Coeval intermediate-mass star formation in N4W
  Protostars are mostly found in star-forming regions, where the natal
molecular gas still remains. In about 5' west of the molecular bubble N4, N4W
is identified as a star-forming clump hosting three Class II (IRS\,1\,--\,3),
and one Class I (IRS\,4) young stellar objects (YSOs), as well as a
submillimeter source SMM1. The near-IR polarization imaging data of N4W reveal
two infrared reflection nebulae close to each other, which are in favor of the
outflows of IRS\,1 and IRS\,2. The bipolar mid-IR emission centered on IRS\,4
and the arc-like molecular gas shell are lying on the same axis, indicating a
bipolar molecular outflow from IRS\,4. There are two dust temperature
distributions in N4W. The warmer one is widely distributed and has a
temperature $T_\mathrm{d}\gtrsim28\,\mathrm{K}$, with the colder one from the
embedded compact submillimeter source SMM1. N4W's mass is estimated to be
$\sim2.5\times10^3\,M_\odot$, and the mass of SMM1 is
$\sim5.5\times10^2\,M_\odot$ at $T_\mathrm{d}=15\,\mathrm{K}$, calculated from
the CO\,$1-0$ emission and $870\,\mu$m dust continuum emission, respectively.
Based on the estimates of bolometric luminosity of IRS\,1\,--\,4, these four
sources are intermediate-mass YSOs at least. SMM1 is gravitationally bound, and
is capable of forming intermediate-mass stars or even possibly massive stars.
The co-existence of the IR bright YSOs and the submillimeter source represents
potential sequential star formation processes separated by $\sim0.5$\,Myr in
N4W. This small age spread implies that the intermediate-mass star formation
processes happening in N4W are almost coeval.
