Robust composition signatures in bolometric kilonova light curves

Sarkar, Arnab (2021) Robust composition signatures in bolometric kilonova light curves. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS Dissertation of Arnab Sarkar (16MS088)) 16MS088_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (2MB)

Abstract

It is known that during the Big Bang, mostly hydrogen an helium were created. Heavier elements like carbon and oxygen are created in the interiors of stars through nuclear fusion and when these stars end their lives in the form of a violent explosion called supernova, elements till the iron peak are created. But the question pertaining to the creation of elements in the Universe heavier than the iron group has been quite perplexing. Although it is known that slow and rapid neutron capture (s- and r-) process are the processes involved in the creation of heavy elements in the Universe, the astrophysical site for the r-process is still debated. In the recent years, a promising site for r-process nucleosynthesis has been proposed to be the merger of neutron stars. The recent discovery of gravitational wave (GW) emission from a binary neutron star merger, GW170817, was followed by the detection of an electromagnetic counterpart, AT2017gfo. The electromagnetic counterpart to such mergers is known to give rise to an expanding ejecta heated by the decay of unstable isotopes created by the r-process, back to stability, known as a kilonova. Analyzing kilonova light curves may enable one to determine its composition and neutron richness. This however, requires the determination of the values of various micro-physics parameters like the !- opacity (!), # particle opacity (Fie) and their associated uncertainty. These uncertainties have not yet been quantified for unusual compositions. Kilonova observations will enable us to determine the ! producing elements and constrain the ejecta structure, thus constraining neutron star merger dynamics. This work is focused on obtaining ! opacity for moderate to low degrees of neutron richness, that is, Ye ! 0.25, where fission reactions do not play a significant role in governing temporal abundances. With a simple kilonova model we demonstrate the importance of determining the values and uncertainties of !- and # particle opacity. We consider uncertainties stemming from the ! spectra, and most importantly due to nuclear reaction rates. We find that various ! databases are consistent with each other, and for every degree of neutron richness these databases give good coverage of all relevant isotopes 1 hour after the merger event. We report that for weakly neutron rich ejecta (Ye = 0.4), ! rises from its Compton value (0.048 ± 4.5 ⇥ 10−5 cm2/g) at 1 hour to 0.1 ± 0.006 cm2/g at 10 days. For moderately neutron rich ejecta (Ye = 0.25) however, ! is nearly time-independent at 0.11 ± 0.02 cm2/g. We also report that the uncertainties in ! are not large despite large uncertainties stemming from theoretical nuclear v reaction rates. We outline our work for the near future, which includes considering uncertainties stemming from nuclear mass models that propagates through the evaluation of Nuclear Statistical Equilibrium (NSE), and determining the values of e with uncertainties. We hope that this work and the subsequent continuation in the near future will be useful in understanding kilonovae for the broader community of astronomers and astrophysicists. Keywords: nucleosynthesis, kilonova, opacity, isotopic abundance, SkyNet, r-process, nuclear statistical equilibrium, electron fraction, Monte Carlo method.

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