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We study kilonova emission from binary neutron star (BNS) mergers for the case that a remnant massive neutron star (MNS) forms and collapses to a black hole within $20$ ms after the onset of the merger (which we refer to as "a short-lived case") by consistently employing numerical-relativity and nucleosynthesis results. We find that such kilonovae are fainter and last shorter than those for BNSs resulting in the formation of long-lived ($\gg 1\,{\rm s}$) MNSs, in particular in the optical band. The resulting light curves are too faint and last for a too short duration to explain the kilonova observation for the BNS associated with GW170817, indicating that the merger remnant formed in GW170817 is unlikely to have collapsed to a black hole within a short period of time ($\sim 20$ ms) after the onset of the merger. Our present result implies that early observation is necessary to detect kilonovae associated with BNSs leading to short-lived MNS formation in particular for the optical blue band as well as that kilonovae could be hidden by the gamma-ray burst afterglow for nearly face-on observation. We provide a possible approximate scaling law for near-infrared light curves with the given reference time and magnitude when the decline power of the ${\it z}$-band magnitude, $d M_{\it z}/d{\rm log}_{10}t$, reaches $2.5$. This scaling law suggests that the ${\it HK}$-band follow-up observation should be at least $1$ mag deeper than that for the ${\it z}$-band reference magnitude and earlier than 4 times the reference time., Comment: 15 pages, 13 figures, submitted to MNRAS

The innermost ejecta of core-collapse supernovae are considered to be the sources of some iron-group and heavier nuclei. The ejecta are predominantly driven by neutrino heating, principally due to neutrino capture on free neutrons and protons. Such neutrino interaction plays a crucial role for setting neutron richness in the ejecta. Recent hydrodynamics work with sophisticated neutrino transport indicates that the ejecta are only mildly neutron rich or even proton rich. In such conditions a wide variety of trans-iron isotopes are synthesized, while the neutron richness is insufficient for the production of r-process nuclei. In this capter, basic concepts of nucleosynthesis in neutrino-heated ejecta and neutrino-driven winds of core-collapse supernovae are presented along with latest studies. Neutrino-heated ejecta indicate the early ejecta component within the first few seocnds in which anisotropic convective activities of material above the neutrinosphere become important for nucleosynthesis. Then, neutrino-driven winds follow, which are approximately isotropic outflows emerging from the surface of a proto-neutron star . According to such characteristics, studies of nucleosynthesis here are based on recent multi-dimentional hydrodynamics simulations and semi-alalytic wind solutions, respectively. These studies suggest that trans-iron species up to the atomic mass number of 90, as well as some rare isotopes such as 48Ca and 92Mo, are produced in the neutrino-heated ejecta. Neutrino-driven winds are unlikely sources of r-process nuclei, but rather promising sources of proton-rich isotopes up to the atomic number of 110., Comment: 33 pages, 18 figures, chapter to appear in Handbook of Nuclear Physics (Springer)

The radioactive decay of the freshly synthesized $r$-process nuclei ejected in compact binary mergers power optical/infrared macronovae (kilonovae) that follow these events. The light curves depend critically on the energy partition among the different products of the radioactive decay and this plays an important role in estimates of the amount of ejected $r$-process elements from a given observed signal. We study the energy partition and $��$-ray emission of the radioactive decay. We show that $20$-$50\%$ of the total radioactive energy is released in $��$-rays on timescales from hours to a month. The number of emitted $��$-rays per unit energy interval has roughly a flat spectrum between a few dozen keV and $1$ MeV so that most of this energy is carried by $\sim 1$ MeV $��$-rays. However at the peak of macronova emission the optical depth of the $��$-rays is $\sim 0.02$ and most of the $��$-rays escape. The loss of these $��$-rays reduces the heat deposition into the ejecta and hence reduces the expected macronova signals if those are lanthanides dominated. This implies that the ejected mass is larger by a factor of $2$-$3$ than what was previously estimated. Spontaneous fission heats up the ejecta and the heating rate can increase if a sufficient amount of transuranic nuclei are synthesized. Direct measurements of these escaping $��$-rays may provide the ultimate proof for the macronova mechanisms and an identification of the $r$-process nucleosynthesis sites. However, the chances to detect these signals are slim with current X-ray and $��$-ray missions. New detectors, more sensitive by at least a factor of ten, are needed for a realistic detection rate., 10 pages, 5 figures, submitted to MNRAS

An electromagnetic transient powered by the radioactive decay of r-process elements, a so-called kilonova/macronova, is one of the possible observable consequences of compact binary mergers including at east one neutron star. Recent observations strongly suggest the first discovery of the electromagnetic transient, which is associated with the short GRB 130603B. We explore a possible progenitor of this event combining the numerical-relativity simulations and radiative transfer simulations of the dynamical ejecta of binary neutron star and black hole - neutron star mergers. We show that the ejecta models within a realistic parameter range consistently reproduce the observed near-infrared excess. We also show that the soft equation of state models for binary neutron star mergers and the stiff equation of state models for black hole - neutron star mergers are favored to reproduce the observed luminosity., Comment: 6 pages, 3 figures, submitted to ApJL

Recent observations have revealed that Type Ia supernovae (SNe Ia) are not perfect standard candles but show some variations in their absolute magnitudes, light curve shapes, and spectra. The C/O ratio in the SNe Ia progenitors (C-O white dwarfs) may be related to this variation. In this work, we systematically investigate the effects of stellar mass (M) and metallicity (Z) on the C/O ratio and its distribution in the C-O white dwarfs by calculating stellar evolution from the main-sequence through the end of the second dredge-up for M=3-9 Mo and Z=0.001-0.03. We find that the total carbon mass fraction just before SN Ia explosion varies in the range 0.36 -- 0.5. We also calculate the metallicity dependence of the main-sequence-mass range of the SN Ia progenitor white dwarfs. Our results show that the maximum main-sequence mass to form C-O white dwarfs decreases significantly toward lower metallicity, and the number of SN Ia progenitors may be underestimated if metallicity effectis neglected. We discuss the implications of these results on the variation of SNe Ia, determination of cosmological parameters, luminosity function of white dwarfs, and the galactic chemical evolution., Comment: Added references and corrected typos. To appear in the Astrophysical Journal 1999 March 10 issue