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Occultation of a star by the asteriod Egeria

In January 2021, a double (actually triple as we shall see) star was occulted by the asteriode Egeria.


This post explains my experience measuring the occultation, which was long overdue for a blog post. But this week, on 23/10 another asteriod, Eurybates, produces the occultation of a star visible near my observatory, and this has made me remember that I had to write this entry and here is the summary of the experience.

Egeria is an asteriod discovered in 1850 by Annibale Gasparis. It orbits the Sun at an average distance of 2.57 au and completes its orbit in 1511 days. It has a diameter of about 228 km.


The star it occulted, a phenomenon visible from my observatory in January 2021, was HIP 25974, a double star with a separation of 0.2 arcsec between its components. The occultation of a double star generates two steps in the light curve as seen in the light curve I captured:




But the magnitude drop of the main component was not 2.4 mag as predicted, but 1.5, which can be explained if the main component is itself double, this third component not being occulted. According to this model, the stars in the system would have the following magnitudes: A=8.9 B=9.3 and C=10.0. This last star is the one that has been added, not by direct measurement but by establishing a model on the basis of the measurements, resulting in the stellar system being actually triple.


One of the advantages of obtaining the light curve of the occultation is that the exact measurement of the moments of brightness dip and brightness recovery, together with the precise position of the observer, allows us to derive a " chord" of the asteriod, which is ultimately a line of the 2D projection we see of the asteriod. By putting together the chords of the different observers, its shape can be reconstructed, which is also modelled with physical parameters that are known from the asteriod.


As a result of the measurements of the different observers, the following model was obtained:





Finally, comment on the details of the observation:


- Observatory L46

- Telescope: SC 8" with focal reducer of x0.7

- EQ6 mount

- ASI1600mmPro camera with 100ms of exposure

- Video recording with Sharpcap

- Time base: local GPS-based stratum 1 server

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