Astrometric and photometric initial mass functions from the UKIDSS Galactic Clusters Survey: II The Alpha Persei open cluster

We present the results of a deep (J=19.1 mag) infrared (ZYJHK) survey over the full Alpha Persei open cluster extracted from the Data Release 9 of the UKIRT Infrared Deep Sky Survey Galactic Clusters Survey. We have selected ~700 cluster member candidates in ~56 square degrees in \APer{} by combining photometry in five near-infrared passbands and proper motions derived from the multiple epochs provided by the UKIDSS GCS DR9. We also provide revised membership for all previously published APer low-mass stars and brown dwarfs recovered in GCS based on the new photometry and astrometry provided by DR9. We find no evidence of $K$-band variability in members of APer with dispersion less than 0.06-0.09 mag. We employed two independent but complementary methods to derive the cluster luminosity and mass functions: a probabilistic analysis and a more standard approach consisting of stricter astrometric and photometric cuts. We find that the resulting luminosity and mass functions obtained from both methods are consistent. We find that the shape of the APer mass function is similar to that of the Pleiades although the characteristic mass may be higher after including higher mass data from earlier studies (the dispersion is comparable). We conclude that the mass functions of APer, the Pleiades, and Praesepe are best reproduced by a log-normal representation similar to the system field mass function although with some variation in the characteristic mass and dispersion values.