Delete core.xxxxxx files?

While searching through my …/stats folder, I noticed that I had nine files that are 144Mb each.
As you can imagine, they are causing me some grief when it comes to backing up my site because they take so long and so much space.
These are all core.xxxxxx files
for example core.1040277 which is 143.73 Mb was modified (created?) on 20 June
I have three such core files for 24 June (an hour apart)
Does anyone know what these are and if they can be deleted?
And, where do I go in the Piwik admin to turn their creation off?
Any help greatly appreciated.

Nick

these files are not created by Piwik and can be safely deleted. I’m not sure where they are from?