Lowell Observatory

Abstract Longitudinal density fluctuations observed in stellar streams can result from gravitational interactions with massive perturbers in the Milky Way, such as dark matter subhalos. Analysing these density variations provides a powerful probe of properties (motion, mass, size, etc.) of the perturbing objects. However, caution is needed because density variations may arise naturally from internal dynamics of streams, namely epicycles. In this work, we focus on the Indus stellar stream, a remnant of an ancient dwarf satellite of the Galaxy. An Indus stream spanning ∼90○ is revealed in the southern Galactic sky using a comprehensive matched-filter analysis utilizing data from the Gaia mission. A spatial density model is fitted to the filtered map to quantitatively characterize the morphology, which demonstrates episodic density peaks and gaps in the stream. Through N-body simulations, we show that there are strong epicyclic motions of stars happening during tidal disruptions. The present-day longitudinal densities from simulations are comparable to the measurement from data, with similar numbers and locations of peaks and gaps, suggesting that the observed density should mainly be caused by epicycles. We also find that a cuspy dark matter halo for the Indus dwarf is likely to produce milder stellar epicyclic peaks compared to a cored halo which results in steeper peaks. This arises from different instantaneous mass loss due to distinct central mass distributions of halos, where a cored halo usually leads to severer tidal stripping. The observed density exhibits moderate peak sharpness, implying that Indus may have originally possessed a cuspy halo.

Abstract JWST imagery of the approaching Kreutz sungrazer C/2026 A1 (MAPS) appears sufficient to distinguish the comet’s nucleus from the surrounding dust. Coma model fitting indicates the morphology is compatible with a ∼0.4 km diameter nucleus—likely larger than those of the minor Kreutz fragments routinely discovered with coronagraphs, and comparable to or slightly smaller than that of C/2011 W3 (Lovejoy). However, since C/2026 A1 is the only Kreutz sungrazer whose nucleus has been directly observed, these comparisons should be treated with caution.

Abstract We report observations of main-belt asteroid 2025 VZ 8 using multiple telescopes from UT 2025 November 24 to UT 2025 December 23 that confirm the presence of visible comet-like activity. These observations were motivated by online reports that an abrupt brightening event may have led to the object’s discovery on UT 2025 November 9. In all observations reported here, the object displays a tapered tail approximately 3″ long aligned with the antisolar direction. The object has an asteroidal Tisserand parameter value with respect to Jupiter ( T J = 3.371), and is thus considered an active asteroid. We measure an average apparent r ′ -band magnitude of m r ∼ 21.6 mag over the reported observing period. Due to the sudden appearance of activity far from perihelion, we suggest that the activity is more likely due to an impact or rotational destabilization, rather than sublimation.

Abstract Stellar activity remains one of the primary challenges in the detection and characterization of low-mass exoplanets, as it can induce radial velocity (RV) variations that mask or mimic planetary signals. Identifying reliable activity proxies is essential in order to distinguish stellar variability from genuine planetary signatures. In this study, we examine the variability of the chromospheric He I D3 line in high-resolution solar spectra and assess its potential as an activity indicator. We find a strong correlation between the He I D3 line intensity variation and the Sun’s unsigned magnetic flux derived from Solar Dynamics Observatory/Helioseismic and Magnetic Imager data as well as with the solar RVs. Our results suggest that the He I D3 line offers a promising and straightforward proxy for magnetic activity, which may complement existing stellar activity indicators. Its inclusion could help disentangle stellar signals in RV measurements and ultimately improve the detection of Earth-like exoplanets.

Abstract We report on roughly 16 yr of photometric monitoring of the trans-Neptunian binary system (120347) Salacia–Actaea, which provides significant evidence that Salacia and Actaea are tidally locked to the mutual orbital period in a fully synchronous configuration. The orbit of Actaea is updated, followed by a Lomb–Scargle periodogram analysis of the ground-based photometry, which reveals a synodic period similar to the orbital period and a peak-to-peak lightcurve amplitude of Δ m = 0.0900 ± 0.0036 mag (1 σ uncertainty). Incorporating archival Hubble Space Telescope photometry that resolves each component, we argue that the periodicity in the unresolved data is driven by a longitudinally varying surface morphology on Salacia, and we derive a sidereal rotation period that is within 1 σ of the mutual orbital period. A rudimentary tidal evolution model is invoked that suggests synchronization occurred within 1.1 Gyr after Actaea was captured/formed.

Abstract We present the discovery of 30 transiting giant planets that were initially detected using data from NASA’s Transiting Exoplanet Survey Satellite mission. These new planets orbit relatively bright ( G ≤ 12.5) FGK host stars with orbital periods between 1.6 and 8.2 days, and have radii between 0.9 and 1.7 Jupiter radii. We performed follow-up ground-based photometry, high angular resolution imaging, high-resolution spectroscopy, and radial velocity monitoring for each of these objects to confirm that they are planets and determine their masses and other system parameters. The planets’ masses span more than an order of magnitude (0.17 M J &lt; M p &lt; 3.3 M J ). For two planets, TOI-3593 b and TOI-4961 b, we measured significant nonzero eccentricities of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>0.1</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.03</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.05</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>0.1</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.05</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.04</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , respectively, while for the other planets, the data typically provide a 1 σ upper bound of 0.15 on the eccentricity. These discoveries represent a major step toward assembling a complete, magnitude-limited sample of transiting hot Jupiters around FGK stars.

Abstract We present the JWST/NIRSpec G395H transmission spectrum of the young (10–30 Myr) transiting planet V1298 Tau b (9.85 ± 0.35 R ⊕, T eq = 670 K). Combined Hubble Space Telescope and JWST observations reveal a haze-free, H/He-dominated atmosphere with a large scale height (∼1500 km), allowing detection of CO2 (35σ), H2O (30σ), CO (10σ), CH4 (6σ), SO2 (4σ), and OCS (3.5σ). Our observations probe several scale heights (∼4.4 in the CO2 4.3 μm band and ∼3 in the 2.7 μm water band). The planet’s mass, inferred from atmospheric scale height using free retrieval and grid modeling, is 12 ± 1 M ⊕ and 15 ± 1.7 M ⊕, respectively, which is significantly lower than previous radial velocity estimates and confirms it as “gas-dwarf” sub-Neptune progenitor. We find an atmospheric metallicity (log Z = 0.6 − 0.6 + 0.4 × solar) and subsolar C/O ratio (0.22 − 0.05 + 0.06 ). The atmospheric metallicity is low compared to mature sub-Neptunes by an order of magnitude. The CH4 abundance ([CH4] = −6.2 − 0.5 + 0.3 ]) is ∼7σ lower than the equilibrium chemistry prediction. To adjust for the low methane abundance, the self-consistent grids favor a high internal temperature (∼500 K) and vertical mixing (K zz ∼ 107−108 cm2 s–1). These internal temperatures are inconsistent with predictions from evolutionary models, which expect ∼100–200 K at the current system age. We estimate a gas-to-core mass fraction between 0.1% and 8%, with a core mass of 11–12 M ⊕, consistent with in-situ gas-dwarf formation. A deep atmospheric metallicity gradient may explain both the high internal temperature and low observable metallicity. Over time, mass loss from such an atmosphere could enhance its metallicity, potentially reconciling V1298 Tau b with mature sub-Neptunes.

Abstract Features that change in appearance between different NASA New Horizons images of Pluto are analyzed. Contrast reversal features (CRs) appear to transition from darker to brighter than their surroundings, more likely due to changes of imaging geometry, specifically increasing solar phase angle, than temporal change. Contrast emergent features (CEs) are not apparent in low- and intermediate-solar-phase-angle images but brighter than their surroundings in high-solar-phase-angle image(s), also more likely due to the increase of solar phase angle than temporal change. Hypotheses for Pluto’s CRs and CEs include plumes, clouds, wind streaks, snow, frost, precipitated haze, lags, pits, glints, slopes, and particle size, compaction, and geometric effects. The CRs and CEs could be on and/or above Pluto’s surface and/or within its shallow subsurface. These hypotheses are investigated via mapping, color, morphology, photometry, and comparison with Triton’s fans. Pluto’s CRs and CEs are likely different manifestations of the same or similar phenomena. They are likely not diffuse deposits of Pluto’s dark red equatorial material(s) on or above or within volatile-ice-rich regions, sunglints, or geometric effects from spatially resolved slopes. They are consistent with less backward-scattering and more forward-scattering surfaces and shallow subsurfaces than their surroundings, as well as optically thin, preferentially forward scattering between the surface and global haze, including atmospheric (e.g., cloud) and surface-coating (e.g., condensation deposit) features. Surface and/or shallow subsurface features, possibly atmospheric derived, are favored over atmospheric features.

We are searching for new active asteroids using data from the Subaru Hyper Suprime-Cam (HSC) Public Data Release 3 (PDR3). The active asteroids are a poorly understood class of asteroids, which have orbits similar to other asteroids but exhibit tails or comae like comets. Of the over 1.4 million catalogued minor planets, most of which are main belt asteroids, fewer than 100 of the asteroids have exhibited activity, and roughly half of those are thought to be driven by water ice. The true fraction and solar system distribution of active asteroids is poorly known as most have been discovered serendipitously by a variety of methods. In this work, we search for activity among the known asteroids using one of the deepest wide-area public datasets available, the HSC PDR3. The goal of this work is to discover new active asteroid candidates and recurring activity in known active asteroids in archival data, collect contemporary images of those asteroids using ground based telescopes, and assess the true rate of active asteroid activity in a consistent manner using a large dataset. This will lead to insights into the present day distribution of water ice in our solar system.The HSC PDR3 has been collected using one of the largest ground-based optical telescopes, the 8.2 m Subaru telescope atop Maunakea in Hawaii which regularly achieves image quality better than 0.7 arcseconds yielding single-image depths of 25 to 25.5 magnitudes in the typical exposure times used for the PDR3. In addition, HSC itself has an extremely wide field of view, about 1.8 square degrees, for such a large telescope. Over 13,000 fields were imaged in good conditions with HSC as part of the PDR3, representing over 23,000 square degrees. The PDR3 dataset was collected for scientific purposes unrelated to the asteroids but many fields contain serendipitous images of asteroids. We have cross-correlated the PDR3 fields and the Minor Planet Database using the SkyBoT project and find that over 230,000 minor planets (over 15% of all minor planets and the vast majority asteroids) were serendipitously imaged in the dataset for a total of over 1.6 million minor planet images. We have begun the process of constructing thumbnail images of these minor planets from the full HSC PDR3 data release and expect to finish this year.Initially, we are examining images by eye ourselves to determine activity candidates. However, we will launch a Citizen Science campaign akin to activeasteroids.net, which has to date only used data from the DECam public archive. This will allow us to mobilize volunteers to examine more images than we can reliably categorize ourselves. We present first results from this work including an assessment of activity levels for thousands of asteroids most likely to exhibit activity including: previously known active asteroids, distant C-type main belt asteroids, and Centaurs on unstable orbits that could have recently moved inward from the trans-Neptunian region.Figure 1: One of 4 images from the HSC PDR3 archive of known active outer main belt asteroid 331P/Gibbs, exhibiting a faint linear tail.Figure 2: One of 17 images from the HSC PDR3 archive of known Jupiter family comet 242P/Spahr, exhibiting a tail. While not a primary target of this work, many comets are present in the HSC PDR3.

We have observed the Galilean moons to determine their surface composition, any differences with longitude, and whether there are any temporal changes. One of the primary components we are trying to understand is the presence of salts. Salts play an important role in habitability because they can affect the stability of liquid water. Chlorine salts lower the freezing point of water significantly, while sulfate salts create a milder environment. For Europa, most of the “non-icy” spectra from NIMS are very similar to each other and have been suggested to be composed of heavily hydrated sulfate salts (McCord et al, 1999). However, Ligier et al., 2016 and King et al., 2022 observed Europa with VLT/SINFONI and SPHERE, respectively, and utilized recent NIR laboratory spectra (Hanley et al., 2014) to model spectral features, finding that Mg-bearing chlorinated species provide better spectral fits than sulfates in some areas. Additionally, studies suggest that magnesium is originally brought to the surface as magnesium chloride (Brown and Hand, 2013), and NaCl has been detected on the surface (Trumbo et al., 2019). The correlation with lineae and darker units suggests an endogenic origin for these salts.Any detection of chlorine salts at the surface would further constrain theories that the dark surface material might in fact be emplaced by movement of the ice sheet and possible subsurface ocean interaction and/or cryovolcanism, rather than implantation from Io’s torus, as could be the case for sulfates. This is especially strengthened by observations of plumes on Europa. Identifying the primary constituent of Europa’s ocean salts would lead to greater understanding of the ocean temperature and the thickness of the ice shell. The composition of the ocean puts limits on the habitability of ocean worlds.We have observed Europa, Io, Ganymede, Callisto and Titan with Lowell’s 4.3 m Discovery Telescope (LDT) with the NIHTS and EXPRES instruments. The Near-Infrared High-Throughput Spectrograph (NIHTS) is a low-resolution (R ~ 200) near-infrared (NIR) spectrograph, covering 0.86 - 2.4 µm. The EXtreme PREcision Spectrometer (EXPRES) observations measure the visible spectrum from ~0.35 - 0.85 µm at a resolution of R ~ 137,500. A unique setup of EXPRES is that it is simultaneously connected to a Solar telescope which observes the Sun daily. Thus we are able to correct for actual Solar features in our spectra. The observations are disk-averaged, and those of Europa and Titan are centered around six different longitude bins to enable us to look for longitudinal differences. We also have access to telescope time every semester, so we will be able to monitor for any temporal variations. The wavelength regions covered by our instruments cover the necessary wavelengths for previous chlorine salt identifications. This work allows for monitoring of any temporal changes in Europa’s surface composition, especially if plumes are depositing new material.Initial analysis of the NIHTS data for Europa shows water ice/hydration features in the NIR at 1.25, 1.5, 1.65, 2.0 and 2.4 µm. The 1.65 µm water ice band can be used to determine the temperature of the ice, while the others can be compared to literature spectra to determine composition. Preliminary analysis of the EXPRES data centered around 310° longitude do not show salt color centers, as expected, but do show a red slope at lower wavelengths (0.4 – 0.55 µm) followed by a slight blue slope from 0.55 – 0.8 µm. We do see transient molecular oxygen features on Ganymede as well (Figure 1). We will present these observations, as well as those to be collected in June 2025, along with analysis at various longitudes for both NIHTS and EXPRES, for all the Galilean Moons. These ongoing observations will be useful to the upcoming JUICE and Clipper missions to monitor the surface compositions over time. Figure 1: Visible Spectra of Ganymede from EXPRES. All data are disk-averaged, centered on the labelled longitude. Spectra have been normalized and offset for clarity. Dashed line represents the 0.5773 µm molecular oxygen feature.