  Direct detection of gravitational waves (GW) on Aug. 17, 2017, propagating
from a binary neutron star merger, opened the era of multimessenger astronomy.
The ejected material from neutron star mergers, or kilonova, is a good
candidate for optical and near infrared follow-up observations after the
detection of GWs. The kilonova from the ejecta of GW1780817 provided the first
evidence for the astrophysical site of the synthesis of heavy nuclei through
the rapid neutron capture process or rprocess. Since properties of the emission
are largely affected by opacities of the ejected material, enhancements in the
available, but limited rprocess atomic data have been motivated recently.
However, given the complexity of the electronic structure of these heavy
elements, considerable efforts are still needed to converge to a reliable set
of atomic structure data. The aim of this work is to alleviate this situation
for low charge state elements in the Os-like isoelectronic sequence. In this
regard, the general purpose relativistic atomic structure packages (GRASP0 and
GRASP2K) were used to obtain energy levels and transition probabilities (E1 and
M1). We provide line lists and expansion opacities for a range of r-process
elements. We focus here on the Os isoelectronic sequence (Os I, Ir II, Pt III,
Au IV, Hg V). The results are benchmarked against existing experimental data
and prior calculations, and predictions of emission spectra relevant to
kilonovae are provided. Finestructure (M1) lines in the infrared potentially
observable by the James Webb Space Telescope are highlighted.
