Keywords

We search for superflares from 4068 cool stars in 2+ yr of Evryscope photometry, focusing on those with high-cadence data from both Evryscope and the Transiting Exoplanet Survey Satellite (TESS). The Evryscope array of small telescopes observed 575 flares from 284 stars, with a median energy of 10^34.0 erg. Since 2016, Evryscope has enabled the detection of rare events from all stars observed by TESS through multi-year, high-cadence continuous observing. We report around twice the previous largest number of 10³⁴ erg high-cadence flares from nearby cool stars. We find eight flares with amplitudes of 3+ g' magnitudes, with the largest reaching 5.6 mag and releasing 10^36.2 erg. We observe a 10³⁴ erg superflare from TOI-455 (LTT 1445), a mid-M with a rocky planet candidate. We measure the superflare rate per flare-star and quantify the average flaring of active stars as a function of spectral type, including superflare rates, flare frequency distributions, and typical flare amplitudes in g'. We confirm superflare morphology is broadly consistent with magnetic reconnection. We estimate starspot coverage necessary to produce superflares, and hypothesize maximum allowed superflare energies and waiting times between flares corresponding to 100% coverage of the stellar hemisphere. We observe decreased flaring at high Galactic latitudes. We explore the effects of superflares on ozone loss to planetary atmospheres: we observe one superflare with sufficient energy to photodissociate all ozone in an Earth-like atmosphere in one event. We find 17 stars that may deplete an Earth-like atmosphere via repeated flaring. Of the 1822 stars around which TESS may discover temperate rocky planets, we observe 14.6% ± 2% emit large flares.

GJ 1132b, which orbits an M dwarf, is one of the few known Earth-sized planets, and at 12 pc away it is one of the closest known transiting planets. Receiving roughly 19× Earth's insolation, this planet is too hot to be habitable but can inform us about the volatile content of rocky planet atmospheres around cool stars. Using Hubble Space Telescope Imaging Spectrograph spectra, we search for a transit in the Lyα line of neutral hydrogen (Lyα). If we were to observe a deep Lyα absorption signature, that would indicate the presence of a neutral hydrogen envelope flowing from GJ 1132b. On the other hand, ruling out deep absorption from neutral hydrogen may indicate that this planet does not have a detectable amount of hydrogen loss, is not losing hydrogen, or has lost hydrogen and other volatiles early in the star's life. We do not detect a transit and determine a 2σ upper limit on the effective envelope radius of 0.36 R_* in the red wing of the Lyα line, which is the only portion of the spectrum we detect after absorption by the ISM. We analyze the Lyα spectrum and stellar variability of GJ1132, which is a slowly rotating 0.18 solar mass M dwarf with previously uncharacterized UV activity. Our data show stellar variabilities of 5%–22%, which is consistent with the M dwarf UV variabilities of up to 41% found by Loyd & France. Understanding the role that UV variability plays in planetary atmospheres is crucial to assess atmospheric evolution and the habitability of cooler rocky exoplanets.

Following our previous detection of ubiquitous ${{\rm{H}}}_{2}{\rm{O}}$ and ${{\rm{O}}}_{2}$ absorption against the far-ultraviolet continuum of stars located near the nucleus of Comet 67P/Churyumov–Gerasimenko, we present a serendipitously observed stellar occultation that occurred on 2015 September 13, approximately one month after the comet's perihelion passage. The occultation appears in two consecutive 10-minute spectral images obtained by Alice, Rosetta's ultraviolet (700–2100 Å) spectrograph, both of which show ${{\rm{H}}}_{2}{\rm{O}}$ absorption with column density >10^17.5 cm⁻² and significant ${{\rm{O}}}_{2}$ absorption (${{\rm{O}}}_{2}$/${{\rm{H}}}_{2}{\rm{O}}$ ≈ 5%–10%). Because the projected distance from the star to the nucleus changes between exposures, our ability to study the ${{\rm{H}}}_{2}{\rm{O}}$ column density profile near the nucleus (impact parameters <1 km) is unmatched by our previous observations. We find that the ${{\rm{H}}}_{2}{\rm{O}}$ and ${{\rm{O}}}_{2}$ column densities decrease with increasing impact parameter, in accordance with expectations, but the ${{\rm{O}}}_{2}$ column decreases ∼3 times more quickly than ${{\rm{H}}}_{2}{\rm{O}}$. When combined with previously published results from stellar appulses, we conclude that the ${{\rm{O}}}_{2}$ and ${{\rm{H}}}_{2}{\rm{O}}$ column densities are highly correlated, and ${{\rm{O}}}_{2}$/${{\rm{H}}}_{2}{\rm{O}}$ decreases with the increasing ${{\rm{H}}}_{2}{\rm{O}}$ column.

In order to reveal variations of days to weeks in the brightness distribution of Jovian Synchrotron Radiation (JSR), we made simultaneous radio and ultraviolet observations using the Giant Metrewave Radio Telescope (GMRT) and the Hisaki EXtreme ultraviolet spectrosCope for ExosphEric Dynamics (EXCEED). It is known from visible and ultraviolet observations that Io plasma torus (IPT) has dawn–dusk asymmetry, and that this asymmetry is believed to be due to the dawn-dusk electric field. Continuous ultraviolet observation by Hisaki reveals that dawn–dusk asymmetry of IPT changes in days to weeks, therefore, if this global electric field around Io's orbit (5.9 Jovian radii) could penetrate the radiation belt region (<2 Jovian radii), the variations in brightness distribution of JSR and IPT are expected to be correlated. The GMRT observations were made from 2013 December 31 to 2014 January 16 at 610 MHz and 2016 March 14–June 23 at 1390 MHz, while Hisaki continuously monitored IPT. The statistical analysis indicates that JSR and IPT do not have a significant correlation. Although these results do not support our hypothesis, we cannot rule out the possibility that the dawn-dusk electric field was masked by some other process, including the conductivity variation and/or the time-variable longitudinal asymmetry of JSR.

KELT-9b is a recently discovered exoplanet with a 1.49 day orbit around a B9.5/A0-type star. The unparalleled levels of ultraviolet irradiation that it receives from its host star put KELT-9b in its own unique class of ultra-hot Jupiters, with an equilibrium temperature >4000 K. The high quantities of dissociated hydrogen and atomic metals present in the dayside atmosphere of KELT-9b bear more resemblance to a K-type star than a gas giant. We present a single observation of KELT-9b during its secondary eclipse, taken with the Wide Field Camera on the Isaac Newton Telescope (INT). This observation was taken in the U-band, a window particularly sensitive to Rayleigh scattering. We do not detect a secondary eclipse signal, but our 3σ upper limit of 181 ppm on the depth allows us to constrain the dayside temperature of KELT-9b at pressures of ∼30 mbar to 4995 K (3σ). Although we can place an observational constraint of A_g < 0.14, our models suggest that the actual value is considerably lower than this due to H⁻ opacity. This places KELT-9b squarely in the albedo regime populated by its cooler cousins, almost all of which reflect very small components of the light incident on their daysides. This work demonstrates the ability of ground-based 2 m class telescopes like the INT to perform secondary eclipse studies in the near-ultraviolet, which have previously only been conducted from space-based facilities.

Thanks to the Cassini–Huygens mission, it is now established that the first aerosols in Titan's upper atmosphere are found from an altitude of ∼1200 km. Once they are formed and through their descent toward the surface, these nanoparticles are submitted to persistent far-ultraviolet (FUV) radiation that can reach lower atmospheric layers. Such an interaction has an impact, especially on the chemistry and charge budget of the atmospheric compounds. Models are useful to understand this photoprocessing, but they lack important input data such as the photoemission threshold or the absolute photoabsorption/emission cross sections of the aerosols. In order to quantify the photoemission processes, analogs of Titan's aerosols have been studied with the DESIRS FUV beamline at the synchrotron SOLEIL as isolated substrate-free nanoparticles. We present here the corresponding angle-resolved photoelectron spectroscopy data recorded at different FUV photon energies. The results show a very low photoionization threshold (6.0 ± 0.1 eV ∼ 207 nm) and very high absolute ionization cross sections (∼10⁶ Mb), indicating that FUV photoemission from aerosols is an intense source of slow electrons that has to be taken into account in photochemical models of Titan's atmosphere.

Proxima b is a terrestrial-mass planet in the habitable zone of Proxima Centauri. Proxima Centauri's high stellar activity, however, casts doubt on the habitability of Proxima b: sufficiently bright and frequent flares and any associated proton events may destroy the planet's ozone layer, allowing lethal levels of UV flux to reach its surface. In 2016 March, the Evryscope observed the first naked-eye-brightness superflare detected from Proxima Centauri. Proxima increased in optical flux by a factor of ∼68 during the superflare and released a bolometric energy of 10^33.5 erg, ∼10× larger than any previously detected flare from Proxima. Over the last two years the Evryscope has recorded 23 other large Proxima flares ranging in bolometric energy from 10^30.6 to 10^32.4 erg; coupling those rates with the single superflare detection, we predict that at least five superflares occur each year. Simultaneous high-resolution High Accuracy Radial velocity Planet Searcher (HARPS) spectroscopy during the Evryscope superflare constrains the superflare's UV spectrum and any associated coronal mass ejections. We use these results and the Evryscope flare rates to model the photochemical effects of NO_x atmospheric species generated by particle events from this extreme stellar activity, and show that the repeated flaring may be sufficient to reduce the ozone of an Earth-like atmosphere by 90% within five years; complete depletion may occur within several hundred kyr. The UV light produced by the Evryscope superflare would therefore have reached the surface with ∼100× the intensity required to kill simple UV-hardy microorganisms, suggesting that life would have to undergo extreme adaptations to survive in the surface areas of Proxima b exposed to these flares.

The Alice ultraviolet spectrograph on the European Space Agency Rosetta spacecraft observed comet 67P/Churyumov–Gerasimenko in its orbit around the Sun for just over two years. Alice observations taken in 2015 October, two months after perihelion, show large increases in the comet's Lyβ, O i 1304, O i 1356, and C i 1657 Å atomic emission that initially appeared to indicate gaseous outbursts. However, the Rosetta Plasma Consortium instruments showed a coronal mass ejection (CME) impact at the comet coincident with the emission increases, suggesting that the CME impact may have been the cause of the increased emission. The presence of the semi-forbidden O i 1356 Å emission multiplet is indicative of a substantial increase in dissociative electron impact emission from the coma, suggesting a change in the electron population during the CME impact. The increase in dissociative electron impact could be a result of the interaction between the CME and the coma of 67P or an outburst coincident with the arrival of the CME. The observed dissociative electron impact emission during this period is used to characterize the O₂ content of the coma at two peaks during the CME arrival. The mechanism that could cause the relationship between the CME and UV emission brightness is not well constrained, but we present several hypotheses to explain the correlation.

The oxidation of rocky planet surfaces and atmospheres, which arises from the twin forces of stellar nucleosynthesis and gravitational differentiation, is a universal process of key importance to habitability and exoplanet biosignature detection. Here we take a generalized approach to this phenomenon. Using a single parameter to describe the redox state, we model the evolution of terrestrial planets around nearby M stars and the Sun. Our model includes atmospheric photochemistry, diffusion and escape, line-by-line climate calculations, and interior thermodynamics and chemistry. In most cases, we find abiotic atmospheric ${{\rm{O}}}_{2}$ buildup around M stars during the pre-main-sequence phase to be much less than calculated previously, because the planet's magma ocean absorbs most oxygen liberated from ${{\rm{H}}}_{2}{\rm{O}}$ photolysis. However, loss of noncondensing atmospheric gases after the mantle solidifies remains a significant potential route to abiotic atmospheric ${{\rm{O}}}_{2}$ subsequently. In all cases, we predict that exoplanets that receive lower stellar fluxes, such as LHS1140b and TRAPPIST-1f and g, have the lowest probability of abiotic ${{\rm{O}}}_{2}$ buildup and hence may be the most interesting targets for future searches for biogenic ${{\rm{O}}}_{2}$. Key remaining uncertainties can be minimized in future by comparing our predictions for the atmospheres of hot, sterile exoplanets such as GJ1132b and TRAPPIST-1b and c with observations.

Since its launch in 1990, the Hubble Space Telescope (HST) has served as a platform with unique capabilities for remote observations of comets in the far-ultraviolet region of the spectrum. Successive generations of imagers and spectrographs have seen large advances in sensitivity and spectral resolution enabling observations of the diverse properties of a representative number of comets during the past 25 years. To date, four comets have been observed in the far-ultraviolet by the Cosmic Origins Spectrograph (COS), the last spectrograph to be installed in HST, in 2009: 103P/Hartley 2, C/2009 P1 (Garradd), C/2012 S1 (ISON), and C/2014 Q2 (Lovejoy). COS has unprecedented sensitivity, but limited spatial information in its 25 diameter circular aperture, and our objective was to determine the CO production rates from measurements of the CO Fourth Positive system in the spectral range of 1400–1700 Å. In the two brightest comets, 19 bands of this system were clearly identified. The water production rates were derived from nearly concurrent observations of the OH (0,0) band at 3085 Å by the Space Telescope Imaging Spectrograph. The derived CO/${{\rm{H}}}_{2}{\rm{O}}$ production rate ratio ranged from ∼0.3% for Hartley 2 to ∼22% for Garradd. In addition, strong partially resolved emission features due to multiplets of S i, centered at 1429 Å and 1479 Å, and of C i at 1561 Å and 1657 Å, were observed in all four comets. Weak emission from several lines of the ${{\rm{H}}}_{2}$ Lyman band system, excited by solar Lyα and Lyβ pumped fluorescence, were detected in comet Lovejoy.