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1. First Detection of CO 2 Emission in a Centaur: JWST NIRSpec Observations of 39P/Oterma

   https://doi.org/10.3847/PSJ/acf928  

   Pinto, O. Harrington ; Kelley, M. S. P. ; Villanueva, G. L. ; Womack, M. ; Faggi, S. ; McKay, A. ; DiSanti, M. A. ; Schambeau, C. ; Fernandez, Y. ; Bauer, J. ; et al ( November 2023 , The Planetary Science Journal)

   Centaurs are minor solar system bodies with orbits transitioning between those of trans-Neptunian scattered disk objects and Jupiter-family comets (JFCs). 39P/Oterma (39P) is a frequently active centaur that has recently held both centaur and JFC classifications and was observed with the JWST NIRSpec instrument on 2022 July 27 UTC while it was 5.82 au from the Sun. For the first time, CO₂gas emission was detected in a centaur, with a production rate of$Q_{{\mathrm{CO}}_2}$= (5.96 ± 0.80) × 10²³molecules s⁻¹. This is the lowest detection of CO₂of any centaur or comet. CO and H₂O were not detected down to constraining upper limits. Derived mixing ratios ofQ_CO/Q${}_{{\mathrm{CO}}_2}$≤ 2.03 and$Q_{{\mathrm{CO}}_2}$/Q${}_{{\mathrm{H}}_2\mathrm{O}}$≥ 0.60 are consistent with CO₂and/or CO outgassing playing large roles in driving the activity, but not water, and show a significant difference between the coma abundances of 29P/Schwassmann–Wachmann 1, another centaur at a similar heliocentric distance, which may be explained by thermal processing of 39P’s surface during its previous JFC orbit. To help contextualize the JWST data we also acquired visible CCD imaging data on two dates in 2022 July (Gemini-North) and September (Lowell Discovery Telescope). Image analysis and photometry based on these data are consistent with a point-source detection and an estimated effective nucleus radius of 39P in the range ofR_nuc= 2.21–2.49 km.

    

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2. The Active Asteroids Citizen Science Program: Overview and First Results

   https://doi.org/10.3847/1538-3881/ad1de2  

   Chandler, Colin Orion ; Trujillo, Chadwick A. ; Oldroyd, William J. ; Kueny, Jay K. ; Burris, William A. ; Hsieh, Henry H. ; DeSpain, Jarod A. ; Sedaghat, Nima ; Sheppard, Scott S. ; Farrell, Kennedy A. ; et al ( March 2024 , The Astronomical Journal)

   We present the Citizen Science program Active Asteroids and describe discoveries stemming from our ongoing project. Our NASA Partner program is hosted on the Zooniverse online platform and launched on 2021 August 31, with the goal of engaging the community in the search for active asteroids—asteroids with comet-like tails or comae. We also set out to identify other unusual active solar system objects, such as active Centaurs, active quasi-Hilda asteroids (QHAs), and Jupiter-family comets (JFCs). Active objects are rare in large part because they are difficult to identify, so we ask volunteers to assist us in searching for active bodies in our collection of millions of images of known minor planets. We produced these cutout images with our project pipeline that makes use of publicly available Dark Energy Camera data. Since the project launch, roughly 8300 volunteers have scrutinized some 430,000 images to great effect, which we describe in this work. In total, we have identified previously unknown activity on 15 asteroids, plus one Centaur, that were thought to be asteroidal (i.e., inactive). Of the asteroids, we classify four as active QHAs, seven as JFCs, and four as active asteroids, consisting of one main-belt comet (MBC) and three MBC candidates. We also include our findings concerning known active objects that our program facilitated, an unanticipated avenue of scientific discovery. These include discovering activity occurring during an orbital epoch for which objects were not known to be active, and the reclassification of objects based on our dynamical analyses.

    

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3. Cometary Activity Discovered on Vacationing Centaur 2019 OE31

   https://doi.org/10.3847/2515-5172/ad14f5  

   Oldroyd, William J. ; Chandler, Colin Orion ; Trujillo, Chadwick A. ; Deen, Sam ; Hsieh, Henry H. ; Farrell, Kennedy A. ; DeSpain, Jarod A. ; Kueny, Jay K. ; Burris, William A. ; Sheppard, Scott S. ; et al ( December 2023 , Research Notes of the AAS)

   We have detected cometary activity on minor planet 2019 OE₃₁through both theActive AsteroidsCitizen Science program and an independent archival search. Before 2013, 2019 OE₃₁was on a Centaur orbit, between the orbits of Jupiter and Neptune. Centaurs are objects in transition from the outer solar system to the inner solar system. They play a vital role in the understanding of the Kuiper Belt and comets. In 2013 October, following a close encounter with Jupiter, 2019 OE₃₁moved to an orbit entirely interior to that of Jupiter. This reduced orbital distance and, hence, increased temperature is likely the cause of the observed activity. Through a suite of orbital dynamics simulations, we find that 2019 OE₃₁will experience many more similar encounters and is statistically likely to return to a Centaur orbit, potentially within the next 80 yr, from its current “vacation.”

    

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4. Semimajor-axis Jumps as the Activity Trigger in Centaurs and High-perihelion Jupiter-family Comets

   https://doi.org/10.3847/2041-8213/ad1606  

   Lilly, Eva ; Jevčák, Peter ; Schambeau, Charles ; Volk, Kat ; Steckloff, Jordan ; Hsieh, Henry ; Fernandez, Yanga R. ; Bauer, James ; Weryk, Robert ; Wainscoat, Richard J. ( January 2024 , The Astrophysical Journal Letters)

   We present a dynamical study of 39 active Centaurs and 17 high-perihelion (q> 4.5 au) Jupiter-family comets (JFCs) with a focus on investigating recent orbital changes as potential triggers for comet-like activity. We have identified a common feature in the recent dynamical histories of all active Centaurs and JFCs in our sample that is not present in the history of the majority of inactive population members: a sharp decrease in semimajor axis and eccentricity occurring within the past several hundred years prior to observed activity. We define these rapid orbital changes as “a-jumps.” Our results indicate that these orbital reshaping events lead to shorter orbital periods and subsequently greater average per-orbit heating of Centaur nuclei. We suggest that thea-jumps could therefore be a major trigger of cometary activity on Centaurs and JFCs. Our results further imply that analyses of the recent dynamical histories could be used to identify objects that are currently active or may become active soon, where we have identified three such Centaurs with recenta-jumps that should be considered high-priority targets for observational monitoring to search for activity.

    

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5. A Search of the Full Six Years of the Dark Energy Survey for Outer Solar System Objects

   https://doi.org/10.3847/1538-4365/ac3914  

   Bernardinelli, Pedro H. ; Bernstein, Gary M. ; Sako, Masao ; Yanny, Brian ; Aguena, M. ; Allam, S. ; Andrade-Oliveira, F. ; Bertin, E. ; Brooks, D. ; Buckley-Geer, E. ; et al ( February 2022 , The Astrophysical Journal Supplement Series)

   Abstract We present a search for outer solar system objects in the 6 yr of data from the Dark Energy Survey (DES). The DES covered a contiguous 5000 deg 2 of the southern sky with ≈80,000 3 deg 2 exposures in the grizY filters between 2013 and 2019. This search yielded 812 trans-Neptunian objects (TNOs), one Centaur and one Oort cloud comet, 458 reported here for the first time. We present methodology that builds upon our previous search on the first 4 yr of data. All images were reprocessed with an optimized detection pipeline that leads to an average completeness gain of 0.47 mag per exposure, as well as improved transient catalog production and algorithms for linkage of detections into orbits. All objects were verified by visual inspection and by the “sub-threshold significance,” the signal-to-noise ratio in the stack of images in which its presence is indicated by the orbit, but no detection was reported. This yields a pure catalog complete to r ≈ 23.8 mag and distances 29 < d < 2500 au. The TNOs have minimum (median) of 7 (12) nights’ detections and arcs of 1.1 (4.2) yr, and will have grizY magnitudes available in a further publication. We present software for simulating our observational biases for comparisons of models to our detections. Initial inferences demonstrating the catalog’s statistical power are: the data are inconsistent with the CFEPS-L7 model for the classical Kuiper Belt; the 16 “extreme” TNOs ( a > 150 au, q > 30 au) are consistent with the null hypothesis of azimuthal isotropy; and nonresonant TNOs with q > 38 au, a > 50 au show a significant tendency to be sunward of major mean-motion resonances. 

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