A survey for occultation astrometry of Main Belt: expected astrometric performances

TL;DR

This survey evaluates the potential of occultation astrometry for precise asteroid position measurements, demonstrating that with Gaia data and a robotic telescope, sub-milliarcsecond accuracy comparable to Gaia's performance can be achieved.

Contribution

It provides a systematic analysis of occultation astrometry's accuracy, highlighting the importance of asteroid shape knowledge and demonstrating potential precision levels with a specific instrument setup.

Findings

Single chord astrometry can reach Gaia-like precision.

Astrometric errors are mainly due to asteroid shape uncertainties.

Occultation timing uncertainties translate into sub-milliarcsecond positional accuracy.

Abstract

Context: Occultations of stars by asteroids are an efficient method to study the properties of minor bodies, and can be exploited as tools to derive very precise asteroid astrometry relative to the target star. With the availability of stellar astrometry thanks to the ESA mission Gaia, the frequency of good predictions and the quality of the astrometry have been strongly enhanced. Aims: Our goal is to evaluate the astrometric performance of a systematic exploitation of stellar occultations, with a homogeneous data set and a given instrument setup. As a reference instrument, we adopt the example of a robotic 50 cm telescope, which is under construction at the Observatoire de la C\^ote d Azur. We focus in particular on single chord occultations. Methods: We created a data set of simulated light curves, that are modelled by a Bayesian approach. To build the final statistics, we considered a list of predicted events over a long time span, and stellar astrometry from Gaia data release 2. Results: We derive an acceptable range of observability of the events, with clear indications of the expected errors in terms of timing uncertainties. By converting the distribution of such errors to astrometric uncertainties, we show that the precision on a single chord can reach levels equivalent to the performance of Gaia (sub milli arcseconds). The errors on the asteroid position are dominated by the uncertainty on the position of the occultation chord with respect to the barycentre of the object. Conclusions: The limiting factor in the use of occultation astrometry is not the light curve uncertainty, but our knowledge of the shape and size of the asteroid. This conclusion is valid in a wide range of flux drops and magnitudes of the occulted star. The currently increasing knowledge of the shape, spin properties, and size, must be used to mitigate this source of error.