# UCAC5: New Proper Motions using Gaia DR1

## TL;DR

This paper presents UCAC5, a new star catalog with improved proper motions derived from Gaia DR1 and UCAC data, achieving high accuracy for over 107 million stars and extending precise measurements to fainter stars.

## Contribution

The paper introduces UCAC5, a star catalog with enhanced proper motions using Gaia DR1 data, significantly improving accuracy and extending coverage to fainter stars compared to previous catalogs.

## Key findings

- Proper motions for over 107 million stars with 1-2 mas/yr accuracy.
- Significant improvement in astrometric solutions over previous catalogs.
- UCAC5 proper motions are more accurate than Gaia data for most TGAS stars.

## Abstract

New astrometric reductions of the US Naval Observatory CCD Astrograph Catalog (UCAC) all-sky observations were performed from first principles using the TGAS stars in the 8 to 11 magnitude range as reference star catalog. Significant improvements in the astrometric solutions were obtained and the UCAC5 catalog of mean positions at a mean epoch near 2001 was generated. By combining UCAC5 with Gaia DR1 data new proper motions on the Gaia coordinate system for over 107 million stars were obtained with typical accuracies of 1 to 2 mas/yr (R = 11 to 15 mag), and about 5 mas/yr at 16th mag. Proper motions of most TGAS stars are improved over their Gaia data and the precision level of TGAS proper motions is extended to many millions more, fainter stars. External comparisons were made using stellar cluster fields and extragalactic sources. The TGAS data allow us to derive the limiting precision of the UCAC x,y data, which is significantly better than 1/100 pixel.