Sh2-138: Physical environment around a small cluster of massive stars
  We present a multi-wavelength study of the Sh2-138, a Galactic compact H II
region. The data comprise of optical and near-infrared (NIR) photometric and
spectroscopic observations from the 2-m Himalayan Chandra Telescope, radio
observations from the Giant Metrewave Radio Telescope (GMRT), and archival data
covering radio through NIR wavelengths. A total of 10 Class I and 54 Class II
young stellar objects (YSOs) are identified in a 4'.6$\times$4'.6 area of the
Sh2-138 region. Five compact ionized clumps, with four lacking of any optical
or NIR counterparts, are identified using the 1280 MHz radio map, and
correspond to sources with spectral type earlier than B0.5. Free-free emission
spectral energy distribution fitting of the central compact H II region yields
an electron density of ~2250$\pm$400 cm$^{-3}$. With the aid of a wide range of
spectra, from 0.5-15 $\mu m$, the central brightest source - previously
hypothesised to be the main ionizing source - is characterized as a Herbig Be
type star. At large scale (15'$\times$15'), the Herschel images (70-500 $\mu
m$) and the nearest neighbour analysis of YSOs suggest the formation of an
isolated cluster at the junction of filaments. Furthermore, using a greybody
fit to the dust spectrum, the cluster is found to be associated with the
highest column density (~3$\times$10$^{22}$ cm$^{-2}$) and high temperature
(~35 K) regime, as well as with the radio continuum emission. The mass of the
central clump seen in the column density map is estimated to be ~3770
$M_\odot$.
