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.
•Data on rotations of 46 binary and triple asteroid systems were collected.•An anti-correlation of secondary synchroneity with orbital eccentricity observed.•Libration angles of synchronous ...secondaries are less than 20° on most epochs.•A paucity of chaotic rotations among asynchronous secondaries is apparent.•An upper limit on the secondary equatorial axis ratios of 1.5 was found.
We collected data on rotations and elongations of 46 secondaries of binary and triple systems among near-Earth, Mars-crossing and small main belt asteroids. 24 were found or are strongly suspected to be synchronous (in 1:1 spin–orbit resonance), and the other 22, generally on more distant and/or eccentric orbits, were found or are suggested to have asynchronous rotations. For 18 of the synchronous secondaries, we constrained their librational angles, finding that their long axes pointed to within 20° of the primary on most epochs. The observed anti-correlation of secondary synchroneity with orbital eccentricity and the limited librational angles agree with the theories by Ćuk and Nesvorný (Ćuk, M., Nesvorný, D. 2010. Icarus 207, 732–743) and Naidu and Margot (Naidu, S.P., Margot, J.-L. 2015. Astron. J. 149, 80). A reason for the asynchronous secondaries being on wider orbits than synchronous ones may be longer tidal circularization time scales at larger semi-major axes. The asynchronous secondaries show relatively fast spins; their rotation periods are typically <10 h. An intriguing observation is a paucity of chaotic secondary rotations; with an exception of (35107) 1991 VH, the secondary rotations are single-periodic with no signs of chaotic rotation and their periods are constant on timescales from weeks to years. The secondary equatorial elongations show an upper limit of a2/b2∼1.5. The lack of synchronous secondaries with greater elongations appears consistent, considering uncertainties of the axis ratio estimates, with the theory by Ćuk and Nesvorný that predicts large regions of chaotic rotation in the phase space for a2/b2≳2. Alternatively, secondaries may not form or stay very elongated in gravitational (tidal) field of the primary. It could be due to the secondary fission mechanism suggested by Jacobson and Scheeres (Jacobson, S.A., Scheeres, D.J. 2011. Icarus 214, 161–178), as its efficiency is correlated with the secondary elongation. Sharma (Sharma, I. 2014. Icarus 229, 278–294) found that rubble-pile satellites with a2/b2≲1.5 are more stable to finite structural perturbations than more elongated ones. It appears that more elongated secondaries, if they originally formed in spin fission of parent asteroid, are less likely to survive intact and they more frequently fail or fission.
Context.
We present results from the first recorded stellar occultation by the large trans-Neptunian object (174567) Varda that was observed on September 10, 2018. Varda belongs to the ...high-inclination dynamically excited population, and has a satellite, Ilmarë, which is half the size of Varda.
Aims.
We determine the size and albedo of Varda and constrain its 3D shape and density.
Methods.
Thirteen different sites in the USA monitored the event, five of which detected an occultation by the main body. A best-fitting ellipse to the occultation chords provides the instantaneous limb of the body, from which the geometric albedo is computed. The size and shape of Varda are evaluated, and its bulk density is constrained using Varda’s mass as is known from previous works.
Results.
The best-fitting elliptical limb has semi-major (equatorial) axis of (383 ± 3) km and an apparent oblateness of 0.066 ± 0.047, corresponding to an apparent area-equivalent radius
R
′
equiv
= (370±7) km and geometric albedo
p
v
= 0.099 ± 0.002 assuming a visual absolute magnitude
H
V
= 3.81 ± 0.01. Using three possible rotational periods for the body (4.76, 5.91, and 7.87 h), we derive corresponding MacLaurin solutions. Furthermore, given the low-amplitude (0.06 ± 0.01) mag of the single-peaked rotational light-curve for the aforementioned periods, we consider the double periods. For the 5.91 h period (the most probable) and its double (11.82 h), we find bulk densities and true oblateness of
ρ
= (1.78 ± 0.06) g cm
−3
,
ɛ
= 0.235 ± 0.050, and
ρ
= (1.23 ± 0.04) g cm
−3
,
ɛ
= 0.080 ± 0.049. However, it must be noted that the other solutions cannot be excluded just yet.
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.
We present results derived from the first multi-chord stellar occultations by the transneptunian object (50000) Quaoar, observed on 2011 May 4 and 2012 February 17, and from a single-chord ...occultation observed on 2012 October 15. If the timing of the five chords obtained in 2011 were correct, then Quaoar would possess topographic features (crater or mountain) that would be too large for a body of this mass. An alternative model consists in applying time shifts to some chords to account for possible timing errors. Satisfactory elliptical fits to the chords are then possible, yielding an equivalent radius R sub(equiv) = 555+ or -2.5 km and geometric visual albedo p sub(v) = 0.109+ or -0.007. Assuming that Quaoar is a Maclaurin spheroid with an indeterminate polar aspect angle, we derive a true oblateness of member of = 0.087 super(+0.0268) sub(-0.0175), an equatorial radius of 569 super(+24) sub(-17)km, and a density of 1.99 + or - 0.46 g cm super(-3). The orientation of our preferred solution in the plane of the sky implies that Quaoar's satellite Weywot cannot have an equatorial orbit. Finally, we detect no global atmosphere around Quaoar, considering a pressure upper limit of about 20 nbar for a pure methane atmosphere.
Pluto and Eris are icy dwarf planets with nearly identical sizes, comparable densities and similar surface compositions as revealed by spectroscopic studies. Pluto possesses an atmosphere whereas ...Eris does not; the difference probably arises from their differing distances from the Sun, and explains their different albedos. Makemake is another icy dwarf planet with a spectrum similar to Eris and Pluto, and is currently at a distance to the Sun intermediate between the two. Although Makemake's size (1,420 ± 60 km) and albedo are roughly known, there has been no constraint on its density and there were expectations that it could have a Pluto-like atmosphere. Here we report the results from a stellar occultation by Makemake on 2011 April 23. Our preferred solution that fits the occultation chords corresponds to a body with projected axes of 1,430 ± 9 km (1σ) and 1,502 ± 45 km, implying a V-band geometric albedo p(V) = 0.77 ± 0.03. This albedo is larger than that of Pluto, but smaller than that of Eris. The disappearances and reappearances of the star were abrupt, showing that Makemake has no global Pluto-like atmosphere at an upper limit of 4-12 nanobar (1σ) for the surface pressure, although a localized atmosphere is possible. A density of 1.7 ± 0.3 g cm(-3) is inferred from the data.
Convective storms occur regularly in Saturn's atmosphere. Huge storms known as Great White Spots, which are ten times larger than the regular storms, are rarer and occur about once per Saturnian year ...(29.5 Earth years). Current models propose that the outbreak of a Great White Spot is due to moist convection induced by water. However, the generation of the global disturbance and its effect on Saturn's permanent winds have hitherto been unconstrained by data, because there was insufficient spatial resolution and temporal sampling to infer the dynamics of Saturn's weather layer (the layer in the troposphere where the cloud forms). Theoretically, it has been suggested that this phenomenon is seasonally controlled. Here we report observations of a storm at northern latitudes in the peak of a weak westward jet during the beginning of northern springtime, in accord with the seasonal cycle but earlier than expected. The storm head moved faster than the jet, was active during the two-month observation period, and triggered a planetary-scale disturbance that circled Saturn but did not significantly alter the ambient zonal winds. Numerical simulations of the phenomenon show that, as on Jupiter, Saturn's winds extend without decay deep down into the weather layer, at least to the water-cloud base at pressures of 10-12 bar, which is much deeper than solar radiation penetrates.
Spin rate distribution of small asteroids Pravec, P.; Harris, A.W.; Vokrouhlický, D. ...
Icarus (New York, N.Y. 1962),
10/2008, Letnik:
197, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The spin rate distribution of main belt/Mars crossing (MB/MC) asteroids with diameters 3–15 km is uniform in the range from
f
=
1
to 9.5 d
−1, and there is an excess of slow rotators with
f
<
1
d
−1. ...The observed distribution appears to be controlled by the Yarkovsky–O'Keefe–Radzievskii–Paddack (YORP) effect. The magnitude of the excess of slow rotators is related to the residence time of slowed down asteroids in the excess and the rate of spin rate change outside the excess. We estimated a median YORP spin rate change of
≈
0.022
d
−1
/
Myr
for asteroids in our sample (i.e., a median time in which the spin rate changes by 1 d
−1 is
≈
45
Myr
), thus the residence time of slowed down asteroids in the excess is
≈
110
Myr
. The spin rate distribution of near-Earth asteroids (NEAs) with sizes in the range 0.2–3 km (∼5 times smaller in median diameter than the MB/MC asteroids sample) shows a similar excess of slow rotators, but there is also a concentration of NEAs at fast spin rates with
f
=
9
–
10
d
−1
. The concentration at fast spin rates is correlated with a narrower distribution of spin rates of primaries of binary systems among NEAs; the difference may be due to the apparently more evolved population of binaries among MB/MC asteroids.
Observation of stellar occultation by objects of the Solar System is a powerful technique that allows measurements of size and shape of the small bodies with accuracies in the order of the kilometre. ...In addition, the occultation star probes the surroundings of the object, allowing the study of putative rings/debris or atmosphere around it. Since 2009, more than 60 events by trans-Neptunian and Centaur objects have been detected, involving more than 34 different bodies. Some remarkable results were achieved, such as the discovery of rings around Chariklo and Haumea, or the high albedo of Eris, the lack of global atmosphere around Makemake and the discovery of the double shape of 2014 MU69, among others. After the release of Gaia catalogues, predictions became more accurate, leading to an increasing number of successful observations of occultation events. To keep track of the results achieved with this technique, we created a database to gather all the detected events worldwide. The database is presented as an electronic table (http://occultations.ct.utfpr.edu.br/), where the main information obtained from any occultation by small outer solar system objects are listed. The structure and term definitions used in the database are presented here, as well as some simple statistics that can be done with the available results.
► We present observations in December 2010 and January 2011 of Saturn’s GWS. ► We model the vertical cloud structure (10−3 to 1.4bar) using these visible images. ► An ascent of the top level of the ...bottom cloud is found. ► Particles in the storm disturbed region are more reflecting than in the surroundings.
We present a study of the vertical cloud structure for the initial stage of the Great White Spot (GWS), a giant storm that developed in Saturn in December 2010, using ground-based visual images. We focus in the characterization of the undisturbed atmosphere preceding the storm and the disturbed region in the wake of the GWS. The observations were taken at Calar Alto (Spain) and Pic du Midi (France) observatories on 27 December 2010 and 13 January 2011, about 1month after the detection of the outbreak. They cover a spectral range from the ultraviolet at 375nm to the near infrared at 954nm, including the deep methane absorption band at 890nm and a number of increasingly weaker methane absorption bands. Limb to limb scans of the absolute reflectivity of the regions preceding and following the storm at different wavelengths are compared to those produced by a radiative transfer model atmosphere. Our model assumes three layers of gas and particles: stratospheric and tropospheric hazes and a deep cloud. We find that the most notorious changes in the wake of the GWS occurred in the top level of the semi-infinite bottom cloud which ascended from an altitude level P>1bar in the undisturbed region to P=300+300-100mbar in the stormy area, representing a rise of more than 40km. The density of the tropospheric haze does not change substantially but tropospheric particles are found to be more reflectant at all wavelengths, suggesting that they are coated by fresh material, putatively coming from deeper levels of the atmosphere.
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