United States Naval Observatory

Solar Ultraviolet (UV) radiation plays a key role in many chemical and biological processes, and affects significantly human health. Excessive UV exposure may lead to adverse health effects, including sunburns, skin cancer, and cataracts, whereas moderate exposure is beneficial, e.g., by supporting vitamin D production and promoting mental well-being, among other benefits. UV radiation interacts with various atmospheric components before reaching the Earth’s surface. Photons with shorter wavelengths are absorbed at higher atmospheric layers by oxygen and tropospheric ozone, and practically only UV-A and a small part of the UV-B irradiance reach the troposphere. In the troposphere, UV is scattered by air molecules and is further attenuated by aerosols and clouds. Interactions between UV radiation and aerosols are not yet completely understood, and their parameterization constitutes a major uncertainty factor in models and satellite retrieval algorithms. Understanding these interactions is thus essential for accurately assessing UV exposure using modeled UV irradiance.Τhe purpose of this study is to evaluate satellite- and reanalysis-based retrievals of the effective dose for the cutaneous vitamin D synthesis using ground-based measurements over Athens and Thessaloniki, Greece. We evaluate data that are derived (1) using the methodology described in Fragkos et al., (2024, https://doi.org/10.3390/rs16111878), based on CAMS information in combination with satellite data from OMI and MSG, and (2) the UV climatology of which is also based on data from various sensors analyzing air quality. Ground-based spectral solar UV irradiance measurements performed with a MKIV single monochromator Brewer spectrophotometer in Athens, and a MKIII double monochromator Brewer spectrophotometer in Thessaloniki are used to validate the Satellite-based retrievals. AOD measurements from co-located CIMEL sun-photometers, part of the AERONET network are used to assess the effect of aerosols. The evaluation has been performed for the period 2004 - 2024. Further analysis yielded positive trends in the effective dose for vitamin D production in the last two decades, mainly due the decreasing trends in aerosols.

Abstract Due to the rapid timescales of solar flares, the observed response of the upper atmosphere of Mars during a solar flare event depends on the timing within the flare event at which measurements are made. To accurately constrain the impact that solar flares may cause to the Martian upper atmosphere, the full time‐extent of a solar flare's atmospheric effects must as such be understood. In this paper, the full duration of changes to the Martian upper atmosphere due to the 10 September 2017 X8.2‐class solar flare are investigated using M‐GITM simulations. Analyses characterize the Martian upper atmosphere's electron, ion, and neutral densities and composition over time at the fixed location of MAVEN spacecraft periapsis observations during this flare event. In particular, the effects of the atmospheric processes that drive these changes—which are uniquely accessible during a solar flare's rapid changes due to their differing timescales—are investigated to provide insight into these time‐dependent changes. Results indicate rapid initial plasma density changes across the vertical extent of the ionosphere, plasma and neutral density changes with strong altitude‐ and species‐specific dependence, and a long duration of maximum perturbed upper atmospheric conditions due to this flare over time. The neutral thermosphere's thermal expansion over time is found to extend and magnify many of the density changes produced by photochemistry. Constraining solar flares' time‐dependent atmospheric effects provides context to future observation‐based comparisons of flare events and investigations into the role of solar irradiance in shaping Mars' early climate and impacting technology.

Abstract Studying cold brown dwarfs is key to understanding the diverse characteristics of cold giant exoplanets atmospheres. COCONUTS-2 is a wide binary system composed of a T9 brown dwarf and an M3 star, which presents a unique opportunity to characterize a cold benchmark brown dwarf. As part of a JWST program to study the range of physical and atmospheric properties of the coldest brown dwarfs, we obtained NIRSpec G395H spectra ( R ∼ 2700, 2.87−5.13 μ m) and MIRI F1000W, F1280W, and F1800W photometry for COCONUTS-2 b. In this work, we find a 99% probability of the system belonging to the Corona of Ursa Major moving group (414 ± 23 Myr) using BANYAN Σ and its full kinematics. We also reestimate the astrometry of COCONUTS-2b using the MIRI data. We support this membership with a comparison of the rotation period, metallicity, and C/O ratio of the group with those of the COCONUTS-2 system. We also calculate its bolometric luminosity, which, combined with our age estimation, allows us to derive its mass, effective temperature, surface gravity, and radius with high precision. As a result of our analysis, we support the conclusion that COCONUTS-2 b is a planetary-mass object (7.5 ± 0.4 M Jup ), which was likely formed via the same mechanism as stars. In addition we compare the JWST spectrum to another object in the sample, J082507.35+280548.5 (0825+2805), a Y0.5 brown dwarf, which is a candidate member of the same moving group, but has a lower mass (3.7 ± 0.2 M Jup ). We identify absorption feature differences, which could indicate that 0825+2805 has stronger vertical mixing.

Abstract. Dust particles originating from desert areas of the planet have significant radiative impacts on the ground and atmospheric column, changing the energy distribution of the entire earth system, which cannot be underestimated, as dust is considered a climatic regulator. The magnitude of the dust radiative effect is dependent on the optical properties of desert dust aerosols, which in turn is regulated by the composition and mineralogical content of desert dust plumes. The mineralogical composition in atmospheric models is commonly related to the soil type at dust sources and the optical properties needed are provided by observational campaigns and dedicated measurements. In this work we upgrade the METAL-WRF model to incorporate the direct radiative impact of the minerals in dust and test the impact of the direct radiative feedback on a 2-month simulation period, namely March and April 2022, when consecutive intense dust outbursts affected the Mediterranean Basin. The simulation results were compared against Global Horizontal Irradiance data and AERONET optical properties that were measured at five sites. An improvement was observed in all sites, especially close to the sources, when the minerals and dust are treated as interacting in the radiative transfer calculations, thus improving the capabilities of METAL-WRF to simulate the chemical composition of dust particles in the atmosphere along with their contribution to radiative transfer processes.

Studying cold brown dwarfs is key to understanding the diverse characteristics of cold giant exoplanets atmospheres. COCONUTS-2, is a wide binary system composed of a T9 brown dwarf and an M3 star, which presents a unique opportunity to characterize a cold benchmark brown dwarf. As part of a JWST program to study the range of physical and atmospheric properties of the coldest brown dwarfs, we obtained NIRSpec G395H spectra (R~2700, 2.87-5.13 um) and MIRI F1000W, F1280W, and F1800W photometry for COCONUTS-2b. In this work, we find a 99% probability of the system belonging to the Corona of Ursa Major (414+/-23 Myr) using BANYAN Sigma and its full kinematics. We also re-estimate the astrometry of COCONUTS-2b using the MIRI data. We support the membership with a comparison of rotation period, metallicity and C/O ratio of the group with those of the COCONUTS-2 system. We also calculate its bolometric luminosity, which combined with our age estimation, allows us to derive its mass, effective temperature, surface gravity, and radius with high precision. As a result of our analysis, we support the conclusion that COCONUTS-2b is a planetary mass object (7.5+/-0.4 MJup) which was likely formed via the same mechanism as stars. In addition we compare the JWST spectrum to another object in the sample, J082507.35+280548.5 (0825+2805), a Y0.5 brown dwarf, which is a candidate member of the same moving group, but has a lower mass (3.7+/-0.2 MJup). We identify absorption feature differences which could indicate that 0825+2805 has stronger vertical mixing.

Abstract Be stars are widely considered to be the product of binary interaction. However, whether all Be stars are formed via binary interaction is unclear, and detailed estimates of the multiplicity of Be stars and characterization of their components are required. In this study, we present speckle observations of 76 Be stars taken using the Gemini North and South speckle imagers spanning angular separations of 20 mas–1 . ″ 2, reaching contrasts Δ m ∼ 5–6 mag at separations around 0 . ″ 1. We identify 11 (6 previously unreported) binaries having separations in the 10–1000 au range, and Δ m between 0.8 and 5 mag in our sample. Using archival data to search for components outside our visibility range, we add further multiples (16), which include three triples, leading to a total of 24 multiple systems. Our findings rule out a multiplicity fraction >27% at the 3 σ level within the speckle observations separation range and detection limits. Future homogeneous spectroscopic/interferometric observations are essential to probe the inner separations, and along with analysis of available astrometry can cover the entire separation range to characterize the multiplicity fraction, and evolutionary scenario of Be stars.

Abstract We present a near-complete spectral energy distribution (SED) for an extrasolar world: the T8 brown dwarf 2MASS J04151954−0935066. Spanning from optical to mid-infrared (0.7–20.4 μm) wavelengths, the SED for this substellar atmosphere is constructed from new James Webb Space Telescope (JWST) NIRSpec G395H (R ∼ 2700) and Magellan Folded-port InfraRed Echellete (FIRE) echelle (R ∼ 8000) near-infrared spectra, along with MIRI mid-infrared photometry complemented by spectra from Keck I, Infrared Telescope Facility, Magellan, AKARI, Spitzer, and photometry from various surveys and missions. The NIRSpec G395H spectrum reveals strong molecular absorptions from NH3, CH4, H2S, CO2, and H2O at approximately 3.00, 3.35, 3.95, 4.25, and 5.00 μm, respectively, along with the presence of a CO absorption feature detected mainly at ∼4.6 μm. We detect no absorption of near-infrared K i doublets in the R ∼ 8000 FIRE spectra. In the mid-infrared Infrared Spectrograph spectrum, we tentatively identify a new CO2 feature at 14–16 μm. The comprehensive SED allows us to empirically constrain bolometric luminosity, effective temperature, mass, and radius. Additionally, we demonstrate that the NIRSpec G395H resolution, the highest allowable by JWST, enables a precise radial velocity measurement of 47.1 ± 1.8 km s−1 for the object, in agreement with previous measurements.

Abstract Local low-metallicity galaxies with signatures of possible accretion activity are ideal laboratories in which to search for the lowest-mass black holes and study their impact on the host galaxy. Here we present the first JWST NIRSpec IFS observations of SDSS J120122.30+021108.3, a nearby (z = 0.00354) extremely metal-poor dwarf galaxy with no optical signatures of accretion activity but identified by the Wide-field Infrared Survey Explorer to have red mid-infrared (MIR) colors consistent with active galactic nuclei (AGNs). We identify over 100 lines between ∼1.7 and 5.2 μm, an unresolved nuclear continuum source with an extremely steep spectral slope consistent with hot dust from an AGN (F ν ≈ ν −1.5), and a plethora of H i, He i, and H2 lines, with no lines from heavier elements, CO or ice absorption features, or polycyclic aromatic hydrocarbons (PAHs). While the bright central unresolved source (≲5 pc) is suggestive of an AGN, there are no He ii lines or coronal lines identified in the spectrum, and, importantly, there is no evidence that the radiation field is harder in the nuclear source compared with surrounding regions. The emission-line spectrum can be explained by a young (<5 Myr) nuclear star cluster with stellar mass ∼3 × 104 M ⊙, but the unresolved continuum source is more typical for a deeply embedded AGN powered by a black hole of minimum mass ∼1450 M ⊙. These observations reveal that either a metal-poor stellar population can heat the dust to extremely high temperatures, a result that would have significant impact on the reliability of MIR color selection in AGN surveys and our understanding of the properties of the interstellar medium and young stars in metal-poor galaxies, or an accreting intermediate-mass black hole is deeply embedded and hidden even at near-infrared wavelengths.

Abstract Sub-Neptunes—volatile-rich exoplanets smaller than Neptune—are intrinsically the most common type of planet known. However, the formation and nature of these objects, as well as the distinctions between subclasses (if any), remain unclear. Two powerful tools to tease out the secrets of these worlds are measurements of (i) atmospheric composition and structure revealed by transit and/or eclipse spectroscopy, and (ii) mass, radius, and density revealed by transit photometry and Doppler spectroscopy. Here, we present OrCAS, a survey to better elucidate the origins, compositions, and atmospheres of sub-Neptunes. This radial velocity survey uses a repeatable, quantifiable metric to select targets suitable for subsequent transmission spectroscopy and address key science themes about the atmospheric and internal compositions and architectures of these systems. Our survey targets 26 systems with transiting sub-Neptune planet candidates, with the overarching goal of increasing the sample of such planets suitable for subsequent atmospheric characterization. This paper lays out our survey's science goals, defines our target prioritization metric, and performs light-curve fits and statistical validation using existing TESS photometry and ground-based follow-up observations. Our survey serves to continue expanding the sample of small exoplanets with well-measured properties orbiting nearby bright stars, ensuring fruitful studies of these systems for many years to come.

Abstract The coldest Y spectral type brown dwarfs are similar in mass and temperature to cool and warm (∼200–400 K) giant exoplanets. We can therefore use their atmospheres as proxies for planetary atmospheres, testing our understanding of physics and chemistry for these complex, cool worlds. At these cold temperatures, their atmospheres are cold enough for water clouds to form, and chemical timescales increase, increasing the likelihood of disequilibrium chemistry compared to warmer classes of planets. JWST observations are revolutionizing the characterization of these worlds with high signal-to-noise, moderate-resolution near- and mid-infrared spectra. The spectra have been used to measure the abundances of prominent species, like water, methane, and ammonia; species that trace chemical reactions, like carbon monoxide; and even isotopologues of carbon monoxide and ammonia. Here, we present atmospheric retrieval results using both published fixed-slit (Guaranteed Time Observation program 1230) and new averaged time series observations (GO program 2327) of the coldest known Y dwarf, WISE 0855–0714 (using NIRSpec G395M spectra), which has an effective temperature of ∼264 K. We present a detection of deuterium in an atmosphere outside of the solar system via a relative measurement of deuterated methane (CH3D) and standard methane. From this, we infer the D/H ratio of a substellar object outside the solar system for the first time. We also present a well-constrained part-per-billion abundance of phosphine (PH3). We discuss our interpretation of these results and the implications for brown dwarf and giant exoplanet formation and evolution.