Origin of heavy elements:

"The  Cosmos are within us. We are all made of star-stuff"
- Carl Sagan

We are all made of elements and are surrounded by elements. But do we ponder about where do the elements come from?
If we go back to the time of the Big-Bang, we see that the baby Universe is filled with only the lightest of the elements like Hydrogen, Helium, and Lithium. So naturally the question arises where do the heavier elements we are made of and are surrounded by come from?

A part of the question has been solved in the past century. The origin of the elements up to iron is known to be stellar burning or the violent death of the stars (Supernovae). However, the origin of heavier elements such as gold, platinum, and rare earth elements lanthanides has been a long-standing mystery in astrophysics.

My research revolves around the question of the cosmic origin of heavy elements. I study binary neutron star (BNS) mergers, which are the only confirmed sources for heavy element synthesis so far via the rapid neutron capture (r-process) nucleosynthesis.

Kilonovae from BNS mergers:

The radioactive decay of the freshly synthesized heavy elements in the BNS merger ejecta produces emission in the ultraviolet-optical-infrared range, which is called a kilonova (Li and Paczynsky 1998, ApJ). Moreover, BNS mergers are associated with short gamma-ray bursts (GRB) and are also the ideal source for gravitational waves (GW). The first kilonova associated with the BNS merger is observed as a follow-up observation of GW170817.

I use the observables of kilonova (light curves and spectra) since the elemental abundances affect the light curves and spectra and hence, can be tracked from the observations. In my previous research, I have focused on extracting the abundance pattern indirectly from the kilonova light curves. For this purpose, I derive the kilonova starting from early time (t ∼ hours, where t is the time after the merger, Banerjee et al., 2020, 2022, Banerjee et al. 2023, submitted to ApJ). As a continuation of my journey to understand the cosmic origin of heavy elements using kilonova from BNS mergers as a probe, I decide to use a more direct way to obtain the abundances, i.e., by using the spectra of kilonova from BNS merger. Towards this goal, I focus on the spectra from the kilonova at t ≥ a few weeks, i.e., the kilonova spectra at the nebular phase.

Kilonova light curves data available on request

Atomic data for kilonovae from BNS mergers:

The observables of kilonova (light curves and spectra) are determined by the different atomic processes, which in turn are dependent on the atomic properties of the elements. However, for most of the heavy elements, no reliable atomic data are available, which hinders the progress in the modeling kilonova light curves and spectra. Hence, I calculate the atomic data (energy levels and transitions to calculate opacity, and recombination cross-sections for nebular Spectra) to determine the realistic kilonova spectra at the nebular phase.

Atomic opacity data for highly ionized (V -XI) heavy elements (Ca - Ra, Z = 20 -88), available on request