The synthesis of polynitrogen compounds is of great importance due to their potential as high-energy-density materials (HEDM), but because of the intrinsic instability of these compounds, their synthesis and stabilization is a fundamental challenge. Polymeric nitrogen units which may be stabilized in compounds with metals at high pressure are now restricted to non-branched chains with an average N-N bond order of 1.25, limiting their HEDM performances. Herein, we demonstrate the synthesis of a novel polynitrogen compound TaN5 via a direct reaction between tantalum and nitrogen in a diamond anvil cell at circa 100 GPa. TaN5 is the first example of a material containing branched all-single-bonded nitrogen chains [N-5(5-)](infinity). Apart from that we discover two novel Ta-N compounds: TaN4 with finite N-4(4-) chains and the incommensurately modulated compound TaN2-x, which is recoverable at ambient conditions.

We present a detailed characterization of the extremely dusty main-sequence star TYC 8830 410 1. This system hosts inner planetary system dust (T (dust) approximate to 300 K) with a fractional infrared luminosity of similar to 1%. Mid-infrared spectroscopy reveals a strong, mildly crystalline solid-state emission feature. TYC 8830 410 1 (spectral type G9 V) has a 49.5 '' separation M4-type companion comoving and co-distant with it, and we estimate a system age of similar to 600 Myr. TYC 8830 410 1 also experiences "dipper"-like dimming events as detected by the All-Sky Automated Survey for Supernovae, Transiting Exoplanet Survey Satellite, and characterized in more detail with the Las Cumbres Observatory Global Telescope. These recurring eclipses suggest at least one roughly star-sized cloud of dust orbits the star in addition to assorted smaller dust structures. The extreme properties of the material orbiting TYC 8830 410 1 point to dramatic dust-production mechanisms that likely included something similar to the giant impact event thought to have formed the Earth-Moon system, although hundreds of millions of years after such processes are thought to have concluded in the solar system. TYC 8830 410 1 holds promise to deliver significant advances in our understanding of the origin, structure, and evolution of extremely dusty inner planetary systems.

Following the discovery of high-temperature superconductivity in the La-H system, we studied the formation of new chemical compounds in the barium-hydrogen system at pressures from 75 to 173GPa. Using in situ generation of hydrogen from NH3BH3, we synthesized previously unknown superhydride BaH12 with a pseudocubic (fcc) Ba sublattice in four independent experiments. Density functional theory calculations indicate close agreement between the theoretical and experimental equations of state. In addition, we identified previously known P6/mmm-BaH2 and possibly BaH10 and BaH6 as impurities in the samples. Ab initio calculations show that newly discovered semimetallic BaH12 contains H-2 and H-3(-) molecular units and detached H-12 chains which are formed as a result of a Peierls-type distortion of the cubic cage structure. Barium dodecahydride is a unique molecular hydride with metallic conductivity that demonstrates the superconducting transition around 20K at 140GPa. Metallization of pure hydrogen via overlapping of electronic bands requires high pressure above 3 Mbar. Here the authors study the Ba-H system and discover a unique superhydride BaH12 that contains molecular hydrogen, which demonstrates metallic properties and superconductivity below 1.5 Mbar.

We present near-infrared Large Binocular Telescope LMIRCam imagery of the disk around the Herbig Ae/Be star AB Aurigae. A comparison of the surface brightness at K-s (2.16 itm), H2O narrowband (3.08 mu m), and L' (3.7 mu m) allows us to probe the presence of icy grains in this (pre)transitional disk environment. By applying reference differential imaging point-spread function subtraction, we detect the disk at high signal-to-noise ratios in all three bands. We find strong morphological differences between the bands, including asymmetries consistent with the observed spiral arms within 100 au in . An apparent deficit of scattered light at 3.08 mu m relative to the bracketing wavelengths (K-s and L') is evocative of ice absorption at the disk surface layer. However, the Delta(K-s - H2O) color is consistent with grains with little to no ice (0%-5% by mass). The Delta(H2O - L') color, conversely, suggests grains with a much higher ice mass fraction (-0.68), and the two colors cannot be reconciled under a single grain population model. Additionally, we find that the extremely red Delta(K-s - L') disk color cannot be reproduced under conventional scattered light modeling with any combination of grain parameters or reasonable local extinction values. We hypothesize that the scattering surfaces at the three wavelengths are not colocated, and that the optical depth effects in each wavelength result from probing the grain population at different disk surface depths. The morphological similarity between K-s and H2O suggests that their scattering surfaces are near one another, lending credence to the Delta(K-s - H2O) disk color constraint of <5% ice mass fraction for the outermost scattering disk layer.

Carbon-bearing phases show a rich variety of structural transitions as an adaptation to pressure. Of particular interest is the crossover from sp(2) carbon to sp(3) carbon, as physical and chemical properties of carbon in these distinct electronic configurations are very different. In this chapter we review pressure-induced sp(2)-sp(3) transitions in elemental carbon, carbonates, and hydrocarbons.

We present follow-up photometry and spectroscopy of ZTF J0328-1219, strengthening its status as a white dwarf exhibiting transiting planetary debris. Using TESS and Zwicky Transient Facility photometry, along with follow-up high-speed photometry from various observatories, we find evidence for two significant periods of variability at 9.937 and 11.2 hr. We interpret these as most likely the orbital periods of different debris clumps. Changes in the detailed dip structures within the light curves are observed on nightly, weekly, and monthly timescales, reminiscent of the dynamic behavior observed in the first white dwarf discovered to harbor a disintegrating asteroid, WD 1145+017. We fit previously published spectroscopy along with broadband photometry to obtain new atmospheric parameters for the white dwarf, with M-star = 0.731 +/- 0.023 M-circle dot, T-eff = 7630 +/- 140 K, and [Ca/He] = - 9.55 +/- 0.12. With new high-resolution spectroscopy, we detect prominent and narrow Na D absorption features likely of circumstellar origin, with velocities 21.4 +/- 1.0 km s(-1) blueshifted relative to atmospheric lines. We attribute the periodically modulated photometric signal to dusty effluents from small orbiting bodies such as asteroids or comets, but we are unable to identify the most likely material that is being sublimated, or otherwise ejected, as the environmental temperatures range from roughly 400 to 700 K.

The application of pressure has been speculated to boost the search for high-temperature superconductors, especially in superhydrides. However, the applied pressure as high as hundreds of GPa needed to create superconductivity in those materials limits their technological application. Finding a route to achieve the high-temperature superconductivity at near-ambient conditions is attractive. By choosing a phase-change alloy Ge2Sb2Te5, we study the phase evolution of this material with pressure from the trigonal phase through the amorphous to the body-centered cubic one by the measurements of x-ray diffraction, Raman scattering, resistivity, and Hall coefficient. Superconductivity is observed to take place in the last two phases and can maintain at nearly ambient pressure in the decompression run. Pressure-induced disorder is found to be the key for holding superconductivity in the compressed phase-change alloy.

We present ALMA 1.3 mm observations of the HD 53143 debris disk-the first infrared or millimeter image produced of this similar to 1 Gyr old solar analog. Previous HST STIS coronagraphic imaging did not detect flux along the minor axis of the disk, which could suggest a face-on geometry with two clumps of dust. These ALMA observations reveal a disk with a strikingly different structure. In order to fit models to the millimeter visibilities and constrain the uncertainties on the disk parameters, we adopt a Markov Chain Monte Carlo approach. This is the most eccentric debris disk observed to date with a forced eccentricity of 0.21 +/- 0.02, nearly twice that of the Fomalhaut debris disk, and also displays an apocenter glow. Although this eccentric model fits the outer debris disk well, significant interior residuals remain, which may suggest a possible edge-on inner disk, which remains unresolved in these observations. Combined with the observed structure difference between HST and ALMA, these results suggest a potential previous scattering event or dynamical instability in this system. We also note that the stellar flux changes considerably over the course of our observations, suggesting flaring at millimeter wavelengths. Using simultaneous TESS observations, we determine the stellar rotation period to be 9.6 +/- 0.1 days.

Nitrogen and water are very abundant in nature; however, the way they chemically react at extreme pressure-temperature conditions is unknown. Below 6 GPa, they have been reported to form clathrate compounds. Here, we present Raman spectroscopy and x-ray diffraction studies in the H2O-N-2 system at high pressures up to 140 GPa. We find that clathrates, which form locally in our diamond cell experiments above 0.3 GPa, transform into a fine grained state above 6 GPa, while there is no sign of formation of mixed compounds. We point out size effects in fine grained crystallites, which result in peculiar Raman spectra in the molecular regime, but x-ray diffraction shows no additional phase or deviation from the bulk behavior of familiar solid phases. Moreover, we find no sign of ice doping by nitrogen, even in the regimes of stability of nonmolecular nitrogen.

Companions embedded in the cavities of transitional circumstellar disks have been observed to exhibit excess luminosity at H alpha, an indication that they are actively accreting. We report 5 yr (2013-2018) of monitoring of the position and H alpha excess luminosity of the embedded, accreting low-mass stellar companion HD 142527 B from the MagAO/VisAO instrument. We use pyklip, a Python implementation of the Karhunen-Loeve Image Processing algorithm, to detect the companion. Using pyklip forward modeling, we constrain the relative astrometry to 1-2 mas precision and achieve sufficient photometric precision (+/- 0.2 mag, 3% error) to detect changes in the H alpha contrast of the companion over time. In order to accurately determine the relative astrometry of the companion, we conduct an astrometric calibration of the MagAO/VisAO camera against 20 yr of Keck/NIRC2 images of the Trapezium cluster. We demonstrate agreement of our VisAO astrometry with other published positions for HD 142527 B, and use orbitize! to generate a posterior distribution of orbits fit to the relative astrometry of HD 142527 B. Our data suggest that the companion is close to periastron passage, on an orbit significantly misaligned with respect to both the wide circumbinary disk and the recently observed inner disk encircling HD 142527 A. We translate observed H alpha contrasts for HD 142527 B into mass accretion rate estimates on the order of 4-9 x 10(-10) M (circle dot) yr(-1). Photometric variation in the H alpha excess of the companion suggests that the accretion rate onto the companion is variable. This work represents a significant step toward observing accretion-driven variability onto protoplanets, such as PDS 70 b&c.