arXiv subjects

Estela Fernández-Valenzuela

Publications and source records attributed to Estela Fernández-Valenzuela.

14 recordsLinked to original sources

arXiv2026

Triaxial shapes and densities of G!kún||'hòmdímà, Haumea, and Varda from stellar occultations

The shapes and densities of mid-sized and large trans-Neptunian objects (TNOs) are pivotal for understanding a variety of important aspects of planet formation. In this work, we present a Bayesian shape modeling method which combines constraints from rotational light curves and satellite orbits to construct three-dimensional shape models of TNOs. We use it to reanalyze three stellar occultations of the TNOs (229762) G!kún||'hòmdímà (2007 UK$_{126}$), (136108) Haumea, and (174567) Varda. By assuming that their satellites (or ring) orbit in their respective equatorial planes, we are able to derive unique shape models for both G!kún||'hòmdímà and Haumea. Our derived shape for G!kún||'hòmdímà is spheroidal with $a = b = 329^{+4}_{-3}$ km and $c = 294^{+11}_{-10}$ km, with a system density $ρ= 1007^{+50}_{-49}$ kg m$^{-3}$. For Haumea, we find $a = 1061^{+87}_{-71}$ km, $b = 844^{+5}_{-7}$ km, and $c = 514^{+18}_{-19}$ km, providing $ρ= 2050^{+157}_{-152}$ kg m$^{-3}$. For Varda, after updating its mutual orbit with its satellite Ilmarë, we find that currently published data are unable to fully constrain its three-dimensional shape. Intriguingly, Varda's elongated limb appears to point towards its satellite at the time of the occultation. With a $\sim$2\% chance of such an alignment happening randomly, this may be suggestive of a frozen-in tidal and/or rotational bulge. Our work emphasizes the importance of how external constraints can improve occultation analyses. With continued observations of rotational light curves, stellar occultations, and satellite orbits, these and other TNOs can have their shapes and densities further refined.

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arXiv2026

JWST reveals anomalously enhanced methane outgassing from below Chiron's water ice and carbon dioxide bearing surface

Centaurs are inward-scattered Kuiper belt objects, with some exhibiting comet-like activity. The physical mechanisms powering this activity remain poorly understood, with carbon monoxide (CO) sublimation or the crystallization of amorphous water ice commonly invoked as the dominant drivers. Here we present high-resolution JWST spectroscopy of 2060 Chiron, one of the largest known Centaurs, revealing methane and carbon dioxide gas emission with distinct coma spatial morphologies and production rates of $Q_{\rm CH_4}=(1.55\pm0.04)\times10^{27}$ molecules s$^{-1}$ and $Q_{\rm CO_2}=(1.01\pm0.06)\times10^{26}$ molecules s$^{-1}$. The surface spectrum displays spectral signatures attributed to water ice, carbon dioxide, CO, and refractory organic-rich material, while lacking detectable methane ice absorption bands. These findings suggest that carbon dioxide production is sustained by direct surface sublimation, whereas methane originates from the subsurface. The absence of measurable CO emission despite the presence of solid-state CO implies that any surviving primordial CO reservoir remains thermally inaccessible at greater depth below the methane, while irradiation-produced near-surface CO may be inefficiently released from the surface matrix. This inferred volatile stratification may result from long-term thermal evolution or potentially partial differentiation. Chiron differs markedly from other active small bodies, where CO production typically dominates over methane, indicating that Centaur activity may be driven by a broader range of volatile and thermophysical processes than predicted by canonical models.

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arXiv2025

JWST occultation reveals unforeseen complexity in Chariklo's ring system

Ring systems have been discovered around several small bodies in the outer Solar System through stellar occultations. While such measurements provide key information about ring geometry and dynamical interactions, little is known about their origins, lifetimes, evolutionary pathways, or compositions. Here we report near-infrared observations with the James Webb Space Telescope (JWST) of a stellar occultation by (10199) Chariklo, a Centaur known to host a double-ring system. Our JWST measurements show that Chariklo's inner dense ring has become significantly more opaque than in previous observations, pointing to ongoing replenishment processes or dynamical restructuring. In contrast, the outer ring exhibits a much weaker near-infrared occultation signature than seen in earlier visible-light detections. This discrepancy may reflect material loss, suggesting that the outer ring could be transient, or may arise from wavelength-dependent opacity. These scenarios, which are not mutually exclusive, point to an unprecedented level of complexity in small-body ring systems, distinct from those observed around any other minor bodies in the Solar System.

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arXiv2025

JWST/NIRSpec Observations of Salacia-Actaea and Máni: Exploring Population-level Trends among Water-ice-rich Kuiper Belt Objects

We present observations of the midsized Kuiper Belt objects (KBOs) Salacia$-$Actaea and Máni, obtained with the Near-Infrared Spectrograph on JWST. The satellite Actaea was fully blended with Salacia at the spatial resolution of the integral field unit, and we extracted the combined spectrum. The 0.7$-$5.1 $μ$m reflectance spectra of Salacia$-$Actaea and Máni display prominent water-ice absorption bands at 1.5, 2, 3, and 4$-$5 $μ$m. The $ν_{3}$ fundamental vibrational band of carbon dioxide ice at 4.25 $μ$m is present in both spectra. From a quantitative band-depth analysis of the entire current JWST spectroscopic sample of water-ice-rich KBOs, we find strong evidence for a positive covariance between relative water-ice abundance and size, which may indicate the emergent impacts of internal differentiation and cryovolcanic production of surface water ice on midsized KBOs. A detailed look at the distribution of 2 and 3 $μ$m band depths suggests additional sources of variability, such as different water-ice grain sizes. In addition, we report an apparent transition in the carbon dioxide band depth at object diameters of roughly 300$-$500 km, with larger objects showing systematically weaker absorptions, although selection effects within the sample do not allow us to confidently distinguish between a size-dependent phenomenon and a correlation with dynamical class.

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arXiv2025

Beyond Point Masses. V. Weywot's Non-Keplerian Orbit

We present a detailed dynamical analysis of the Quaoar-Weywot system based on nearly 20 years of high-precision astrometric data, including new HST observations and stellar occultations. Our study reveals that Weywot's orbit deviates significantly from a purely Keplerian model, requiring the inclusion of Quaoar's non-spherical gravitational field and center-of-body-center-of-light (COB-COL) offsets in our orbit models. We place a robust upper limit on Weywot's orbital eccentricity ($e<0.02$), substantially lower than previous estimates, which has important implications for the strength of mean motion resonances (MMRs) acting on Quaoar's ring system. Under the assumption that Quaoar's rings lie in its equatorial plane, we detect Quaoar's dynamical oblateness, $J_2$, at $\sim$2$σ$ confidence. The low $J_2$ value found under that assumption implies Quaoar is differentiated, with a total bulk density of $1751\pm13$ (stat.) kg m$^{-3}$. Additionally, we detect significant COB-COL offsets likely arising from latitudinal albedo variations across Quaoar's surface. These offsets are necessary to achieve a statistically robust orbit fit and highlight the importance of accounting for surface heterogeneity when modeling the orbits of dwarf planet moons. These findings improve our understanding of Quaoar's interior and surface while providing key insights into the stability and confinement mechanisms of its rings.

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arXiv2025

Orbital Characterization of a Newly Discovered Small Satellite Around Quaoar

Recent observations of a stellar occultation have revealed the presence of a previously undiscovered small satellite around Quaoar. Orbiting near Quaoar's unusual ring system, this new satellite has the potential to provide significant insights into the formation and evolution of Quaoar and its ring system. In this letter, we characterize the orbit of this newly discovered satellite, finding that it is likely on a $3.6^{+0.5}_{-0.3}$-day orbit, plausibly placing it near a 5:3 mean motion resonance with Quaoar's outermost known ring. Examining the possibility of observing the newly discovered satellite with further stellar occultations, we estimate that $\sim$hundreds of observing stations are required for recovery, since phase information about its orbit was rapidly lost after the lone detection. We also attempted to recover the satellite in JWST NIRCam imaging of Quaoar, but find no convincing detection. This non-detection is limited by the accuracy of the available NIRCam PSF models, as well as the satellite's extreme faintness and close-in orbital separation. Therefore, current-generation telescopes will likely struggle to directly image this new satellite, but near-future 30-meter-class telescopes should prove capable of detecting it. Discovery of such a satellite provides evidence that the rings around Quaoar may have been part of an initially broad collisional disk that has evolved considerably since its formation. To further explore this hypothesis, we encourage follow-up observations of the rings and satellites with stellar occultations and direct imaging, as well as updated hydrodynamical, collisional, and tidal modeling of the system.

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arXiv2025

Constraints on Quaoar's rings and atmosphere from JWST/NIRCam observations of a stellar occultation

Observations of stellar occultations have revealed that small bodies are capable of hosting ring systems. The trans-Neptunian object (TNO) Quaoar, is the host of an enigmatic ring system, with two rings located well-outside the Roche limit. To better understand these structures, we observed a stellar occultation by Quaoar and its rings using the James Webb Space Telescope's (JWST) NIRCam instrument. Our observations detect both known rings, although Q2R -- the inner known ring -- is not detected on both sides of Quaoar, showing that it has substantial azimuthal variations similar to Q1R -- the outer ring. We also fit a model of the ring radii and pole orientation of the ring system, which confirms that Quaoar's spin-orbit and Weywot's mean motion resonances (especially Weywot's 6:1) may play a role in the rings' confinement and stability. In addition to examination of Quaoar's ring system, we also use our observations to place upper limits on a putative CH$_4$ atmosphere around Quaoar, finding that no global atmosphere with surface pressure $>1$ nbar can exist (at 3$σ$ significance). The lack of atmosphere supports the hypothesis that atmospheric processes are not the source of Quaoar's recently discovered inventory of light hydrocarbons.

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arXiv2024

Prominent mid-infrared excess of the dwarf planet (136472) Makemake discovered by JWST/MIRI indicates ongoing activity

We report on the discovery of a very prominent mid-infrared (18-25 μm) excess associated with the trans-Neptunian dwarf planet (136472) Makemake. The excess, detected by the MIRI instrument of the James Webb Space Telescope, along with previous measurements from the Spitzer and Herschel space telescopes, indicates the occurrence of temperatures of about 150 K, much higher than what solid surfaces at Makemake's heliocentric distance could reach by solar irradiation. We identify two potential explanations: a continuously visible, currently active region, powered by subsurface upwelling and possibly cryovolcanic activity, covering <1% of Makemake's surface, or an as yet undetected ring containing very small carbonaceous dust grains, which have not been seen before in trans-Neptunian or Centaur rings. Both scenarios point to unprecedented phenomena among trans-Neptunian objects and could greatly impact our understanding of these distant worlds.

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arXiv2022

The LCO Outbursting Objects Key Project: Overview and Year 1 Status

The LCO Outbursting Objects Key (LOOK) Project uses the telescopes of the Las Cumbres Observatory (LCO) Network to: (1) to systematically monitor a sample of Dynamically New Comets over the whole sky, and (2) use alerts from existing sky surveys to rapidly respond to and characterize detected outburst activity in all small bodies. The data gathered on outbursts helps to characterize each outburst's evolution with time, assess the frequency and magnitude distribution of outbursts in general, and contributes to the understanding of outburst processes and volatile distribution in the Solar System. The LOOK Project exploits the synergy between current and future wide-field surveys such as ZTF, PanSTARRS, and LSST as well as rapid-response telescope networks such as LCO, and serves as an excellent testbed for what will be needed the much larger number of objects coming from Rubin Observatory. We will describe the LOOK Project goals, the planning and target selection (including the use of NEOexchange as a Target and Observation Manager or "TOM"), and results from the first phase of observations, including the detection of activity and outbursts on the giant comet C/2014 UN271 (Bernardinelli-Bernstein) and the discovery and follow-up of outbursts on comets. Within these outburst discoveries, we present a high cadence of 7P/Pons-Winnecke with days, a large outburst on 57P/duToit-Neujmin-Delporte, and evidence that comet P/2020 X1 (ATLAS) was in outburst when discovered.

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arXiv2022

Modeling long-term photometric data of trans-Neptunian objects and Centaurs

Trans-Neptunian objects and Centaurs are small solar system bodies that reside in the outer parts of the solar system. These objects present photometric behaviours that are influenced due to a change in their aspect angle. Using absolute photometric measurements and rotational light-curves at different location on their orbits allow to model their photometric behaviour and obtain physical properties such as pole orientation, shape, density, and even detecting different peculiarities as departure from hydrostatic equilibrium or rings. In this work, I present how these models are performed in order to extract different physical properties. A review of the objects for which their long-term photometric behaviour has been modeled is also given.

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arXiv2019

The changing rotational light-curve amplitude of Varuna and evidence for a close-in satellite

From CCD observations carried out with different telescopes, we present short-term photometric measurements of the large trans-Neptunian object Varuna in 10 epochs, spanning around 19 years. We observe that the amplitude of the rotational light-curve has changed considerably during this period of time from 0.41 to 0.55 mag. In order to explain this variation, we constructed a model in which Varuna has a simple triaxial shape, assuming that the main effect comes from the change of the aspect angle as seen from Earth, due to Varuna's orbital motion in the 19-year time span. The best fits to the data correspond to a family of solutions with axial ratios b/a between 0.56 and 0.60. This constrains the pole orientation in two different ranges of solutions presented here as maps. Apart from the remarkable variation of the amplitude, we have detected changes in the overall shape of the rotational light-curve over shorter time scales. After the analysis of the periodogram of the residuals to a 6.343572 h double-peaked rotational light-curve fit, we find a clear additional periodicity. We propose that these changes in the rotational light-curve shape are due to a large and close-in satellite whose rotation induces the additional periodicity. The peak-to-valley amplitude of this oscillation is in the order of 0.04 mag. We estimate that the satellite orbits Varuna with a period of 11.9819 h (or 23.9638 h), assuming that the satellite is tidally locked, at a distance of ~ 1300 km (or ~ 2000 km) from Varuna, outside the Roche limit.

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arXiv2018

Activity of (2060) Chiron possibly caused by impacts?

The centaur 95P/(2060) Chiron is showing comet-like activity since its discovery, but the mass-loss mechanisms triggering its activity remained unexplained. Although the collision rates in the centaur region are expected to be very low, and impacts are thought not to be responsible for the mass-loss, since the recent indications that Chiron might possess a ring similar to Chariklo's, and assuming that there is debris orbiting around, the impact triggered mass-loss mechanism should not be excluded as a possible cause of its activity. From time series observations collected on Calar Alto Observatory in Spain between 2014 and 2016, we found that the photometric scatter in Chiron's data is larger than a control star's scatter, indicating a possible microactivity, possibly caused by debris falling back to Chiron's surface and lifting small clouds of material. We also present rotational light curves, and measurements of Chiron's absolute magnitudes, that are consistent with the models supporting the presumption that Chiron possesses rings. By co-adding the images acquired in 2015, we have detected a $ \sim $5 arcsec long tail, showing a surface brightness of 25.3 mag(V)/arcsec$^{2}$.

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arXiv2015

2008 OG$_{19}$: A highly elongated Trans-Neptunian Object

From two observing runs during the 2014 summer at Calar Alto Observatory in Almería (Spain) and at Sierra Nevada Observatory in Granada (Spain), we were able to derive CCD photometry of the Trans-Neptunian Object 2008 OG$_{19}$. We analyzed the time series and obtained a double-peaked light curve with a peak to valley amplitude of (0.437 $\pm$ 0.011) mag and a rotational period of (8.727$\pm$ 0.003) h. This implies that this object is very elongated, closely resembling Varuna's case. The photometry also allowed us to obtain an absolute magnitude in R-band of (4.39 $\pm$ 0.07) mag. From this result we estimated an equivalent diameter of 2008 OG$_{19}$ which is 619$^{+56}_{-113}$ km using an average albedo for Scattered Disk Objects. Finally we interpreted the results under the assumption of hydrostatic equilibrium and found a lower limit for the density of 544$^{+42}_{-4}$ kg$\,$m$^{-3}$. However, a more likely density is (609 $\pm$ 4) kg$\,$m$^{-3}$ using an aspect angle of 60$^\circ$, which corresponds to the most likely configuration for the spin axis with respect to the observer assuming random orientations.

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arXiv2015

Short-term variability of comet C/2012 S1 (ISON) at 4.8 AU from the Sun

We observed comet C/2012 S1 (ISON) during six nights in February 2013 when it was at 4.8 AU from the sun. At this distance and time the comet was not very active and it was theoretically possible to detect photometric variations likely due to the rotation of the cometary nucleus. The goal of this work is to obtain differential photometry of the comet inner coma using different aperture radii in order to derive a possible rotational period. Large field of view images were obtained with a 4k x 4k CCD at the f/3 0.77m telescope of La Hita Observatory in Spain. Aperture photometry was performed in order to get relative magnitude variation versus time. Using calibrated star fields we also obtained ISON's R-magnitudes versus time. We applied a Lomb-Scargle periodogram analysis to get possible periodicities for the observed brightness variations, directly related with the rotation of the cometary nucleus. The comet light curve obtained is very shallow, with a peak-to-peak amplitude of 0.03 $\pm$ 0.02 mag. A tentative synodic rotational period (single-peaked) of 14.4 $\pm$ 1.2 hours for ISON's nucleus is obtained from our analysis, but there are other possibilities. We studied the possible effect of the seeing variations in the obtained periodicities during the same night, and from night to night. These seeing variations had no effect on the derived periodicity. We discuss and interpret all possible solutions for the rotational period of ISON's nucleus.

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