He II recombination lines as a test of the nature of SN Ia progenitors in elliptical galaxies

To date, the question of which progenitor channel can reproduce the observed rate of Type Ia supernovae (SNe Ia) remains unresolved, with the single and double degenerate scenarios remaining the leading contenders. The former implies a large population of hot accreting white dwarfs with photospheric temperatures of T ~ 10^5-10^6 K during some part of their accretion history. We show that in early-type galaxies, a population of accreting white dwarfs large enough to reproduce the SN Ia rate would contribute significantly to the ionizing UV radiation expected from the stellar population. For mean stellar ages < ~5 Gyr, single degenerate progenitors would dominate the ionizing background produced by stars, increasing the continuum beyond the He II-ionizing limit more than ten-fold. This opens a new avenue for constraining the progenitors of SNe Ia, through consideration of the spatially extended low-ionization emission-line regions now found in many early-type galaxies. Modelling the expected emission, we show that one can constrain the contribution of the single degenerate channel to the SN Ia rate in E/S0 galaxies from upper limits on the luminosity of He II recombination lines in the optical and FUV. We discuss future directions, as well as possible implications for the evolution of SNe Ia in old stellar populations.Comment: 12 pages, 13 figures, MNRA

Tandem catalysis for the production of alkyl lactates from ketohexoses at moderate temperatures

Retro-aldol reactions have been implicated as the limiting steps in catalytic routes to convert biomass-derived hexoses and pentoses into valuable C_2, C_3, and C_4 products such as glycolic acid, lactic acid, 2-hydroxy-3-butenoic acid, 2,4-dihydroxybutanoic acid, and alkyl esters thereof. Due to a lack of efficient retro-aldol catalysts, most previous investigations of catalytic pathways involving these reactions were conducted at high temperatures (≥160 °C). Here, we report moderate-temperature (around 100 °C) retro-aldol reactions of various hexoses in aqueous and alcoholic media with catalysts traditionally known for their capacity to catalyze 1,2-intramolecular carbon shift (1,2-CS) reactions of aldoses, i.e., various molybdenum oxide and molybdate species, nickel(II) diamine complexes, alkali-exchanged stannosilicate molecular sieves, and amorphous TiO_2–SiO_2 coprecipitates. Solid Lewis acid cocatalysts that are known to catalyze 1,2-intramolecular hydride shift (1,2-HS) reactions that enable the formation of α-hydroxy carboxylic acids from tetroses, trioses, and glycolaldehyde, but cannot readily catalyze retro-aldol reactions of hexoses and pentoses at these moderate temperatures, are shown to be compatible with the aforementioned retro-aldol catalysts. The combination of a distinct retro-aldol catalyst with a 1,2-HS catalyst enables lactic acid and alkyl lactate formation from ketohexoses at moderate temperatures (around 100 °C), with yields comparable to best-reported chemocatalytic examples at high temperature conditions (≥160 °C). The use of moderate temperatures enables numerous desirable features such as lower pressure and significantly less catalyst deactivation

Diffuse Gas in Retired Galaxies: Nebular Emission Templates and Constraints on the Sources of Ionization

We present emission line templates for passively evolving ("retired") galaxies, useful for investigation of the evolution of the ISM in these galaxies, and characterization of their high-temperature source populations. The templates are based on high signal-to-noise ($>800$) co-added spectra ($3700-6800$\AA) of $\sim11500$ gas-rich Sloan Digital Sky Survey galaxies devoid of star-formation and active galactic nuclei. Stacked spectra are provided for the entire sample and sub-samples binned by mean stellar age. In Johansson~et al (2014), these spectra provided the first measurements of the He II 4686\AA\ line in passively-evolving galaxies, and the observed He II/H$\beta$ ratio constrained the contribution of accreting white dwarfs (the "single-degenerate" scenario) to the type Ia supernova rate. In this paper, the full range of unambiguously detected emission lines are presented. Comparison of the observed [O I] 6300\AA/H$\alpha$ ratio with photoionization models further constrains any high-temperature single-degenerate scenario for type Ia supernovae (with 1.5 $\lesssim$ T/$10^{5}K$ $\lesssim$ 10) to $\lesssim$3-6\% of the observed rate in the youngest age bin (i.e. highest SN Ia rate). Hence, for the same temperatures, in the presence of an ambient population of post-AGB stars, we exclude additional high-temperature sources with a combined ionizing luminosity of $\approx 1.35\times 10^{30} L_{\odot}/M_{\odot,*}$ for stellar populations with mean ages of 1 - 4 Gyrs. Furthermore, we investigate the extinction affecting both the stellar and nebular continuum. The latter shows about five times higher values. This contradicts isotropically distributed dust and gas that renders similar extinction values for both cases.Comment: Accepted for publication in MNRAS, 16 pages, 12 figure

1/N_c corrections to the magnetic susceptibility of the QCD vacuum

We investigate the magnetic susceptibility of the QCD vacuum with the $1/N_c$ corrections taken into account, based on the instanton vacuum. Starting from the instanton liquid model we derive the gauged light-quark partition function in the presence of the current quark mass as well as of external Abelian vector and tensor fields. We consider the $1/N_c$ meson-loop corrections which are shown to contribute to the magnetic susceptibility by around 15% for the up (and down) quarks. We also take into account the tensor terms of the quark-quark interaction from the instanton vacuum as well as the finite-width effects, both of which are of order $\mathcal{O}(1/N_c)$. The effects of the tensor terms and finite width turn out to be negligibly small. The final results for the up-quarks are given as: $\chi_0 \simeq 35-40 \mathrm{MeV}$ with the quark condensate $_0$. We also discuss the pion mass dependence of the magnetic susceptibility in order to give a qualitative guideline for the chiral extrapolation of lattice data.Comment: 18 pages, 5 figures. Final version to appear in Phys. Rev.