Sh2-138: Physical environment around a small cluster of massive stars

TL;DR

This study provides a comprehensive multi-wavelength analysis of the Sh2-138 region, revealing the physical environment, young stellar objects, ionized clumps, and the nature of the main ionizing source, a Herbig Be star, in a compact Galactic H II region.

Contribution

It offers the first detailed multi-wavelength characterization of Sh2-138, identifying YSOs, ionized clumps, and the properties of the central ionizing source, advancing understanding of small cluster environments.

Findings

Identification of 10 Class I and 54 Class II YSOs.

Detection of five compact ionized clumps, four without optical/NIR counterparts.

Characterization of the main ionizing source as a Herbig Be star.

Abstract

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 $M_\odot$.