This week I’ve continued the task of assembling new targets for ESPRI. We require a “secondary” or “reference” star next to each star that we survey for planets. That way, if the target star hosts a giant planet, then the secondary star serves as the reference point by which we can measure the orbit recoil motion.
Even though the secondary is next to the target star on the sky, in most cases the alignment is only an illusion, and it is actually at a great physical distance in the background.
To look for secondary stars, I’m using public data archives to dig up images already taken of our target candidates. In the case above, I happened to find two useful archive images of the same target. There’s a strong difference in image quality between the two, mostly because they were made with different sized telescopes. The one on the left was taken with the 4-meter VISTA observatory, while the one on the right was taken with the 1.3-meter, southern 2MASS telescope. It’s the same wavelength however, the near-infrared K band.
Sometimes, this kind of difference in quality (in terms of angular resolution and dynamic range) makes the difference between spotting a secondary star and missing it. Below is an example of a target for which the better VISTA image reveals the star, while it is completely lost in the 2MASS data. The two images are matched in terms of spatial scale on the sky.
The downside of the VISTA’s archive, however, is that it doesn’t cover the entire sky. The VISTA observing teams just haven’t gotten around to capturing all the stars, even though that is one of their goals. So in practice we are forced to look across several archives to pick out the best images available.