Search

Binary neutron star mergers are the first confirmed site of element nucleosynthesis by the rapid neutron-capture process (r -process). The kilonova AT2017gfo is the only electromagnetic counterpart of a neutron star merger spectroscopically observed. We analyse the entire spectral sequence of AT2017gfo (from merger to +10.4 d) and identify seven emission-like features. We confirm that the prominent 1.08  |$\mu{\text{m}}$| feature can be explained by the Sr  ii near-infrared triplet evolving from a P-Cygni profile through to pure emission. We calculate the expected strength of the [Sr  ii ] doublet and show that its absence requires highly clumped ejecta. Near-infrared features at 1.58 and 2.07  |$\mu {\text{m}}$| emerge after three days and become more prominent as the spectra evolve. We model these as optically thick P-Cygni profiles and alternatively as pure emission features (with FWHM ≃ 35 600 ± 6600 km s−1) and favour the latter interpretation. The profile of the strong 2.07  |$\mu {\text{m}}$| emission feature is best reproduced with two lines, centred at 2.059 and 2.135  |$\mu {\text{m}}$|⁠. We search for candidate ions for all prominent features in the spectra. Strong, permitted transitions of La  iii , Ce  iii , Gd  iii , Ra  ii , and Ac  i are plausible candidates for the emission features. If any of these features are produced by intrinsically weak, forbidden transitions, we highlight candidate ions spanning the three r -process peaks. The second r -process peak elements Te and I have plausible matches to multiple features. We highlight the need for more detailed and quantitative atomic line transition data. [ABSTRACT FROM AUTHOR]

The kilonova (KN) associated with the binary neutron star (BNS) merger GW170817 is the only known electromagnetic counterpart to a gravitational wave source. Here we produce a sequence of radiative transfer models (using tardis) with updated atomic data, and compare them to accurately calibrated spectra. We use element compositions from nuclear network calculations based on a realistic hydrodynamical simulation of a BNS merger. We show that the blue spectrum at +1.4 d after merger requires a nucleosynthetic trajectory with a high electron fraction. Our best-fitting model is composed entirely of first r -process peak elements (Sr and Zr) and the strong absorption feature is reproduced well by Sr  ii absorption. At this epoch, we set an upper limit on the lanthanide mass fraction of |$X_{{\small LN}} \lesssim 5 \times 10^{-3}$|⁠. In contrast, all subsequent spectra from +2.4 to 6.4 d require the presence of a modest amount of lanthanide material (⁠|$X_{{\small LN}} \simeq 0.05^{+0.05}_{-0.02}$|⁠), produced by a trajectory with Y e = 0.29. This produces lanthanide-induced line blanketing below 6000 Å, and sufficient light r -process elements to explain the persistent strong feature at ∼0.7–1.0  |$\mu$| m (Sr  ii). The composition gives good matches to the observed data, indicating that the strong blue flux deficit results in the near-infrared (NIR) excess. The disjoint in composition between the first epoch and all others indicates either ejecta stratification, or the presence of two distinct components of material. This further supports the 'two-component' KN model, and constrains the element composition from nucleosynthetic trajectories. The major uncertainties lie in availability of atomic data and the ionization state of the expanding material. [ABSTRACT FROM AUTHOR]

Binary neutron star mergers are thought to be one of the dominant sites of production for rapid neutron capture elements, including platinum and gold. Since the discovery of the binary neutron star merger GW170817, and its associated kilonova AT2017gfo, numerous works have attempted to determine the composition of its outflowing material, but they have been hampered by the lack of complete atomic data. Here, we demonstrate how inclusion of new atomic data in synthetic spectra calculations can provide insights and constraints on the production of the heaviest elements. We employ theoretical atomic data (obtained using |$\small {\rm GRASP}^{0}$|⁠) for neutral, singly and doubly ionized platinum and gold, to generate photospheric and simple nebular phase model spectra for kilonova-like ejecta properties. We make predictions for the locations of strong transitions, which could feasibly appear in the spectra of kilonovae that are rich in these species. We identify low-lying electric quadrupole and magnetic dipole transitions that may give rise to forbidden lines when the ejecta becomes optically thin. The strongest lines lie beyond 8000 Å, motivating high quality near-infrared spectroscopic follow-up of kilonova candidates. We compare our model spectra to the observed spectra of AT2017gfo, and conclude that no platinum or gold signatures are prominent in the ejecta. From our nebular phase modelling, we place tentative upper limits on the platinum and gold mass of ≲ a few 10−3 M⊙, and ≲ 10−2 M⊙, respectively. This work demonstrates how new atomic data of heavy elements can be included in radiative transfer calculations, and motivates future searches for elemental signatures. [ABSTRACT FROM AUTHOR]