Dear user of UCAC4,
it came to our attention that for some high proper motion (HPM) stars the published proper motion in the UCAC4 catalog is pointing in the wrong direction. This has been tracked down to a bug in preparing some external HPM catalog data, which later were merged with UCAC4.
Affected are 3416 stars and 416 binary zone files between zones 6 and 449. All affected stars have object type flag=4, i.e. stars observed with the astrograph and mean position and proper motion taken from external catalogs. Only the proper motion columns are affected by this bug, all other data items of those stars remain as they are in the UCAC4.
A Python script, modifs.py (Python V2.x) or modif3.py (Python V3.x, to be used in Windows), can be used for patching the original zone files. It can be downloaded from CDS or USNO. Put the script inside the folder which contains the binary zone files. The zone files need to have write permissions set to allow changing of the data. Run the script (takes only a few seconds) and your UCAC4 is up to date. No changes are needed to any code which accesses UCAC4 data.
The CDS which serves the UCAC4 online catalog did apply the corrections for those 3416 stars proper motions on 2013-01-30.
Many thanks to Brian Skiff (Lowell Observatory) and Rae Stiening (2MASS) for pointing out this problem. We are grateful to Francois Ochsenbein at CDS for providing the script and support in this matter.
Norbert Zacharias and Charlie Finch (USNO)
2013 January 29