Selection Effects, Biases, and Constraints in the Calan/Tololo Supernova Survey

We use Monte Carlo simulations of the Calan/Tololo photographic supernova survey to show that a simple model of the survey's selection effects accounts for the observed distributions of recession velocity, apparent magnitude, angular offset, and projected radial distance between the supernova and the host galaxy nucleus for this sample of Type Ia supernovae (SNe Ia). The model includes biases due to the flux-limited nature of the survey, the different light curve morphologies displayed by different SNe Ia, and the difficulty of finding events projected near the central regions of the host galaxies. From these simulations we estimate the bias in the zero-point and slope of the absolute magnitude-decline rate relation used in SNe Ia distance measurements. For an assumed intrinsic scatter of 0.15 mag about this relation, these selection effects decrease the zero-point by 0.04 mag. The slope of the relation is not significantly biased. We conclude that despite selection effects in the survey, the shape and zero-point of the relation determined from the Calan/Tololo sample are quite reliable. We estimate the degree of incompleteness of the survey as a function of decline rate and estimate a corrected luminosity function for SNe Ia in which the frequency of SNe appears to increase with decline rate (the fainter SNe are more common). Finally, we compute the integrated detection efficiency of the survey in order to infer the rate of SNe Ia from the 31 events found. For a value of Ho=65 km/sec/Mpc we obtain a SN Ia rate of 0.21(+0.30)(-0.13) SNu. This is in good agreement with the value 0.16+/-0.05 SNu recently determined by Capellaro et al. (1997).Comment: 36 pages, 19 figures as extra files, to appear in the A

Positron Escape from Type Ia Supernovae

We generate bolometric light curves for a variety of type Ia supernova models at late times, simulating gamma-ray and positron transport for various assumptions about the magnetic field and ionization of the ejecta. These calculated light curve shapes are compared with light curves of specific supernovae for which there have been adequate late observations. %The selection of models is generally not based upon the %ability to fit the late observations, but rather because the %model has been demonstrated by other authors to approximate the spectra %and early light curves of that specific SN. From these comparisons we draw two conclusions: whether a suggested model is an acceptable approximation of a particular event, and, given that it is, the magnetic field characteristics and degree of ionization that are most consistent with the observed light curve shape. For the ten SNe included in this study, five strongly suggest $^{56}$Co positron escape as would be permitted by a weak or radially-combed magnetic field. Of the remaining five SNe, none clearly show the upturned light curve expected for positron trapping in a strong, tangled magnetic field. Chandrasekhar mass models can explain normally, sub-, and super- luminous supernova light curves; sub-Chandrasekhar mass models have difficulties with sub- (and potentially normally) luminous SNe. An estimate of the galactic positron production rate from type Ia SNe is compared with gamma-ray observations of Galactic 511 keV annihilation radiation. Additionally, we emphasize the importance of correctly treating the positron transport for calculations of spectra, or any properties, of type Ia SNe at late epochs ($\geq$ 200 d).Comment: 82 pages including 25 figure

Measurement of the Slope Parameter for the eta->3pi0 Decay in the pp->pp eta Reaction

The CELSIUS/WASA setup is used to measure the 3pi0 decay of eta mesons produced in pp interactions with beam kinetic energies of 1.36 and 1.45 GeV. The efficiency-corrected Dalitz plot and density distributions for this decay are shown, together with a fit of the quadratic slope parameter alpha yielding alpha = -0.026 +/- 0.010(stat) +/- 0.010(syst). This value is compared to recent experimental results and theoretical predictions.Comment: 4 pages, 7 Postscript figures, uses revtex4.st

Measurement of the eta->pi+pi-e+e- decay branching ratio

The reaction pd->3He eta at threshold was used to provide a clean source of eta mesons for decay studies with the WASA detector at CELSIUS. The branching ratio of the decay eta->pi+pi-e+e- is measured to be (4.3+/-1.3+/-0.4)x10^-4.Comment: 10 pages,6 figures revised versio

The origin of intergalactic thermonuclear supernovae

The population synthesis method is used to study the possibility of explaining the appreciable fraction (20^+12_15%) of the intergalactic (no-host) type Ia supernovae observed in galaxy clusters (Gal-Yam ete al. 2003) by binary whote dwarf merginngs in the cores of globular clusters. In a typical globular cluster, the number of merging double white dwarfs is fount to be smaller than 10^{-13} per year per average cluster star during the entire evolution of the cluster, which is a factor of 3 higher than in a Milky-Way-type galaxy. From 5 to 30% of the merging white dwarfs are dynamically expelled from the cluster with barycenter velocities up to 150 km/s. SN Ia explosions during the mergers of binary white dwarfs in dense star clusters may account for \sim 1% of the total rate of SN Ia in the central parts of galaxy clusters if the baryon mass fraction in such star clusters is \sim 0.3%.Comment: 8 pages, 3 figs. Astronomy Letters (in press

The pp -> pp pi pi pi reaction channels in the threshold region

The cross section for prompt neutral and charged three pion production in pp interactions was measured at excess energies in the range 160 - 217 MeV. That comprises the first measurement of the pp->pp pi0pi0pi0 reaction and the comparison with the pp->pp pi+pi-pi0 reaction, in a very direct way. The experiment was performed above the eta meson production threshold and the cross section normalization was obtained from a concurrent measurement of the reaction pp->pp eta with the eta decaying into 3 pions. Since the same final states are selected, the measurement has a low systematical error. The measured cross section ratio sigma(pp->pp pi+pi-pi0)/sigma(pp->pp pi0\pi0\pi0) is compared to predictions of dominance of different isobars in the intermediate state.Comment: 12 pages, 4 figures New discussion on the pp->pp3pi reaction mechanis

Expression of HPCA‐1 and HLA‐DR antigens on growth factor‐ and stroma‐dependent colony forming cells

The expression of HLA‐DR and HPCA‐1 antigens (recognized by the L243 and BL3C5 antibodies respectively) on adult human bone marrow cells was examined by fluorescence activated cell sorting and colony assays. Nearly all the (day 14) lineage restricted and multipotential colony forming cells analysed in methylcellulose cultures in the presence of added growth factors express HLA‐DR and HPCA‐1 determinants. Two colour cell sorting reveals that the lineage restricted HLA‐DR positive progenitors express variable levels of BI.3C5 positivity whereas most of the multipotential progenitors, the multi‐CFC or CFU‐GEMM, are highly BI.3C5 positive. The isolated HLA‐DR and BI.3C5 positive populations also contain haemopoietic precursors which adhere to and form colonies on pre‐formed stromal layers. Thus, haemopoietic progenitors assayed in both types of culture system can be analysed and enriched by simultaneous two‐colour sorting using anti‐HLA‐DR and BI.3C5 monoclonal antibodies. Similarities in the antigenic phenotype of such cells, however, precludes the use of these reagents for segregating growth factor‐dependent from stroma‐dependent progenitors