In reply to Loralee.

Most likely, these are caused by some plugins you use. I doubt it was caused by WordPress itself. My suggestion is to check error logs on the server, to see if you have errors logged, and that would point to specific plugins. Probably, you are using one or more plugins that are not compatible with WordPress 4.9.x.

If you update/replace/fix these plugins, you should monitor these core.xxxx files, my guess would be that they would disappear.