The Structure of the Outer Galactic Disc as revealed by IPHAS early A
  Stars
  This study is an investigation of the stellar density profile of the Galactic
disc in the Anticentre direction. We select over 40,000 early A stars from
IPHAS photometry in the Galactic longitude range 160 < l < 200 close to the
equatorial plane (-1 < b < +1). We then compare their observed
reddening-corrected apparent magnitude distribution with simulated photometry
obtained from parameterised models in order to set constraints on the
Anticentre stellar density profile. By selecting A stars, we are appraising the
properties of a population only ~100 Myrs old. We find the stellar density
profile of young stars is well fit to an exponential with length scale of (3020
\pm 120_{statistical} \pm 180_{systematic}) pc, which is comparable to that
obtained in earlier studies, out to a Galactocentric radius of R_T = (13.0 \pm
0.5_{statistical} \pm 0.6_{systematic}) kpc. At larger radii the rate of
decline appears to increase with the scale length dropping to (1200 \pm
300_{statistical} \pm 70_{systematic}) pc. This result amounts to a refinement
of the conclusions reached in previous studies that the stellar density profile
is abruptly truncated. The IPHAS A star data are not compatible with models
that propose a sudden change in metallicity at R_G = 10 kpc.
