About the NIST-LANL Opacity Database

Karen Olsen1, Christopher J. Fontes2, C.L. Fryer2, A.L. Hungerford2, R.T. Wollaeger2, O. Korobkin2, Yuri Ralchenko1

Kilonovae (or macronovae) are astronomical transient events that produce electromagnetic emission from matter ejected by the merger of two neutron stars or a neutron star and black hole. This ejected matter is thought to be roughly 1/1000 to a few times 1/100 the mass of the Sun and move at about 1/10 the speed of light. The first kilonova associated with gravitational waves (detected by LIGO/Virgo) was observed in 2017, in signals that spanned the electromagnetic spectrum. This was a significant discovery in astronomy, galvanizing research into the broad range of physics underlying neutron star mergers.

Radiative opacities for a significant fraction of the elements in the periodic table are required to model the light curves and spectra produced by kilonovae. A first set of opacities has been calculated, under the assumption of local thermodynamic equilibrium (LTE), for the lanthanide elements with atomic number 57 ≤ Z ≤ 70. A paper describing these opacities has been published by Fontes et al, Mon. Not. Roy. Astron. Soc. 493, 4143 (2020). The second set of opacities for actinidies with 89 ≤ Z ≤ 102 was published by Fontes et al, Mon. Not. Roy. Astron. Soc. 519, 2862 (2023).

The number of (bound-bound) absorption features, or lines, associated with these f-shell elements can render the spectral simulations computationally intractable. Therefore, it is common to consider methods that group many lines within a particular photon-energy bin. The present opacities were generated with a line-binned treatment that produces tabular results that are a function of temperature and density, but independent of the particular form of the hydrodynamic expansion chosen to model a kilonova. This independence eliminates the computational expense of repeatedly calculating opacities at specific conditions within the spectral simulations of kilonovae (see Fontes et al (2020) for more details).

The total opacities presented here are split into various contributions according to the following formulae:

Κtot() = Κabs() + Κscat(),

where is the photon energy, Κscat() is the scattering contribution due to Compton scattering and Κabs() is the absorption contribution given by

Κabs() = Κbb() + Κbf() + Κff(),

where Κbb() is the bound-bound contribution due to photo-excitation, Κbf() is the bound-free contribution due to photo-ionization, and Κff() is the free-free contribution due to inverse bremsstrahlung.

The bound-bound opacities presented here are computed with the line-binned treatment, i.e. they are obtained via a discrete sum of all lines that occur between two points in the prescribed photon-energy grid, according to Eq. (2) in Fontes et al, MNRAS 493, 4143 (2020).

Those lanthanide and uranium opacities are provided here for use in kilonova modeling. A description of the relevant temperature, density and photon-energy grids was provided in the aforementioned MNRAS article by Fontes et al (2020). We reproduce that description here for convenience. The temperature grid consists of 27 values (in eV): 0.01, 0.07, 0.1, 0.14, 0.17, 0.2, 0.22, 0.24, 0.27, 0.3, 0.34, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.2, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, and 5.0. Due to the fact that only the first four ion stages were considered for each element, the opacities are most accurate for temperatures below about 2 eV, even though data are provided up to a maximum temperature of 5 eV. The density grid contains 17 values ranging from 10-20 to 10-4 g/cm3, with one value per decade. The photon energy grid contains 14,900 points for the (dimensionless) temperature-scaled quantity u = /kT, where kT is the temperature expressed in the same units as the photon energy. A description of this u grid is available in Table 1 of Frey et al, Astro. Phys. J. Supp. Ser. 204, 16 (2013).

Version history

Example of how to reference this online database:
K. Olsen, C.J. Fontes, C.L. Fryer, A.L. Hungerford, R.T. Wollaeger, O. Korobkin, Yu. Ralchenko, NIST-LANL Lanthanide Opacity Database (ver. 1.2), [Online]. Available: https://nlte.nist.gov/OPAC [1/1/2022] National Institute of Standards and Technology, Gaithersburg, MD 20899. DOI: https://doi.org/10.18434/mds2-2375
Citation Form:
Author(s)/editor(s) (Year). Title (edition), [Type of medium]. Available: URL [Access date].

v1.2, January 2023: Opacities for actinides added
v1.1, September 2021: Database description updated
v1.0: December 2020