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HST-COS Transit Spectroscopy of KELT-20b: First Detection of Excess Far-ultraviolet Absorption from an Ultrahot Jupiter
KELT-20 b is an ultrahot Jupiter with an equilibrium temperature of 2260 K orbiting a bright (V = 7.6), fast-rotating ( vsini = 117 km s−1) A2 V star. The atmosphere of KELT-20 b has been studied extensively via transmission spectroscopy at optical wavelengths, showing strong hydrogen absorption and metals such as Na i, Ca ii, Fe i, Fe ii, Mg i, Si i, and Cr ii. The atmospheric and ionization conditions of this planet may differ from Jupiter-mass exoplanets owing to the relatively weak extreme-ultraviolet radiation from its host star, as the stellar dynamo that generates chromospheric and coronal activity is thought to shut down at spectral types earlier than A4. We present the first spectroscopic observations of KELT-20 b in the far-ultraviolet (FUV) using the Hubble Space Telescope Cosmic Origins Spectrograph, searching for previously undetected low-ionization and neutral atoms in the upper atmosphere. We find that the FUV transit depth increases with decreasing wavelengths, from 1.88% ± 0.04% at 1600–1760 Å to 2.28% ± 0.04% at 1410–1570 Å, yielding planetary radii of 0.1139R* ± 0.06R* and 0.1222R* ± 0.07R*, respectively. We report tentative detections of Fe ii and N i at 2.4σ each and nondetections of C i, S i, Al ii, and Si ii. We find no evidence for molecular absorption from CO or H2 and no sign of hydrodynamic escape.
Journal Article
Stellar Library of Differential Emission Measures and Extreme Ultraviolet Spectra: Dwarf Stars Observed by the Extreme Ultraviolet Explorer
Extreme ultraviolet (EUV; 100–912 Å) photons influence the formation and evolution of planets by ionizing hydrogen and other species, but the EUV radiation of most exoplanet host stars is poorly constrained. This work applies the differential emission measure (DEM) technique to infer the EUV spectra of eight stars previously observed with high signal-to-noise ratios by the Extreme Ultraviolet Explorer (EUVE; 1992–2002) spanning spectral types from M to F: AD Leo (MV), E Eri (KV), κ1 Ceti (GV), Procyon (FIV–V), α Cen A + B (GV + KV), and ξ Boo A + B (GV + KV). The model spectra are accurate to within a factor of 3 for the majority of individual EUVE flux density data points and accurate to within 30% of the integrated 100–300 Å fluxes. We provide the atomic data used for each star’s calculations, X-ray/EUV/far-ultraviolet (noncontemporaneous) observational inputs, the DEM models, and the model-predicted EUV spectra (extending beyond the 90–510 Å range of the EUVE spectra) and compare the results to archival EUVE data. We also find that the relative contributions of the transition region and corona to a star’s EUV emission vary significantly among the few stars analyzed here. The flare-driven variability of the corona is greater than the transition region, so the EUV spectrum of each star will have stochastic flare excess contributions at different wavelengths depending on the star’s temperature structure, flare behavior, and activity cycle. We conclude with a discussion of the implications of this variability for analyses that use EUV estimates to study planetary atmospheric evolution.
Journal Article
Early-time Ultraviolet Spectroscopy and Optical Follow-up Observations of the Type IIP Supernova 2021yja
We present three epochs of early-time ultraviolet (UV) and optical HST/STIS spectroscopy of the young, nearby Type IIP supernova (SN) 2021yja. We complement the HST data with two earlier epochs of Swift UVOT spectroscopy. The HST and Swift UVOT spectra are consistent with those of other well-studied Type IIP SNe. The UV spectra exhibit rapid cooling at early times, while less dramatic changes are seen in the optical. We also present Lick/KAIT optical photometry up to the late-time tail phase, showing a very long plateau and shallow decline compared with other SNe IIP. Our modeling of the UV spectrum with the TARDIS radiative transfer code produces a good fit for a high-velocity explosion, a low total extinction E(B − V) = 0.07 mag, and a subsolar metallicity. We do not find a significant contribution to the UV flux from an additional heating source, such as interaction with the circumstellar medium, consistent with the observed flat plateau. Furthermore, the velocity width of the Mg ii λ2798 line is comparable to that of the hydrogen Balmer lines, suggesting that the UV emission is confined to a region close to the photosphere.
Journal Article
Late-time Hubble Space Telescope Ultraviolet Spectra of SN 2023ixf and SN 2024ggi Show Ongoing Interaction with Circumstellar Material
We present far- and near-ultraviolet (UV) spectra of the type II supernovae (SNe) SN 2023ixf from days 199 to 722 and SN 2024ggi at days 41 and 232. Both SNe show broad, blueshifted, and asymmetric UV emission lines with an initial maximum velocity of ∼9000 km s−1 and narrow unresolved emission in C ivλλ1548.9, 1550.8. We compare the optical and UV emission-line profiles, showing that they evolve from two distinct velocity profiles to a single profile tracing the UV emission. We interpret this as shock power from interaction with circumstellar material coming to dominate over the radioactive-decay power from the inner ejecta. Comparing our observations to radiative transfer models with injected shock power, we find SN 2024ggi is best matched by Pshock,abs = 1 × 1041 erg s−1 at day 40; SN 2023ixf at day 300 and SN 2024ggi at day 200 are best matched by Pshock,abs = 1 × 1040 erg s−1; and SN 2023ixf at day 600 is best matched by Pshock,abs = 5 × 1039 erg s−1. From these models, we find that the mass-loss rate of both SNe increased just before the explosion. For SN 2023ixf, our mass-loss rates go from 4 × 10−5M⊙ yr−1 at 600 yr before explosion to 2 × 10−2M⊙ yr−1 at 15 yr prior to explosion. For SN 2024ggi, we find a mass-loss rate of 9 × 10−5M⊙ yr−1 at 150 yr before explosion and 1 × 10−3M⊙ yr−1 at 30 yr before explosion.
Journal Article
Ultraviolet Radiation From a Plant Perspective: The Plant-Microorganism Context
Ultraviolet (UV) radiation directly affects plants and microorganisms, but also alters the species-specific interactions between them. The distinct bands of UV radiation, UV-A, UV-B, and UV-C have different effects on plants and their associated microorganisms. While UV-A and UV-B mainly affect morphogenesis and phototropism, UV-B and UV-C strongly trigger secondary metabolite production. Short wave (<350 nm) UV radiation negatively affects plant pathogens in direct and indirect ways. Direct effects can be ascribed to DNA damage, protein polymerization, enzyme inactivation and increased cell membrane permeability. UV-C is the most energetic radiation and is thus more effective at lower doses to kill microorganisms, but by consequence also often causes plant damage. Indirect effects can be ascribed to UV-B specific pathways such as the UVR8-dependent upregulated defense responses in plants, UV-B and UV-C upregulated ROS accumulation, and secondary metabolite production such as phenolic compounds. In this review, we summarize the physiological and molecular effects of UV radiation on plants, microorganisms and their interactions. Considerations for the use of UV radiation to control microorganisms, pathogenic as well as non-pathogenic, are listed. Effects can be indirect by increasing specialized metabolites with plant pre-treatment, or by directly affecting microorganisms.
Journal Article
Far-ultraviolet Emission Line Investigation of Flares on AU Mic
The role of nonthermal proton energy transportation during solar and stellar flares is largely unknown; a better understanding of this physical process will allow us to rectify longstanding deficiencies in flare models. One way to detect the presence of nonthermal protons during flares is through the Orrall–Zirker (OZ) effect, proposed by F. Q. Orrall & J. B. Zirker, whereby an enhanced red wing appears in hydrogen emission lines (e.g., Lyman-α at 1215.67 Å). We analyze archival Hubble Space Telescope/Cosmic Origins Spectrograph G130M (1060–1360 Å) observations of the young M dwarf AU Microscopii (AU Mic) to search for evidence of the OZ effect during the impulsive phase of six stellar flares with Eflare≈1030−31 erg. While we found nondetections of the OZ effect, we note there is a pronounced blue enhancement in several C ii and C iii emission lines during one of the high-energy flares. We propose that either filament eruptions or chromospheric evaporation could be the mechanism driving this observed blue enhancement. We compare the far-ultraviolet (FUV) spectra to one-dimensional radiative-hydrodynamic stellar flare models, which are unable to reproduce the blue enhancement and broadening in these cool flare lines. By completing a line-by-line analysis of the FUV spectrum of AU Mic, we provide further constraints on the physical mechanisms producing stellar flares on M dwarfs.
Journal Article
The Coronal Veil
Coronal loops, seen in solar coronal images, are believed to represent emission from magnetic flux tubes with compact cross sections. We examine the 3D structure of plasma above an active region in a radiative magnetohydrodynamic simulation to locate volume counterparts for coronal loops. In many cases, a loop cannot be linked to an individual thin strand in the volume. While many thin loops are present in the synthetic images, the bright structures in the volume are fewer and of complex shape. We demonstrate that this complexity can form impressions of thin bright loops, even in the absence of thin bright plasma strands. We demonstrate the difficulty of discerning from observations whether a particular loop corresponds to a strand in the volume, or a projection artifact. We demonstrate how apparently isolated loops could deceive observers, even when observations from multiple viewing angles are available. While we base our analysis on a simulation, the main findings are independent from a particular simulation setup and illustrate the intrinsic complexity involved in interpreting observations resulting from line-of-sight integration in an optically thin plasma. We propose alternative interpretation for strands seen in Extreme Ultraviolet images of the corona. The “coronal veil” hypothesis is mathematically more generic, and naturally explains properties of loops that are difficult to address otherwise—such as their constant cross section and anomalously high density scale height. We challenge the paradigm of coronal loops as thin magnetic flux tubes, offering new understanding of solar corona, and by extension, of other magnetically confined bright hot plasmas.
Journal Article
Circumstellar Interaction in the Ultraviolet Spectra of SN 2023ixf 14–66 Days After Explosion
SN 2023ixf was discovered in M101 within a day of the explosion and rapidly classified as a Type II supernova with flash features. Here we present ultraviolet (UV) spectra obtained with the Hubble Space Telescope 14, 19, 24, and 66 days after the explosion. Interaction between the supernova ejecta and circumstellar material (CSM) is seen in the UV throughout our observations in the flux of the first three epochs and asymmetric Mg ii emission on day 66. We compare our observations to CMFGEN supernova models that include CSM interaction ( Ṁ<10−3 M ⊙ yr−1) and find that the power from CSM interaction is decreasing with time, from L sh ≈ 5 × 1042 erg s−1 to L sh ≈ 1 × 1040 erg s−1 between days 14 and 66. We examine the contribution of individual atomic species to the spectra on days 14 and 19, showing that the majority of the features are dominated by iron, nickel, magnesium, and chromium absorption in the ejecta. The UV spectral energy distribution of SN 2023ixf sits between that of supernovae, which show no definitive signs of CSM interaction, and those with persistent signatures assuming the same progenitor radius and metallicity. Finally, we show that the evolution and asymmetric shape of the Mg ii λ λ 2796, 2802 emission are not unique to SN 2023ixf. These observations add to the early measurements of dense, confined CSM interaction, tracing the mass-loss history of SN 2023ixf to ∼33 yr prior to the explosion and the density profile to a radius of ∼5.7 × 1015 cm. They show the relatively short evolution from a quiescent red supergiant wind to high mass loss.
Journal Article
The UV Legacy Library of Young Stars as Essential Standards (ULLYSES) Large Director’s Discretionary Program with Hubble. I. Goals, Design, and Initial Results
Specifically selected to leverage the unique ultraviolet capabilities of the Hubble Space Telescope, the Hubble Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES) is a Director’s Discretionary program of approximately 1000 orbits—the largest ever executed—that produced a UV spectroscopic library of O and B stars in nearby low-metallicity galaxies and accreting low-mass stars in the Milky Way. Observations from ULLYSES combined with archival spectra uniformly sample the fundamental astrophysical parameter space for each mass regime, including spectral type, luminosity class, and metallicity for massive stars, and the mass, age, and disk accretion rate for low-mass stars. The ULLYSES spectral library of massive stars will be critical to characterize how massive stars evolve at different metallicities; to advance our understanding of the production of ionizing photons, and thus of galaxy evolution and the re-ionization of the Universe; and to provide the templates necessary for the synthesis of integrated stellar populations. The massive-star spectra are also transforming our understanding of the interstellar and circumgalactic media of low-metallicity galaxies. On the low-mass end, UV spectra of T Tauri stars contain a plethora of diagnostics of accretion, winds, and the warm disk surface. These diagnostics are crucial for evaluating disk evolution and provide important input to assess atmospheric escape of planets and to interpret powerful probes of disk chemistry, as observed with the Atacama Large Millimeter Array and the James Webb Space Telescope. In this paper, we motivate the design of the program, describe the observing strategy and target selection, and present initial results.
Journal Article