Near-infraread observation of supernovae with the Nordic Optical Telescope (NOT)

Supernova (SN) is an explosion of a star at the end of its lifetime. SNe are classified to two types, namely type I and II through the optical spectra. They have been categorised based on their explosion mechanism, to core collapse supernovae (CCSNe) and thermonuclear supernovae. The CCSNe group which includes types IIP, IIn, IIL, IIb, Ib, and Ic are produced when a massive star with initial mass more than 8 M⊙ explodes due to a collapse of its iron core. On the other hand, thermonuclear SNe originate from white dwarfs (WDs) made of carbon and oxygen, in a binary system. Infrared astronomy covers observations of astronomical objects in infrared radiation. The infrared sky is not completely dark and it is variable. Observations of SNe in the infrared give different information than optical observations. Data reduction is required to correct raw data from for example unusable pixels and sky background. In this project, the NOTCam package in the IRAF was used for the data reduction. For measuring magnitudes of SNe, the aperture photometry method with the Gaia program was used. In this Master’s thesis, near-infrared (NIR) observations of three supernovae of type IIn (namely LSQ13zm, SN 2009ip and SN2011jb), one type IIb (SN2012ey), in addition to one type Ic (SN2012ej) and type IIP (SN 2013gd) are studied with emphasis on luminosity and colour evolution. All observations were done with the Nordic Optical Telescope (NOT). Here, we used the classification by Mattila & Meikle (2001) [76], where the SNe are differentiated by the infrared light curves into two groups, namely ’ordinary’ and ’slowly declining’. The light curves and colour evolution of these supernovae were obtained in J, H and Ks bands. In this study, our data, combined with other observations, provide evidence to categorize LSQ13zm, SN 2012ej and SN 2012ey as being part of the ordinary type. We found interesting NIR behaviour of SN 2011jb, which lead it to be classified as a slowly declining type.

Investigating the cosmic-ray ionization rate in the galactic interstellar medium through observations of H3+

Observations of H3+ in the Galactic diffuse interstellar medium (ISM) have led to various surprising results, including the conclusion that the cosmic-ray ionization rate (zeta_2) is about 1 order of magnitude larger than previously thought. The present survey expands the sample of diffuse cloud sight lines with H3+ observations to 50, with detections in 21 of those. Ionization rates inferred from these detections are in the range (1.7+-1.0)x10^-16 s^-1 < zeta_2 < (10.6+-6.8)x10^-16 s^-1 with a mean value of zeta_2=(3.3+-0.4)x10^-16 s^-1. Upper limits (3sigma) derived from non-detections of H3+ are as low as zeta_2 < 0.4x10^-16 s^-1. These low upper-limits, in combination with the wide range of inferred cosmic-ray ionization rates, indicate variations in zeta_2 between different diffuse cloud sight lines. Calculations of the cosmic-ray ionization rate from theoretical cosmic-ray spectra require a large flux of low-energy (MeV) particles to reproduce values inferred from observations. Given the relatively short range of low-energy cosmic rays --- those most efficient at ionization --- the proximity of a cloud to a site of particle acceleration may set its ionization rate. Variations in zeta_2 are thus likely due to variations in the cosmic-ray spectrum at low energies resulting from the effects of particle propagation. To test this theory, H3+ was observed in sight lines passing through diffuse molecular clouds known to be interacting with the supernova remnant IC 443, a probable site of particle acceleration. Where H3+ is detected, ionization rates of zeta_2=(20+-10)x10^-16 s^-1 are inferred, higher than for any other diffuse cloud. These results support both the concept that supernova remnants act as particle accelerators, and the hypothesis that propagation effects are responsible for causing spatial variations in the cosmic-ray spectrum and ionization rate. Future observations of H3+ near other supernova remnants and in sight lines where complementary ionization tracers (OH+, H2O+, H3O+) have been observed will further our understanding of the subject.

Beyond the Cosmological Principle

Simplifying the Einstein field equation by assuming the cosmological principle yields a set of differential equations which governs the dynamics of the universe as described in the cosmological standard model. The cosmological principle assumes the space appears the same everywhere and in every direction and moreover, the principle has earned its position as a fundamental assumption in cosmology by being compatible with the observations of the 20th century. It was not until the current century when observations in cosmological scales showed significant deviation from isotropy and homogeneity implying the violation of the principle. Among these observations are the inconsistency between local and non-local Hubble parameter evaluations, baryon acoustic features of the Lyman-α forest and the anomalies of the cosmic microwave background radiation. As a consequence, cosmological models beyond the cosmological principle have been studied vastly; after all, the principle is a hypothesis and as such should frequently be tested as any other assumption in physics. In this thesis, the effects of inhomogeneity and anisotropy, arising as a consequence of discarding the cosmological principle, is investigated. The geometry and matter content of the universe becomes more cumbersome and the resulting effects on the Einstein field equation is introduced. The cosmological standard model and its issues, both fundamental and observational are presented. Particular interest is given to the local Hubble parameter, supernova explosion, baryon acoustic oscillation, and cosmic microwave background observations and the cosmological constant problems. Explored and proposed resolutions emerging by violating the cosmological principle are reviewed. This thesis is concluded by a summary and outlook of the included research papers.

Cosmic Explosions: Observations Of Infant Hydrogen-Free Supernovae Towards An Understanding Of Their Parent Systems

Radiation in the first days of supernova explosions contains rich information about physical properties of the exploding stars. In the past three years, I used the intermediate Palomar Transient Factory to conduct one-day cadence surveys, in order to systematically search for infant supernovae. I show that the one-day cadences in these surveys were strictly controlled, that the realtime image subtraction pipeline managed to deliver transient candidates within ten minutes of images being taken, and that we were able to undertake follow-up observations with a variety of telescopes within hours of transients being discovered. So far iPTF has discovered over a hundred supernovae within a few days of explosions, forty-nine of which were spectroscopically classified within twenty-four hours of discovery.

Our observations of infant Type Ia supernovae provide evidence for both the single-degenerate and double-degenerate progenitor channels. On the one hand, a low-velocity Type Ia supernova iPTF14atg revealed a strong ultraviolet pulse within four days of its explosion. I show that the pulse is consistent with the expected emission produced by collision between the supernova ejecta and a companion star, providing direct evidence for the single degenerate channel. By comparing the distinct early-phase light curves of iPTF14atg to an otherwise similar event iPTF14dpk, I show that the viewing angle dependence of the supernova-companion collision signature is probably responsible to the difference of the early light curves. I also show evidence for a dark period between the supernova explosion and the first light of the radioactively-powered light curve. On the other hand, a peculiar Type Ia supernova iPTF13asv revealed strong near-UV emission and absence of iron in the spectra within the first two weeks of explosion, suggesting a stratified ejecta structure with iron group elements confined to the slow-moving part of the ejecta. With its total ejecta mass estimated to exceed the Chandrasekhar limit, I show that the stratification and large mass of the ejecta favor the double-degenerate channel.

In a separate approach, iPTF found the first progenitor system of a Type Ib supernova iPTF13bvn in the pre-explosion HST archival mages. Independently, I used the early-phase optical observations of this supernova to constrain its progenitor radius to be no larger than several solar radii. I also used its early radio detections to derive a mass loss rate of 3e-5 solar mass per year for the progenitor right before the supernova explosion. These constraints on the physical properties of the iPTF13bvn progenitor provide a comprehensive data set to test Type Ib supernova theories. A recent HST revisit to the iPTF13bvn site two years after the supernova explosion has confirmed the progenitor system.

Moving forward, the next frontier in this area is to extend these single-object analyses to a large sample of infant supernovae. The upcoming Zwicky Transient Facility with its fast survey speed, which is expected to find one infant supernova every night, is well positioned to carry out this task.

The locations of recent supernovae near the Sun from modelling 60Fe transport

The signature of 60Fe in deep-sea crusts indicates that one or more supernovae exploded in the solar neighbourhood about 2.2 million years ago1–4. Recent isotopic analysis is consistent with a core-collapse or electron-capture supernova that occurred 60 to 130 parsecs from the Sun5. Moreover, peculiarities in the cosmic ray spectrum point to a nearby supernova about two million years ago6. The Local Bubble of hot, diffuse plasma, in which the Solar System is embedded, originated from 14 to 20 supernovae within a moving group, whose surviving members are now in the Scorpius– Centaurus stellar association7,8. Here we report calculations of the most probable trajectories and masses of the supernova progenitors, and hence their explosion times and sites. The 60Fe signal arises from two supernovae at distances between 90 and 100 parsecs. The closest occurred 2.3 million years ago at present-day galactic coordinates l = 327°, b = 11°, and the second-closest exploded about 1.5 million years ago at l = 343°, b = 25°, with masses of 9.2 and 8.8 times the solar mass, respectively. The remaining supernovae, which formed the Local Bubble, contribute to a smaller extent because they happened at larger distances and longer ago (60Fe has a half- life of 2.6 million years9,10). There are uncertainties relating to the nucleosynthesis yields and the loss of 60Fe during transport, but they do not influence the relative distribution of 60Fe in the crust layers, and therefore our model reproduces the measured relative abundances very well.

Principali reazioni termo-nucleari nelle stelle

Le reazioni termo-nucleari sono un particolare tipo di reazioni nucleari che avvengono nelle stelle e che ne assicurano il rifornimento energetico. Esse sono la principale fonte di energia nelle stelle, in quanto la fusione di elementi leggeri (in particolare fino al ferro) risulta essere esotermica. La fusione di elementi leggeri produce elementi più pesanti, a partire dalla fusione dell'idrogeno, che produce elio, fino alla fusione del silicio, che produce nichel. Solamente le stelle con massa maggiore di 8 masse solari, tuttavia, hanno massa necessaria a raggiungere la combustione del silicio. Le stelle meno massive sono destinate ad arrestarsi alla fusione di elementi più leggeri e a diventare delle nane bianche. Oltre alla fusione di elementi leggeri, esistono altre reazioni negli interni stellari in grado di fornire energia e formare nuovi elementi. Esse sopraggingono in larga parte nelle fasi finali della vita di una stella, quando le temperature all'interno dei nuclei stellari sono particolarmente elevate. Ne sono un esempio le catture alfa, le catture neutroniche e la fotodisintegrazione. L'insieme delle reazioni termo-nucleari che avvengono nel corso della vita di una stella permettono la creazioni di nuovi elementi, che vengono poi riemessi nel mezzo interstellare tramite esplosioni di supernova e fungono da materiale fondante per la nascita di nuove stelle. Nella seguente tesi verranno affrontate le principali reazioni termo-nucleari che avvengono negli interni stellari, dalla fusione di elementi leggeri fino alle fasi finali della vita di una stella.

Emissione di sincrotrone e applicazioni astrofisiche

Nel 1932 l'ingegnere e fisico Karl Jansky progettò un’antenna in grado di rilevare onde radio alla frequenza di 20.5 MHz, con la quale notò un'emissione diffusa che proveniva da ogni zona del cielo e si intensificava verso la costellazione del Sagittario. Oggi sappiamo che quella osservata da Jansky è radiazione di sincrotrone. Il meccanismo di emissione di sincrotrone affonda le sue radici nelle leggi dell'elettromagnetismo: quando una particella carica attraversa una regione di spazio in cui è presente un campo magnetico, viene accelerata dalla forza di Lorentz e comincia ad irraggiare in virtù dell'accelerazione subita, come previsto dalla formula di Larmor. A seconda che il moto avvenga a velocità non relativistiche, relativistiche o ultrarelativistiche, l’emissione è chiamata rispettivamente radiazione di ciclotrone, ciclotrone relativistico e sincrotrone. L’emissione diffusa osservata da Jansky, allora, può essere interpretata come radiazione di sincrotrone prodotta dall’interazione delle particelle ultrarelativistiche dei raggi cosmici con il campo magnetico che permea la Via Lattea, mentre l’emissione più intensa nel Sagittario è oggi identificata con la radiosorgente Sagittarius A*, localizzata in corrispondenza del buco nero supermassiccio al centro della Galassia. L’emissione di sincrotrone rappresenta uno dei processi di emissione più rilevanti in Astrofisica ed è in grado di spiegare l’origine di gran parte della radiazione osservata nella banda radio, tanto di quella diffusa quanto di quella generata da radiosorgenti individuali, come radiogalassie e resti di supernova. Le proprietà e la peculiare distribuzione spettrale della radiazione di sincrotrone consentono di ricavare una serie di informazioni sulla sorgente da cui è stata emessa. Per via dello stretto legame con il campo magnetico, inoltre, la radiazione di questo tipo è uno strumento d’indagine fondamentale per la ricostruzione del campo magnetico galattico ed extragalattico.

Materia degenere: fisica e applicazioni astrofisiche

La Degenerazione è un determinato stato della materia causato da particolari condizioni di temperatura e densità. E' un fenomeno che è necessario valutare quando la fisica che descrive un sistema non può fare a meno di considerare la trattazione quantistica per caratterizzare le sue proprietà. Proprio per questo motivo è necessario abbandonare l'approccio deterministico della Meccanica Classica e abbracciare quello probabilistico della Meccanica Quantistica, che vede le particelle dividersi in due categorie: Fermioni e Bosoni. Per entrambe le specie, la materia, mediante specifiche condizioni, può dunque ritrovarsi ad essere in uno stato degenere, presentando diversi fenomeni a seconda della tipologia di particelle che compongono un gas in analisi. Tale fisica della materia degenere, in particolare dei Fermioni degeneri, ha importanti applicazioni nel campo dell'astrofisica: il regime che domina il comportamento del gas interno ad una struttura stellare determina completamente lo sviluppo della sua evoluzione, per via dei differenti contributi di pressione che ogni stato apporta al sostenimento di questi corpi celesti. La degenerazione della materia riveste un ruolo fondamentale anche negli stadi evolutivi finali delle stelle: é il caso delle Nane Bianche e delle Stelle di Neutroni, nelle quali la sola pressione di degenerazione fermionica, entro certi limiti, contrasta la pressione gravitazionale che guida la loro contrazione, mantenendo così l'Equilibrio Idrostatico di queste strutture stellari.