- ID:
- ivo://CDS.VizieR/J/AJ/146/12
- Title:
- Occultations through Saturn's C ring
- Short Name:
- J/AJ/146/12
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Saturn's C ring contains multiple spiral patterns that appear to be density waves driven by periodic gravitational perturbations. In other parts of Saturn's rings, such waves are generated by Lindblad resonances with Saturn's various moons, but most of the wave-like C-ring features are not situated near any strong resonance with any known moon. Using stellar occultation data obtained by the Visual and Infrared Mapping Spectrometer on board the Cassini spacecraft, we investigate the origin of six unidentified C-ring waves located between 80900 and 87200km from Saturn's center. By measuring differences in the waves' phases among the different occultations, we are able to determine both the number of arms in each spiral pattern and the speeds at which these patterns rotate around the planet. We find that all six of these waves have between two and four arms and pattern speeds between 1660{deg}/day and 1861{deg}/day. These speeds are too large to be attributed to any satellite resonance. Instead, they are comparable to the predicted pattern speeds of waves generated by low-order normal-mode oscillations within the planet. The precise pattern speeds associated with these waves should therefore provide strong constraints on Saturn's internal structure. Furthermore, we identify multiple waves with the same number of arms and very similar pattern speeds, indicating that multiple m=3 and m=2 sectoral (l=m) modes may exist within the planet.
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- ID:
- ivo://CDS.VizieR/J/A+A/515/A32
- Title:
- Pluto, Charon, Nix, Hydra occultations 2008-15
- Short Name:
- J/A+A/515/A32
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We made the astrometry of 110GB of images with the Platform for Reduction of Astronomical Images Automatically (PRAIA). By relatively simple astrometric techniques, we treated the overlapping observations and derived a field distortion pattern for the WFI mosaic of CCDs to within 50mas precision. Positions were obtained in the UCAC2 frame with errors of 50mas for stars up to magnitude R=19, and 25mas up to R=17. New stellar proper motions were also determined with 2MASS and the USNO B1.0 catalog positions as first epoch. The catalog contains 2.24 million stars with proper motions, covering Pluto's sky path with 30 arcmin width. Its magnitude completeness is about R=18 with a limit about R=21. The catalog of star positions along Pluto's 2008-2015 sky path was built in the context of the investigation of Pluto and its satellites, aiming to support precise astrometric predictions for stellar occultations by Pluto, Charon, Hydra and Nix for the period 2008-2015. For that, we have carried out in 2007 an observational program at the ESO2p2/WFI instrument covering the sky path of Pluto from 2008 to 2015. We highlight the usefulness of this catalog as a reference frame for star/body astrometric follow-up before and after future stellar occultations involving the Pluto system. In this way, updates for the ephemeris offsets of these bodies and for the position of star candidates for occultations can be enhanced. Besides, the catalog also furnishes useful photometric information for field stars in the flux calibration of observed light curves. The prediction tables of stellar occultations by Pluto, Charon, Nix and Hydra for 2008-2015 were built for investigating the physical properties of Pluto and its satellites, for understanding in a broader sense transneptunian objects - keystones in the study of structure, origin and evolution of the Solar System. Our aim was to derive precise, astrometric predictions. We then constructed an astrometric star catalog in the UCAC2 system covering Pluto's sky path. For that, we have carried out in 2007 an observational program at the ESO2p2/WFI instrument covering the sky path of Pluto from 2008 to 2015. Based on the past 2005-8 occultations successfully predicted, recorded and fitted, a linear drift with time in declination with regard to DE418/plu017 ephemerides was determined for Pluto and used in the current predictions. For offset (mas)=A(t(yr)-2005.0)+B, we find A=+30.5+/-4.3mas/yr and B=-31.5+/-11.3mas, with standard deviation of 14.4mas for the offsets. For these past occultations, predictions and follow-up observations were made with the 0.6m and 1.6m telescopes at the Laboratorio Nacional de Astrofisica/Brazil. No ephemeris correction was applied for right ascension. Although an oscillation pattern related to an error in Pluto's heliocentric distance (geocentric parallax error) cannot be ruled out, none of the attempted models for this scenario could fit the more dispersed right ascension ephemeris offsets derived from the studied past occultations, at least not well below 50mas, in contrast with the standard deviation of only 14.4mas achieved for declination. The candidate stars listed in the prediction tables were searched using a proximity radius of 335mas with the geocentric apparent orbit (corrected by ephemeris offsets) of the body considered. This radius is about the apparent radius of Pluto (50mas) plus the apparent Earth radius (285mas) as projected in the sky plane at 31AU (Pluto-Earth distance for 2008-2015). No threshold in R magnitude was used in the search for candidates. Pluto is crossing interstellar clouds, so relatively faint R objects may turn out to be bright infrared stars, perfect targets for the SOFIA observatory and for ground-based instruments well equipped with H, J or K band detectors (H, J and K magnitudes are promptly available in the tables if the star belongs to the 2MASS). Besides, events may be also favored by slow shadow speeds of less than 20km/s. Also, no constraint on a geographic place was applied, as in principle SOFIA observations can be done from any sub-solar point on Earth. Events in daylight at sub-planet point were not excluded either, as they could yet be observable in the dark, right above the horizon, from places near the Earth terminator. We furnish here prediction tables for future and also for past stellar occultations covering the sky path by Pluto between 2008-2015. The importance of predictions for occultations still to come is obvious. But the predictions of past occultations are also useful for at least three reasons. First, they can be used by anyone as reference for ongoing fittings of light curves of recent past observed events. Second, they serve to derive ephemeris drifts by comparing expected and observed central instants and C/A values. Finally, they can be used as an external check for the accuracy and precision of our prediction tables. In a general sense, assuming a bulk error of 30mas for C/A from the estimated errors of the catalog star positions and from the errors of the derived ephemeris offsets, we can state that the shadow path uncertainties over Earth are on the order of less than 800km for the stellar occultation predictions of Pluto and Charon. Considering orbital issues and the smaller sizes of Nix and Hydra, the probability of success quoted for these two satellites are about 10-25%. This is not as high as hoped, but not despairingly small, especially if the event occurs above a dense, populated region in terms of astronomers, including amateurs.
- ID:
- ivo://CDS.VizieR/J/AJ/103/983
- Title:
- Stellar occultation candidates of Saturn
- Short Name:
- J/AJ/103/983
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a list of 203 potential occultations by Saturn and its rings of stars from the Hubble Space Telescope Guide Star Catalog (GSC), during the years 1991-1999. Because the GSC is not a complete catalog, this is not an exhaustive list of Saturn occultations. In particular, stars brighter than magnitude 8 are not included. However, this list does include many fainter candidates than do current occultation candidate lists for Saturn; these fainter stars also can provide a high signal-to-noise ratio if observed with a large telescope or in the infrared where Saturn and its rings have absorption bands. We list the occultation circumstances, as well as star information found in the GSC.
- ID:
- ivo://CDS.VizieR/J/A+A/639/A134
- Title:
- The large TNO 2002 TC302
- Short Name:
- J/A+A/639/A134
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- On 28th January 2018, the large Trans-Neptunian Object 2002 TC_302_ occulted a m_v_~15.3 star with designation 593-005847 in the UCAC4 stellar catalog, corresponding to Gaia source 130957813463146112. Twelve positive occultation chords were obtained from Italy, France, Slovenia and Switzerland. Also, four negative detections were obtained near the north and south limbs. This represents the best observed stellar occultation by a TNO other than Pluto in terms of the number of chords published thus far. From the twelve chords, an accurate elliptical fit to the instantaneous projection of the body can be obtained, compatible with the near misses. The resulting ellipse has major and minor axes of 543+/-18km and 460+/-11km, respectively, with a position angle of 3+/-1 degrees for the minor axis. This information, combined with rotational light curves obtained with the 1.5-m telescope at Sierra Nevada Observatory and the 1.23-m telescope at Calar Alto observatory, allows us to derive possible three-dimensional shapes and density estimations for the body based on hydrostatic equilibrium assumptions. The effective diameter in equivalent area is around 84km smaller than the radiometrically derived diameter using thermal data from Herschel and Spitzer Space Telescopes. This might indicate the existence of an unresolved satellite of up to ~300km in diameter, to account for all the thermal flux, although the occultation and thermal diameters are compatible within their error bars given the considerable uncertainty of the thermal results. The existence of a potential satellite also appears to be consistent with other ground-based data presented here. From the effective occultation diameter combined with absolute magnitude measurements we derive a geometric albedo of 0.147+/-0.005, which would be somewhat smaller if 2002 TC_302_ has a satellite. The best occultation light curves do not show any signs of ring features or any signatures of a global atmosphere.
- ID:
- ivo://CDS.VizieR/J/AJ/119/936
- Title:
- Triton stellar occultation candidates
- Short Name:
- J/AJ/119/936
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- As part of our ongoing program of predictions and observations of stellar occultations by solar system bodies, we have completed a search for candidates for occultations by Triton over the decade 2000 to 2009. Star positions near Triton's projected orbit as determined by the DE405 ephemeris and NEP016 orbit model were measured on (unfiltered) CCD strip scans recorded with the 0.6 m telescope at the George R. Wallace Astrophysical Observatory to a depth of 16th to 18th magnitude, depending on the quality of individual strip scans. Within 1.0" of the predicted orbit of Triton during this period, 128 stars were found, including 12 stars brighter than 14th magnitude. Only appulses with geocentric minimum separations of less than about 0.37" will result in an occultation visible from Earth, but potential errors in the ephemeris and in the positions of our candidates preclude accurate prediction of actual occultation events without further astrometry.
- ID:
- ivo://CDS.VizieR/J/AJ/109/1352
- Title:
- Triton stellar occultation candidates
- Short Name:
- J/AJ/109/1352
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have completed a search for candidates for stellar occultations by Triton over the years 1995-1999 CCD strip scan images provided star positions in the relevant sky area to a depth of about 17.5Rmag. Over this time period, we find that Triton passes within 1.0arcsec of 75 stars. Appulses with geocentric minimum separations of less than 0.35arcsec will result in stellar occultations, but further astrometry and photometry is necessary to refine individual predictions for identification of actual occultations. Finder charts are included to aid in further studies and prediction refinement. The two most promising potential occultations, Tr176 and Tr180, occur in 1997.
- ID:
- ivo://CDS.VizieR/J/AJ/104/862
- Title:
- Triton stellar occultation candidates
- Short Name:
- J/AJ/104/862
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- A search for Triton stellar occultation candidates for the period 1992-1994 has been completed with CCD strip-scanning observations. The search reached an R magnitude of about 17.4 and found 129 candidates within 1.5arcsec of Triton's ephemeris during this period. Of these events, we expect around 30 occultations to be visible from the Earth, indicating that a number of Triton occultation events should be visible from major observatories. Even the faintest of our candidate events could produce useful occultation data if observed with a large enough telescope. Our astrometric accuracy is inadequate to identify which of these appulse events will produce occultations on the Earth; further astrometry is needed to refine the predictions for positive occultation identification. To aid in selecting candidates for additional astrometric and photometric studies, we include finder charts and Earth-based visibility charts for each event.
