– University of Copenhagen

Dust outside our local group of galaxies is still surprisingly poorly understood, especially given its fundamental importance in the determination of dark energy acceleration via Type Ia supernova extinction corrections, its contribution to biasing our understanding of the history of star‐formation, its effects on the formation of early stars, and its role in the formation of planets and hence life.

Dust forms during the late stages of stellar evolution and is important in the mass loss of lower mass stars (when they reach the asymptotic giant branch, AGB). For more massive stars, dust is likely able to form in the supernova remnant if the conditions aren't too harsh. In any scenario, dust production will only be possible for a relative short period of stellar evolution, on time scales of a few to a few thousand years. The necessary conditions for dust formation are modest temperature (< 2000 K), high supers aturation pressure needed for nucleation directly from the gas phase, sufficient densities for grain growth, and limited dust destruction by evaporation, sputtering and/or shocks.

From observations, we know that dust must form on short timescales in the early universe, simply because significant dust is observed at epochs less than about one billion years after the Big Bang and there is some evidence of a change in the dominant dust production mechanism at high redshift, as hinted at by unusual extinction curves. In the present universe, long lived stars seem to be the main dust producers but this cannot have been the case early on. Until now, it has been assumed that due to this time constraint and the prediction that early stars (population III and II.5) were exclusively very high, that there was no alternative to supernovae. This is in spite of observations of supernovae and their remnants indicating that they fall short by at least one to two orders of magnitude in the total mass of dust they form.

A radical solution is that intermediate mass stars (the high mass end of the asymptotic giant branch) may contribute very substantially to dust mass at early times, but it is very uncertain how much dust can be formed from high‐mass AGB stars, and whether enough intermediate mass stars can be produced from population III and population II.5 stars. Very little is solidly known about the production and destruction of dust in the high‐mass end of late stage stellar evolution compared to the low‐mass end, and the effectiveness of these high‐mass AGB stars in injecting dust into the interstellar medium at different metallicities.