We report on H-band spectra of chemically peculiar Mercury–Manganese (HgMn) stars obtained via the SDSS/APOGEE survey. As opposed to other varieties of chemically peculiar stars such as classical Ap/Bp stars, HgMn stars lack strong magnetic fields and are defined by extreme overabundances of Mn, Hg, and other heavy elements. A satisfactory explanation for the abundance patterns remains to be determined, but low rotational velocity is a requirement and involvement in binary/multiple systems may be as well. The APOGEE HgMn sample currently consists of 269 stars that were identified among the telluric standard stars as those whose metallic absorption content is limited to or dominated by the H-band Mn ii lines. Due to the fainter magnitudes probed by the APOGEE survey as compared to past studies, only 9/269 stars in the sample were previously known as HgMn types. The 260 newly identified HgMn stars represents a more than doubling of the known sample. At least 32 per cent of the APOGEE sample are found to be binary or multiple systems, and from multi-epoch spectroscopy, we were able to determine orbital solutions for at least one component in 32 binaries. Many of the multilined systems include chemically peculiar companions, with noteworthy examples being the HgMn+Ap/Bp binary HD 5429, the HgMn+HgMn binary HD 298641, and the HgMn+Bp Mn + Am triple system HD 231263. As a further peculiarity, roughly half of the sample produces narrow emission in the C i 16895 Å line, with widths and radial velocities that match those of the Mn ii lines.

CO2 ice is present on the trailing hemisphere of Ariel but is mostly absent from its leading hemisphere. The leading/trailing hemispherical asymmetry in the distribution of CO2 ice is consistent with radiolytic production of CO2, formed by charged particle bombardment of H2O ice and carbonaceous material in Ariel’s regolith. This longitudinal distribution of CO2 on Ariel was previously characterized using 13 near-infrared reflectance spectra collected at “low” sub-observer latitudes between 30°S and 30°N. Here we investigated the distribution of CO2 ice on Ariel using 18 new spectra: 2 collected over low sub-observer latitudes, 5 collected at “mid” sub-observer latitudes (31°N–44°N), and 11 collected over “high” sub-observer latitudes (45°N–51°N). Analysis of these data indicates that CO2 ice is primarily concentrated on Ariel’s trailing hemisphere. However, CO2 ice band strengths are diminished in the spectra collected over mid and high sub-observer latitudes. This sub-observer latitudinal trend may result from radiolytic production of CO2 molecules at high latitudes and subsequent migration of this constituent to low-latitude cold traps. We detected a subtle feature near 2.13 μm in two spectra collected over high sub-observer latitudes, which might result from a “forbidden” transition mode of CO2 ice that is substantially stronger in well-mixed substrates composed of CO2 and H2O ice, consistent with regolith-mixed CO2 ice grains formed by radiolysis. Additionally, we detected a 2.35 μm feature in some low sub-observer latitude spectra, which might result from CO formed as part of a CO2 radiolytic production cycle.

Many tidally locked icy satellites in the outer solar system show leading/trailing hemispherical asymmetries in the strength of near-infrared (NIR) H2O ice absorption bands, in which the absorption bands are stronger on the leading hemisphere. This is often attributed to a combination of magnetospheric irradiation effects and impact gardening, which can modify grain size, expose fresh ice, and produce dark contaminating compounds that reduce the strength of absorption features. Previous research identified this leading/trailing asymmetry on the four largest classical Uranian satellites but did not find a clear leading/trailing asymmetry on Miranda, the smallest and innermost classical moon. We undertook an extensive observational campaign to investigate variations of the NIR spectral signature of H2O ice with longitude on Miranda’s northern hemisphere. We acquired 22 new spectra with the TripleSpec spectrograph on the ARC 3.5 m telescope and four new spectra with GNIRS on Gemini North. Our analysis also includes three unpublished and seven previously published spectra taken with SpeX on the 3 m IRTF. We confirm that Miranda has no substantial leading/trailing hemispherical asymmetry in the strength of its H2O ice absorption features. We additionally find evidence for an anti-Uranus/sub-Uranus asymmetry in the strength of the 1.5 μm H2O ice band that is not seen on the other Uranian satellites, suggesting that additional endogenic or exogenic processes influence the longitudinal distribution of H2O ice band strengths on Miranda.

Near-infrared spectra of Umbriel and the other classical Uranian moons exhibit 2.2 μm absorption bands that could result from ammonia (NH3) bearing species, possibly exposed in the geologically recent past. However, Umbriel has an ancient surface with minimal evidence for recent endogenic activity, raising the possibility that more refractory species are present, and/or that NH3 is retained over long timescales. We analyzed 33 spectra of Umbriel to investigate its 2.2 μm band, along with three other absorption features we identified near 2.14, 2.22, and 2.24 μm. We assessed the subobserver longitudinal distributions of these four bands, finding that they are present across Umbriel and may be spatially associated with geologic features such as craters and large basins. We compared the bands to 15 candidate constituents. We found that Umbriel’s 2.14 μm and 2.22 μm bands are most consistent with the spectral signature of organics, its 2.24 μm band is best matched by NH3 ice, and its 2.2 μm band is consistent with the signatures of NH3–H2O mixtures, aluminum-bearing phyllosilicates, and sodium-bearing carbonates. However, some of these candidate constituents do not match Umbriel’s spectral properties in other wavelength regions, highlighting the gaps in our understanding of the Uranian moons’ surface compositions. Umbriel’s 2.14 μm band may alternatively result from a 2ν 3 overtone mode of CO2 ice. If present on Umbriel, these candidate constituents could have formed in contact with an internal ocean and were subsequently exposed during Umbriel’s early history. Alternatively, these constituents might have originated in an undifferentiated crust or were delivered by impactors.

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