Half of all the elements in the universe heavier than iron were created by rapid neutron capture. The theory for this astrophysical 'r-process' was worked out six decades ago and requires an enormous neutron flux to make the bulk of these elements. 1 Where this happens is still debated. 2 A key piece of missing evidence is the identification of freshly-synthesised r-process elements in an astrophysical site. Current models 3-5 and circumstantial evidence 6 point to neutron star mergers as a probable r-process site, with the optical/infrared 'kilonova' emerging in the days after the merger a likely place to detect the spectral signatures of newly-created neutron-capture elements. 7-9 The kilonova, AT2017gfo, emerging from the gravitational-wave-discovered neutron star merger, GW170817, 10 was the first kilonova where detailed spectra were recorded. When these spectra were first reported 11, 12 it was argued that they were broadly consonant with an outflow of radioactive heavy elements, however, there was no robust identification of any element. Here we report the identification of the neutron-capture element strontium in a re-analysis of these spectra. The detection of a neutron-capture element associated with the collision of two extreme-density stars establishes the origin of r-process elements in neutron star mergers, and demonstrates that neutron stars comprise neutron-rich matter 13 .The most detailed information available for a kilonova comes from a series of spectra of AT2017gfo taken over several weeks with the medium resolution, ultraviolet (320 nm) to near-infrared (2,480 nm) spectrograph, X-shooter, mounted at the Very Large Telescope at the European Southern Observatory. These spectra 11, 12 , allow us to track the evolution of the kilonova's primary electromagnetic output from 1.5 days until 10 days after the event. Detailed modelling of these spectra has yet to be done owing to the limited understanding of the phenomenon and the expectation that a very large number of moderate to weak lanthanide lines with unknown oscillator strengths would dominate the spectra 14,15 . Despite the expected complexity, we sought to identify individual elements in the early spectra because these spectra are well-reproduced by relatively simple models 11 .The first epoch spectrum can be reproduced over the entire observed spectral range with a single-temperature blackbody with an observed temperature 4, 800 K. The two major deviations short of 1 µm from a pure blackbody are due to two very broad (∼ 0.2c) absorption components. These components are observed centred at about 350 nm and 810 nm (Fig. 1). The shape of the ultraviolet absorption component is not well constrained because it lies close to the edge of our sensitivity limit and may simply be cut off below about 350 nm. The presence of the absorption feature at 810 nm at this epoch has been noted in earlier publications 11,12 .The fact that the spectrum is very well reproduced by a single temperature blackbody in the first epoch suggests a population of states 0.3...

We present mid-infrared (MIR) observations of the Type II-plateau supernova (SN) 2004et, obtained with the Spitzer Space Telescope between 64 and 1406 days past explosion. Late-time optical spectra are also presented. For the period 300-795 days past explosion, we argue that the spectral energy distribution (SED) of SN 2004et comprises (1) a hot component due to emission from optically thick gas, as well as free-bound radiation; (2) a warm component due to newly formed, radioactively heated dust in the ejecta; and (3) a cold component due to an IR echo from the interstellar-medium dust of the host galaxy, NGC 6946. There may also have been a small contribution to the IR SED due to free-free emission from ionized gas in the ejecta. We reveal the first-ever spectroscopic evidence for silicate dust formed in the ejecta of a supernova. This is supported by our detection of a large, but progressively declining, mass of SiO. However, we conclude that the mass of directly detected ejecta dust grew to no more than a few times 10 −4 M . We also provide evidence that the ejecta dust formed in comoving clumps of fixed size. We argue that, after about two years past explosion, the appearance of wide, box-shaped optical line profiles was due to the impact of the ejecta on the progenitor circumstellar medium and that the subsequent formation of a cool, dense shell was responsible for a later rise in the MIR flux. This study demonstrates the rich, multifaceted ways in which a typical core-collapse supernova and its progenitor can produce and/or interact with dust grains. The work presented here adds to the growing number of studies that do not support the contention that SNe are responsible for the large mass of observed dust in high-redshift galaxies.

With the aim of investigating whether stellar sources can account for the ≥10 8 M dust masses inferred from mm/sub-mm observations of samples of 5 < z < 6.4 quasars, we develop a chemical evolution model which follows the evolution of metals and dust on the stellar characteristic lifetimes, taking into account dust destruction mechanisms. Using a grid of stellar dust yields as a function of the initial mass and metallicity over the range 1-40 M and 0-1 Z , we show that the role of asymptotic giant branch (AGB) stars in cosmic dust evolution at high redshift might have been overlooked. In particular, we find that (i) for a stellar population forming according to a present-day Larson initial mass function (IMF) with m ch = 0.35 M , the characteristic time-scale at which AGB stars dominate dust production ranges between 150 and 500 Myr, depending both on the assumed star formation history and on the initial stellar metallicity; (ii) this result is only moderately dependent on the adopted stellar lifetimes, but it is significantly affected by variations of the IMF: for a m ch = 5 M , dust from AGB starts to dominate only on time-scales larger than 1 Gyr and SNe are found to dominate dust evolution when m ch ≥10 M . We apply the chemical evolution model with dust to the host galaxy of the most distant quasar at z = 6.4, SDSS J1148+5251. Given the current uncertainties on the star formation history of the host galaxy, we have considered two models: (i) the star formation history obtained in a numerical simulation by Li et al. which predicts that a large stellar bulge is already formed at z = 6.4, and (ii) a constant star formation rate of 1000 M yr −1 , as suggested by the observations if most of the far-infrared luminosity is due to young stars. The total mass of dust predicted at z = 6.4 by the first model is 2 × 10 8 M , within the range of values inferred by observations, with a substantial contribution (∼80 per cent) of AGB dust. When a constant star formation rate is adopted, the contribution of AGB dust decreases to ∼50 per cent but the total mass of dust formed is a factor of 2 smaller. Both models predict a rapid enrichment of the interstellar medium with metals and a relatively mild evolution of the carbon abundance, in agreement with observational constraints. This supports the idea that stellar sources can account for the dust observed but show that the contribution of AGB stars to dust production cannot be neglected, even at the most extreme redshifts currently accessible to observations.

The Lyman α emission line is an essential diagnostic tool for probing galaxy formation and evolution. Not only is it commonly the strongest observable line from high-redshift galaxies but from its shape detailed information about its host galaxy can be revealed. However, due to the scattering nature of Lyα photons increasing their path length in a non-trivial way, if dust is present in the galaxy the line may be severely suppressed and its shape altered. In order to interpret observations correctly, it is thus of crucial significance to know how much of the emitted light actually escapes the galaxy.In the present work, using a combination of high-resolution cosmological hydro-simulations and an adaptively refinable Monte Carlo Lyα radiative transfer code including an advanced model of dust, the escape fractions f esc of Lyα radiation from high-redshift (z = 3.6) galaxies are calculated. In addition to the average escape fraction, the variation of f esc in different directions and from different parts of the galaxies is investigated, as well as the effect on the emergent spectrum.Escape fractions from a sample of simulated galaxies of representative physical properties are found to decrease for increasing galaxy virial mass M vir , from f esc approaching unity for M vir ∼ 10 9 M ⊙ to f esc less than 10% for M vir ∼ 10 12 M ⊙ . In spite of the dust being almost grey, it is found that the emergent spectrum is affected non-uniformly, with the escape fraction of photons close to the line center being much higher than of those in the wings, thus effectively narrowing the Lyα line.

The large amounts of dust detected in sub-millimeter galaxies and quasars at high redshift pose a challenge to galaxy formation models and theories of cosmic dust formation. At z > 6 only stars of relatively high mass (> 3 Msun) are sufficiently short-lived to be potential stellar sources of dust. This review is devoted to identifying and quantifying the most important stellar channels of rapid dust formation. We ascertain the dust production efficiency of stars in the mass range 3-40 Msun using both observed and theoretical dust yields of evolved massive stars and supernovae (SNe) and provide analytical expressions for the dust production efficiencies in various scenarios. We also address the strong sensitivity of the total dust productivity to the initial mass function. From simple considerations, we find that, in the early Universe, high-mass (> 3 Msun) asymptotic giant branch stars can only be dominant dust producers if SNe generate <~ 3 x 10^-3 Msun of dust whereas SNe prevail if they are more efficient. We address the challenges in inferring dust masses and star-formation rates from observations of high-redshift galaxies. We conclude that significant SN dust production at high redshift is likely required to reproduce current dust mass estimates, possibly coupled with rapid dust grain growth in the interstellar medium.Comment: 72 pages, 9 figures, 5 tables; to be published in The Astronomy and Astrophysics Revie