An Optical and Infrared Photometric Study of the Young Open Cluster IC
  1805 in the Giant H II Region W4
  We present deep wide-field optical CCD photometry and mid-infrared
Spitzer/IRAC and MIPS 24micron data for about 100,000 stars in the young open
cluster IC 1805. The members of IC 1805 were selected from their location in
the various color-color and color-magnitude diagrams, and the presence of
Halpha emission, mid-infrared excess emission, and X-ray emission. The
reddening law toward IC 1805 is nearly normal (R_V = 3.05+/-0.06). However, the
distance modulus of the cluster is estimated to be 11.9+/-0.2 mag (d =
2.4+/-0.2 kpc) from the reddening-free color-magnitude diagrams, which is
larger than the distance to the nearby massive star-forming region W3(OH)
measured from the radio VLBA astrometry. We also determined the age of IC 1805
(tau_MSTO = 3.5 Myr). In addition, we critically compared the age and mass
scale from two pre-main-sequence evolution models. The initial mass function
with a Salpeter-type slope of Gamma = -1.3+/-0.2 was obtained and the total
mass of IC 1805 was estimated to be about 2700+/-200 M_sun. Finally, we found
our distance determination to be statistically consistent with the Tycho-Gaia
Astrometric Solution Data Release 1, within the errors. The proper motion of
the B-type stars shows an elongated distribution along the Galactic plane,
which could be explained by some of the B-type stars being formed in small
clouds dispersed by previous episodes of star formation or supernova
explosions.
