Lanthanides or dust in kilonovae: lessons learned from GW170817

TL;DR

This study investigates whether dust formation could explain the near-infrared emission in GW170817's kilonova, concluding that dust is unlikely the main cause based on lightcurve analysis and nucleosynthesis models.

Contribution

The paper tests the dust extinction hypothesis for kilonova emission and finds it inconsistent with observed lightcurves and nucleosynthesis predictions.

Findings

Approximately 10^{-5} solar masses of carbon dust could reproduce lightcurves.

Dust cooling from 2000 K to 1500 K over a week was observed.

Nucleosynthesis models predict at most 10^{-9} solar masses of carbon formation.

Abstract

The unprecedented optical and near-infrared lightcurves of the first electromagnetic counterpart to a gravitational wave source, GW170817, a binary neutron star merger, exhibited a strong evolution from blue to near-infrared (a so-called 'kilonova' or 'macronova'). The emerging near-infrared component is widely attributed to the formation of r-process elements which provide the opacity to shift the blue light into the near infrared. An alternative scenario is that the light from the blue component gets extinguished by dust formed by the kilonova and subsequently is re-emitted at near-infrared wavelengths. We here test this hypothesis using the lightcurves of AT2017gfo, the kilonova accompanying GW170817. We find that of order 10$^{-5}$ $M_\odot$ of carbon is required to reproduce the optical/near-infrared lightcurves as the kilonova fades. This putative dust cools from $\sim$ 2000 K at $\sim$ 4 d after the event to $\sim$ 1500 K over the course of the following week, thus requiring dust with a high condensation temperature, such as carbon. We contrast this with the nucleosynthetic yields predicted by a range of kilonova wind models. These suggest that at most 10$^{-9}$ $M_\odot$ of carbon is formed. Moreover, the decay in the inferred dust temperature is slower than that expected in kilonova models. We therefore conclude that in current models of the blue component of the kilonova, the near-infrared component in the kilonova accompanying GW170817 is unlikely to be due to dust.