Understanding formation of young, distributed low-mass stars and
  clusters in the W4 cloud complex
  It is well known that most of the stars form in rich clusters. However,
recent $Spitzer$ observations have shown that a significant number of stars
also form in distributed mode, origin of which is not well understood. In this
work, we aim to investigate clustered and distributed mode of star formation in
the W4 complex. To do so, we identified and characterized the young stellar
population associated with the region using homogeneous infra-red data-sets
obtained from 2MASS, GLIMPSE, MIPS and WISE surveys. We make stellar surface
density and minimum spanning tree maps to identify young clusters, and use {\it
Spitzer} images to identify irradiated structures, such as elephant trunk-like
structures (ETLSs) and pillars in the region. The surface density distribution
of the young stellar objects (YSOs) reveals three new clusterings and $\sim$
50\% distributed protostars in the H{\sc ii} region. The clusters are of
low-mass nature but significantly younger than the central cluster IC~1805. We
identified $\sim$ 38 ETLSs in the region, a majority of which consist of one or
a few stars at their tips. We find these stars are low-mass ($<$ 2~M$_\odot$)
YSOs, located at the outskirts ($>$ 17 pc) of the cluster IC~1805 and are part
of scattered distributed population. We argued that the star formation in the
ETLSs of W4 is going on possibly due to triggering effect of expanding W4
bubble. Although high-resolution photometric and spectroscopic data would be
required to confirm the scenario, nonetheless, we discuss the implications of
this scenario for our understanding of distributed low-mass star formation in
cloud complexes as opposed to other mechanisms such as turbulent fragmentation
and dynamical ejection.
