Search

Context. Quaoar is a classical trans-Neptunian object (TNO) with an area-equivalent diameter of 1100 km and an orbital semi-major axis of 43.3 astronomical units. Based on stellar occultations observed between 2018 and 2021, an inhomogeneous ring (Q1R, i.e., Quaoar’s first ring) has been detected around this body. Aims. A new stellar occultation by Quaoar was observed on August 9, 2022, with the aim of improving Quaoar’s shape models and the physical parameters of Q1R, while searching for additional material around the body. Methods. The occultation provided nine effective chords across Quaoar, pinning down its size, shape, and astrometric position. Large facilities, such as Gemini North and the Canada-France-Hawaii Telescope (CFHT), were used to obtain high acquisition rates and signal-to-noise ratios. The light curves were also used to characterize the Q1R ring (radial profiles and orbital elements). Results. Quaoar’s elliptical fit to the occultation chords yields the limb with an apparent semi-major axis of 579.5 ± 4.0 km, apparent oblateness of 0.12 ± 0.01, and area-equivalent radius of 543 ± 2 km. Quaoar’s limb orientation is consistent with Q1R and Weywot orbiting in Quaoar’s equatorial plane. The orbital radius of Q1R is refined to a value of 4057 ± 6 km. The radial opacity profile of the more opaque ring profile follows a Lorentzian shape that extends over 60 km, with a full width at half maximum (FWHM) of ∼5 km and a peak normal optical depth of 0.4. Besides the secondary events related to the already reported rings, new secondary events detected during the August 2022 occultation in three different data sets are consistent with another ring around Quaoar with a radius of 2520 ± 20 km, assuming the ring is circular and co-planar with Q1R. This new ring has a typical width of 10 km and a normal optical depth of ∼0.004. Just as Q1R, it also lies outside Quaoar’s classical Roche limit. © The Authors 2023.

Within our program of physical characterization of trans-Neptunian objects and centaurs, we predicted a stellar occultation by the centaur (54598) Bienor to occur on January 11, 2019, with good observability potential. We obtained high accuracy astrometric data to refine the prediction, resulting in a shadow path favorable for the Iberian Peninsula. This encouraged us to carry out an occultation observation campaign that resulted in five positive detections from four observing sites. This is the fourth centaur for which a multichord (more than two chords) stellar occultation has been observed so far, the other three being (2060) Chiron, (10199) Chariklo, and (95626) 2002 GZ32. From the analysis of the occultation chords, combined with the rotational light curve obtained shortly after the occultation, we determined that Bienor has an area-equivalent diameter of 150 ± 20 km. This diameter is ~30 km smaller than the one obtained from thermal measurements. The position angle of the short axis of the best fitting ellipse obtained through the analysis of the stellar occultation does not match that of the spin axis derived from long-term photometric models. We also detected a strong irregularity in one of the minima of the rotational light curve that is present no matter the aspect angle at which the observations were done. We present different scenarios to reconcile the results from the different techniques. We did not detect secondary drops related to potential rings or satellites. Nonetheless, similar rings in size to that of Chariklo's cannot be discarded due to low data accuracy.

Context. A stellar occultation by Neptune’s main satellite, Triton, was observed on 5 October 2017 from Europe, North Africa, and the USA. We derived 90 light curves from this event, 42 of which yielded a central flash detection. Aims. We aimed at constraining Triton’s atmospheric structure and the seasonal variations of its atmospheric pressure since the Voyager 2 epoch (1989). We also derived the shape of the lower atmosphere from central flash analysis. Methods. We used Abel inversions and direct ray-tracing code to provide the density, pressure, and temperature profiles in the altitude range ~8 km to ~190 km, corresponding to pressure levels from 9 µbar down to a few nanobars. Results. (i) A pressure of 1.18 ± 0.03 µbar is found at a reference radius of 1400 km (47 km altitude). (ii) A new analysis of the Voyager 2 radio science occultation shows that this is consistent with an extrapolation of pressure down to the surface pressure obtained in 1989. (iii) A survey of occultations obtained between 1989 and 2017 suggests that an enhancement in surface pressure as reported during the 1990s might be real, but debatable, due to very few high S/N light curves and data accessible for reanalysis. The volatile transport model analysed supports a moderate increase in surface pressure, with a maximum value around 2005-2015 no higher than 23 µbar. The pressures observed in 1995-1997 and 2017 appear mutually inconsistent with the volatile transport model presented here. (iv) The central flash structure does not show evidence of an atmospheric distortion. We find an upper limit of 0.0011 for the apparent oblateness of the atmosphere near the 8 km altitude. © J. Marques Oliveira et al. 2022.

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 timescales. After the analysis of the periodogram of the residuals to a 6.343572 hr 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 hr (or 23.9638 hr), assuming that the satellite is tidally locked, at a distance of ∼1300 km (or ∼2000 km) from Varuna, outside the Roche limit.© 2019. The American Astronomical Society..

The combination of visible and thermal data from the ground and astrophysics space missions is key to improving the scientific understanding of near-Earth, main-belt, trojans, centaurs, and trans-Neptunian objects. To get full information on a small sample of selected bodies we combine different methods and techniques: lightcurve inversion, stellar occultations, thermophysical modelling, radiometric methods, radar ranging and adaptive optics imaging. The SBNAF project will derive size, spin and shape, thermal inertia, surface roughness, and in some cases bulk densities and even internal structure and composition, for objects out to the most distant regions in the Solar System. The applications to objects with ground-truth information allows us to advance the techniques beyond the current state-of-the-art and to assess the limitations of each method. We present results from our project's first phase: the analysis of combined Herschel-KeplerK2 data and Herschel-occultation data for TNOs; synergy studies on large MBAs from combined high-quality visual and thermal data; establishment of well-known asteroids as celestial calibrators for far-infrared, sub-millimetre, and millimetre projects; first results on near-Earth asteroids properties from combined lightcurve, radar and thermal measurements, as well as the Hayabusa-2 mission target characterisation. We also introduce public web-services and tools for studies of small bodies in general. © 2017 COSPAR

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, which are consistent with the models supporting the presumption that Chiron possesses rings. By co-adding the images acquired in 2015, we have detected an similar to 5 arcsec long tail, showing a surface brightness of 25.3 mag(V) arcsec(-2).© The Author 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society.

Los objetos transneptunianos (TNOs) son aquellos cuerpos del Sistema Solar con órbitas cuyo semieje mayor es más grande que el de la órbita de Neptuno. Estos objetos se formaron en regiones exteriores a la línea de hielo, es decir, en zonas de la nebulosa protoplanetaria donde los elementos volátiles (compuestos químicos con bajos puntos de sublimación) tenían cabida, ya que las temperaturas eran suficientemente bajas como para permitir la formación de los mismos. Los TNOs poseen por tanto, de manera similar a los cometas, una composición basada en una mezcla de rocas y hielos (es decir, fases sólidas de los elementos volátiles). Debido a las grandes distancias que los separan del Sol, los materiales de sus superficies han sufrido menos procesos químicos en comparación con los de otros cuerpos del Sistema Solar. En cierto sentido, estos objetos son cápsulas del tiempo que contienen y mantienen prácticamente intacto el material del disco protoplanetario. Es por este motivo por el que el estudio de TNOs aporta información fundamental a cerca de la nebulosa solar primitiva y los materiales a partir de los cuales se formó el Sistema Solar. Esta observación incluye también a los centauros: objetos que provienen del cinturón transneptuniano, los cuales han sido inyectados a zonas más internas debido a encuentros planetarios, principalmente con Neptuno., Esta tesis se ha llevado a cabo en el marco de un programa dedicado al estudio de las propiedades físicas de objetos transneptunianos y otros cuerpos pequeños del Sistema Solar, que lleva en curso más de dos décadas en el Instituto de Astrofísica de Andalucía, con financiación de entidades públicas nacionales, autonómicas y europeas. Dentro de este programa se utilizan técnicas variadas para el estudio de estos objetos, tanto observacionales como teóricas. Entre ellas, la técnica de la fotometría ha sido una de las herramientas destacadas, en la que el grupo ha hecho un gran número de contribuciones científicas y en la que esta tesis se encuadra. Durante el desarrollo de esta tesis se ha abordado un estudio sistemático de un conjunto numeroso de objetos, si bien las conclusiones principales han podido derivarse de manera más concluyente para cuatro de ellos. Los resultados sobre estos objetos han sido publicados en 4 artículos recogidos en esta memoria de tesis, que se presenta en la modalidad de compendio de artículos. Análisis adicionales, así como la preparación de las correspondientes publicaciones, se encuentran en desarrollo. El primer capítulo versa sobre el objeto transneptuniano 2008 OG19. Durante los primeros pasos de la investigación, quedó patente que se trataba de un cuerpo de unas características fuera de lo común. Resultó ser un objeto que presentaba una variabilidad fotométrica de más de 0.43 magnitudes, lo cual es algo extraordinariamente inusual, ya que solo existen menos de 10 objetos conocidos con estas características en el cinturón transneptuniano, en comparación con los ~150 objetos de los que se conoce su curva de luz. Identificar uno de estos objetos tan singulares es ya importante, pero además, del estudio fotométrico detallado se pudieron obtener características importantes tales como su periodo de rotación, forma y su densidad (suponiendo equilibrio hidrostático). Esto es también un logro considerable ya que la determinación de densidades, En el capítulo de (2060) Quirón se reinterpretan los resultados, publicados en la bibliografía, de ocultaciones estelares producidas por el centauro. Esta nueva interpretación propone la existencia de material en forma de anillo, similar al que se encuentra en Cariclo que, además, es confirmada mediante los estudios fotométricos de largo término, donde se modela la evolución temporal de la amplitud de la curva de luz y la magnitud absoluta. A través de la modelización también se obtiene el eje de rotación del objeto que encaja con el del anillo. La monitorización posterior del objeto verifica la validez de los modelos en los que se incluyen los efectos fotométricos producidos por los anillos del centauro. Finalmente, el último capítulo de resultados está dedicado al análisis de los datos fotométricos del centauro (54 598) Bienor, el cual presenta características muy similares a las exhibidas por los centauros Cariclo y Quirón. Se obtienen nuevos datos observacionales de la amplitud de la curva de luz y la magnitud absoluta que serán utilizados, junto con los publicados en la bibliografía, para modelar conjuntamente su evolución temporal. Se proponen varios modelos para explicar la variación de ambos resultados fotométricos, entre ellos: (i) relajación de la suposición de equilibrio hidróstatico, (ii) variabilidad del albedo superficial y (iii) un sistema de cuerpo + anillo. Si bien no pueden descartase ninguno de ellos, el que produce mejores resultados en términos estadísticos es el que describe un anillo similar a los encontrados en los centauros Cariclo y Quirón. Gracias a los modelos fotométricos de evolución temporal de la magnitud absoluta y la amplitud de la curva de luz, se han obtenido forma y dirección del eje de rotación para los tres centauros. Para Cariclo y Bienor se ha obtenido también la densidad. Con anterioridad a este trabajo, estas tres propiedades sólo han sido extraídas para un centauro más, (5145) Pholus.

From two observing runs during the 2014 summer at the Calar Alto Observatory in Almeŕa (Spain) and at the Sierra Nevada Observatory in Granada (Spain), we were able to derive CCD photometry of the Trans-Neptunian object 2008 OG. We analysed the time series and obtained a double-peaked light curve with a peak-to-valley amplitude of 0.437 ± 0.011 mag and a rotational period of 8.727 ± 0.003 h. This implies that this object is very elongated closely resembling the case of Varuna. The photometry also allowed us to obtain an absolute magnitude in the R band of 4.39 ± 0.07 mag. From this result, we estimated an equivalent diameter of 2008 OG of 619 km using an average albedo for scattered disc objects. Finally, we interpreted the results under the assumption of hydrostatic equilibrium and found a lower limit for the density of 544 kg m. However, a more likely density is 609 ± 4 kg m using an aspect angle of 60°, which corresponds to the most likely configuration for the spin axis with respect to the observer assuming random orientations.© 2015 The Authors.