Type IA supernovae from very long delayed explosion of core - WD merger

We study the spinning down time scale of rapidly rotating white dwarfs (WDs) in the frame of the core-degenerate (CD) scenario for type Ia supernovae (SNe Ia). In the CD scenario the Chandrasekhar or super-Chandrasekhar mass WD is formed at the termination of the common envelope phase or during the planetary nebula phase, from a merger of a WD companion with the hot core of a massive asymptotic giant branch star. In the CD scenario the rapidly rotating WD is formed shortly after the stellar formation episode, and the delay from stellar formation to explosion is basically determined by the spin-down time of the rapidly rotating merger remnant. We find that gravitational radiation is inefficient in spinning down WDs, while the magneto-dipole radiation torque can lead to delay times that are required to explain SNe Ia.Comment: MNRAS, in pres

3D seismic isolation of the European extremely large telescope using elastomeric bearings

LAUREA MAGISTRALEL'isolamento della base sismica rappresenta un metodo innovativo per la protezione antisismica, che è stato utilizzato con grande successo nel ridurre il pericolo dai movimenti orizzontali del terreno. Questa tesi indaga la capacità di un sistema di isolamento basato su cuscinetti elastomerici in termini di isolamento sia nella direzione orizzontale che in quella verticale. A tale scopo, i cuscinetti elastomerici in questione sono stati modellati con un basso valore del fattore di forma (S<5), che dovrebbe abbassare la rigidità verticale e fornire un sufficiente isolamento dal carico verticale indotto dal terremoto. Il processo di progettazione è conforme agli standard europei, con un'eccezione nella definizione del carico assiale di progettazione, che viene eseguito attraverso un approccio semplificato che consente un rilassamento dei severi requisiti degli standard europei, al costo di una ragionevole riduzione del fattore di sicurezza. L'efficienza dei cuscinetti elastomerici viene testata attraverso la loro implementazione in una struttura con proporzioni e massa considerevoli, in particolare il Telescopio Europeo – Estremamente Grande, destinato a essere il più grande telescopio al mondo. Questo edificio è scelto, a parte le sue dimensioni, per la sua sensibilità alle vibrazioni ad alta frequenza. La convalida del design dei cuscinetti viene eseguita attraverso analisi negli Opensee, utilizzando L'oggetto elemento elastomericx di Kumar [2016], che modella il cuscinetto come un elemento a due nodi, 12 gradi di libertà con sei molle che rappresentano il comportamento meccanico in tutte le direzioni. Le analisi dell'intera struttura vengono eseguite utilizzando i movimenti del terreno registrati dal terremoto di El Centro (1940), che mostrava uno spettro di risposta elastica orizzontale simile a quello di El Cerro Armazones, dove si trova la struttura. I risultati delle analisi producono informazioni utili sul comportamento dei cuscinetti, come la chiara risposta elastica nella direzione verticale. Inoltre, la risposta orizzontale presenta forti somiglianze con il comportamento bi-lineare idealizzato. Tuttavia, i risultati Dell'analisi di OpenSees indicano la difficoltà di prevedere con precisione il comportamento di un sistema di isolamento sismico 3D, che rimarrà un campo aperto per futuri studi e sviluppo.Seismic base isolation represents an innovative method for earthquake protection, that has been utilized with great success in reducing the danger from the horizontal ground motions. This thesis investigates the capability of an elastomeric bearing-based isolation system in terms of isolation in both the horizontal and the vertical direction. To this aim, the elastomeric bearings in question have been modelled with a low value of the shape factor (S<5), which is expected to lower the vertical stiffness and provide sufficient isolation from the vertical earthquake-induced load. The design process is in accordance with the European Standards, with an exception in the defining of the design axial load, which is performed through a simplified approach that allows a relaxation of the strict requirements of the European Standards, at the cost of a reasonable reduction in the safety factor. The efficiency of the elastomeric bearings is tested through their implementation in a structure with considerable proportions and mass, in particular the European – Extremely Large Telescope, set to be the largest telescope in the world. This building is chosen, apart from its size, due to its sensitivity to high frequency vibrations. Validation of the bearings’ design is performed through analyses in the OpenSees, using the ElastomericX element object by Kumar [2016], that models the bearing as a two-node, 12 degree-of-freedom element with six springs that represent the mechanical behavior in all directions. Analyses of the entire structure are performed using the recorded ground motions from the El Centro (1940) earthquake, which exhibited a similar horizontal elastic response spectrum as the one of El Cerro Armazones, where the structure is located. The analyses results produce useful insight concerning the bearings’ behavior, such as the clear elastic response in the vertical direction. Moreover, the horizontal response exhibits strong similarities with the idealized bi-linear behavior. However, the results from the OpenSees analysis indicate the difficulty in accurately prediction the behavior of a 3D seismic isolation system, which will remain a field open for future studies and development

Merger by Migration at the Final Phase of Common Envelope Evolution

I find the common envelope (CE) energy formalism, the CE \alpha-prescription, to be inadequate to predict the final orbital separation of the CE evolution in massive envelopes. I find that when the orbital separation decreases to ~10 times the final orbital separation predicted by the CE \alpha-prescription, the companion has not enough mass in its vicinity to carry away its angular momentum. The core-secondary binary system must get rid of its angular momentum by interacting with mass further out. The binary system interacts gravitationally with a rapidly-rotating flat envelope, in a situation that resembles planet-migration in protoplanetary disks. The envelope convection of the giant carries energy and angular momentum outward. The basic assumption of the CE \alpha-prescription, that the binary system's gravitational energy goes to unbind the envelope, breaks down. Based on that, I claim that merger is a common outcome of the CE evolution of AGB and red super-giants stars with an envelope to secondary mass ratio of (M_env/M_2)>~5. I discuss some other puzzling observations that might be explained by the migration and merger processes.Comment: New Astronomy, in pres

The delay-time distribution of type-Ia supernovae from Sloan II

We derive the delay-time distribution (DTD) of type-Ia supernovae (SNe Ia) using a sample of 132 SNe Ia, discovered by the Sloan Digital Sky Survey II (SDSS2) among 66,000 galaxies with spectral-based star-formation histories (SFHs). To recover the best-fit DTD, the SFH of every individual galaxy is compared, using Poisson statistics, to the number of SNe that it hosted (zero or one), based on the method introduced in Maoz et al. (2011). This SN sample differs from the SDSS2 SN Ia sample analyzed by Brandt et al. (2010), using a related, but different, DTD recovery method. Furthermore, we use a simulation-based SN detection-efficiency function, and we apply a number of important corrections to the galaxy SFHs and SN Ia visibility times. The DTD that we find has 4-sigma detections in all three of its time bins: prompt (t < 420 Myr), intermediate (0.4 2.4 Gyr), indicating a continuous DTD, and it is among the most accurate and precise among recent DTD reconstructions. The best-fit power-law form to the recovered DTD is t^(-1.12+/-0.08), consistent with generic ~t^-1 predictions of SN Ia progenitor models based on the gravitational-wave induced mergers of binary white dwarfs. The time integrated number of SNe Ia per formed stellar mass is N_SN/M = 0.00130 +/- 0.00015 Msun^-1, or about 4% of the stars formed with initial masses in the 3-8 Msun range. This is lower than, but largely consistent with, several recent DTD estimates based on SN rates in galaxy clusters and in local-volume galaxies, and is higher than, but consistent with N_SN/M estimated by comparing volumetric SN Ia rates to cosmic SFH.Comment: MNRAS, in pres

Space-charge dynamics in microdiodes

Space-charge limited emission in vacuum microdiodes can go through transverse modulation under certain conditions. The frequency of this modulated current is in the terahertz band and it can be tuned by changing either the the gap spacing or the gap voltage of the diode. Doing the latter is the preferred mode of operation during the simulations which we did. As the power of such diode radiators would be very small, we coupled arrays of such diodes and showed that such an array, which synchronizes itself, can give powerful enough radiation for practical usage. For the emission process, it was first assumed that electrons would be emitted with zero initial velocity, as such an assumption is readily used in most emission calculations in the room temperature range. To further the research, we assumed non-zero initial velocities for the photoemitted electrons and found that, although, not without effect, the non-zero initial velocity does not significantly reduce the power of the radiator. The spatial structure of the beams was examined as well. Finally, a code for simulation of the space-charge effects in solar cells was tested, and although at present no claims can be made about these effects, the code proved to be a very good tool which replicated results from experiments and theory alike. Further experimental research on the detection of space-charge limited currents and the associated Coulomb oscillations is needed.Losun rafeinda frá díóðu í lofttæmi getur orðið fyrir láréttri mótun undir vissumskilyrðum. Tíðnin á þessum mótaða straumi getur verið á terahertz tíðni sviðinuog hægt er að stilla hana með því að breyta fjarlægð díóðu frá skauti og spennuniþar á milli. Seinni kosturinn er ákjósanlegri og sá möguleiki rannsakaður hér meðtölvulíkani.Aflið frá einni díóðu er frekar lítið og því eru mörgum díóðum raðað upp. Díóð-unar samstilla sig og geta saman gefið frá sér nóg afl til að verahagnýtanlegar. Ílosunar ferli rafeindana var fyrst gert ráð fyrir að þær hefðu engan upphafshraða.Slík nálgun er oft gerð jafnavel við herbergishita. En til aðganga lengra gerðumvið ráð fyrir að upphafshraða fyrir rafeindirnar og fundum að hann minnkar ekkimikið aflið. Dreifing rafeinda geislans var líka skoðuð. Í lokinn var tölvulíkanifyrir sólarrafhlöður notaður til að skoða áhrif hleðslannaá starfrækslu rafhlöðunn-ar. Kóðinn reyndist vera ágæt tæki og gat hermt eftir niðurstöðum úr tilraunum ogsvipuðum líkönum. Frekari rannsóknar er þörf á mettunar strauminn og Coulombsveiflunum í slíkum kerfum.This PhD project was financially supported by the IcelandicResearch Fund grant number 120009021. Additional funding for solar cellsimulations was received from EEA Financial Mechanism 2009-2014 project8SEE/30.06.2014

Discovery of 90 Type Ia supernovae among 700,000 Sloan spectra: the Type-Ia supernova rate versus galaxy mass and star-formation rate at redshift ~0.1

Using a method to discover and classify supernovae (SNe) in galaxy spectra, we find 90 Type Ia SNe (SNe Ia) and 10 Type II SNe among the ~700,000 galaxy spectra in the Sloan Digital Sky Survey Data Release 7 that have VESPA-derived star-formation histories (SFHs). We use the SN Ia sample to measure SN Ia rates per unit stellar mass. We confirm, at the median redshift of the sample, z = 0.1, the inverse dependence on galaxy mass of the SN Ia rate per unit mass, previously reported by Li et al. (2011b) for a local sample. We further confirm, following Kistler et al. (2011), that this relation can be explained by the combination of galaxy "downsizing" and a power-law delay-time distribution (DTD; the distribution of times that elapse between a hypothetical burst of star formation and the subsequent SN Ia explosions) with an index of -1, inherent to the double-degenerate progenitor scenario. We use the method of Maoz et al. (2011) to recover the DTD by comparing the number of SNe Ia hosted by each galaxy in our sample with the VESPA-derived SFH of the stellar population within the spectral aperture. In this galaxy sample, which is dominated by old and massive galaxies, we recover a "delayed" component to the DTD of 4.5 +/- 0.6 (statistical) +0.3 -0.5 (systematic) X 10^-14 SNe Msun^-1 yr^-1 for delays in the range > 2.4 Gyr. The mass-normalized SN Ia rate, averaged over all masses and redshifts in our galaxy sample, is R(Ia,M,z=0.1) = 0.10 +/- 0.01 (statistical) +/- 0.01 (systematic) SNuM, and the volumetric rate is R(Ia,V,z=0.1) = 0.247 +0.029 -0.026 (statistical) +0.016 -0.031 (systematic) X 10^-4 SNe yr^-1 Mpc^-3. This rate is consistent with the rates and rate evolution from other recent SN Ia surveys, which together also indicate a ~t^-1 DTD.Comment: MNRAS accepted. 20 pages, 12 figures, 5 tables. Revised following referee report. A full version of figure 8 can be found at http://www.astro.tau.ac.il/~orgraur/Graur_SDSS_SNe_full.pd

On Intimacy

I offer a general philosophical account of intimacy in four chapters. I argue that all intimacy is a matter of taking what I call ‘the intimate stance’ towards another person, object, or thing. To take the intimate stance is to treat the other (person, object, or thing) as though they are engaged in the cooperative activity of joint authorship over the narrative one uses to make sense of oneself. In other words, to take the intimate stance is to treat the other as co-author in answering the question ‘who am I?’ When one takes this stance towards another on a particular occasion that individual is engaged in an intimate interaction with the other, and when one takes this stance towards a particular relationship, then that individual is engaged in an intimate relationship. I also argue that if intimacy is a matter of taking the intimate stance then we have good reason to that in certain circumstances it may be good to be intimate, in others it may be a very risky thing, and in others yet, something quite dangerous.Doctor of Philosoph