Strong Ultraviolet Pulse From a Newborn Type Ia Supernova

Type Ia supernovae are destructive explosions of carbon oxygen white dwarfs. Although they are used empirically to measure cosmological distances, the nature of their progenitors remains mysterious, One of the leading progenitor models, called the single degenerate channel, hypothesizes that a white dwarf accretes matter from a companion star and the resulting increase in its central pressure and temperature ignites thermonuclear explosion. Here we report observations of strong but declining ultraviolet emission from a Type Ia supernova within four days of its explosion. This emission is consistent with theoretical expectations of collision between material ejected by the supernova and a companion star, and therefore provides evidence that some Type Ia supernovae arise from the single degenerate channel.Comment: Accepted for publication on the 21 May 2015 issue of Natur

Three Hypervelocity White Dwarfs in Gaia DR2: Evidence for Dynamically Driven Double-Degenerate Double-Detonation Type Ia Supernovae

Double detonations in double white dwarf (WD) binaries undergoing unstable mass transfer have emerged in recent years as one of the most promising Type Ia supernova (SN Ia) progenitor scenarios. One potential outcome of this "dynamically driven double-degenerate double-detonation" (D^6) scenario is that the companion WD survives the explosion and is flung away with a velocity equal to its > 1000 km/s pre-SN orbital velocity. We perform a search for these hypervelocity runaway WDs using Gaia's second data release. In this paper, we discuss seven candidates followed up with ground-based instruments. Three sources are likely to be some of the fastest known stars in the Milky Way, with total Galactocentric velocities between 1000 and 3000 km/s, and are consistent with having previously been companion WDs in pre-SN Ia systems. However, although the radial velocity of one of the stars is > 1000 km/s, the radial velocities of the other two stars are puzzlingly consistent with 0. The combined five-parameter astrometric solutions from Gaia and radial velocities from follow-up spectra yield tentative 6D confirmation of the D^6 scenario. The past position of one of these stars places it within a faint, old SN remnant, further strengthening the interpretation of these candidates as hypervelocity runaways from binary systems that underwent SNe Ia.Comment: Accepted for publication in ApJ. Minor corrections for clarity. D6 spectra are available as ancillary data file

Evidence for Type Ia Supernova Diversity from Ultraviolet Observations with the Hubble Space Telescope

We present ultraviolet (UV) spectroscopy and photometry of four Type Ia supernovae (SNe 2004dt, 2004ef, 2005M, and 2005cf) obtained with the UV prism of the Advanced Camera for Surveys on the Hubble Space Telescope, This dataset provides unique spectral time series down to 2000 A. Significant diversity is seen in the near-maximum-light spectra (approx.2000-3500 A) for this small sample. The corresponding photometric data, together with archival data from Swift Ultraviolet/Optical Telescope observations, provide further evidence of increased dispersion in the UV emission with respect to the optical. The peak luminosities measured in the uvw lIF250W filter are found to correlate with the B-band light-curve shape parameter .(Delta)m15(B), but with much larger scatter relative to the correlation in the broad-band B band (e.g., approx. 0.4 mag versus approx. 0.2 mag for those with 0.8 3(sigma), being brighter than normal SNe Ia such as SN 2005cf by approx. 0,9 mag and approx. 2.0 mag in the uvwl1F250W and uvm2/F220W filters, respectively. We show that different progenitor metallicity or line-expansion velocities alone cannot explain such a large discrepancy. Viewing-angle effects, such as due to an asymmetric explosion, may have a significant influence on the flux emitted in the UV region. Detailed modeling is needed to disentangle and quantify the above effect

Evidence for Type Ia Supernova Diversity from Ultraviolet Observations with the Hubble Space Telescope

We present ultraviolet (UV) spectroscopy and photometry of four Type Ia supernovae (SNe 2004dt, 2004ef, 2005M, and 2005cf) obtained with the UV prism of the Advanced Camera for Surveys on the Hubble Space Telescope. This dataset provides unique spectral time series down to 2000 Angstrom. Significant diversity is seen in the near maximum-light spectra (~ 2000--3500 Angstrom) for this small sample. The corresponding photometric data, together with archival data from Swift Ultraviolet/Optical Telescope observations, provide further evidence of increased dispersion in the UV emission with respect to the optical. The peak luminosities measured in uvw1/F250W are found to correlate with the B-band light-curve shape parameter dm15(B), but with much larger scatter relative to the correlation in the broad-band B band (e.g., ~0.4 mag versus ~0.2 mag for those with 0.8 < dm15 < 1.7 mag). SN 2004dt is found as an outlier of this correlation (at > 3 sigma), being brighter than normal SNe Ia such as SN 2005cf by ~0.9 mag and ~2.0 mag in the uvw1/F250W and uvm2/F220W filters, respectively. We show that different progenitor metallicity or line-expansion velocities alone cannot explain such a large discrepancy. Viewing-angle effects, such as due to an asymmetric explosion, may have a significant influence on the flux emitted in the UV region. Detailed modeling is needed to disentangle and quantify the above effects.Comment: 17 pages, 13 figures, accepted by Ap

Absence of fast-moving iron in an intermediate type ia supernova between normal and super-chandrasekhar

The American Astronomical Society. All rights reserved.In this paper, we report observations of a peculiar SN Ia iPTF13asv (a.k.A., SN2013cv) from the onset of the explosion to months after its peak. The early-phase spectra of iPTF13asv show an absence of iron absorption, indicating that synthesized iron elements are confined to low-velocity regions of the ejecta, which, in turn, implies a stratified ejecta structure along the line of sight. Our analysis of iPTF13asv's light curves and spectra shows that it is an intermediate case between normal and super-Chandrasekhar events. On the one hand, its light curve shape (B-band ) and overall spectral features resemble those of normal SNe Ia. On the other hand, its large peak optical and UV luminosity (, ) and its low but almost constant Si ii velocities of about 10,000 km s-1 are similar to those in super-Chandrasekhar events, and its persistent carbon signatures in the spectra are weaker than those seen commonly in super-Chandrasekhar events. We estimate a 56Ni mass of and a total ejecta mass of . The large ejecta mass of iPTF13asv and its stratified ejecta structure together seemingly favor a double-degenerate origin. © 2016

Theoretical and experimental investigation of heterogeneous catalytic reactions: Two case studies

The overall goal of this work is to utilize multi scale modeling techniques as well as various experimental methods to understand the fundamentals of two important catalytic reactions. The reactions studied were the combustion of methane on PdO catalysts and the reduction of NOx compounds in diesel exhaust using a Pt/Ba/Al2O3 catalyst. Both reactions are important from an environmental standpoint due to strict regulations on reducing the emissions of harmful NOx compounds released into the atmosphere. The goal of the study on methane combustion was to understand the fundamental nature of the chemistry that actually occurs on the catalyst surface. To accomplish this, the reaction steps and catalyst surface were modeled through the use of density functional theory (DFT). Density functional theory is a quantum mechanics based formalism used to calculate the wavefunctions and energies of various quantum states that the system can be in. DFT can be used to examine various stable adsorption states on the surface as well as study the fundamentals of the reaction mechanism through the use of transition state calculations and statistical mechanics. DFT was used to examine the plausibility of the commonly proposed Mars-van Krevelen mechanism for methane combustion through the use of various adsorption calculations as well as reaction pathway studies. The most important step in this mechanism is the abstraction of the first hydrogen from the methane molecule. This step is well known as the rate limiting step in the reaction mechanism. The results show that methane will activate on the saturated PdO(100) surface and does not necessarily require an oxygen vacancy or a defect in the surface structure. The activation energy for this step was found to be 28.5 kcal mol-1, which is within the range seen in the experimental literature. Besides the rate determining step, other important steps such as oxygen dissociation were investigated. Also, a large library of adsorption calculations were completed in order to consider all of the intermediate species thought to be involved in the reaction. The results ultimately show that the Mars-van Krevelen mechanism is not sufficient to describe the complex nature of the methane combustion reaction on PdO surfaces. Species such as O 2, H2O, CO2, and CH4 were all shown to behave differently than the mechanism proposes. The results also indicate the there are multiple pathways that the reaction steps can occur through and that the mechanism is most likely too complicated to be described by a simple set of elementary steps. In contrast to the atomistic modeling of the methane combustion work, the NOx reduction in diesel exhaust was studied through the use of macro scale reactor modeling with the purpose of being able to predict the behavior of the catalytic reactor under varying operating conditions. The goal of the work was to be able to develop a simple model that could be solved in real time in order to be able to control the operation of the engine/catalyst system in an actual on road application. The catalyst studied was Pt/Ba/Al 2O3 which is known as a NOx trap. It is operated in a cyclic manner in which NO is oxidized to NO2 and NOx is stored on the catalyst surface under an oxidizing environment. Once the surface is saturated, a short burst of a reducing atmosphere is used to desorb and reduce the NO x to N2 and H2O. One dimensional models were developed to describe each of the steps in the cycle. The storage model included mechanisms for describing diffusion into the bulk of the catalyst as well as Pt/Ba proximity effects. The results show that the simple modeling approach used worked very well for describing the NOx trap reactor under various inlet concentrations as well as varying temperatures. The model is able to make accurate predictions as to when the storage component of the catalyst is saturated. The separate models were also designed for easy coupling so that a model describing the entire cycle can be put together. In addition to the modeling, some experimental scanning tunneling microscopy (STM) studies were performed on the NO oxidation reaction. The goal of this work was to use STM to understand the structural changes of the catalyst surface after being exposed to oxidizing conditions. The catalyst used was a single crystal Pt(100) surface. The studies were performed to support the hypothesis that larger particles are more active toward NO oxidation because the large particles consist mostly of stable low index planes of the catalyst and are resistant to forming a surface oxide. The results confirm that the surface does not go under any major topological changes even in the presence of extreme oxidizing environments which support the hypothesis from other reaction studies

Workshop Session One: Reto Kromer

Click on the first URL link below in the "External Files" section to play the authoritative published version of the streaming video accompanying this transcript.The second URL link provided below in the "External Files" section forwards to a second version of the streaming video file associated with this transcript. This streaming video, hosted on Indiana University's Avalon Media System, represents the efforts of the Black Film Center/Archive to ensure the long-term preservation of the digital file associated with this transcript