* Astronomy

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RE: Type Ia supernovae

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NASA's Hubble Finds Supernovae in 'Wrong Place at Wrong Time'

What happens when you find something in the wrong place at the wrong time? That's a question astronomers have been trying to answer after finding several exploding stars outside the cozy confines of galaxies, where most stars reside. These wayward supernovae also have puzzled astronomers because they exploded billions of years before their predicted detonations. Astronomers using archived observations from several telescopes, including the Hubble Space Telescope, have developed a theory for where these doomed stars come from and how they arrived at their current homes.
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Title: Strong Ultraviolet Pulse From a Newborn Type Ia Supernova
Author: Yi Cao, S. R. Kulkarni, D. Andrew Howell, Avishay Gal-Yam, Mansi M. Kasliwal, Stefano Valenti, J. Johansson, R. Amanullah, A. Goobar, J. Sollerman, F. Taddia, Assaf Horesh, Ilan Sagiv, S. Bradley Cenko, Peter E. Nugent, Iair Arcavi, Jason Surace, P. R. Woniak, Daniela I. Moody, Umaa D. Rebbapragada, Brian D. Bue, Neil Gehrels

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.

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Astronomers Upgrade Their Cosmic Light Bulbs

The brilliant explosions of dead stars have been used for years to illuminate the far-flung reaches of our cosmos. The explosions, called Type Ia supernovae, allow astronomers to measure the distances to galaxies and measure the ever-increasing rate at which our universe is stretching apart.
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UK physicists getting closer to reading the inside of supernovae

UK nuclear physicists are one step closer to being able to read the inside of stars and discover new elements that exist for only a trillionth of a trillionth of a second inside exploding supernovae.
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Type Ia supernovae stem from the explosion of white dwarfs coupled with twin stars

Study discards possibility that type Ia supernovae might stem from explosions of white dwarfs nourished by normal stars. Were these conclusions to become generalized, type Ia supernovae might no longer serve as "standard candles" to measure astronomical distances
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Title: Host Galaxies of Type Ia Supernovae from the Nearby Supernova Factory
Authors: M. J. Childress, G. Aldering, P. Antilogus, C. Aragon, S. Bailey, C. Baltay, S. Bongard, C. Buton, A. Canto, F. Cellier-Holzem, N. Chotard, Y. Copin, H. K. Fakhouri, E. Gangler, J. Guy, E. Y. Hsiao, M. Kerschhaggl, A. G. Kim, M. Kowalski, S. Loken, P. Nugent, K. Paech, R. Pain, E. Pecontal, R. Pereira, S. Perlmutter, D. Rabinowitz, M. Rigault, K. Runge, R. Scalzo, G. Smadja, C. Tao, R. C. Thomas, B. A. Weaver, C. Wu

We present photometric and spectroscopic observations of galaxies hosting Type Ia supernovae (SNe Ia) observed by the Nearby Supernova Factory (SNfactory). Combining GALEX UV data with optical and near infrared photometry, we employ stellar population synthesis techniques to measure SN Ia host galaxy stellar masses, star-formation rates (SFRs), and reddening due to dust. We reinforce the key role of GALEX UV data in deriving accurate estimates of galaxy SFRs and dust extinction. Optical spectra of SN Ia host galaxies are fitted simultaneously for their stellar continua and emission lines fluxes, from which we derive high precision redshifts, gas-phase metallicities, and Halpha-based SFRs. With these data we show that SN Ia host galaxies present tight agreement with the fiducial galaxy mass-metallicity relation from SDSS for stellar masses log(M_*/solar masses)>8.5 where the relation is well-defined. The star-formation activity of SN Ia host galaxies is consistent with a sample of comparable SDSS field galaxies, though this comparison is limited by systematic uncertainties in SFR measurements. Our analysis indicates that SN Ia host galaxies are, on average, typical representatives of normal field galaxies.

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Title: The Progenitors of Type Ia Supernova Explosions are Head-On Collisions of White Dwarfs in Triple Systems
Authors: Doron Kushnir, Boaz Katz, Subo Dong, Eli Livne, Rodrigo Fernández

We argue that type Ia supernovae (SNe Ia) are the result of head-on collisions of White Dwarfs (WDs) in triple systems. The thermonuclear explosions resulting from the zero-impact-parameter collisions of WDs are calculated from first principles by using 2D hydrodynamical simulations. Collisions of typical WDs with masses 0.5-0.9 solar masses result in explosions that synthesise Ni56 masses in the range of 0.15-0.8 solar masses, spanning the wide distribution of yields observed for the majority of SNe Ia. The robustness of the shock ignition process is verified with a detailed study using a one-dimensional toy model and analytic tools. The late-time (~50 days after peak) bolometric light curve is equal to the instantaneous energy deposition and is calculated exactly, by solving the transport of gamma-rays emitted by the decay of Ni56 using a Monte-Carlo code. All collisions are found to have the same late-time light curves, when normalized to the amount of synthesized Ni56. This universal light curve is shown to agree with the majority of the supernovae in the compilation made by M. Stritzinger to an accuracy of better than 30% in the range 40<t<80 days after bolometric peak. The widths of the Ni56-mass-weighted-line-of-sight velocity distributions are correlated with the Ni56 yield and in agreement with the observed Mazzali relation. The continuous distribution of observed SN Ia features, is naturally reproduced with the distribution of WD masses involved in the collisions. The effect of a non-zero impact parameter requires further studies, using 3D hydrodynamical codes.

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Title: Type Ia Supernova Models and Progenitor Scenarios
Authors: Ken'ichi Nomoto, Yasuomi Kamiya, Naohito Nakasato

We review some recent developments in theoretical studies on the connection between the progenitor systems of Type Ia supernovae (SNe Ia) and the explosion mechanisms. (1) DD-subCh: In the merging of double C+O white dwarfs (DD scenario), if the carbon detonation is induced near the white dwarf (WD) surface in the early dynamical phase, it could result in the (effectively) sub-Chandrasekhar mass explosion. (2) DD-Ch: If no surface C-detonation is ignited, the WD could grow until the Chandrasekhar mass is reached, but the outcome depends on whether the quiescent carbon shell burning is ignited and burns C+O into O+Ne+Mg. (3) DD-subCh: In the single degenerate (SD) scenario, if the He shell-flashes grow strong to induce a He detonation, it leads to the sub-Chandra explosion. (4) DD-Ch: If the He-shell flashes are not strong enough, they still produce interesting amount of Si and S near the surface of C+O WD before the explosion. In the Chandra mass explosion, the central density is high enough to produce electron capture elements, e.g., stable Ni-58. Observations of the emission lines of Ni in the nebular spectra provides useful diagnostics of the sub-Chandra vs. Chandra issue. The recent observations of relatively low velocity carbon near the surface of SNe Ia provide also interesting constraint on the explosion models.

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Title: An exact integral relation between the Ni56 mass and the bolometric light curve of a type Ia supernova
Authors: Boaz Katz, Doron Kushnir, Subo Dong (IAS)

An exact relation between the Ni56 mass and the bolometric light curve of a type Ia supernova can be derived as follows, using the following excellent approximations: 1. the emission is powered solely by Ni56-> Co56 ->Fe56; 2. each mass element propagates at a non-relativistic velocity which is constant in time (free coasting); and 3. the internal energy is dominated by radiation. Under these approximations, the energy E(t) carried by radiation in the ejecta satisfies: dE/dt=-E(t)/t-L(t)+Q(t), where Q(t) is the deposition of energy by the decay which is precisely known and L(t) is the bolometric luminosity. By multiplying this equation by time and integrating over time we find: E(t)*t=\int_0^t Q(t')t'dt' -\int_0^t L(t')t'dt'. At late time, t>> t_peak, the energy inside the ejecta decreases rapidly due to its escape, and thus we have \int_0^t Q(t')t'dt'=\int_0^t L(t')t'dt'. This relation is correct regardless of the opacities, density distribution or Ni56 deposition distribution in the ejecta and is very different from "Arnett's rule", L_peak ~ Q(t_peak). By comparing \int_0^t Q(t')t'dt' with \int_0^t L(t')t'dt' at t~40 day after the explosion, the mass of Ni56 can be found directly from UV, optical and infrared observations with modest corrections due to the unobserved gamma-rays and due to the small residual energy in the ejecta, E(t)*t>0.

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