Haumea-one of the four known trans-Neptunian dwarf planets-is a very elongated and rapidly rotating body. In contrast to other dwarf planets, its size, shape, albedo and density are not well ...constrained. The Centaur Chariklo was the first body other than a giant planet known to have a ring system, and the Centaur Chiron was later found to possess something similar to Chariklo's rings. Here we report observations from multiple Earth-based observatories of Haumea passing in front of a distant star (a multi-chord stellar occultation). Secondary events observed around the main body of Haumea are consistent with the presence of a ring with an opacity of 0.5, width of 70 kilometres and radius of about 2,287 kilometres. The ring is coplanar with both Haumea's equator and the orbit of its satellite Hi'iaka. The radius of the ring places it close to the 3:1 mean-motion resonance with Haumea's spin period-that is, Haumea rotates three times on its axis in the time that a ring particle completes one revolution. The occultation by the main body provides an instantaneous elliptical projected shape with axes of about 1,704 kilometres and 1,138 kilometres. Combined with rotational light curves, the occultation constrains the three-dimensional orientation of Haumea and its triaxial shape, which is inconsistent with a homogeneous body in hydrostatic equilibrium. Haumea's largest axis is at least 2,322 kilometres, larger than previously thought, implying an upper limit for its density of 1,885 kilograms per cubic metre and a geometric albedo of 0.51, both smaller than previous estimates. In addition, this estimate of the density of Haumea is closer to that of Pluto than are previous estimates, in line with expectations. No global nitrogen- or methane-dominated atmosphere was detected.
ABSTRACT
Recently, it has been reported the discovery of a dense ring around the trans-Neptunian object 50000 Quaoar. The ring particles seem to be very close to the 6/1 mean motion resonance with ...Weywot, the only known satellite in the system. In this work, we investigate the dynamical environment in the close vicinity of the 6/1 orbital resonance in the context of the restricted three-body problem. We aim to analyse whether, in view of observational constraints, the ring could be effectively evolving in resonant motion with the satellite. Through the technique of dynamical maps we identify and characterize the 6/1 mean motion resonance, finding that the main location of the resonance deviates by only 29 km from the central part of the ring. This difference lies within the 3σ confidence level, considering the uncertainties in the observational parameters. We also show that the Weywot’s eccentricity plays a significant role in the dynamical structure of the 6/1 resonance. The results show that the resonance width is smaller than the estimated ring’s width. Under assumption of a ring with eccentricity smaller than 0.05, clumping of test particles appears at the position of the different resonant multiplets, considering the nominal value of Weywot’s eccentricity. This is in agreement with observations, which indicate that the estimated resonance width (≤10 km) is comparable with the narrow and dense arc of material within Quaoar’s ring. Our results may be an indicative that the 6/1 resonance plays a key role in confining the arc ring.
SORA: Stellar occultation reduction and analysis Gomes-Júnior, A R; Morgado, B E; Benedetti-Rossi, G ...
Monthly notices of the Royal Astronomical Society,
02/2022, Letnik:
511, Številka:
1
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
The stellar occultation technique provides competitive accuracy in determining the sizes, shapes, astrometry, etc., of the occulting body, comparable to in-situ observations by spacecraft. ...With the increase in the number of known Solar system objects expected from the LSST, the highly precise astrometric catalogs, such as Gaia, and the improvement of ephemerides, occultations observations will become more common with a higher number of chords in each observation. In the context of the Big Data era, we developed sora, an open-source python library to reduce and analyse stellar occultation data efficiently. It includes routines from predicting such events up to the determination of Solar system bodies’ sizes, shapes, and positions.
Gomes-Júnior et al. published 3613 positions for the eight largest irregular satellites of Jupiter and 1787 positions for the largest irregular satellite of Saturn, Phoebe. These observations were ...made between 1995 and 2014 and have an estimated error of about 60–80 mas. Based on this set of positions, we derived new orbits for the eight largest irregular satellites of Jupiter: Himalia, Elara, Pasiphae, Carme, Lysithea, Sinope, Ananke and Leda. For Phoebe we updated the ephemeris from Desmars et al. using 75 per cent more positions than the previous one. Because of their orbital characteristics, it is common belief that the irregular satellites were captured by the giant planets in the early Solar system, but there is no consensus for a single model explaining where they were formed. Size, shape, albedo and composition would help to trace back their true origin, but these physical parameters are yet poorly known for irregular satellites. The observation of stellar occultations would allow for the determination of such parameters. Indeed Jupiter will cross the galactic plane in 2019–2020 and Saturn in 2018, improving a lot the chances of observing such events in the near future. Using the derived ephemerides and the UCAC4 catalogue we managed to identify 5442 candidate stellar occultations between 2016 January and 2020 December for the nine satellites studied here. We discussed how the successful observation of a stellar occultation by these objects is possible and present some potential occultations.
Abstract
We use data from five stellar occultations observed between 2013 and 2016 to constrain Chariklo’s size and shape, and the ring reflectivity. We consider four possible models for Chariklo ...(sphere, Maclaurin spheroid, triaxial ellipsoid, and Jacobi ellipsoid), and we use a Bayesian approach to estimate the corresponding parameters. The spherical model has a radius
R
= 129 ± 3 km. The Maclaurin model has equatorial and polar radii
and
, respectively, with density
. The ellipsoidal model has semiaxes
,
, and
. Finally, the Jacobi model has semiaxes
a
= 157 ± 4 km,
b
= 139 ± 4 km, and
c
= 86 ± 1 km, and density
. Depending on the model, we obtain topographic features of 6–11 km, typical of Saturn icy satellites with similar size and density. We constrain Chariklo’s geometric albedo between 3.1% (sphere) and 4.9% (ellipsoid), while the ring
I
/
F
reflectivity is less constrained between 0.6% (Jacobi) and 8.9% (sphere). The ellipsoid model explains both the optical light curve and the long-term photometry variation of the system, giving a plausible value for the geometric albedo of the ring particles of 10%–15%. The derived mass of Chariklo of 6–8 × 10
18
kg places the rings close to 3:1 resonance between the ring mean motion and Chariklo’s rotation period.
ABSTRACT
Doing high-precision astrometry on Uranus’ moons is currently quite challenging. No probes will orbit the system before 2040. New high-precision mutual phenomena measurements will only occur ...in 2050. Besides, Uranus is slowly passing through a sky region without many stars, which makes it difficult to map field of view (FOV) distortions below 50 mas. In this context, the new astrometric technique of mutual approximations comes in handy. It measures central instants at the closest approach between two moving satellites in the sky plane. Measurements are made on small portions of the FOV, benefiting from the so-called precision premium. Approximations and mutual phenomena share geometric principles and parameters, with similar precision in the central instant as indicated by first applications to the Jovian moons. However, mutual phenomena can only be observed at the planet’s equinoxes, while approximations always occur. Central instants do not depend on reference stars and are useful in orbit and ephemeris fittings. Here, we present results for 23 mutual approximations between the five main Uranus satellites observed in Brazil during 2015–2018 with a 1.6 m aperture telescope. Digital coronagraphy mitigated Uranus’ scattered light, improving measurements for Miranda, Ariel and Umbriel. We measured the impact parameter and relative velocity in milliarcseconds for the first time by using a variant of the method. Relative position errors, including Miranda, were 45 mas per coordinate, twice as good as in classical CCD astrometry for this satellite, and comparable to mutual phenomena. This shows the potential of mutual approximations for improving the current orbits and ephemerides of Uranus’ moons.
ABSTRACT
Recording a stellar occultation is one powerful method that gives direct information about the physical properties of the occulting Solar system object. In order to obtain reliable and ...accurate results, simultaneous observations from different locations across-track of the projected path are of great importance. However, organizing all the observing stations, aggregating, and analysing the data is time-consuming and not that easy. We have developed a web portal named Occultation Portal (OP) to manage all those occultation observation campaigns from a central server. With this portal, the instrumental and observational information of all observers participating in a stellar occultation campaign and the concerned data are archived systematically in a standard format. The researchers can then visualize the archived data on an event basis. The investigators can also extract the light curve for each data set with the added reduction pipeline to the portal base. This paper describes in detail the portal structure and the developed features.
ABSTRACT
Astrometric studies and orbital modelling of planetary moons have contributed significantly to advancing our understanding of their orbital dynamics. These studies require precise positions ...measured over extended periods. In this paper, we present the results of the 2021 Brazilian Jovian mutual phenomena campaign. The data correspond to eight events between Galilean satellites, in addition to a rare eclipse of Thebe, an inner satellite, totalling nine events. A geometric model along with the DE440/JUP365 ephemerides was used to reproduce the events and simulate the light curves. A Monte Carlo method and chi-squared statistics were used to fit the simulated light curves to the observations. The reflectance model adopted for our simulations was the complete version of the Oren–Nayer model. The average uncertainty of the relative positions of the Galilean satellites was 5 mas (15 km) and for the inner Thebe satellite 32 mas (96 km). The seven mutual events (nine independent observations) here analysed represent an addition of 17 per cent events (10 per cent light curves) with respect to the 2021 international mutual phenomena campaign.
Furthermore, our result of Thebe eclipse is only the second measurement published to date. Our results contribute to the ephemeris data base, being fundamental to improving satellite orbits and thus minimizing their uncertainties.
ABSTRACT
We report six stellar occultations by Phoebe (Saturn IX), an irregular satellite of Saturn, obtained between mid-2017 and mid-2019. The 2017 July 6 event was the first stellar occultation by ...an irregular satellite ever observed. The occultation chords were compared to a 3D shape model of the satellite obtained from Cassini observations. The rotation period available in the literature led to a sub-observer point at the moment of the observed occultations where the chords could not fit the 3D model. A procedure was developed to identify the correct sub-observer longitude. It allowed us to obtain the rotation period with improved precision compared to the currently known value from literature. We show that the difference between the observed and the predicted sub-observer longitude suggests two possible solutions for the rotation period. By comparing these values with recently observed rotational light curves and single-chord stellar occultations, we can identify the best solution for Phoebe’s rotational period as 9.27365 ± 0.00002 h. From the stellar occultations, we also obtained six geocentric astrometric positions in the ICRS as realized by the Gaia DR2 with uncertainties at the 1-mas level.
We were able to accurately predict the shadow path and successfully observe an occultation of a bright star by Chiron on December 15, 2022. The Kottamia Astronomical Observatory in Egypt did not ...detect the occultation by the solid body, but we found three extinction features in the light curve that had symmetrical counterparts with respect to the central time of the occultation. One of the features is broad and shallow, whereas the other two features are sharper, with a maximum extinction of ∼25% at the achieved spatial resolution of 19 km per data point. From the Wise Observatory in Israel, we detected the occultation caused by the main body and several extinction features surrounding the body. When all the secondary features are plotted in the sky plane, we find that they can be caused by a broad ∼580 km disk with concentrations at radii of 325 ± 16 km and 423 ± 11 km surrounding Chiron. At least one of these structures appears to be outside the Roche limit. The ecliptic coordinates of the pole of the disk are
λ
= 151° ±8° and
β
= 18° ±11°, in agreement with previous results. We also reveal our long-term photometry results, indicating that Chiron had suffered a brightness outburst of at least 0.6 mag between March and September 2021 and that Chiron was still somewhat brighter at the occultation date than at its nominal pre-outburst phase. The outermost extinction features might be consistent with a bound or temporarily bound structure associated with the brightness increase. However, the nature of the brightness outburst is unclear, and it is also unclear whether the dust or ice released in the outburst could be feeding a putative ring structure or whether it is emanating from it.
PDF
