James Gillanders
Gather.town id
CCE23
Poster Title
Spectroscopic modelling of the kilonova AT2017gfo
Institution
Queen's University Belfast
Abstract (short summary)
Binary neutron star (BNS) mergers are thought to be the dominant, or one of the dominant, production sites of rapid-neutron capture (r-process) elements in the Universe. Approximately half of all elements heavier than iron are synthesised via this process. Despite the r-process being extremely important for heavy element production, there is much we still do not understand about this production mechanism in BNS mergers. To date, we have observed one confirmed optical counterpart to a BNS merger - the kilonova (KN), AT2017gfo. Since its discovery in 2017, there has been several attempts to directly identify different r-process species in the observed spectra (strontium, caesium and tellurium). This proves to be difficult, due to the lack of complete atomic data for the heavy elements that would be of interest in this type of study. Additionally, the works to date have not considered realistic compositions from theoretical nucleosynthesis calculations. In this talk, I will present the results of our modelling of AT2017gfo, with compositions obtained from such a calculation. From this, we can make more robust predictions about the composition of the ejecta. Additionally, I will present the results of an analysis we performed with a new atomic dataset we generated for neutral, singly- and doubly-ionised platinum and gold (both 3rd r-peak elements). From these types of study, we can attempt to directly identify features corresponding to individual species. From this, we can infer properties about the KN ejecta, such as electron fraction, which helps to inform us about which r-peak is favoured in the elemental synthesis in KN ejecta.
Plain text (extended) Summary
Poster Abstract:
Binary neutron star (BNS) mergers are thought to be the dominant production site of rapid-neutron capture (r-process) elements in the Universe. To date, we have observed one confirmed optical counterpart to a BNS merger - the kilonova (KN), AT2017gfo. Since its discovery in 2017, there has been several attempts to directly identify different r-process species in the observed spectra. This proves to be difficult, due to the lack of complete atomic data for the heavy elements. Additionally, the works to date have not considered realistic compositions from theoretical nucleosynthesis calculations. Here I present the results of an analysis we performed with a new atomic data set we generated for neutral, singly- and doubly ionised platinum and gold (both third r-peak elements). Additionally, I present preliminary results of our modelling of AT2017gfo, with a realistic composition. From these types of study, we can attempt to directly identify features corresponding to individual species. From this, we can infer properties about the KN ejecta, which helps to inform us about which r-process peak is favoured in the elemental synthesis in KN ejecta.
Plain text summary:
Slide 1:
“Spectroscopic modelling of the kilonova AT2017gfo”
Slide contains a summary of the abstract.
Plot:
TARDIS models to illustrate the effect a realistic mass (0.001 solar masses) of Pt & Au has on the spectral energy distribution at various ejecta temperatures. There is clear line blanketing in the UV, but no identifiable strong transitions in the optical or NIR.
Slide 2:
“Photospheric stage models with TARDIS”
Plot:
High-mass pure Pt and Au TARDIS models to highlight the locations of the strongest features that would appear for these elements in an early phase spectrum.
In both cases, the model spectra are compared with observed spectra of AT2017gfo to compare locations of the strongest features. We don’t see good agreement between the strongest features of Pt/Au and the observed spectra.
Slide 3:
“Simple late stage nebular models”
Assuming a KN becomes optically thin, we can then predict the strongest emission features for Pt and Au, using the new atomic data set (McCann et al. in prep). Assuming LTE level populations, we were able to use the Einstein A-values for the transitions to calculate the strengths of features. From these, we can identify where these species would be expected to influence spectra.
Plot 1:
Here we show a sequence of synthetic emission spectra for the individual ions of Pt and Au under investigation.
Plot 2:
Comparison of our synthetic LTE nebular phase spectra for Pt I, II, III and Au I, II, III and two late-phase spectra of AT2017gfo. For our models, there are features of comparable strength to those of AT2017gfo, for reasonable masses of Pt and Au.
Slide 4:
“Realistic composition models with TARDIS”
Previous works have attempted to identify specific features in the spectra of AT2017gfo (Smartt et al. 2017; Watson et al. 2019; Perego et al. 2020). These have focussed on fitting the data empirically. In this work, we are using realistic compositions that have been computed from BNS merger simulations and nucleosynthesis calculations (Bauswein et al. 2013; Goriely et al. 2013, 2015). Here we are taking these realistic compositions and attempting to forward model to match the observed sequence of spectra.
Plot:
TARDIS model for an early spectrum of AT2017gfo. The model spectrum with different contributions to the spectrum are highlighted. There is significant contribution from photons that do not interact with the ejecta. We find Sr II produces strong absorption at 8500 angstroms, and that the spectrum is dominated by line blanketing from a multitude of heavy elements at shorter wavelengths.
Binary neutron star (BNS) mergers are thought to be the dominant production site of rapid-neutron capture (r-process) elements in the Universe. To date, we have observed one confirmed optical counterpart to a BNS merger - the kilonova (KN), AT2017gfo. Since its discovery in 2017, there has been several attempts to directly identify different r-process species in the observed spectra. This proves to be difficult, due to the lack of complete atomic data for the heavy elements. Additionally, the works to date have not considered realistic compositions from theoretical nucleosynthesis calculations. Here I present the results of an analysis we performed with a new atomic data set we generated for neutral, singly- and doubly ionised platinum and gold (both third r-peak elements). Additionally, I present preliminary results of our modelling of AT2017gfo, with a realistic composition. From these types of study, we can attempt to directly identify features corresponding to individual species. From this, we can infer properties about the KN ejecta, which helps to inform us about which r-process peak is favoured in the elemental synthesis in KN ejecta.
Plain text summary:
Slide 1:
“Spectroscopic modelling of the kilonova AT2017gfo”
Slide contains a summary of the abstract.
Plot:
TARDIS models to illustrate the effect a realistic mass (0.001 solar masses) of Pt & Au has on the spectral energy distribution at various ejecta temperatures. There is clear line blanketing in the UV, but no identifiable strong transitions in the optical or NIR.
Slide 2:
“Photospheric stage models with TARDIS”
Plot:
High-mass pure Pt and Au TARDIS models to highlight the locations of the strongest features that would appear for these elements in an early phase spectrum.
In both cases, the model spectra are compared with observed spectra of AT2017gfo to compare locations of the strongest features. We don’t see good agreement between the strongest features of Pt/Au and the observed spectra.
Slide 3:
“Simple late stage nebular models”
Assuming a KN becomes optically thin, we can then predict the strongest emission features for Pt and Au, using the new atomic data set (McCann et al. in prep). Assuming LTE level populations, we were able to use the Einstein A-values for the transitions to calculate the strengths of features. From these, we can identify where these species would be expected to influence spectra.
Plot 1:
Here we show a sequence of synthetic emission spectra for the individual ions of Pt and Au under investigation.
Plot 2:
Comparison of our synthetic LTE nebular phase spectra for Pt I, II, III and Au I, II, III and two late-phase spectra of AT2017gfo. For our models, there are features of comparable strength to those of AT2017gfo, for reasonable masses of Pt and Au.
Slide 4:
“Realistic composition models with TARDIS”
Previous works have attempted to identify specific features in the spectra of AT2017gfo (Smartt et al. 2017; Watson et al. 2019; Perego et al. 2020). These have focussed on fitting the data empirically. In this work, we are using realistic compositions that have been computed from BNS merger simulations and nucleosynthesis calculations (Bauswein et al. 2013; Goriely et al. 2013, 2015). Here we are taking these realistic compositions and attempting to forward model to match the observed sequence of spectra.
Plot:
TARDIS model for an early spectrum of AT2017gfo. The model spectrum with different contributions to the spectrum are highlighted. There is significant contribution from photons that do not interact with the ejecta. We find Sr II produces strong absorption at 8500 angstroms, and that the spectrum is dominated by line blanketing from a multitude of heavy elements at shorter wavelengths.
URL
jgillanders01@qub.ac.uk
Poster file