arXiv subjects

Alan Fitzsimmons

Publications and source records attributed to Alan Fitzsimmons.

At least 19 recordsLinked to original sources

arXiv2026

Multi-year Ground-Based Survey Photometry of Active Comet 103P/Hartley 2 and Centaur (2060) Chiron: A Tale of Two Comets in the Pre-LSST Era

Comets and Centaurs trace the evolution of trans-Neptunian objects (TNOs) into the inner solar system. Their activity reflects the interplay between volatile sublimation, dust dynamics, and ring scattering. Yet the long-term behavior of individual objects is less constrained. To probe this evolutionary transition, we use wide-field survey photometry from the Asteroid Terrestrial-impact Last Alert System, Zwicky Transient Facility, and Las Cumbres Observatory observations of the Jupiter-family comet (JFC) 103P/Hartley 2 during its 2023/24 apparition, and the Centaur (2060) Chiron across 2020-2025, including its 2021 outburst. For 103P, heliocentric activity slopes are asymmetric about perihelion, with a steep inbound index ($n_{r,\rm pre}=-3.48\pm0.08$) and flatter outbound value ($n_{r,\rm post}=-1.16\pm0.04$), consistent with enhanced relative dust contribution post-perihelion. Reduced brightness versus prior apparitions matches reported secular fading trends. Dust mass-loss rates are $\sim4$-16 kg s$^{-1}$ for assumed grain properties. Colors exhibit a blueward trend near perihelion, consistent with enhanced gas contamination of the $g$-band, with possible phase-dependent scattering. A periodogram recovers a $\sim18.7$ hr activity-linked period near perihelion. For Chiron, subtracting a quiescent baseline reveals exponential decay from the 2021 outburst on a $\sim1.4$ yr timescale. Seasonal phase curves flatten from $\beta_o=0.150\pm0.034$ mag deg$^{-1}$ in 2021 to $\lesssim0.09$ mag deg$^{-1}$ by 2023-2025, converging with quiescent behavior. Broad-band colors remain unchanged at ATLAS ($c-o$)=0.22$\pm$0.09 mag. This extended activity suggests a new epoch of persistent, low-level activity and/or evolving ring-scattering. These objects bracket the TNO-to-JFC evolutionary sequence, with 103P near the volatile-depleted end, and Chiron still volatile-rich and capable of episodic activity.

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arXiv2025

On the Border: Searching for Cometary Activity Near the Centaur-JFC Transition Line

Current wide-field surveys discover ~15 Jupiter-family comets (JFCs) each year, typically identified via visual detection of a dust coma or tail. The same surveys also discover many asteroids that have distant JFC-like orbits, but with no reported activity. We observed asteroids on Jupiter-crossing orbits beyond the depth of typical survey imaging using the 2.5 m Isaac Newton Telescope. We used deep imaging to observe 16 asteroids in this region, plus 7 known comets for comparison. Three asteroids (2011 WM183, (669525) 2012 XO144, and 2020 RX133) showed surface brightness profiles consistent with low-level activity, equating to ~19% of our total sample. We note that 2020 RX133 is a Jupiter Trojan. When we considered the heliocentric distance range of the asteroids at the time they showed activity, this fraction increased to 33% of the targets in the 3.16 \leq Rh \leq 4.56 au region, and therefore it is possible to infer that at least ~30 asteroids with Tj \leq 3.05 and in the 4.05 < a < 5.05 au parameter space may potentially exhibit low-level activity. We also estimated nuclear radii for the three active targets of r_n = 1.8 \pm 0.2 km, r_n \leq 0.8 km, and r_n \leq 0.5 km for (669525) 2012 XO144, 2011 WM183, and 2020 RX133 respectively. The median color index for the observed asteroids is (g-r)_{PS1} = 0.52 \pm 0.13, aligning with those expected for D-type asteroids.

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arXiv2025

ATLAS Photometry of Interstellar Object 3I/ATLAS

We present calibrated ATLAS photometry of the interstellar comet 3I/ATLAS (C/2025 N1) from 28 March through 29 Aug 2025, obtained with the five-site, robotic ATLAS network in the c (420-650~nm), o (560-820~nm), and Teide w (420-720~nm) bands. Stacked difference images yield reliable light curves measured in four fixed apertures that capture the evolving coma. We observe 3I/ATLAS transitioning in color from red (c-o)~0.7 before MJD 60860 to near-solar (c-o)~0.3 after MJD 60870, coincident with the appearance of a prominent anti-solar tail. The absolute magnitude curve H(t) shows a slope break near MJD 60890 at r~3.3 au from -0.035 to -0.012 mag/day, or in terms of coma cross section as a function of heliocentric distance, r^{-3.9} to r^{-1.1}.

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arXiv2025

A portrait throughout perihelion of the NH$_2$-rich interstellar comet 2I/Borisov

The interstellar comet 2I/Borisov is the first interstellar object where compositional characterisation was possible throughout its entire perihelion passage. We report all 16 epochs of a comprehensive optical observation campaign with ESO VLT's integral field spectrograph MUSE, spanning 126 days from 2019 November 14 to 2020 March 19. The spatial dust emission of 2I/Borisov was predominantly smooth, with no seasonal effect. A jet-like feature was consistently visible. The gas production morphology of its coma was also smooth and similar for C$_2$, NH$_2$, and CN: symmetric around the photocentre. The production rates of these species gently declined into and beyond perihelion, until 2I's outburst and splitting event in early 2020 March. C$_2$, NH$_2$, and CN production rates all increased, with NH$_2$ being the most significant; the dust emission also slightly reddened. 2I/Borisov is a carbon-depleted, relatively NH$_2$-rich comet when compared to those comets yet measured in the Solar System.

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arXiv2025

Dust production rates in Jupiter-family comets II: Trends and population insights from ATLAS photometry of 116 JFCs

Jupiter-family comets (JFCs) have orbital periods of less than 20 years and therefore undergo more frequent sublimation compared to other comet populations. The JFCs therefore represent the ideal dynamical population for investigating the dust production rates at high-cadence. We analyzed observations by the Asteroid Terrestrial-impact Last Alert System (ATLAS) of 74 JFCs that reached perihelion in 2022 and 2023. The work contained in this study builds upon our previous work (Gillan et al. 2024), for a total of 116 JFCs over a four-year period. Using the Afrho parameter, we measured the dust production rates of each JFC as a function of heliocentric distance. We found that there remained a clear preference for JFCs to reach their maximum A(0)frho post-perihelion, with 170P/Christensen, 254P/McNaught and P/2020 WJ5 (Lemmon) reaching a maximum A(0)frho between 200-400 days after perihelion. However, all JFCs reached their maximum dust production within 10% of their orbital period relative to perihelion. Fitting A(0)fp as a function of Rh^n, we measured statistically significant differences in the distribution of pre-perihelion and post-perihelion activity index n, with average activity indices of -5.2 +/- 4.5 and -3.2 +/- 2.7 respectively. We derived upper limits for the nuclear radii of comets 444P/WISE-PANSTARRS and 459P/Catalina as Rn \leq 1.5 +/- 0.2 km and Rn \leq 1.7 +/- 0.1 km respectively. We measured six outbursts in comets 97P/Metcalf-Brewington, 99P/Kowal 1, 118P/Shoemaker-Levy 4, 285P/LINEAR and 382P/Larson. From our four years of observing JFC outbursts in the ATLAS data, the average increase in magnitude was - 1.3 +/- 0.8.

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arXiv2024

Analysing the Onset of Cometary Activity by the Jupiter-Family Comet 2023 RN3

We utilize serendipitous observations from the Asteroid Terrestrial-impact Last Alert System (ATLAS) and the Zwicky Transient Facility (ZTF) in addition to targeted follow-up observations from the Las Cumbres Observatory (LCO) and Liverpool Telescope to analyze the first observed instance of cometary activity by the newly-discovered Jupiter-family comet C/2023 RN3 (ATLAS), whose orbital dynamics place it close to residing on a Centaur-like orbit. Across our 7-month baseline, we observe an epoch of cometary activity commencing in August 2023 with an increase in brightness of >5.4 mag. The lightcurve of 2023 RN3 indicates the presence of continuous cometary activity across our observations, suggesting the onset of a new period of sustained activity. We find no evidence of any outbursts on top of the observed brightening, nor do we find any significant color evolution across our observations. 2023 RN3 is visibly extended in LCO and Liverpool Telescope observations, indicating the presence of a spatially-extended coma. Numerical integration of 2023 RN3's orbit reveals the comet to have recently undergone a slight increase in semimajor axis due to a planetary encounter with Jupiter, however whether this orbital change could trigger 2023 RN3's cometary activity is unclear. Our estimate for the maximum dust production metric of Afrho ~400 cm is consistent with previous measurements for the Jupiter-family comet and Centaur populations.

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arXiv2024

The Discovery and Evolution of a Possible New Epoch of Cometary Activity by the Centaur (2060) Chiron

Centaurs are small Solar System objects on chaotic orbits in the giant planet region, forming an evolutionary continuum with the Kuiper belt objects and Jupiter-family comets. Some Centaurs are known to exhibit cometary activity, though unlike comets this activity tends not to correlate with heliocentric distance and the mechanism behind it is currently poorly understood. We utilize serendipitous observations from the Asteroid Terrestrial-impact Last Alert System (ATLAS), Zwicky Transient Facility (ZTF), Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), Dark Energy Survey (DES), and Gaia in addition to targeted follow-up observations from the Las Cumbres Observatory, TRAnsiting Planets and PlanetesImals Small Telescope South (TRAPPIST-South), and Gemini North telescope to analyze an unexpected brightening exhibited by the known active Centaur (2060) Chiron in 2021. This is highly indicative of a cometary outburst. As of 2023 February, Chiron has still not returned to its pre-brightening magnitude. We find Chiron's rotational lightcurve, phase curve effects, and possible high-albedo surface features to be unlikely causes of this observed brightening. We consider the most likely cause to be an epoch of either new or increased cometary activity, though we cannot rule out a possible contribution from Chiron's reported ring system, such as a collision of as-yet unseen satellites shepherding the rings. We find no evidence for coma in our Gemini or TRAPPIST-South observations, though this does not preclude the possibility that Chiron is exhibiting a coma that is too faint for observation or constrained to the immediate vicinity of the nucleus.

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arXiv2024

Main-belt and Trojan Asteroid Phase Curves from the ATLAS Survey

Sparse and serendipitous asteroid photometry obtained by wide field surveys such as the Asteroid Terrestrial-impact Last Alert System (\ATLAS) is a valuable resource for studying the properties of large numbers of small Solar System bodies. We have gathered a large database of \ATLAS photometry in wideband optical cyan and orange filters, consisting of 9.6\e{7} observations of 4.5\e{5} main belt asteroids and Jupiter Trojans. We conduct a phase curve analysis of these asteroids considering each apparition separately, allowing us to accurately reject outlying observations and to remove apparitions and asteroids not suitable for phase curve determination. We obtain a dataset of absolute magnitudes and phase parameters for over 100,000 selected asteroids observed by \ATLAS, $\sim66,000$ of which had sufficient measurements to derive colours in the \ATLAS filters. To demonstrate the power of our dataset we consider the properties of the Nysa-Polana complex, for which the \ATLAS colours and phase parameters trace the S-like and C-like compositions amongst family members. We also compare the properties of the leading and trailing groups of Jupiter Trojans, finding no significant differences in their phase parameters or colours as measured by \ATLAS, supporting the consensus that these groups were captured from a common source population during planetary migration. Furthermore, we identify $\sim9000$ asteroids that exhibit large shifts in derived absolute magnitude between apparitions, indicating that these objects have both elongated shapes and spin axes with obliquity $\sim 90$ degrees.

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arXiv2023

Dust production rates in Jupiter-family Comets: A two-year study with ATLAS photometry

Jupiter-family Comets (JFCs) exhibit a wide range of activity levels and mass-loss over their orbits. We analyzed high-cadence observations of 42 active JFCs with the wide-field Asteroid Terrestrial-impact Last Alert System (ATLAS) survey in 2020-2021. We measured dust production rates of the JFCs using the Af\rho parameter and its variation as a function of heliocentric distance. There is a tendency for our JFC sample to exhibit a maximum Af\rho after perihelion, with 254P/McNaught and P/2020 WJ5 (Lemmon) having their maximum Af\rho over a year after perihelion. On average, the rate of change of activity post-perihelion was shallower than that pre-perihelion. We also estimated the mass maximum loss rate for 17 of the JFCs in our sample, finding 4P/Faye to be the most active. We present a subset of comets whose measured Af\rho have been interpolated and extrapolated to a common distance of 2 au pre-perihelion and post-perihelion. From these measurements we found no correlation of intrinsic activity with current perihelion distance. For three of the JFCs in our sample, 6P/d'Arrest, 156P/Russell-LINEAR and 254P/McNaught, there was no visible coma but a constant absolute magnitude which we attributed to a probable detection of the nucleus. We derived upper limits for the nuclear radii of \leq 2.1 +/- 0.3 km, \leq 2.0 +/- 0.2 km and \leq 4.0 +/- 0.8 km respectively. Finally, we found that 4P/Faye, 108P/Ciffreo, 132P/Helin-Roman-Alu 2, 141P/Machholz 2, and 398P/Boattini experienced outbursts between 2020 and 2022.

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arXiv2023

Ejecta Evolution Following a Planned Impact into an Asteroid: The First Five Weeks

The impact of the DART spacecraft into Dimorphos, moon of the asteroid Didymos, changed Dimorphos' orbit substantially, largely from the ejection of material. We present results from twelve Earth-based facilities involved in a world-wide campaign to monitor the brightness and morphology of the ejecta in the first 35 days after impact. After an initial brightening of ~1.4 magnitudes, we find consistent dimming rates of 0.11-0.12 magnitudes/day in the first week, and 0.08-0.09 magnitudes/day over the entire study period. The system returned to its pre-impact brightness 24.3-25.3 days after impact through the primary ejecta tail remained. The dimming paused briefly eight days after impact, near in time to the appearance of the second tail. This was likely due to a secondary release of material after re-impact of a boulder released in the initial impact, through movement of the primary ejecta through the aperture likely played a role.

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arXiv2023

Interstellar Objects and Exocomets

In this chapter we review our knowledge of our galaxy's cometary population outside our Oort Cloud - exocomets and Interstellar Objects (ISOs). We start with a brief overview of planetary system formation, viewed as a general process around stars. We then take a more detailed look at the creation and structure of exocometary belts, as revealed by the unprecedented combination of theoretical and observational advances in recent years. The existence and characteristics of individual exocomets orbiting other stars is summarized, before looking at the mechanisms by which they may be ejected into interstellar space. The discovery of the first two ISOs is then described, along with the surprising differences in their observed characteristics. We end by looking ahead to what advances may take place in the next decade.

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arXiv2023

Phase Curves of Kuiper Belt Objects, Centaurs, and Jupiter Family Comets from the ATLAS Survey

The Kuiper belt objects, the Centaurs, and the Jupiter-family comets form an evolutionary continuum of small outer Solar System objects, and their study allows us to gain insight into the history and evolution of the Solar System. Broadband photometry can be used to measure their phase curves, allowing a first-order probe into the surface properties of these objects, though limited telescope time makes measuring accurate phase curves difficult. We make use of serendipitous broadband photometry from the long-baseline, high-cadence ATLAS survey to measure the phase curves for a sample of 18 Kuiper belt objects, Centaurs, and Jupiter-family comets with unprecedentedly large datasets. We find phase curves with previously reported negative slopes become positive with increased data and are thus due to insufficient sampling of the phase curve profile, and not a real physical effect. We search for correlations between phase curve parameters, finding no strong correlations between any parameter pair, consistent with the findings of previous studies. We search for instances of cometary activity in our sample, finding a previously reported outburst by Echeclus and a new epoch of increased activity by Chiron. Applying the main belt asteroid HG1G2 phase curve model to three Jupiter-family comets in our sample with large phase angle spans, we find their slope parameters imply surfaces more consistent with those of carbonaceous main belt asteroids than silicaceous ones.

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arXiv2023

Ejecta from the DART-produced active asteroid Dimorphos

Some active asteroids have been proposed to be the result of impact events. Because active asteroids are generally discovered serendipitously only after their tail formation, the process of the impact ejecta evolving into a tail has never been directly observed. NASA's Double Asteroid Redirection Test (DART) mission, apart from having successfully changed the orbital period of Dimorphos, demonstrated the activation process of an asteroid from an impact under precisely known impact conditions. Here we report the observations of the DART impact ejecta with the Hubble Space Telescope (HST) from impact time T+15 minutes to T+18.5 days at spatial resolutions of ~2.1 km per pixel. Our observations reveal a complex evolution of ejecta, which is first dominated by the gravitational interaction between the Didymos binary system and the ejected dust and later by solar radiation pressure. The lowest-speed ejecta dispersed via a sustained tail that displayed a consistent morphology with previously observed asteroid tails thought to be produced by impact. The ejecta evolution following DART's controlled impact experiment thus provides a framework for understanding the fundamental mechanisms acting on asteroids disrupted by natural impact.

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arXiv2022

A Targeted Search for Main Belt Comets

Main Belt Comets (MBCs) exhibit sublimation-driven activity while occupying asteroid-like orbits in the Main Asteroid Belt. MBCs and candidates show stronger clustering of their longitudes of perihelion around 15{\deg} than other objects from the Outer Main Belt (OMB). This potential property of MBCs could facilitate the discovery of new candidates by observing objects in similar orbits. We acquired deep r-band images of 534 targeted asteroids using the INT/WFC between 2018 and 2020. Our sample is comprised of OMB objects observed near perihelion, with longitudes of perihelion between 0{\deg} and 30{\deg} and orbital parameters similar to knowns MBCs. Our pipeline applied activity detection methods to 319 of these objects to look for tails or comae, and we visually inspected the remaining asteroids. Our activity detection pipeline highlighted a faint anti-solar tail-like feature around 2001 NL19 (279870) observed on 2018 November 07, six months after perihelion. This is consistent with cometary activity but additional observations of this object will be needed during its next perihelion to investigate its potential MBC status. If it is active our survey yields a detection rate of $\sim$1:300, which is higher than previous similar surveys, supporting the idea of dynamical clustering of MBCs. If not, it is consistent with previously estimated abundance rates of MBCs in the OMB (<1:500).

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arXiv2022

The CUBES Science Case

We introduce the scientific motivations for the development of the Cassegrain U-Band Efficient Spectrograph (CUBES) that is now in construction for the Very Large Telescope. The assembled cases span a broad range of contemporary topics across Solar System, Galactic and extragalactic astronomy, where observations are limited by the performance of current ground-based spectrographs shortwards of 400nm. A brief background to each case is presented and specific technical requirements on the instrument design that flow-down from each case are identified. These were used as inputs to the CUBES design, that will provide a factor of ten gain in efficiency for astronomical spectroscopy over 300-405nm, at resolving powers of R~24,000 and ~7,000. We include performance estimates that demonstrate the ability of CUBES to observe sources that are up to three magnitudes fainter than currently possible at ground-ultraviolet wavelengths, and we place its predicted performance in the context of existing facillities.

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arXiv2022

Physical properties of near-Earth asteroid (2102) Tantalus from multi-wavelength observations

Between 2010 and 2017 we have collected new optical and radar observations of the potentially hazardous asteroid (2102)~Tantalus from the ESO NTT and Danish telescopes at the La Silla Observatory and from the Arecibo planetary radar. The object appears to be nearly spherical, showing a low amplitude light-curve variation and limited large-scale features in the radar images. The spin-state is difficult to constrain with the available data; including a certain light-curve subset significantly changes the spin-state estimates, and the uncertainties on period determination are significant. Constraining any change in rotation rate was not possible, despite decades of observations. The convex lightcurve-inversion model, with rotational pole at ${\lambda}=210{\pm}41${\deg} and ${\beta}=-30{\pm}35${\deg}, is more flattened than the two models reconstructed by including radar observations: with prograde (${\lambda}=36{\pm}23${\deg}, ${\beta}=30{\pm}15${\deg}), and with retrograde rotation mode (${\lambda}=180{\pm}24${\deg}, ${\beta}=-30{\pm}16${\deg}). Using data from WISE we were able to determine that the prograde model produces the best agreement in size determination between radar and thermophysical modelling. Radar measurements indicate possible variation in surface properties, suggesting one side might have lower radar albedo and be rougher at centimetre-to-decimetre scale than the other. However, further observations are needed to confirm this. Thermophysical analysis indicates a surface covered in fine-grained regolith, consistent with radar albedo and polarisation ratio measurements. Finally, geophysical investigation of the spin-stability of Tantalus shows that it could be exceeding its critical spin-rate via cohesive forces.

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arXiv2022

Surface Properties of Near-Sun Asteroids

Near-Earth Asteroids (NEAs) with small perihelion distances reach sub-solar temperatures of > 1000 K. They are hypothesized to undergo "super-catastrophic" disruption, potentially caused by near-Sun processes such as thermal cracking, spin-up, meteoroid impacts, and subsurface volatile release; all of which are likely to cause surface alteration, which may change the spectral slope of the surface. We attempted to observe 35 of the 53 known near-Sun asteroids with q < 0.15 au from January 2017 to March 2020 to search for trends related to near-Sun processes. We report the optical colors and spectral slopes of 22 objects that we successfully observed and the measured rotation periods for three objects. We find the distribution of colors to be overall bluer than the color distribution of NEAs, though there is large overlap. We attribute large scatter to unknown dynamical histories and compositions for individual objects, as well as competing surface altering processes. We also investigated potential correlations between colors and other properties (e.g., perihelion distance, Tisserand parameter, rotation period), and searched for evidence of activity. Finally, we have compiled all known physical and dynamical properties of these objects, including probabilistic source regions and dwell times with q < 0.15 au.

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arXiv2022

Cometary science with CUBES

The proposed CUBES spectrograph for ESO's Very Large Telescope will be an exceptionally powerful instrument for the study of comets. The gas coma of a comet contains a large number of emission features in the near-UV range covered by CUBES (305-400 nm), which are diagnostic of the composition of the ices in its nucleus and the chemistry in the coma. Production rates and relative ratios between different species reveal how much ice is present and inform models of the conditions in the early solar system. In particular, CUBES will lead to advances in detection of water from very faint comets, revealing how much ice may be hidden in the main asteroid belt, and in measuring isotopic and molecular composition ratios in a much wider range of comets than currently possible, provide constraints on their formation temperatures. CUBES will also be sensitive to emissions from gaseous metals (e.g., FeI and NiI), which have recently been identified in comets and offer an entirely new area of investigation to understand these enigmatic objects.

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