Angelo Pidatella

Heterogeneous composition of compact binary ejecta, arising from merging of binary neutron stars being observed in recent gravitational-wave event (GW170818) and having electromagnetic transient counterparts, known as Kilonovae (KN), makes problematic the resolution of both KN light-curve and the composition of the ejecta. This is due to the ejecta opacity, whose complexity is far to be trivially solved by only using available theoretical atomic structure models. Since these ejecta are thought to be among the major r-process nucleosynthesis cosmic sites, exploring opacity of these systems is a new challenge in the multi-messenger astronomy era. We propose to re-create plasma conditions of early stage KN event in laboratory plasmas, attempting to measure the opacity of these environment, trying to close the gap between theory and observations. At the INFN – LNS the new facility PANDORA (Plasmas for Astrophysics Nuclear Decay Observation and Radiation for Archaeometry) is under construction. It is a plasma trap, exploiting the electron cyclotron resonance (ECR) heating as energy sustainment of plasma, which is confined in a magnetic field. The facility will allow to finely tune electron density and temperature by moving in the experimental-parameter space (gas pressure, microwave power, magnetic field, heating frequency) and to simultaneously monitor density and temperature via a multi-diagnostic setup, e.g., by means of indirect diagnostics such as those based on the optical emission spectroscopy (OES) and microwave interefero-polarimetry. A feasibility study has been carried out, starting from astrophysical models of the ejecta adiabatic expansion as a function of starting masses, temperature, velocity, and composition. It turns out that plasma density and temperature reproducible in the ECR plasma trap (1E10-14 cm-3, 1-2 eV) closely corresponds to the blue-KN emission (up to few days after the merging), emitting in the visible range and being related to a higher electron fraction content. From the latter feature, the expected abundances from r-process elements yields can be constrained to few light species (e.g., Strontium, Selenium, Zirconium, Niobium, Molybdenum, Technetium, Rhodium, Ruthenium). Combining their abundances to their partial opacity, the most interesting and suitable for experiments relates to Se and Sr. Our first task has been then to reproduce stable plasma in trap, having density and temperature of blue-KN stage. Plasma characterization in the currently operative ECR plasma trap at the INFN – LNS, named Flexible Plasma Trap, has been performed as a function of several combination of experimental configurations, aiming at achieving a stable and reproducible plasma. Monitoring of plasma density and temperature has been carried out via the simultaneous employment of OES and polarimetry, obtaining many different stable plasma conditions in a range of pressure and power of 7E-03 – 2E-02 mbar and 150 – 450 W, respectively. These ranges are required for tending to higher densities and lower temperature. Self-emitted plasma light in the visible has been collected through a dedicated optical setup (collimation and lens) and send in fiber to a high-resolution spectrometer. Integrated emission lines are extracted, and ratios are then evaluated to estimate average values of density and temperature, according to the line-ratio method. The analysis of OES data is still in progress, as well as the completion of characterization, the optimization of the spectral analysis code, and further improvements of the experimental setup for the incoming opacity measurements.