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Register a new userComposition Effects on Kilonova Spectra and Light Curves: I
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The merger of neutron star binaries is believed to eject a wide range of heavy elements into the universe. By observing the emission from this ejecta, scientists can probe the ejecta properties (mass, velocity and composition distributions). The emission (a.k.a. kilonova) is powered by the radioactive decay of the heavy isotopes produced in the merger and this emission is reprocessed by atomic opacities to optical and infra-red wavelengths. Understanding the ejecta properties requires calculating the dependence of this emission on these opacities. The strong lines in the optical and infra-red in lanthanide opacities have been shown to significantly alter the light-curves and spectra in these wavelength bands, arguing that the emission in these wavelengths can probe the composition of this ejecta. Here we study variations in the kilonova emission by varying individual lanthanide (and the actinide uranium) concentrations in the ejecta. The broad forest of lanthanide lines makes it difficult to determine the exact fraction of individual lanthanides. Nd is an exception. Its opacities above 1 micron are higher than other lanthanides and observations of kilonovae can potentially probe increased abundances of Nd. Similarly, at early times when the ejecta is still hot (first day), the U opacity is strong in the 0.2-1 micron wavelength range and kilonova observations may also be able to constrain these abundances.
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Neutron star mergers offer unique conditions for the creation of the heavy elements and additionally provide a testbed for our understanding of this synthesis known as the $r$-process. We have performed dynamical nucleosynthesis calculations and identified a single isotope, $^{254}$Cf, which has a particularly high impact on the brightness of electromagnetic transients associated with mergers on the order of 15 to 250 days. This is due to the anomalously long half-life of this isotope and the efficiency of fission thermalization compared to other nuclear channels. We estimate the fission fragment yield of this nucleus and outline the astrophysical conditions under which $^{254}$Cf has the greatest impact to the light curve. Future observations in the middle-IR which are bright during this regime could indicate the production of actinide nucleosynthesis.
The mergers of binary neutron stars, as well as black hole-neutron star systems, are expected to produce an electromagnetic counterpart that can be analyzed to infer the element synthesis that occurred in these events. We investigate one source of uncertainties pertinent to lanthanide-rich outflows: the nuclear inputs to rapid neutron capture nucleosynthesis calculations. We begin by examining thirty-two different combinations of nuclear inputs: eight mass models, two types of spontaneous fission rates, and two types of fission daughter product distributions. We find that such nuclear physics uncertainties typically generate at least one order of magnitude uncertainty in key quantities such as the nuclear heating (one and a half orders of magnitude at one day post-merger), the bolometric luminosity (one order of magnitude at five days post-merger), and the inferred mass of material from the bolometric luminosity (factor of eight when considering the eight to ten days region). Since particular nuclear processes are critical for determining the electromagnetic signal, we provide tables of key nuclei undergoing $beta$-decay, $alpha$-decay, and spontaneous fission important for heating at different times, identifying decays that are common among the many nuclear input combinations.
Compact object mergers can produce a thermal electromagnetic counterpart (a kilonova) powered by the decay of freshly synthesized radioactive isotopes. The luminosity of kilonova light curves depends on the efficiency with which beta-decay electrons are thermalized in the ejecta. Here we derive a simple analytic solution for thermalization by calculating how electrons accumulate in the ejecta and lose energy adiabatically and via plasma losses. We find that the time-dependent thermalization efficiency is well described by $f(t) approx (1 + t/t_e)^{-n}$ where $n approx 1$ and the timescale $t_e$ is a function of the ejecta mass and velocity. For a statistical distribution of r-process isotopes with radioactive power $dot{Q} propto t^{-4/3}$, the late time kilonova luminosity asymptotes to $L propto t^{-7/3}$ and depends super-linearly on the ejecta mass, $L propto M^{5/3}$. If a kilonova is instead powered by a single dominate isotope, we show that the late time luminosity can deviate substantially from the underlying exponential decay and eventually become brighter than the instantaneous radioactivity due to the accumulation of trapped electrons. Applied to the kilonova associated with the gravitational wave source GW170817, these results imply that a possible steepening of the observed light curve at $gtrsim 7$ days is unrelated to thermalization effects and instead could mark the onset of translucency in a high opacity component of ejecta. The analytic results should be convenient for estimating the properties of observed kilonovae and assessing the potential late time detectability of future events.
The light curves of type-II supernovae (SNe) are believed to be highly affected by recombination of hydrogen that takes place in their envelopes. In this work, we analytically investigate the transition from a fully ionized envelope to a partially recombined one and its effects on the SN light curve. The motivation is to establish the underlying processes that dominate the evolution at late times when recombination takes place in the envelope, yet early enough so that $^{56}$Ni decay is a negligible source of energy. We consider the diffusion of photons through the envelope while analyzing the ionization fraction and the coupling between radiation and gas, and find that the main effect of recombination is on the evolution of the observed temperature. Before recombination the temperature decreases relatively fast, while after recombination starts it significantly reduces the rate at which the observed temperature drops with time. This behaviour is the main cause for the observed flattening in the optical bands, where for a typical red supergiant explosion, the recombination wave affects the bolometric luminosity only mildly during most of the photospheric phase. Moreover, the plateau phase observed in some type-II SNe is not a generic result of recombination, and it also depends on the density structure of the progenitor. This is one possible explanation to the different light curve decay rates observed in type II (P and L) SNe.
Supernovae of type IIP are marked by the long plateau seen in their optical light curves. The plateau is believed to be the result of a recombination wave that propagates through the outflowing massive hydrogen envelope. Here, we analytically investigate the transition from a fully ionized envelope to a partially recombined one and its effects on the SN light curve. The motivation is to establish the underlying processes which dominate the evolution at late times when recombination takes place in the envelope, yet early enough so that $^{56}$Ni decay is a negligible source of energy. We assume a simple, yet adequate, hydrodynamic profile of the envelope and study the mechanisms which dominate the energy emission and the observed temperature. We consider the diffusion of photons through the envelope while analyzing the ionization fraction and the coupling between radiation and gas. We find that once recombination starts, the observed temperature decreases slowly in time. However, in a typical red supergiant (RSG) explosion, the recombination wave does not affect the bolometric luminosity immediately. Only at later times, the cooling wave may reach layers that are deep enough to affect the luminosity. We find that the plateau is not a generic result of a recombination process in expanding gas. Instead it depends on the density profile of the parts of the envelope which undergo recombination. Our results are useful to investigate the light curves of RSG explosions. We show the resulting light curves of two examples of RSG explosions according to our model and discuss their compatibility with observations. In addition, we improve the analytical relations between the plateau luminosity and plateau duration to the properties of the pre-explosion progenitor (Arnett 1980; Popov 1993).
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