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Exploring the outer solar system beyond Neptune using stellar occultations

Periodic Reporting for period 4 - LUCKY STAR (Exploring the outer solar system beyond Neptune using stellar occultations)

Okres sprawozdawczy: 2020-05-01 do 2021-10-31

Astronomers have discovered in the early 1990's a new class of remote bodies in our solar system, beyond the giant planet Neptune. Those objects are detected between ~30 and ~100 astronomical units (au) from the Sun, where 1 au is the mean distance between Earth and the Sun.

As of today, more than 4,000 such objects have been discovered, and many more await detections. Those objects (called Trans-Neptunian Objects, or TNOs) are composed of largely unaltered material from the primordial solar system. Moreover, they kept the memory of the early planetary migrations, and thus contains essential information on the origin and evolution of our planetary system.

The Lucky Star project tackles the problem of studying the the TNOs using the stellar occultation technique. It consists in observing the passage of remote TNOs in front of those “lucky stars”. Those occultations reveal the sizes, shapes, densities, reflective properties, atmospheres and/or rings of those bodies, from sub-km to thousand-km in size. Those parameters are largely out of reach of other observing methods, in particular classical imaging techniques, that have spatial resolutions several orders of magnitude coarser than occultations.

Those parameters are of broad interest for society, as they permit to write the big book of the solar system history, and therefore, our origins. The project can be viewed as an exploration of the solar system from our home Earth.

The overall objectives are divided in tasks:

(1) Discover and study rings systems around small bodies. Tackle theoretically the rings’ origins and evolutions around small bodies, and link them with satellite formation.

(2) Detect very small (sub-km) TNOs using serendipitous occultations. Then constrain the collisional history of our early outer solar system, and possibly detect Oort Cloud objects (much farther away than TNO's).

(3) Discover and scrutinize tenuous atmospheres like those of Pluto and Triton. Monitor their atypical seasonal cycles, search for atmospheres around other TNOs.

(4) Explore specific, large TNOs – Provide their sizes, shapes, albedos and densities.

These programs are timely in view of the recent NASA/New Horizons Pluto flyby (July 2015) that is now providing a wealth of complementary information on Pluto's system, unreachable through ground-based occultations.

Moreover, the ESA/GAIA mission is providing an extraordinary jump forward in star position accuracy, with increase by a factor of about 50 in our occultation prediction accuracy. This greatly improves the efficiency of the occultation method, and boost the volume of scientific returns.

Very few teams in the world master this method. The European-led network that I coordinate is now leader in predictions, instrumentation, observations and analysis related to stellar occultations, with innovative approaches and unprecedented results.
- tests of our purchased fast acquisition cameras: sensitivity and timing accuracy validations.

- implementation of the GAIA DR1 and DR2 catalogs in our prediction chain, and opening of a dedicated public page: http://lesia.obspm.fr/lucky-star/predictions/

- discovery of a ring around the dwarf planet Haumea in January 2017. This is the first ring ever detected in the Trans-Neptunian region (published in Nature, 12 Oct. 2017), and the third one co-discovered by the P.I. of the project, after Neptune's ring arcs in 1984 (Nature 1986) and Chariklo' rings in 2013 (Nature 2014).

- detailed study of Chariklo's ring structure and shape of the body from the three best stellar occultations ever observed (2017).

- advances on ring dynamics around small bodies in the solar system, in particular the strong effect of resonances between the orbital period of the ring particles and the rotation period of the body

- monitoring of Pluto's atmosphere until 2016, with climatic implications of our results

- relationship between our occultation results and the detection of CO an HCN in Pluto's atmosphere using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile

- various publications of papers on the size, shape, density and/or rings of large Trans-Neptunian Objects and Centaurs (Chariklo, 2003 AZ84, 2007 UK126)

- observation and analysis of an occultation by Triton's atmosphere on Oct. 5, 2017, successfully observed in Europe, N. Africa and USA. Constraints on the atmospheric structure, seasona evolution and upper limits of winds obtained with un precedented accuracy.

- Serendipitous occultations : various campaigns have been organized in Calar Alto, Spain (2.2m and 1.23m telescopes) with the multi object photometer Miosotys in 2016-2019. Potential occultations by sub-km TNOs found in the data, as well as in the Corot satellite data. A paper on constraints on the collisional history of the Solar System has been revised and re-submitted.

- The stellar occultation method has been extended to objects not initially expected in the list.

One category is the Trojans asteroids that share Jupiter's orbit. This work has been made in the perspectie of the NASA/Lucy mission that will flyby six of those objects
staring in 2027.

Another new kind of targets has been the Near Earth Asteroids. There are very difficult to catch by occultations owing to their small (< km) sizes. However, thanks to the Gaia astrometric catalog and radar detection, we succeeded on observin the object Apophis in March 2021.

Some disseminations and communications are summarized in the corresponding tab.

Everything considered, the Lucky Star project allowed be to become one of the very top leader in occultation works. This is true in terms of number of objects detected and variety of results obtained. They encompass the physical parameters of the observed objects (size, shape, density, albedo, bulk density), the properties and dynamics of their rings, and the structure of their atmospheres.
- the spectacular discovery of a new ring system in the solar system around the dwarf planet Haumea, beyond those of the giant planets and the one co-discovered by my group around the asteroid-like body Chariklo.

- the remarkable and unexpected complexity of seasonal variations of Pluto's and Triton's atmospheres

- the extension of the occultation method to many more objects initially planned, including Trojans of Jupiter ann Near Earth Asteroids
Artist view of a ring discovered around the dwarf planet Haumea, compared with Pluto (right)