International Occultation Timing Association

Montgomery, AL, United States

International Occultation Timing Association

Montgomery, AL, United States
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Gomes-Junior A.R.,Federal University of Rio de Janeiro | Giacchini B.L.,Brazilian Center for Research in Physics (CBPF) | Braga-Ribas F.,Observatorio Nacional MCTI | Braga-Ribas F.,Federal University of Technology of Parana | And 29 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2015

We report the results of two multichord stellar occultations by the dwarf planet (1) Ceres that were observed from Brazil on 2010 August 17, and from the USA on 2013 October 25. Four positive detections were obtained for the 2010 occultation, and nine for the 2013 occultation. Elliptical models were adjusted to the observed chords to obtain Ceres' size and shape. Two limb-fitting solutions were studied for each event. The first one is a nominal solution with an indeterminate polar aspect angle. The second one was constrained by the pole coordinates as given by Drummond et al. Assuming a Maclaurin spheroid, we determine an equatorial diameter of 972 ± 6 km and an apparent oblateness of 0.08 ± 0.03 as our best solution. These results are compared to all available size and shape determinations for Ceres made so far, and shall be confirmed by the NASA's Dawn space mission. © 2015 The Authors.

Person M.J.,Massachusetts Institute of Technology | Dunham E.W.,Lowell Observatory | Bosh A.S.,Massachusetts Institute of Technology | Levine S.E.,Massachusetts Institute of Technology | And 47 more authors.
Astronomical Journal | Year: 2013

On 2011 June 23, stellar occultations by both Pluto (this work) and Charon (future analysis) were observed from numerous ground stations as well as the Stratospheric Observatory for Infrared Astronomy (SOFIA). This first airborne occultation observation since 1995 with the Kuiper Airborne Observatory resulted in the best occultation chords recorded for the event, in three visible wavelength bands. The data obtained from SOFIA are combined with chords obtained from the ground at the IRTF, the U.S. Naval Observatory Flagstaff Station, and Leeward Community College to give the detailed state of the Pluto-Charon system at the time of the event with a focus on Pluto's atmosphere. The data show a return to the distinct upper and lower atmospheric regions with a knee or kink in the light curve separating them as was observed in 1988, rather than the smoothly transitioning bowl-shaped light curves of recent years. The upper atmosphere is analyzed by fitting a model to all of the light curves, resulting in a half-light radius of 1288 ± 1 km. The lower atmosphere is analyzed using two different methods to provide results under the differing assumptions of particulate haze and a strong thermal gradient as causes for the lower atmospheric diminution of flux. These results are compared with those from past occultations to provide a picture of Pluto's evolving atmosphere. Regardless of which lower atmospheric structure is assumed, results indicate that this part of the atmosphere evolves on short timescales with results changing the light curve structures between 1988 and 2006, and then reverting these changes in 2011 though at significantly higher pressures. Throughout these changes, the upper atmosphere remains remarkably stable in structure, again except for the overall pressure changes. No evidence of onset of atmospheric collapse predicted by frost migration models is seen, and the atmosphere appears to be remaining at a stable pressure level, suggesting it should persist at this full level through New Horizon's flyby in 2015. © 2013. The American Astronomical Society. All rights reserved.

Hanus J.,French National Center for Space Studies | Hanus J.,University Of La Cote Dazur | urech J.,Charles University | Oszkiewicz D.A.,Adam Mickiewicz University | And 171 more authors.
Astronomy and Astrophysics | Year: 2016

Context. Asteroid modeling efforts in the last decade resulted in a comprehensive dataset of almost 400 convex shape models and their rotation states. These efforts already provided deep insight into physical properties of main-belt asteroids or large collisional families. Going into finer detail (e.g., smaller collisional families, asteroids with sizes 20 km) requires knowledge of physical parameters of more objects. Aims. We aim to increase the number of asteroid shape models and rotation states. Such results provide important input for further studies, such as analysis of asteroid physical properties in different populations, including smaller collisional families, thermophysical modeling, and scaling shape models by disk-resolved images, or stellar occultation data. This provides bulk density estimates in combination with known masses, but also constrains theoretical collisional and evolutional models of the solar system. Methods. We use all available disk-integrated optical data (i.e., classical dense-in-time photometry obtained from public databases and through a large collaboration network as well as sparse-in-time individual measurements from a few sky surveys) as input for the convex inversion method, and derive 3D shape models of asteroids together with their rotation periods and orientations of rotation axes. The key ingredient is the support of more that 100 observers who submit their optical data to publicly available databases. Results. We present updated shape models for 36 asteroids, for which mass estimates are currently available in the literature, or for which masses will most likely be determined from their gravitational influence on smaller bodies whose orbital deflections will be observed by the ESA Gaia astrometric mission. Moreover, we also present new shape model determinations for 250 asteroids, including 13 Hungarias and three near-Earth asteroids. The shape model revisions and determinations were enabled by using additional optical data from recent apparitions for shape optimization. © 2016 ESO.

Boissel Y.,University Pierre and Marie Curie | Sicardy B.,University Pierre and Marie Curie | Roques F.,University Pierre and Marie Curie | Gaulme P.,New Mexico State University | And 19 more authors.
Astronomy and Astrophysics | Year: 2014

Context. Pluto has five known satellites with diameters ranging from ~1200 km down to ~40 km, a possible outcome of a collisional origin. Smaller objects probably exist and may maintain tenuous rings, thus representing hazards during the New Horizons flyby of July 2015. Aims. The goal is to provide an upper limit for the numbers of unseen small bodies and/or equivalent widths of putative Pluto rings. Methods. We use a Pluto stellar appulse on April 10, 2006, and a stellar occultation by the dwarf planet on June 14, 2007, to scan Pluto's surroundings. Results. Our best data set places a 3σ upper limit of 0.3 km for the radius of isolated moonlets that we can detect. In the absence of detection, we derive an upper limit of 15 000 for the number of such bodies at distances smaller than ~70 000 km from Pluto's system barycenter. We place a 3σ upper limit of typically 30-100 m for the equivalent width of ring material at barycentric distances ranging from 13 000 to 70 000 km. This limit applies for narrow rings only, i.e. less than about 10 km in width. © 2014 ESO.

Christou A.A.,Armagh Observatory | Beisker W.,International Occultation Timing Association | Casas R.,International Occultation Timing Association | Casas R.,Institute Of Ciencies Of L Espai Ieec Csic | And 18 more authors.
Astronomy and Astrophysics | Year: 2013

Aims. Occultations of bright stars by planets provide information on the state of their atmospheres. An occultation of the bright star 45 Capricornii (HIP 107302) by Jupiter occurred on the night of 3/4 August 2009. Methods. The event was observed at multiple sites in Europe, Africa and South America and with instruments ranging in aperture from 0.4 m to 2.2 m. All observations, except one, were carried out in methane absorption bands centred at 0.89 μm and 2.2 μm to minimise the planetary contribution to the measured stellar flux. Following the application of special post-processing techniques, differential photometry was performed. Nearby bright satellites were used as reference sources. Results. Fifteen lightcurves were obtained. The photometric time series for fourteen of these were fitted to a model atmosphere of constant scale height (H). Estimates of H for most lightcurves lie within the range 20-30 km with an inverse-variance weighted mean of 23.6 ± 0.4 km, in good agreement with previous works. A comparison between half-light times at ingress and at egress implies an astrometric offset of 10-15 mas in Jupiter's position relative to the star. Five lightcurves - two for ingress and three for egress - were numerically inverted into profiles of pressure versus temperature. Isothermal, mutually consistent behaviour is observed within the pressure range 3-10 μbar. The inferred temperature of 165 ± 5 K is consistent with, but slightly higher than, that measured by the Galileo Probe at 5 S latitude in 1995 at the same pressure level. Subtraction of isothermal models for nine cases show the presence of at least one, and possibly two, non-isothermal layers a few tens of km below the half-light datum. Their altitudes are similar to those of features previously reported during the occultation of HIP 9369 in 1999. Our temperature estimates are consistent with the expected small magnitude of the perturbation of the atmosphere following the impact event on Jupiter in July 2009. © ESO, 2013.

Make a date with the moon on Saturday, March 4. If you live in the United States and the weather cooperates, chances are good that you can watch a bright star called Aldebaran briefly disappear that evening. What's more, all you need are your naked eyes — no telescope or binoculars required (though they'd definitely help). Most of the contiguous US at night and Hawaii during the day should be able to watch the astronomical event unfold. While the exact moment depends on your location, we've included a rough map and observation times below. We first learned about this stellar occultation, as it's called, from a story by David W. Dunham at Sky & Telescope. While occultations of stars and planets are incredibly common, Dunham says "[i]t can't get much better than this," thanks to the phase of the moon and its lower brightness. Aldebaran is typically the fourteenth-brightest star in the night sky and has an orange hue because it's a red giant nearly 45 times as wide as the sun. (Billions of years in the future, the sun will balloon to roughly the same size and doom Earth in the process.) "The orange star will appear to approach the Moon from the dark side, passing very close to the northern cusp," according to an image posted by the International Occultation Timing Association (IOTA). Those who live along a "graze line" will see Aldebaran kiss the top of the moon instead of making the star entirely disappear. The graze line will make a very thin track across parts of Washington state, Montana, North Dakota, Minnesota, Wisconsin, Michigan, New York, Connecticut, and Rhode Island. North of the graze line, viewers won't see the occultation. (If you live south of the US, the southern graze line of the occultation is shown in this IOTA map.) Here's a rough map of where the occultation will be visible: And here are a few key locations and times for when Aldebaran will disappear and reappear from behind the moon, according to IOTA and Sky & Telescope: For more locations and other information about "Aldebaran's disappearing act" on Saturday, read Dunham's story at Sky & Telescope. IOTA has even more details on the event, including exact time tables, regional weather maps, and more. NOW WATCH: The first nuclear bomb test in history is helping scientists unlock secrets of how the Moon formed

News Article | October 26, 2016

Get Ready For Halloween By Watching The Moon's 'Occultation' Tuesday Night Stargazers, ready your telescopes: An unusual lunar event is going to be visible across large portions of the U.S. It's called an "occultation," in which the waning gibbous moon will pass over the huge, bright orange star of Aldebaran. We thought the phenomenon's spooky name might be just the thing to get geared up for Halloween. The event will be visible across large portions of the South and the Northeastern United States, Mexico, the Caribbean and parts of Canada. "The most interesting views will be from a strip of land only a few hundred yards wide along the occultation's northern limit," where a "grazing occultation" will be visible, according to the International Occultation Timing Association. That's where the Aldebaran star — which is the bull's fiery eye in the Taurus constellation — will appear to graze the top edge of the moon. "Viewed from the narrow graze zone, the giant star should disappear and reappear multiple times as hills and valleys along the Moon's northern limb cover and expose it," the IOTA adds. The area where the grazing occultation is visible passes through or near many highly populated areas, including Los Angeles, Las Vegas, Denver, Minneapolis and Sioux Falls, S.D. Sky & Telescope has this handy map showing where the occultation is visible and where the graze line is: Head here for a detailed list of the exact moment to catch the occultation at more than 1,000 locations. Here's more on what to expect tonight from "The star will disappear on the moon's bright side and reappear on the dark side. "As the moon, three days past full, ascends the eastern sky, Aldebaran will appear to creep up to the moon's bright limb for many minutes, then hang on the sunlit edge of a few seconds — an eerie orange fire among the lunar hills. You'll need a telescope to see it there through the moon's glare. Then, in an instant, it will snap out of view. "Aldebaran will reappear from behind the moon's dark portion (depending on your location) up to 75 minutes later. This time you may be able to see the event with binoculars, since the star will pop back into view farther away from the dazzling lunar surface." If you're having trouble seeing Aldebaran, EarthSky says that covering the moon with your finger could help. Aldebaran is gigantic — according to EarthSky, if it was "placed where the sun is now, its surface would extend almost to the orbit of Mercury." It's also "about 153 times brighter than the sun." Fun fact: EarthSky says "Aldebaran is the name of one of the chariot horses in the movie Ben Hur." "These occultations of Aldebaran have been happening once per lunar month in a series that began in January 2015 and will end in September 2018. But most aren't visible in the U.S.," as USA Today explains. "After this series ends, the next occultation of Aldebaran won't be until 2033."

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