Elementary Composite Binary and Grain Alignment Locked in Dust Growth

Planets are known to grow out of a star-encircling disk of the gas and dust inherited from an interstellar cloud; their formation is thought to begin with coagulation of submicron dust grains into aggregates, the first foundational stage of planet formation. However, with nanoscale and submicron solids unobservable directly in the interstellar medium (ISM) and protoplanetary disks, how dust grains grow is unclear, as are the morphology and structure of interstellar grains and the whereabouts and form of “missing iron.” Here we show an elementary composite binary in 3D sub-10 nm detail—and the alignments of its two subunits and nanoinclusions and a population of elongated composite grains locked in a primitive cosmic dust particle—noninvasively uncovered with phase-contrast X-ray nanotomography. The binary comprises a pair of oblate, quasi-spheroidal grains whose alignment and shape meet the astrophysical constraints on polarizing interstellar grains. Each member of the pair contains a high-density core of octahedral nanocrystals whose twin relationship is consistent with the magnetite’s diagnostic property at low temperatures, with a mantle exhibiting nanoscale heterogeneities, rounded edges, and pitted surfaces. This elongated binary evidently formed from an axially aligned collision of the two similar composite grains whose core–mantle structure and density gradients are consistent with interstellar processes and astronomical evidence for differential depletion. Our findings suggest that the ISM is threaded with dust grains containing preferentially oriented iron-rich magnetic nanocrystals that hold answers to astronomical problems from dust evolution, grain alignment, and the structure of magnetic fields to planetesimal growth.

The ablation of gas clouds by blazar jets

Context. Flaring activity in blazars can last for vastly different timescales, and it may be the result of density enhancements in the jet flow that result from the intrusion of an interstellar cloud into the jet. Aims. We investigate the lightcurves expected from the ablation of gas clouds by the blazar jet under various cloud and jet configurations. Methods. We derived the semi-analytical formulae describing the ablation process of a hydrostatic cloud and performed parameter scans of artificial set-ups over both cloud and jet parameter spaces. We then used parameters obtained from measurements of various cloud types to produce lightcurves of these cloud examples. Results. The parameter scans show that a vast zoo of symmetrical lightcurves can be realized. Both cloud and emission region parameters significantly influence the duration and strength of the flare. The scale height of the cloud is one of the most important parameters as it determines the shape of the lightcurve. In turn, important cloud parameters can be deduced from the observed shape of a flare. The example clouds result in significant flares lasting for various timescales.

What lies immediately outside of the heliosphere in the very local interstellar medium (VLISM): morphology of the Local Interstellar Cloud, its hydrogen hole, Stromgren Shells, and 60Fe accretion

<p> We describe the very local interstellar medium (VLISM)<br>immediately outside of the outer heliosphere. The VLISM consists <br>of four partially ionized clouds - the Local Interstellar Cloud (LIC), <br>G cloud, Blue cloud, and Aql cloud that are in contact with the outer <br>heliosphere, and ionized gas produced by extreme-UV radiation <br>primarily from the star Epsilon CMa. We construct the <br>three-dimensional shape of the LIC based on interstellar line <br>absorption along 62 sightlines and show that in the direction of <br>Epsilon CMa, Beta CMa, and Sirius B the neutral hydrogen column <br>density from the center of the LIC looking outward is a minimum. <br>We call this region the ``hydrogen hole''. In this direction, the <br>presence of Blue cloud absorption and the absence of LIC absorption <br>can be simply explained by the Blue cloud lying just outside of the <br>heliosphere. We propose that the outer edge of the Blue cloud is a <br>Str\"omgren shell driven toward the heliosphere by high pressures in <br>the H II region. The outer edges of other clouds facing Epsilon CMa <br>are likely also Stromgren shells. Unlike previous models, the LIC<br>surrounds less than half of the heliosphere.</p><p>We find that the vectors of neutral and ionized helium flowing<br>through the heliosphere are inconsistent with the mean LIC flow <br>vector and describe several possible explanations. The ionization<br>of nearby intercloud gas is consistent with photo-ionization by <br>Epsilon CMa and hot white dwarfs without requiring additional <br>sources of ionization or million degree plasma. In the upwind <br>direction, the heliosphere is passing through an environment of <br>several LISM clouds, which may explain the recent influx of <br>interstellar grains containing 60Fe from supernova ejecta measured <br>in Antarctica snow. The Sun will leave the outer partof the LIC <br>in less than 1900 years, perhaps this year, to either enter the <br>partially ionized G cloud or a highly ionized intercloud layer. <br>The heliosphere will change in either scenario. An instrumented <br>deep space probe sending back in situ plasma and magnetic field <br>measurements from 500-1,000 AU is needed to understand the <br>heliosphere environment rather than integrated data along the <br>sightlines to stars.  </p>

Ammonium salts are a reservoir of nitrogen on a cometary nucleus and possibly on some asteroids

The measured nitrogen-to-carbon ratio in comets is lower than for the Sun, a discrepancy which could be alleviated if there is an unknown reservoir of nitrogen in comets. The nucleus of comet 67P/Churyumov-Gerasimenko exhibits an unidentified broad spectral reflectance feature around 3.2 micrometers, which is ubiquitous across its surface. On the basis of laboratory experiments, we attribute this absorption band to ammonium salts mixed with dust on the surface. The depth of the band indicates that semivolatile ammonium salts are a substantial reservoir of nitrogen in the comet, potentially dominating over refractory organic matter and more volatile species. Similar absorption features appear in the spectra of some asteroids, implying a compositional link between asteroids, comets, and the parent interstellar cloud.

Modeling sulfur depletion in interstellar clouds

Context. The elemental depletion of interstellar sulfur from the gas phase has been a recurring challenge for astrochemical models. Observations show that sulfur remains relatively non-depleted with respect to its cosmic value throughout the diffuse and translucent stages of an interstellar molecular cloud, but its atomic and molecular gas-phase constituents cannot account for this cosmic value toward lines of sight containing higher-density environments. Aims. We have attempted to address this issue by modeling the evolution of an interstellar cloud from its pristine state as a diffuse atomic cloud to a molecular environment of much higher density, using a gas-grain astrochemical code and an enhanced sulfur reaction network. Methods. A common gas-grain astrochemical reaction network has been systematically updated and greatly extended based on previous literature and previous sulfur models, with a focus on the grain chemistry and processes. A simple astrochemical model was used to benchmark the resulting network updates, and the results of the model were compared to typical astronomical observations sourced from the literature. Results. Our new gas-grain astrochemical model is able to reproduce the elemental depletion of sulfur, whereby sulfur can be depleted from the gas-phase by two orders of magnitude, and that this process may occur under dark cloud conditions if the cloud has a chemical age of at least 106 years. The resulting mix of sulfur-bearing species on the grain ranges across all the most common chemical elements (H/C/N/O), not dissimilar to the molecules observed in cometary environments. Notably, this mixture is not dominated simply by H2S, unlike all other current astrochemical models. Conclusions. Despite our relatively simple physical model, most of the known gas-phase S-bearing molecular abundances are accurately reproduced under dense conditions, however they are not expected to be the primary molecular sinks of sulfur. Our model predicts that most of the “missing” sulfur is in the form of organo-sulfur species that are trapped on grains.