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Polycyclic aromatic hydrocarbon (PAH) molecules are abundant and widespread throughout the Universe, as revealed by their distinctive set of emission bands at 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 μm, which are characteristic of their vibrational modes. They are ubiquitously seen in a wide variety of astrophysical regions, ranging from planet-forming disks around young stars to the interstellar medium of the Milky Way and other galaxies out to high redshifts at z ≳ 4. PAHs profoundly influence the thermal budget and chemistry of the interstellar medium by dominating the photoelectric heating of the gas and controlling the ionization balance. Here I review the current state of knowledge of the astrophysics of PAHs, focusing on their observational characteristics obtained from the Spitzer Space Telescope and their diagnostic power for probing the local physical and chemical conditions and processes. Special attention is paid to the spectral properties of PAHs and their variations revealed by the Infrared Spectrograph onboard Spitzer across a much broader range of extragalactic environments (for example, distant galaxies, early-type galaxies, galactic halos, active galactic nuclei and low-metallicity galaxies) than was previously possible with the Infrared Space Observatory or any other telescope facilities. Also highlighted is the relation between the PAH abundance and the galaxy metallicity established for the first time by Spitzer. Spitzer revealed the power of astrophysical polycyclic aromatic hydrocarbon molecules to probe the local physical and chemical conditions and processes, for example, establishing the relation between their abundance and galaxy metallicity for the first time.

During October 2019 and March 2020, the luminous red supergiant Betelgeuse demonstrated an unusually deep minimum of its brightness. It became fainter by more than one magnitude and this is the most significant dimming observed in the recent decades. While the reason for the dimming is debated, pre-phase of supernova explosion, obscuring dust, or changes in the photosphere of the star were suggested scenarios. Here, we present spectroscopic studies of Betelgeuse using high-resolution and high signal-to-noise ratio near-infrared spectra obtained at Weihai Observatory on four epochs in 2020 covering the phases of during and after dimming. We show that the dimming episode is caused by the dropping of its effective temperature by at least 170 K on 2020 January 31, that can be attributed to the emergence of a large dark spot on the surface of the star. The reason of the dimming of Betelgeuse is debated. Here, the authors show effective temperature decrease that can be explained by a large spot.

First discovered in the Milky Way nearly six decades ago4, the 2,175 A extinction bump is generally thought to be caused by nanoparticles containing aromatic carbon5 - carbon atoms arranged in flat hexagonal rings. Another common type of aromatic material is a polycyclic aromatic hydrocarbon (PAH), which is composed of multiple hexagonal benzene rings that come together to form flat sheets edged with hydrogen atoms (Fig. 1). Supernovae could have provided 'seed' dust to initiate the growth of graphite grains in the interstellar medium, and this graphite could then have shattered, offering a viable mechanism for ma king graphite nanoparticles. A study of the galaxy SPT0418-47 (observed less than 1.5 billion years after the Big Bang) showed that the spatial distribution of light from PAHs differs from that of larger dust particles, suggesting that the two particle types are not found in the same location13.

The obscuration of light from a distant galaxy has raised the possibility that a type of carbon dust existed in the earliest epochs of the Universe — challenging the idea that stars had not yet evolved enough to make such material. Extinction bump detected for a distant Galaxy.

The interstellar traveller, 2I/Borisov, is the first clearly active extrasolar comet ever detected in our Solar System. We obtained high-resolution interferometric observations of 2I/Borisov with the Atacama Large Millimeter/submillimeter Array (ALMA) and multi-colour optical observations with the Very Large Telescope (VLT) to gain a comprehensive understanding of the dust properties of this comet. We found that the dust coma of 2I/Borisov consists of compact ‘pebbles’ of radii exceeding ~1 mm, suggesting that the dust particles have experienced compaction through mutual impacts during the bouncing collision phase in the protoplanetary disk. We derived a dust mass-loss rate of ≳200 kg s −1 and a dust-to-gas ratio ≳3. Our long-term monitoring of 2I/Borisov with the VLT indicates a steady dust mass-loss with no significant dust fragmentation and/or sublimation occurring in the coma. We also detected emissions from carbon monoxide (CO) gas with ALMA and derived the gas production rate of Q (CO) = (3.3 ± 0.8) × 10 26  s −1 . We found that the CO/H 2 O mixing ratio of 2I/Borisov changed drastically before and after perihelion, indicating the heterogeneity of the cometary nucleus, with components formed at different locations beyond the volatile snow-line with different chemical abundances. Our observations suggest that 2I/Borisov’s home system, much like our own system, experienced efficient radial mixing from the innermost parts of its protoplanetary disk to beyond the frost line of CO. The dust in the dust coma of interstellar object 2I/Borisov is large (exceeding ~1 mm radius) and compact, differing from the fluffy aggregates typically found in Solar System comets. This compact dust is presumably a result of impacts in the comet’s home system, and suggests 2I/Borisov formed in a collapsing pebble cloud.

We model the ~ 1–19 μ m infrared (IR) extinction curve toward the Galactic Center (GC) in terms of the standard silicate-graphite interstellar dust model. The grains are taken to have a power law size distribution with an exponential decay above some size. The best-fit model for the GC IR extinction constrains the visual extinction to be A V ~ 38–42 mag. The limitation of the model, i.e., its difficulty in simultaneously reproducing both the steep ~ 1–3 μ m near-IR extinction and the flat ~ 3–8 μ m mid-IR extinction is discussed. We argue that this difficulty could be alleviated by attributing the extinction toward the GC to a combination of dust in different environments: dust in diffuse regions (characterized by small R V and steep near-IR extinction), and dust in dense regions (characterized by large R V and flat UV extinction).

The outburst of a Sun-like star offers a rare opportunity to witness the making of silicate crystals in the star's planet-forming disk, providing key information about the formation of comets and the Solar System. Planet formation: crystals from the dust Protoplanetary disks, the clouds of interstellar gas and dust thought to be the precursors of solar systems, ours included, consist largely of amorphous grains of silicate. Yet the grains found in comets and meteorites (representing the early Solar System), and traced in the spectra of young stars, include large crystalline grains that must have undergone annealing or condensation at temperatures above 1,000 K, despite being surrounded by material that has never experienced such heating. This apparent anomaly has been the subject of much discussion and theorizing. Two papers published in this issue add to the discussion. Ábrahám et al . report mid-infrared features in the outburst spectrum of the young solar-like star EX Lupi that they attribute to crystalline forsterite. These features were not present before EX Lupi's recent outbust, so this may be the first direct observation of the crystal formation process in a celestial object. Annealing by heat from a stellar outburst is a crystal source not previously considered for protoplanetary disks. Dejan Vinković suggests another new mechanism that might produce crystals: infrared light arising from a protoplanetary disk can in theory lift grains bigger than 1 μm out of the inner disk, where they are pushed outwards by stellar radiation pressure while gliding above the disk. Grains re-enter the disk at radii where it is too cold to produce sufficient infrared radiation-pressure support for a given grain size and solid density.