Extreme Infrared Variables from UKIDSS - I. A Concentration in Star
  Forming Regions
  We present initial results of the first panoramic search for high-amplitude
near-infrared variability in the Galactic Plane. We analyse the widely
separated two-epoch K-band photometry in the 5th and 7th data releases of the
UKIDSS Galactic Plane Survey. We find 45 stars with \Delta K > 1 mag, including
2 previously known OH/IR stars and a Nova. Even though the mid-plane is not yet
included in the dataset, we find the majority (66%) of our sample to be within
known star forming regions (SFRs), with two large concentrations in the Serpens
OB2 association (11 stars) and the Cygnus-X complex (12 stars). Sources in SFRs
show spectral energy distributions (SEDs) that support classification as Young
Stellar Objects (YSOs). This indicates that YSOs dominate the Galactic
population of high amplitude infrared variable stars at low luminosities and
therefore likely dominate the total high amplitude population. Spectroscopic
follow up of the DR5 sample shows at least four stars with clear
characteristics of eruptive pre-main-sequence variables, two of which are
deeply embedded. Our results support the recent concept of eruptive variability
comprising a continuum of outburst events with different timescales and
luminosities, but triggered by a similar physical mechanism involving unsteady
accretion. Also, we find what appears to be one of the most variable classical
Be stars.
