10.2.3 Surroundings of bright stars

At an early stage of the processing, false detections due to diffraction spikes on the PSF, etc, were present around bright stars. They were considered as new sources, not being part of the Initial Gaia Source List (IGSL). Thanks to a dedicated cleaning,Figure 10.8 shows that the artefacts present at an early stage of the Gaia DR2 processing seem no more present in Gaia DR2.

Figure 10.8: Around the bright star HR 807: sources in IGSL (blue), new sources (red). In brown, slightly shifted, the clearly false detections that were present at a very early stage of the Gaia DR2 processing have been added on top.

We had developed tests to detect automatically non random features. They actually found real structures (e.g., a dark cloud near HIP 27406, Figure 10.9) rather than artefacts.

Figure 10.9: Around the bright star HIP 27406 in Gaia DR2 detected due to a non random angular distribution of sources in the environment of the star. Left: from top to bottom, for IGSL sources, non IGSL, and all sources, detection test of non randomness as a function of radius squared (left) and versus position angle (right). Right: spatial distribution, histograms of observations, magnitude, fraction of outliers. This star caused suspicion because of the uneven distributions, which, however, were not due to artefacts, but to the presence of an obscure cloud.