Clustering of Emission-line Stars in the W5E HII region

TL;DR

This study surveyed emission-line stars in the W5E HII region, revealing spatial and age distributions that support sequential star formation from the center outward, and suggesting low-mass stars may form before massive OB stars.

Contribution

First detailed analysis of emission-line stars in W5E HII region showing age gradients and spatial clustering related to star formation processes.

Findings

Young stars near the exciting star are older (4 Myr) than those near the bright rims (1 Myr).

Star formation appears to proceed sequentially from the center to the edges.

Low-mass star formation may precede massive OB star formation in this region.

Abstract

We have made a new survey of emission-line stars in the W5E HII region to investigate the population of PMS stars near the OB stars by using the Wide Field Grism Spectrograph 2 (WFGS2). A total of 139 H-alpha emission stars were detected and their g'i'-photometry was performed. The spatial distribution of them shows three aggregates, i.e., two aggregates near the bright-rimmed clouds at the edge of W5E HII region (BRC 13 and BRC 14) and one near the exciting O7V star. The age and mass of each H-alpha star were estimated from the extinction corrected color-magnitude diagram and theoretical evolutionary tracks. We found, for the first time in this region, that the young stars near the exciting star are systematically older (4 Myr) than those near the edge of the HII region (1 Myr). This result supports that the formation of stars proceed sequentially from the center of HII region to the eastern bright rim. We further suggest a possibility that the birth of low mass stars near the exciting star of HII region precede the production of massive OB stars in the pre-existing molecular cloud.