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Abstract Stellar flares are short-duration (< hours) bursts of radiation associated with surface magnetic reconnection events. Stellar magnetic activity generally decreases as a function of both the age and Rossby number,R₀, a measure of the relative importance of the convective and rotational dynamos. Young stars (<300 Myr) have typically been overlooked in population-level flare studies due to challenges with flare-detection methods. Here, we select a sample of stars that are members of 26 nearby moving groups, clusters, or associations with ages <300 Myr that have been observed by the Transiting Exoplanet Survey Satellite at 2 minute cadence. We identified 26,355 flares originating from 3160 stars and robustly measured the rotation periods of 1847 stars. We measure and find the flare frequency distribution slope,α, saturates for all spectral types atα∼ −0.5 and is constant over 300 Myr. Additionally, we find that flare rates for starst_age= 50–250 Myr are saturated belowR₀< 0.14, which is consistent with other indicators of magnetic activity. We find evidence of annual flare rate variability in eleven stars, potentially correlated with long-term stellar activity cycles. Additionally, we crossmatch our entire sample with the Galaxy Evolution Explorer and find no correlation between flare rate and far- and near-ultraviolet flux. Finally, we find the flare rates of planet-hosting stars are relatively lower than comparable, larger samples of stars, which may have ramifications for the atmospheric evolution of short-period exoplanets. 

Abstract 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. 

Abstract The characterization of young planets (<300 Myr) is pivotal for understanding planet formation and evolution. We present the 3–5μm transmission spectrum of the 17 Myr, Jupiter-size (R∼10R_⊕) planet, HIP 67522b, observed with JWST NIRSpec/G395H. To check for spot contamination, we obtain a simultaneousg-band transit with the Southern Astrophysical Research Telescope. The spectrum exhibits absorption features 30%–50% deeper than the overall depth, far larger than expected from an equivalent mature planet, and suggests that HIP 67522b’s mass is <20M_⊕irrespective of cloud cover and stellar contamination. A Bayesian retrieval analysis returns a mass constraint of 13.8 ± 1.0M_⊕. This challenges the previous classification of HIP 67522b as a hot Jupiter and instead, positions it as a precursor to the more common sub-Neptunes. With a density of <0.10 g cm⁻³, HIP 67522 b is one of the lowest-density planets known. We find strong absorption from H₂O and CO₂(≥7σ), a modest detection of CO (3.5σ), and weak detections of H₂S and SO₂(≃2σ). Comparisons with radiative-convective equilibrium models suggest supersolar atmospheric metallicities and solar-to-subsolar C/O ratios, with photochemistry further constraining the inferred atmospheric metallicity to 3 × 10 solar due to the amplitude of the SO₂feature. These results point to the formation of HIP 67522b beyond the water snowline, where its envelope was polluted by icy pebbles and planetesimals. The planet is likely experiencing substantial mass loss (0.01–0.03M_⊕Myr⁻¹), sufficient for envelope destruction within a gigayear. This highlights the dramatic evolution occurring within the first 100 Myr of its existence. 

Abstract The classical T Tauri star (CTTS) stage is a critical phase of the star and planet formation process. In an effort to better understand the mass accretion processes, which can dictate future stellar evolution and planet formation, a multiepoch, multiwavelength photometric and spectroscopic monitoring campaign of four CTTSs (TW Hya, RU Lup, BP Tau, and GM Aur) was carried out in 2021 and 2022/2023 as part of the Outflows and Disks around Young Stars: Synergies for the Exploration of ULLYSES Spectra program. Here we focus on the Hubble Space Telescope (HST) UV spectra obtained by the HST Director’s Discretionary Time UV Legacy Library of Young Stars as Essential Standards (ULLYSES) program. Using accretion shock modeling, we find that all targets exhibit accretion variability, varying from short increases in accretion rate by up to a factor of 3 within 48 hr to longer decreases in accretion rate by a factor of 2.5 over the course of 1 yr. This is despite the generally consistent accretion morphology within each target. Additionally, we test empirical relationships between accretion rate and UV luminosity and find stark differences, showing that these relationships should not be used to estimate the accretion rate for an individual target. Our work reinforces that future multiepoch and simultaneous multiwavelength studies are critical in our understanding of the accretion process in low-mass star formation. 

Abstract We obtained ultraviolet and optical spectra for nine M dwarfs across a range of rotation periods to determine whether they showed stochastic intrinsic variability distinguishable from flares. The ultraviolet spectra were observed during the Far Ultraviolet M-dwarf Evolution Survey Hubble Space Telescope program using the Space Telescope Imaging Spectrograph. The optical observations were taken from the Apache Point Observatory 3.5 m telescope using the Dual Imaging Spectrograph and from the Gemini South Observatory using the Gemini Multi-Object Spectrograph. We used the optical spectra to measure multiple chromospheric lines: the Balmer series from Hαto H10 and the CaiiH and K lines. We find that after excising flares, these lines vary on the order of 1%–20% at minute-cadence over the course of an hour. The absolute amplitude of variability was greater for the faster rotating M dwarfs in our sample. Among the five stars for which we measured the weaker Balmer lines, we note a tentative trend that the fractional amplitude of the variability increases for higher-order Balmer lines. We measured the integrated flux of multiple ultraviolet emission features formed in the transition region: the Nv, Siiv,and Civresonance line doublets, and the Ciiand Heiimultiplets. The signal-to-noise ratio of the UV data was too low for us to detect nonflare variability at the same scale and time cadence as the optical. We consider multiple mechanisms for the observed stochastic variability and propose both observational and theoretical avenues of investigation to determine the physical causes of intrinsic variability in the chromospheres of M dwarfs. 

Abstract Photochemistry is a fundamental process of planetary atmospheres that regulates the atmospheric composition and stability¹. However, no unambiguous photochemical products have been detected in exoplanet atmospheres so far. Recent observations from the JWST Transiting Exoplanet Community Early Release Science Program^2,3found a spectral absorption feature at 4.05 μm arising from sulfur dioxide (SO₂) in the atmosphere of WASP-39b. WASP-39b is a 1.27-Jupiter-radii, Saturn-mass (0.28 M_J) gas giant exoplanet orbiting a Sun-like star with an equilibrium temperature of around 1,100 K (ref. ⁴). The most plausible way of generating SO₂in such an atmosphere is through photochemical processes^5,6. Here we show that the SO₂distribution computed by a suite of photochemical models robustly explains the 4.05-μm spectral feature identified by JWST transmission observations⁷with NIRSpec PRISM (2.7σ)⁸and G395H (4.5σ)⁹. SO₂is produced by successive oxidation of sulfur radicals freed when hydrogen sulfide (H₂S) is destroyed. The sensitivity of the SO₂feature to the enrichment of the atmosphere by heavy elements (metallicity) suggests that it can be used as a tracer of atmospheric properties, with WASP-39b exhibiting an inferred metallicity of about 10× solar. We further point out that SO₂also shows observable features at ultraviolet and thermal infrared wavelengths not available from the existing observations. 

Abstract Carbon dioxide (CO₂) is a key chemical species that is found in a wide range of planetary atmospheres. In the context of exoplanets, CO₂is an indicator of the metal enrichment (that is, elements heavier than helium, also called ‘metallicity’)^1–3, and thus the formation processes of the primary atmospheres of hot gas giants^4–6. It is also one of the most promising species to detect in the secondary atmospheres of terrestrial exoplanets^7–9. Previous photometric measurements of transiting planets with the Spitzer Space Telescope have given hints of the presence of CO₂, but have not yielded definitive detections owing to the lack of unambiguous spectroscopic identification^10–12. Here we present the detection of CO₂in the atmosphere of the gas giant exoplanet WASP-39b from transmission spectroscopy observations obtained with JWST as part of the Early Release Science programme^13,14. The data used in this study span 3.0–5.5 micrometres in wavelength and show a prominent CO₂absorption feature at 4.3 micrometres (26-sigma significance). The overall spectrum is well matched by one-dimensional, ten-times solar metallicity models that assume radiative–convective–thermochemical equilibrium and have moderate cloud opacity. These models predict that the atmosphere should have water, carbon monoxide and hydrogen sulfide in addition to CO₂, but little methane. Furthermore, we also tentatively detect a small absorption feature near 4.0 micrometres that is not reproduced by these models.