROSAT detektierte Röntgenquelle liegt.<br /> <br /> Die in diesem Projekt verwendeten Integral-Field-Instrumente wiesen zwei verschiedene Bauweisen auf, die sich miteinander vergleichen ließen. Ein Hauptkritikpunkt der verwendeten Instrumente war ihre geringe Lichtausbeute. Die gesammelten Erfahrung fanden Eingang in das Konzept des derzeit in Potsdam in der Fertigung befindlichen 3D-Instruments PMAS (Potsdamer Multi - Apertur - Spektrophotometer), welcher zunächst für das 3.5m-Teleskop des Calar - Alto - Observatoriums in Südspanien vorgesehen ist. Um die Effizienz dieses Instrumentes zu verbessern, wurde in dieser Arbeit die Kopplung der zum Bildrasterung verwendeten Optik zu den Lichtleitfasern im Labor untersucht. Die Untersuchungen zur Maximierung von Lichtausbeute und Stabilität zeigen, daß sich die Effizienz durch Auswahl einer geeigneten Koppelmethode um etwa 20 Prozent steigern lässt. / Popular scientific abstract: <br /> <br /> Currently there are two different approaches in the observational optical astronomy: On the first hand objects are imaged with cameras, on the other hand spectra are produced. The integral-field-technique is a relatively new way to combine both branches. The object image in the telecopes focus is sampled spatially and each spatial bin assigned to a spectrograph. Hence the object is not only sampled spatially but the spectral component is achieved as a third dimension, so the name 3D-Method. The result is like an image but each point consists of a whole spectrum. The use of this technique is to sample objects completely in contrast to standard slit spectroscopy. The strength of this method is to deal with high background light contamination. So the 3D method looks convincing for the new branch of extragalactic stellar astronomy with modern large telescopes. The detailled investigation of spatially resolved extragalactic populations in nearby galaxies is a rather new topic demanding newest telescopes and instrumentation. These investigations are very important in future to understand the origin and evolution of galaxies.<br /> <br /> In this thesis the Integral-Field-Spectroscopy was tested for two planetary nebulae in the Andromeda galaxy M31. Observations have been made using the MPFS on the Russian 6m telescope and the INTEGRAL/WYFFOS setup on the 4.2m WHT on La Palma. A surprising result was that one of the two objects was wrongly identified as a planetary nebula. The spatial extension and spectral details excluded this object class. With high probability this object is a supernova remnant.<br /> <br /> The integral-field-instruments used in this thesis had different technical layouts, which were to compare to each other. The main critics is the poor efficiency of both devices. The experience made was utilized to optimize the concept of the recently developed 3D-instrument PMAS (Potsdam Multi- Aperture Spectrophotometer). PMAS will be used for the 3.5 m telescope at the Calar Alto observatory. To improve efficiency, the coupling of the optical fibers used to sample the object and guide the light into the spectrograph was optimized in the laboratory. This investigations showed that an increase of the coupling efficiency by about 20 percent is possible by using immersion coupling between fibers and lenses.

For the main part of this dissertation, I have executed a survey of nearby, main sequence A, F, and G-type stars with the CHARA Array, successfully measuring the angular diameters of forty-four stars to better than 4% accuracy. The results of these observations also yield empirical determinations of stellar linear radii and effective temperatures for the stars observed. In addition, these CHARA-determined temperatures, radii, and luminosities are fit to Yonsei-Yale isochrones to constrain the masses and ages of the stars. These quantities are compared to the results found in Allende Prieto & Lambert (1999), Holmberg et al. (2007), and Takeda (2007), who indirectly determine these same properties by fitting models to observed photometry. I find that for most cases, the models underestimate the radius of the star by ~12%, while in turn they overestimate the effective temperature by ~ 1.5 - 4%, when compared to my directly measured values, with no apparent correlation to the star's metallicity or color index. These overestimated temperatures and underestimated radii in these works appear to cause an additional offset in the star's surface gravity measurements, which consequently yield higher masses and younger ages, in particular for stars with masses greater than ~ 1.3 M_sol. Alternatively, these quantities I measure are also compared to direct measurements from a large sample of eclipsing binary stars in Andersen (1991), and excellent agreement is seen within both data sets. Finally, a multi-parameter solution is found to fit color-temperature-metallicity values of the stars in this sample to provide a new calibration of the effective temperature scale for these types of stars. Published work in the field of stellar interferometry and optical spectroscopy of early-type stars are presented in Appendix D and E, respectively.