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NASA NTRS2011

Atomic Data for Stellar Astrophysics: from the UV to the IR

The study of stars and stellar evolution relies heavily on the analysis of stellar spectra. The need for atomic line data from the ultraviolet (UV) to the infrared (lR) regions is greater now than ever. In the past twenty years, the time since the launch of the Hubble Space Telescope, great progress has been made in acquiring atomic data for UV transitions. The optical wavelength region, now expanded by progress in detector technology, continues to provide motivation for new atomic data. In addition, investments in new instrumentation for ground-based and space observatories has lead to the availability of high-quality spectra at IR wavelengths, where the need for atomic data is most critical. In this review, examples are provided of the progress made in generating atomic data for stellar studies, with a look to the future for addressing the accuracy and completeness of atomic data for anticipated needs.

Wahlgren, Glenn M.↗

NASA NTRS2013

Uv-bright Nearby Early-type Galaxies Observed in the Mid-infrared: Eidence for a Multi-stage Formation History by Way of WISE and GALEX Imaging

In the local universe, 10% of massive elliptical galaxies are observed to exhibit a peculiar property: a substantial excess of ultraviolet emission than what is expected from their old, red stellar populations. Several origins for this ultraviolet excess (UVX) have been proposed including a population of hot young stars and a population of old, blue horizontal branch or extended horizontal branch (BHB or EHB) stars that have undergone substantial mass loss from their outer atmospheres. We explore the radial distribution of UVX in a selection of 49 nearby E/S0-type galaxies by measuring their extended photometry in the UV through mid-infrared (mid-IR) with the Galaxy Evolution Explorer (GALEX), the Sloan Digital Sky Survey, and the Wide-field Infrared Survey Explorer (WISE). We compare UV/optical and UV/mid-IR colors with the Flexible Stellar Population Synthesis models, which allow for the inclusion of EHB stars. We find that combined WISE mid-IR and GALEX UV colors are more effective in distinguishing models than optical colors, and that the UV/mid-IR combination is sensitive to the EHB fraction. There are strong color gradients, with the outer radii bluer than the inner half-light radii by approx.1 mag. This color difference is easily accounted for with an increase in the BHB fraction of 0.25 with radius. We estimated that the average ages for the inner and outer radii are 7.0 +/- 0.3 Gyr, and 6.2 +/- 0.2 Gyr, respectively, with the implication that the outer regions are likely to have formed approx. 1 Gyr after the inner regions. Additionally, we find that metallicity gradients are likely not a significant factor in the color difference. The separation of color between the inner and outer regions, which agrees with a specific stellar population difference (e.g., higher EHB populations), and the approx. 0.5-2 Gyr age difference suggests multi-stage formation. Our results are best explained by inside-out formation: rapid star formation within the core at early epochs (>4 Gyr ago) and at least one later stage starburst event coinciding with z approx. 1.

galaxies: elliptical and lenticular↗

NASA NTRS2011

Deep UV to NIR Space Telescopes and Exoplanet Coronagraphs: A Trade Study on Throughput, Polarization, Mirror Coating Options and Requirements

The NASA Exoplanet program and the Cosmic Origins program are exploring technical options to combine the visible to NIR performance requirements of a space coronagraph with the general astrophysics requirements of a space telescope covering the deep UV spectrum. Are there compatible options in terms of mirror coatings and telescope architecture to satisfy both goals? In this paper, we address some of the main concerns, particularly relating to polarization in the visible and throughput in the UV. Telescope architectures employing different coating options compatible with current technology are considered in this trade study.

coronagraph↗

NASA NTRS2016

Optical Evaluation of DMDs with UV-Grade FS, Sapphire, MgF2 Windows and Reflectance of Bare Devices

Digital Micro-mirror Devices (DMDs) have been identified as an alternative to microshutter arrays for space-based multi-object spectrometers (MOS). Specifically, the MOS at the heart of a proposed Galactic Evolution Spectroscopic Explorer (GESE) that uses the DMD as a reprogrammable slit mask. Unfortunately, the protective borosilicate windows limit the use of DMDs in the UV and IR regimes, where the glass has insufficient throughput. In this work, we present our efforts to replace standard DMD windows with custom windows made from UV-grade fused silica, Low Absorption Optical Sapphire (LAOS) and magnesium fluoride. We present reflectance measurements of the antireflection coated windows and a reflectance study of the DMDs active area (window removed). Furthermore, we investigated the long-term stability of the DMD reflectance and recoating device with fresh Al coatings.

Quijada, Manuel A.↗

NASA NTRS2017

Aerosol Optical Depth as Observed by the Mars Science Laboratory REMS UV Photodiodes

Systematic observations taken by the REMS UV photodiodes on a daily basis throughout the landed Mars Science Laboratory mission provide a highly useful tool for characterizing aerosols above Gale Crater. Radiative transfer modeling is used to model the approximately two Mars Years of observations taken to date taking into account multiple scattering from aerosols and the extended field of view of the REMS UV photodiodes. The retrievals show in detail the annual cycle of aerosol optical depth, which is punctuated with numerous short timescale events of increased optical depth. Dust deposition onto the photodiodes is accounted for by comparison with aerosol optical depth derived from direct imaging of the Sun by Mastcam. The effect of dust on the photodiodes is noticeable, but does not dominate the signal. Cleaning of dust from the photodiodes was observed in the season around Ls=270deg, but not during other seasons. Systematic deviations in the residuals from the retrieval fit are indicative of changes in aerosol effective particle size, with larger particles present during periods of increased optical depth. This seasonal dependence of aerosol particle size is expected as dust activity injects larger particles into the air, while larger aerosols settle out of the atmosphere more quickly leading to a smaller average particle size over time. A full description of these observations, the retrieval algorithm, and the results can be found in Smith et al. (2016).

Aerosol↗

NASA NTRS2016

A Ten-Year Global Record of Absorbing Aerosols Above Clouds from OMI's Near-UV Observations

Aerosol-cloud interaction continues to be one of the leading uncertain components of climate models, primarily due to the lack of an adequate knowledge of the complex microphysical and radiative processes associated with the aerosol-cloud system. The situations when aerosols and clouds are found in the same atmospheric column, for instance, when light-absorbing aerosols such as biomass burning generated carbonaceous particles or wind-blown dust overlay low-level cloud decks, are commonly found over several regional of the world. Contrary to the cloud-free scenario over dark surface, for which aerosols are known to produce a net cooling effect (negative radiative forcing) on climate, the overlapping situation of absorbing aerosols over cloud can potentially exert a significant level of atmospheric absorption and produces a positive radiative forcing at top-of-atmosphere. The magnitude of direct radiative effects of aerosols above cloud depends directly on the aerosol loading, microphysical-optical properties of the aerosol layer and the underlying cloud deck, and geometric cloud fraction. We help in addressing this problem by introducing a novel product of optical depth of absorbing aerosols above clouds retrieved from near-UV observations made by the Ozone Monitoring Instrument (OMI) on board NASA's Aura platform. The presence of absorbing aerosols above cloud reduces the upwelling radiation reflected by cloud and produces a strong 'color ratio' effect in the near-UV region, which can be unambiguously detected in the OMI measurements. Physically based on this effect, the OMACA algorithm retrieves the optical depths of aerosols and clouds simultaneously under a prescribed state of atmosphere. The algorithm architecture and results from a ten-year global record including global climatology of frequency of occurrence and above-cloud aerosol optical depth, and a discussion on related future field campaigns are presented.

Ozone Monitoring Instrument↗

NASA NTRS2016

Aerosol Optical Depth as Observed by the Mars Science Laboratory REMS UV Photodiodes

Systematic observations taken by the REMS (Rover Environmental Monitoring Station) UV (ultraviolet) photodiodes on a daily basis throughout the landed Mars Science Laboratory mission provide a highly useful tool for characterizing aerosols above Gale Crater. Radiative transfer modeling is used to model the approximately 1.75 Mars Years of observations taken to date taking into account multiple scattering from aerosols and the extended field of view of the REMS UV photodiodes. The retrievals show in detail the annual cycle of aerosol optical depth, which is punctuated with numerous short timescale events of increased optical depth. Dust deposition onto the photodiodes is accounted for by comparison with aerosol optical depth derived from direct imaging of the Sun by Mastcam. The effect of dust on the photodiodes is noticeable, but does not dominate the signal. Cleaning of dust from the photodiodes was observed in the season around solar longitude (Ls) equal to 270 degrees, but not during other seasons. Systematic deviations in the residuals from the retrieval fit are indicative of changes in aerosol effective particle size, with larger particles present during periods of increased optical depth. This seasonal dependence of aerosol particle size is expected as dust activity injects larger particles into the air, while larger aerosols settle out of the atmosphere more quickly leading to a smaller average particle size over time.

Smith, Michael D.↗

NASA NTRS2017

Absence of a Detectable Lunar Nanodust Exosphere During a Search with LRO's LAMP UV Imaging Spectrograph

The Lyman-Alpha Mapping Project (LAMP) UV spectrograph on board the Lunar Reconnaissance Orbiter (LRO) performed a campaign to observe the Moon's nanodust exosphere, evidence for which was provided by the Lunar Atmosphere and Dust Environment Explorer (LADEE) Ultraviolet and Visible Spectrometer (UVS) during the 2014 Quadrantid meteoroid stream. These LADEE/UVS observations were consistent with a nanodust exosphere modulated by meteoroid impacts. LRO performed off-nadir maneuvers around the peak of the 2016 Quadrantids, in order to reproduce, as closely as possible, the active meteoroid environment and observing geometry of LADEE/UVS. We analyzed LAMP spectra to search for sunlight backscattering from nanodust. No brightness enhancement attributable to dust, of any size, was observed. We determine an upper limit for dust column concentration of ~10(^ 5) sq. cm for grains of radius ~25 nm, and an upper limit for dust column mass of ~10 11 g sq. cm, nearly independent of grain size for radii <100 nm.

Spectrograph↗

NASA NTRS2018

Bar Code Events in the Juno-UVS Data: Signature ~10 MeV Electron Microbursts at Jupiter

One of the most intriguing discoveries of Juno is the quasi-systematic detection of upgoing electrons above the auroral regions. Here we discuss a by-product of the most energetic component of this population: a contamination resembling bar codes in the Juno-UVS images. This pattern is likely caused by bursts of ∼10 MeV electrons penetrating the instrument. These events are mostly detected when Juno’s magnetic footprint is located poleward of the main emission relative to the magnetic pole. The signal is not periodic, but the bursts are typically 0.1–1 s apart. They are essentially detected when Juno-UVS is oriented toward Jupiter, indicating that the signal is due to upgoing electrons. The event detections occur between 1 and 7 Jovian radii above the 1-bar level, suggesting that the electron acceleration takes place close to Jupiter and is thus both strong and brief.

Microbursts↗

NASA NTRS2014

UV-LED

UV-LED is part of a small satellite technology demonstration mission that will demonstrate non-contacting charge control of an isolated or floating mass using new solid-state ultra-violet light emitting diodes (UV-LEDs). Integrated to the Saudisat-4 spacecraft and launched onboard the Dnepr in June 19, 2014, the project is a collaboration between the NASA Ames Research Center (ARC), Stanford University, and King Abdulaziz City for Science and Technology (KACST). This technology demonstration will validate a novel method of charge control that will improve the performance of drag-free spacecraft allowing for concurrent science collection during charge management operations as well as reduce the mass, power and volume required while increasing lifetime and reliability of a charge management subsystem. These improvements are crucial to the success of ground breaking missions such as LISA and BBO, and demonstrate the ability of low cost small satellite missions to provide technological advances that far exceed mission cost.

Faied, Dohy↗

NASA NTRS2014

UV-LED Project

UV-LED is part of a small satellite technologydemonstration mission that willdemonstrate non-contacting charge controlof an isolated or floating mass usingnew solid-state ultra-violet light emittingdiodes (UV-LEDs). Integrated tothe Saudisat-4 spacecraft and launchedonboard the Dnepr in June 19, 2014,the project is a collaboration betweenthe NASA Ames Research Center (ARC),Stanford University, and King AbdulazizCity for Science and Technology(KACST). This technology demonstrationwill validate a novel method of chargecontrol that will improve the performanceof drag-free spacecraft allowingfor concurrent science collection duringcharge management operations as wellas reduce the mass, power and volumerequired while increasing lifetime and reliabilityof a charge management subsystem.These improvements are crucial tothe success of ground breaking missionssuch as LISA and BBO, and demonstratethe ability of low cost small satellite missionsto provide technological advancesthat far exceed mission cost.

Soulage, Michael↗

NASA NTRS2018

A Mission-Enabling UV Laser for Mass Spectrometry (UVMS) with Continuously Selectable Output for In Situ Planetary Exploration

Ultra-compact, ns-class spaceflight-compatible UV lasers are finding increasing application in laser desorption, excitation, and ionization analytical applications on planetary missions, such as the detection and characterization of potential molecular biosignatures on Mars or icy moon surfaces. A short pulsed, solid state, UV laser is under development with selectable pulse energy capabilities for optimized sample ion production at a planetary surface.

Coyle, D. Barry↗

NASA NTRS2019

The Refractive Index of Amorphous and Crystalline Water Ice in the UV-vis

Amorphous solid water (ASW) is found on icy dust grains in the interstellar medium (ISM), as well as on comets and other icy objects in the outer solar system. The optical properties of ASW are thus relevant for many astrophysical environments, but in the ultraviolet–visible (UV–vis), its refractive index is not well constrained. Here, we introduce a new method based on UV–vis broadband interferometry to measure the wavelength dependent refractive index n(λ) of amorphous water ice from 10 to 130 K, i.e., for different porosities, in the wavelength range of 210–757 nm. We also present n(λ) for crystalline water ice at 150 K, which allows us to compare our new method with literature data. Based on this, a method to calculate n(λ, ρ) as a function of wavelength and porosity is reported. This new approach carries much potential and is generally applicable to pure and mixed ice, both amorphous and crystalline. The astronomical and physical–chemical relevance and future potential of this work are discussed.

Kofman, Vincent↗

NASA NTRS2018

On the Discrepancy Between the X-Ray and UV Absorption Measurements of O VI in the Local ISM

The total amount of Ovi present in the interstellar medium (ISM) obtained via absorption measurements in UV and X-ray spectra is currently in disagreement, with the latter being significantly larger (by a factor of 10 or more) than the former. Previous works have proposed that the blend of the Ovi K line (22.032 Ã) with the Oii K-L12 line (22.04 Ã) could account for the stronger absorption observed in the X-ray spectra. Here, we present a detailed study of the oxygen absorption in the local ISM, implementing our new model IGMabs which includes photoabsorption cross-sections of highly ionized species of abundant elements as well as turbulence broadening. By analysing high-resolution Chandra spectra of 13 low-mass X-ray binaries (LMXBs) and 29 extragalactic sources, we have estimated the column densities of Oi - Oiii and from Ovi - Oviii along multiple line-of-sights. We find that in most cases the Oii K-L12 line accounts for <30 per cent of the total Ovi K a + Oii K b. We conclude that the amount of Oii predicted by our model is still insufficient to explain the discrepancy between X-ray and UV measurements of Ovi column densities.

Gatuzz, Efrain↗

NASA NTRSDate not supplied

Space-based UV Spectropolarimetry for Chromospheric Magnetic Field Measurements

High-resolution observations with space-and ground-based telescopes, along with advanced numerical modeling, have highlighted the intricate coupling between the chromosphere, transition region, and corona, and the critical role the chromosphere plays in the mass and energy balance of the outer solar atmosphere. Despite these recent advances, a major impediment to better understanding the solar atmosphere is our lack of empirical knowledge regarding the direction and strength of the magnetic field in the upper chromosphere (Decadal Survey, 2012). Such measurements are crucial to address several major unresolved issues in solar physics: for example, to constrain the energy flux carried by the Alfvén waves propagating through the chromosphere, and to determine the height at which the plasma β = 1 transition occurs, which has important consequences for the braiding of magnetic fields, for propagation and mode conversion of waves and for non-linear force-free extrapolation methods that are key to determining what drives instabilities such as flares or coronal mass ejections. Probing the magnetic nature of the Sun’s atmosphere requires measurement of the Stokes I, Q, U and V profiles of relevant spectral lines (of which Q, U and V encode the magnetic field information). Many of the magnetically sensitive lines formed in the chromosphere and transition region are in the ultraviolet spectrum, necessitating observations above the absorbing terrestrial atmosphere. The Chromospheric Layer Spectro-Polarimeter (“CLASP2”) sounding rocket was flown successfully in April 2019, as a follow-on to the successful flight in September 2015 of the Chromospheric Lyman-Alpha Spectro-Polarimeter (“CLASP1”). In October of 2021, we re-flew the CLASP2 experiment with a modified observing program to further demonstrate the maturity of the UV spectropolarimetry techniques, and readiness for development into a satellite observatory. During the reflight, called “CLASP2.1”, the spectrograph slit was scanned across an active region plage to acquire a two-dimensional map of Stokes V/I, to demonstrate the ability of UV spectropolarimetry to yield chromospheric magnetic fields over a large area.

David E McKenzie↗

NASA NTRS2021

On Parametric Generation of Deep UV Light in Monolithic Cavities With Cubic Nonlinearity

Coherent ultra violet (UV) light has broad applications in optical atomic clocks and quantum computers. Since availability of semiconductor tunable monochromatic lasers is limited at short wavelengths, optical nonlinear processes are frequently used to produce the desirable radiation wavelength. Optical crystals with quadratic nonlinearity are promising for this purpose because of their transparency and high optical damage threshold. In this paper we discuss a possibility of generation of coherent deep UV light utilizing high quality factor (-Q) monolithic optical microcavities made of crystalline optical materials characterized with cubic nonlinearities. We show that the generation is feasible in spite of the relatively small cubic nonlinearities, due to the ultra-broad-band high optical transparency of the crystals and associated high-Q achievable in the cavities created out of the materials. We found that the birefringence as well as the tensor nature of the nonlinear susceptibility of the crystalline optical materials can simplify the phase matching of the nonlinear processes.

Matsko, A.B.↗

NASA NTRSDate not supplied

Rotational Spectroscopy is SubLIME: Site-Specific Identification of Compounds Formed on UV-photolyzed Cosmcs Ice

A variety of organic compounds has been detected in comets, meteorites, and the cold gas enveloping infant stars. These compounds display a range of complexity and include so-called complex organic molecules that may have astrobiological implications. Laboratory studies of photolyzed and nonirradiated interstellar, planetary, and cometary ices have demonstrated many astrochemical formation routes and require sensitive analytical techniques to identify the composition of ice and gas products formed. The unique identification of organics can be challenging and often requires structure-specific techniques to identify structural isomers, conformers, and isotopologues. We present the approaches used in SubLIME, the Sublimation of Laboratory Ices Millimeter/submillimeter Experiment, which uses the structure-specificity of rotational spectroscopy to identify the composition and abundances of sublimated products of UV-photolyzed ice samples. Ices are formed and photolyzed at low temperature (~10 K) and slowly heated to 300 K to sublimate the volatile products. The gas mixture is then investigated with millimeter/submillimeter spectroscopy, and the resulting spectra compared to observational spectra collected with submillimeter/far-IR telescopes. We present the experimental set-up and results of UV-photolyzed interstellar ice analogues containing water (H O) and carbon monoxide (CO). Formation routes for identified molecules are proposed for follow-up experimental and theoretical investigations.

Olivia H. Wilkins↗

NASA NTRSDate not supplied

Continuation of Solar Absorptivity Degradation of Spacecraft Materials Due to UV and Charged Particles in the Gateway Environment

Gateway is a lunar orbiting platform that will be assembled in space similar to the International Space Station. Modules will arrive following different transit orbits, some of which have an extended duration in the Van Allen Belts. This means the spacecraft is exposed to significant charged particle radiation, after which it must operate in cislunar space for 15 years. Charged particle and UV degradation of potential Gateway materials was identified as a knowledge gap by the Passive Thermal Control System (PTCS) Group as end-of-life solar absorptivity greatly impacts thermal performance. Multiple rounds of ground testing have been completed at NASA Goddard Space Flight Center in which potential Gateway materials were subjected to expected worst-case transit charged particle fluence (9.86x1015 protons/cm2 at 2.5 keV and 3.1x1016 electrons/cm2 at 10 keV) and up to 5093 Equivalent Solar Hours (ESH) of UV exposure. This report builds on previous data from Zinecker et al. 2022 which presented the first round of ground testing. Additional materials have been tested and solar absorptivity degradation along with lessons learned are shared. Comparisons to previous results are also presented.

Brandon Hoffmann↗