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Exploring the outer solar system beyond Neptune using stellar occultations

Periodic Reporting for period 3 - LUCKY STAR (Exploring the outer solar system beyond Neptune using stellar occultations)

Reporting period: 2018-11-01 to 2020-04-30

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 2017, more than 2,300 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.
- finalized tests of our newly 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

- 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)

- prediction of an occultation by Triton's atmosphere on Oct. 5, 2017,
and successful campaign organized in Europe, N. Africa and USA.

- Serendipitous occultations : 3 campaigns have been organized in Calar Alto (2.2m and 1.23m
telescopes) with the multi object photometer Miosotys in 2016 and 2017. The 45 nights data
is currently being analyzed. Potential occultation by sub-km TNOs found in the full
Corot satellite data, are under analysis.
- one of the most spectacular progress has been the 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 our group around the asteroid-like body Chariklo.

- for the immediate future, we foresee very accurate predictions (10-km level
for the shadow on Eart) of occultations. Gaia kept its promises, so that
carefully planned campaigns should allows us to possibly discover tenuous atmospheres
around some large TNO, new ring systems, and accurate measurements of exotic
shapes in some fast-rotation TNO's

Serendipitous occultations : The first campaign of Miosotys led to the discovery of
diffracting occultation profiles. The serendipitous occultations method needs the full
Miosotys campaign to provide statistically reliable results. The full Corot data set
corresponds to 300k star.hours of data with degraded definition has provided complementary
information on the population of small TNOs.
Artist view of a ring discovered around the dwarf planet Haumea, compared with Pluto (right)