Odd files

I've been running postgresql for quite a while now. And it's been
working with minimal errors, leaving me in a fat, dumb and happy state.
Recently though, I ran out of disk space on my 1 partition. Upon
investigation, it appears that one of my databases was to blame. But
here's the odd part:

[base/cm]>ls -l

-rw------- 1 postgres postgres 0 May 28 2000 pg_sorttemp2662.0
-rw------- 1 postgres postgres 0 May 28 2000 pg_sorttemp2662.1
-rw------- 1 postgres postgres 0 May 28 2000 pg_sorttemp2662.2
-rw------- 1 postgres postgres 0 May 28 2000 pg_sorttemp2662.3
-rw------- 1 postgres postgres 0 May 28 2000 pg_sorttemp2662.4
-rw------- 1 postgres postgres 0 May 28 2000 pg_sorttemp2662.5
-rw------- 1 postgres postgres 0 May 28 2000 pg_sorttemp2662.6
-rw------- 1 postgres postgres 0 May 28 2000
pg_sorttemp32606.0
-rw------- 1 postgres postgres 0 May 28 2000
pg_sorttemp32606.1
-rw------- 1 postgres postgres 0 May 28 2000
pg_sorttemp32606.2
-rw------- 1 postgres postgres 0 May 28 2000
pg_sorttemp32606.3
-rw------- 1 postgres postgres 0 May 28 2000
pg_sorttemp32606.4
-rw------- 1 postgres postgres 0 May 28 2000
pg_sorttemp32606.5
-rw------- 1 postgres postgres 0 May 28 2000
pg_sorttemp32606.6
-rw------- 1 postgres postgres 115482624 Dec 18 23:46
pg_sorttemp3946.0
-rw------- 1 postgres postgres 109785088 Dec 18 23:46
pg_sorttemp3946.1
-rw------- 1 postgres postgres 100233216 Dec 18 23:46
pg_sorttemp3946.2
-rw------- 1 postgres postgres 86548480 Dec 18 23:46 pg_sorttemp3946.3
-rw------- 1 postgres postgres 63303680 Dec 18 23:46 pg_sorttemp3946.4
-rw------- 1 postgres postgres 43204608 Dec 18 23:46 pg_sorttemp3946.5
-rw------- 1 postgres postgres 0 Dec 19 09:37 pg_sorttemp3946.6

My questions:
What are these files?
Are they safe to delete?

Thanks for any input

-Rob