Qiushi Chris Tian

The atmospheres of sub-Neptunes provide a window into their internal structure and history, shedding light on the origin of this common, but enigmatic, class of exoplanets. However, the physical and chemical processes that shape sub-Neptunes’ transmission spectra, in particular cloud and haze formation, are not well understood. To identify possible correlations between transmission spectra and UV irradiation, the Sub-Neptune Planetary Atmosphere Characterization Experiment (SPACE) Program observed an array of sub-Neptunes and their host stars using the Hubble Space Telescope (HST), measuring the planets’ transmission spectra between 1.1 and 1.7 μ m with the Wide Field Camera 3 (WFC3) and the stars’ UV spectra with the Space Telescope Imaging Spectrograph (STIS). Here, we present the observations of HD 191939 b carried out as part of the SPACE Program, which reveal no significant spectral features in the transmission spectrum. The data deliver moderate evidence at significance levels between 2.0 σ and 3.2 σ against a cloud-free atmosphere with solar metallicity, rendering this scenario unlikely, but still possible. A supersolar metallicity of HD 191939 b might be consistent with the known trend of increasing atmospheric metallicity with decreasing planet mass. Both hydrocarbon haze formation and cloud condensation can be efficient at HD 191939 b’s zero-albedo equilibrium temperature of (880 ± 20) K, particularly in atmospheres with supersolar metallicity, possibly additionally muting absorption features.

We present results from 28 stellar occultations by the large trans-Neptunian object (50000) Quaoar registered between 2018 and 2025. By performing a joint analysis of this occultation dataset, along with nine other published events, we were able to fit an oblate ellipsoid shape, with equatorial semiaxes a and b of 566 . 1 − 2.2 + 2.5 km, and a polar semiaxis, c , of 511 . 2 − 3.7 + 3.6 km. It provides an equivalent volumetric diameter of 1094.4 ± 4.6 km and polar oblateness of 0.097 ± 0.011. Considering an absolute magnitude of H = 2.79 ± 0.35, we derive a geometric albedo of p _V = 0.125 ± 0.038. We have derived new upper limits to the surface pressure of a CH ₄ atmosphere of 0.15 nbar (1 σ ) and 0.65 nbar (3 σ ). We also provide a table with the 36 new astrometric positions for Quaoar. Using the new system mass derived from Weywot’s orbit around Quaoar, we calculated a density of 1.760 ± 0.109 g cm ⁻³ . Moreover, from the derived size and rotation period (8.8394 ± 0.0002 hr, we calculated that, if Quaoar is in Maclaurin hydrostatic equilibrium state, it would have a density of 1.859 ± 0.200 g cm ⁻³ . This result, within the error bars, is compatible with the value we found. Therefore, this work shows that Quaoar can be a Maclaurin object, being eligible as a dwarf planet.

Newly formed stars profoundly affect their environment by depositing energy and momentum into the surrounding gas. However, only a fraction of the stellar feedback is retained in the cloud, and observational constraints are needed to improve our understanding of this process. In a sample of 19 nearby galaxies, we matched H II regions from PHANGS–MUSE to their ionizing stellar source from PHANGS–HST and measured the percentage of ionizing radiation that leaks into the surrounding diffuse ionised gas (DIG). Based on a catalogue in which each H II region is powered by a single young and massive stellar association, we measure a photon escape fraction of f _esc = 82 ₋₂₄ ⁺¹² %. We obtain comparable results when using different procedures to match the ionised gas to its source. All samples in our study contain a substantial fraction of objects (up to 20%), in which the stellar source is insufficient to produce the H α flux observed from the nebula. Many of these cases are probably related to uncertain age estimates, but we also find numerous regions for which a significant fraction of the ionising photon budget is contributed by stars that reside outside the boundaries of the H II region. This finding motivates the use of an alternative galaxy-wide approach in which we include all H II regions and stellar sources, not just those that show a clear overlap. When we sum the ionisation budget over entire galaxies, we measure slightly lower, but consistent values.

The joint capabilities of the Hubble Space Telescope (HST) and JWST allow for an unparalleled look at the early lives of star clusters at near- and mid-infrared wavelengths. We present here a multiband analysis of embedded young stellar clusters in 11 nearby, star-forming galaxies, using the PHANGS-JWST and PHANGS-HST data sets. We use the Zooniverse citizen science platform to conduct an initial by-eye search for embedded clusters in near-UV/optical/near-infrared images that trace stellar continuum emission, the Paschen α and H α recombination lines, and the 3.3 μ m polycyclic aromatic hydrocarbon feature and its underlying continuum. With this approach, we identify 292 embedded cluster candidates for which we characterize their ages, masses, and levels of line-of-sight extinction by comparing the photometric data to predictions from stellar population models. The embedded cluster candidates have a median age of 4.5 Myr and an average line-of-sight extinction 〈 A _V 〉 = 6.0 mag. We determine lower limits on source stellar masses, resulting in a median stellar mass of 10 ³ M _⊙ . We use this sample of embedded cluster candidates to train multiple convolutional neural network models to carry out deep transfer learning-based searches for embedded clusters. With the aim of optimizing models for future catalog production, we compare results for four variations of training data using two neural networks. Confusion matrices for all eight model configurations, as well as inter-model identification trends, are presented. With refinement of the training sample, we determine that optimized models could serve as a pathway for future embedded cluster identification beyond our 11 galaxy sample.

Sub-Neptune exoplanets are the most abundant type of planet known today. As they do not have a Solar System counterpart, many open questions exist about their composition and formation. Previous spectroscopic studies have ruled out aerosol-free hydrogen-helium dominated atmospheres for many characterized sub-Neptunes but are inconclusive about their exact atmospheric compositions. Here we characterize the hot (T_eq=1311 K) sub-Neptune HD 86226 c (R=2.2 R_⊕, M=7.25 M_⊕), which orbits its G-type host star on a four-day orbit. The planet is located in a special part of the sub-Neptune parameter space: Its high equilibrium temperature prohibits methane-based haze formation, increasing the chances for a clear atmosphere on this planet. We used Hubble Space Telescope data taken with WFC3 and STIS from the Sub-Neptune Planetary Atmosphere Characterization Experiment (SPACE) Program to perform near-infrared (1.1–1.7 μm) transmission spectroscopy and ultraviolet characterization of the host star. We report a featureless transmission spectrum that is consistent within 0.4 σ with a constant transit depth of 418 ± 14 ppm. The amplitude of this spectrum is only 0.01 scale heights for a H/He-dominated atmosphere, excluding a cloud-free solar-metallicity atmosphere on HD 86226 c with a confidence of 6.5 σ. Based on an atmospheric retrieval analysis and forward models of cloud and haze formation, we find that the featureless spectrum could be due to metal enrichment [M/H] > 2.3 (3 σ confidence lower limit) of a cloudless atmosphere, or silicate (MgSiO₃), iron (Fe), or manganese sulfide (MnS) clouds. For these species, we performed a detailed investigation of cloud formation in high metallicity, high-temperature atmospheres. Our results highlight that HD 86226 c does not follow the aerosol trend of sub-Neptunes found by previous studies. Follow-up observations with the James Webb Space Telescope could determine whether this planet aligns with the recent detections of metal-enriched atmospheres or if it harbors a cloud species that is otherwise atypical for sub-Neptunes.

We present the largest catalog to date of star clusters and compact associations in nearby galaxies. We have performed a V-band-selected census of clusters across the 38 spiral galaxies of the PHANGS–Hubble Space Telescope (HST) Treasury Survey, and measured integrated, aperture-corrected near-ultraviolet-U-B-V-I photometry. This work has resulted in uniform catalogs that contain ∼20,000 clusters and compact associations, which have passed human inspection and morphological classification, and a larger sample of ∼100,000 classified by neural network models. Here, we report on the observed properties of these samples, and demonstrate that tremendous insight can be gained from just the observed properties of clusters, even in the absence of their transformation into physical quantities. In particular, we show the utility of the UBVI color–color diagram, and the three principal features revealed by the PHANGS-HST cluster sample: the young cluster locus, the middle-age plume, and the old globular cluster clump. We present an atlas of maps of the 2D spatial distribution of clusters and compact associations in the context of the molecular clouds from PHANGS–Atacama Large Millimeter/submillimeter Array. We explore new ways of understanding this large data set in a multiscale context by bringing together once-separate techniques for the characterization of clusters (color–color diagrams and spatial distributions) and their parent galaxies (galaxy morphology and location relative to the galaxy main sequence). A companion paper presents the physical properties: ages, masses, and dust reddenings derived using improved spectral energy distribution fitting techniques.

Planetary systems in mean motion resonances hold a special place among the planetary population. They allow us to study planet formation in great detail as dissipative processes are thought to have played an important role in their existence. Additionally, planetary masses in bright resonant systems can be independently measured via both radial velocities and transit timing variations. In principle, they also allow us to quickly determine the inclination of all planets in the system since, for the system to be stable, they are likely all in coplanar orbits. To describe the full dynamical state of the system, we also need the stellar obliquity, which provides the orbital alignment of a planet with respect to the spin of its host star and can be measured thanks to the Rossiter–McLaughlin effect. It was recently discovered that HD 110067 harbors a system of six sub-Neptunes in resonant chain orbits. We here analyze an ESPRESSO high-resolution spectroscopic time series of HD 110067 during the transit of planet c. We find the orbit of HD 110067 c to be well aligned, with a sky-projected obliquity of λ =6⁺²⁴_-26 deg. This result indicates that the current architecture of the system was reached through convergent migration without any major disruptive events. Finally, we report transit-timing variation in this system as we find a significant offset of 19 ± 4 min in the center of the transit compared to the published ephemeris.

We present the largest catalog to-date of star clusters and compact associations in nearby galaxies. We have performed a V-band-selected census of clusters across the 38 spiral galaxies of the PHANGS-HST Treasury Survey, and measured integrated, aperture-corrected NUV-U-B-V-I photometry. This work has resulted in uniform catalogs that contain $\sim$20,000 clusters and compact associations which have passed human inspection and morphological classification, and a larger sample of $\sim$100,000 classified by neural network models. Here, we report on the observed properties of these samples, and demonstrate that tremendous insight can be gained from just the observed properties of clusters, even in the absence of their transformation into physical quantities. In particular, we show the utility of the UBVI color-color diagram, and the three principal features revealed by the PHANGS-HST cluster sample: the young cluster locus, the middle-age plume, and the old globular cluster clump. We present an atlas of maps of the 2D spatial distribution of clusters and compact associations in the context of the molecular clouds from PHANGS-ALMA. We explore new ways of understanding this large dataset in a multi-scale context by bringing together once-separate techniques for the characterization of clusters (color-color diagrams and spatial distributions) and their parent galaxies (galaxy morphology and location relative to the galaxy main sequence). A companion paper presents the physical properties: ages, masses, and dust reddenings derived using improved spectral energy distribution (SED) fitting techniques.