Stephen E. Levine

Pluto’s tenuous atmosphere has microbar-level pressure and is composed primarily of N ₂ , with a variable haze. Its eccentric orbit, combined with high obliquity, leads to significant changes in solar insolation throughout the Plutonian year. The atmosphere is supported by vapor pressure equilibrium with the surface ices; thus, surface changes are coupled with the atmospheric properties. Volatile-transport models have anticipated Pluto’s atmospheric evolution: predictions range from collapse over the coming decades to an atmosphere that remains. Previous work claims that Pluto’s atmospheric pressure monotonically increased from 1988 through 2016, that the atmosphere began freezing out in 2018–2019, and that there was a plateau as of 2020. Here we report results from 10 stellar occultations by Pluto between 2017 August and 2023 July. Four events were multichord, while six were from single sites. Our results indicate a pressure plateau between the New Horizons flyby in 2015 through roughly 2021 and suggest that the atmospheric pressure has started to drop. Comparing data taken 2015–2021 and 2022, the clear-atmosphere pressure at 1275 km decreased 7% ± 6%, and it dropped 16% ± 2% for pressure at 1215 km when including haze. From 2017 to 2023, the upper atmospheric structure is consistent, while there is a change in light-curve slope in the lower atmosphere. This change of slope is consistent with haze particles settling over yearly or shorter timescales. Spikes in one light curve are indicative of intermittent buoyancy waves. More data are needed to confirm a recent pressure change.

Weywot is the largest satellite of the candidate dwarf planet Quaoar, which also hosts a remarkable ring system located beyond its Roche limit—a configuration that challenges conventional understanding of ring dynamics. In this work, we predicted three stellar occultations by Weywot occurring in 2023 May and June, of which one was successfully observed on 2023 June 22. This multichord event took place over North America, where observers were deployed across the shadow path from Maryland to California. Five positive detections and two near misses allowed us to accurately constrain Weywot’s projected size and shape at the time of the occultation. We constrained the equivalent diameter to be between 116 and 172 km (with 95% confidence) and estimate a geometric albedo between 0.024 and 0.078—substantially lower than that of Quaoar. These results offer new insights into the physical properties of Quaoar’s system and provide valuable constraints for models of its formation and evolution.

Pluto is the only minor planet (excluding satellites) that is known to host a thin, global atmosphere. The atmosphere has microbar-level surface pressure, is composed primarily of nitrogen, and contains a layered haze made of organic materials (e.g. Gladstone et al., 2016, Science, 351, id. aad8866). Notably, Pluto’s atmosphere is intricately linked to its surface ices through vapor-pressure equilibrium (e.g. Elliot et al., 1989, Icarus, 77, 148). Because of the strong tie between the atmosphere and ices, the surface pressure is highly dependent on ice temperature, which is a function of orbital and seasonal timescales. On Pluto, with an eccentric orbit (e = 0.25) and high obliquity (~123 deg.), these changes are pronounced and the atmospheric properties can vary significantly on timescales of only a few decades. Thermophysical, volatile-transport models have been developed to study Pluto’s atmospheric evolution; predictions range from atmospheric contraction or collapse over the coming decades to an atmosphere that remains throughout Pluto’s entire revolution around the Sun (e.g. Young, 2013, ApJ Lett., 766, L22; Hansen et al. 2015, Icarus, 246, 183; Bertrand et al. 2018, Icarus, 309, 277; Johnson et al. 2021, Icarus, 356, id.114070).Stellar occultation data are the most direct way to measure Pluto’s atmosphere from the Earth. Results from previous occultations reported that Pluto’s atmospheric pressure monotonically increased since its definitive discovery in 1988 through 2016 (Meza et al. 2019, A&A, 625, id.A136) and then that the atmosphere had possibly begun freezing out in 2018-2019 (Arimatsu et al. 2020, A&A 638, L5; Young et al. 2021, AAS DPS Meeting #53, id.307.06). Observations of an occultation in 2020 did not show a pressure drop and were interpreted to be either a continued pressure increase (Poro et al. 2021, A&A, 652, L7) or a plateau phase (Sicardy et al. 2021, ApJ Lett, 923, L31). Here, we report results from ten successfully-observed stellar occultations by Pluto between 2017 August and 2023 July that have not yet been published. The stellar magnitudes ranged from G=12.91 to 18.4 with geocentric relative velocities between 1.7 and 24.5 km/s. Four of these events had successful chords from multiple sites, while six events were from single sites. We carried out atmospheric fits assuming clear, isothermal atmospheres as well as atmospheres with a haze layer. Our results indicate that Pluto’s atmospheric pressure has begun decreasing in recent years.This work is supported by NASA grant 80NSSC21K043.

Astronomy's extensive collections of photographic glass plates contain historical images and spectra of celestial objects, documenting more than a century of the observable cosmos. Many reveal changes, both sudden (explosive), periodic, or gradual (evolutionary), which is material of immense interest for time-domain studies because of the long time-base they cover. Those early photographic observations also furnished all the basic data which supported our early understanding of the universe, and from which modern stellar classifications have been derived. Once the ubiquitous workhorse detector, plates or film are now replaced by electronic detectors, and systems are modified to take advantage of advances in telescope technology. This change poses challenges of preservation and accessibility for the plates, leading administrators to question the usefulness of the older materials in relation to the cost of their care and preservation. The following paper details many examples of reusing or re-purposing those plates, demonstrates their unique value to modern astronomy and the history of science, and makes a strong case for committing resources towards their long-term preservation and ultimately their comprehensive digitization.

Gaussian process (GP) regression is a nonparametric Bayesian approach that has been used successfully in various astronomical domains, especially in time-domain astronomy. The most common applications are the smoothing of data for interpolation and the detection of periodicities. The ability to create unbiased data-driven models without a predefined physical model can be a major advantage over conventional regression methods. Prior knowledge can be included by setting boundary conditions or constraining hyperparameter values, while unknown hyperparameters are optimized during the conditioning of the model. We have adapted and transformed previous approaches of GP regression and introduce three new applications for this regression method, especially in the context of stellar occultations: the modeling of occultation light curves, the correction of public JPL ephemerides of minor planets based on publicly available image data of the Zwicky Transient Facility, and the detection of natural satellites. We used data from observations of stellar occultations to validate the models and achieved promising results in all cases, and thus we confirmed the flexibility of GP regression models. Considering various existing use cases in addition to our novel applications, GP regression can be used to model diverse data sets addressing a wide range of problems. The accuracy of the model depends on the input data and on the set boundary conditions. Generally, high-quality data allow the usage of loose boundary conditions, while low-quality data require more restrictive boundary conditions to avoid overfitting.

A stellar occultation of Gaia DR3 2646598228351156352 by the Centaur (2060) Chiron was observed from the South African Astronomical Observatory on 2018 November 28 UT. Here we present a positive detection of material surrounding Chiron from the 74-inch telescope for this event. Additionally, a global atmosphere is ruled out at the tens of microbars level for several possible atmospheric compositions. There are multiple 3σ drops in the 74-inch light curve: three during immersion and two during emersion. Occulting material is located between 242 and 270 km from the center of the nucleus in the sky plane. Assuming the ring-plane orientation proposed for Chiron from the 2011 occultation, the flux drops are located at 352, 344, and 316 km (immersion) and 357 and 364 km (emersion) from the center, with normal optical depths of 0.26, 0.36, and 0.22 (immersion) and 0.26 and 0.18 (emersion) and equivalent widths between 0.7 and 1.3 km. This detection is similar to the previously proposed two-ring system and is located within the error bars of that ring-pole plane; however, the normal optical depths are less than half of the previous values, and three features are detected on immersion. These results suggest that the properties of the surrounding material have evolved between the 2011, 2018, and 2022 observations.

The distortions of absorption line profiles caused by photospheric brightness variations on the surfaces of cool, main-sequence stars can mimic or overwhelm radial velocity (RV) shifts due to the presence of exoplanets. The latest generation of precision RV spectrographs aims to detect velocity amplitudes ≲ 10 cm s⁻¹, but requires mitigation of stellar signals. Statistical techniques are being developed to differentiate between Keplerian and activity-related velocity perturbations. Two important challenges, however, are the interpretability of the stellar activity component as RV models become more sophisticated, and ensuring the lowest-amplitude Keplerian signatures are not inadvertently accounted for in flexible models of stellar activity. For the K2V exoplanet host ϵ Eridani, we separately used ground-based photometry to constrain Gaussian processes for modeling RVs and TESS photometry with a light-curve inversion algorithm to reconstruct the stellar surface. From the reconstructions of TESS photometry, we produced an activity model that reduced the rms scatter in RVs obtained with EXPRES from 4.72 to 1.98 m s⁻¹. We present a pilot study using the CHARA Array and MIRC-X beam combiner to directly image the starspots seen in the TESS photometry. With the limited phase coverage, our spot detections are marginal with current data but a future dedicated observing campaign should allow for imaging, as well as allow the stellar inclination and orientation with respect to the debris disk to be definitively determined. This work shows that stellar surface maps obtained with high-cadence, time-series photometric and interferometric data can provide the constraints needed to accurately reduce RV scatter.

We observed the occultation of the star Gaia DR2 4056440205544338944 by (28978) Ixion. The event was observed from two Lowell Observatory sites, using the 4.3 m Lowell Discovery Telescope (LDT), near Happy Jack, AZ, USA, and a 0.32 m telescope co-mounted with the Titan Monitoring telescope on Lowell’s Mars Hill campus in Flagstaff, AZ. The LDT chord, at 44.86 s, was roughly 30% longer than the longest predicted possible chord. Under the assumption of a spherical body, Ixion’s fitted diameter D = 709.6 ± 0.2 km. The LDT light-curve profile was used to place an upper limit on the surface pressure P < 2 μbar on any possible atmosphere of Ixion. At the distance of Ixion, the occulted star had a fitted projected diameter of 19.25 ± 0.3 km assuming uniform disk illumination, giving a stellar angular diameter of 0.675 ± 0.010 mas. Using the Gaia EDR3 parallax of 0.565 mas, the stellar radius is . The measured size is consistent with prior spectral classification of this star as a reddened mid-M giant. This is one of only a modest number of M5 III stars to have a directly measured diameter, and is more distant than most.

We present the results from four stellar occultations by (486958) Arrokoth, the flyby target of the New Horizons extended mission. Three of the four efforts led to positive detections of the body, and all constrained the presence of rings and other debris, finding none. Twenty-five mobile stations were deployed for 2017 June 3 and augmented by fixed telescopes. There were no positive detections from this effort. The event on 2017 July 10 was observed by the Stratospheric Observatory for Infrared Astronomy with one very short chord. Twenty-four deployed stations on 2017 July 17 resulted in five chords that clearly showed a complicated shape consistent with a contact binary with rough dimensions of 20 by 30 km for the overall outline. A visible albedo of 10% was derived from these data. Twenty-two systems were deployed for the fourth event on 2018 August 4 and resulted in two chords. The combination of the occultation data and the flyby results provides a significant refinement of the rotation period, now estimated to be 15.9380 ± 0.0005 hr. The occultation data also provided high-precision astrometric constraints on the position of the object that were crucial for supporting the navigation for the New Horizons flyby. This work demonstrates an effective method for obtaining detailed size and shape information and probing for rings and dust on distant Kuiper Belt objects as well as being an important source of positional data that can aid in spacecraft navigation that is particularly useful for small and distant bodies.