Triggered Star Formation and Evolution of T-Tauri stars in and around Bright-Rimmed Clouds

TL;DR

This study provides evidence for small-scale sequential star formation in bright-rimmed clouds, showing age gradients, evolution of T-Tauri stars, and variations in mass functions, supported by photometry and infrared data.

Contribution

It offers a quantitative test of the sequential star formation hypothesis in BRCs, extending previous work with new observations and deeper data, and analyzes star evolution and mass functions.

Findings

Age gradients observed in BRCs support sequential star formation.

WTTSs are generally older than CTTSs, indicating evolutionary progression.

Mass functions vary with BRC morphology, with steeper MF in B/C types.

Abstract

The aim of this paper is to quantitatively testify the "{\it small-scale sequential star formation}" hypothesis in and around bright-rimmed clouds (BRCs). As a continuation of the recent attempt by Ogura et al. (2007, Paper I), we have carried out $BVI_{c}$ photometry of four more BRC aggregates along with deeper re-observations of 2 previously observed BRCs. Again quantitative age gradients are found in almost all the BRCs studied in the present work. Archival Spitzer/IRAC data also support this result. The global distribution of NIR excess stars in each HII region studied here clearly shows evidence that a series of radiation driven implosion (RDI) processes proceeded in the past from near the central O star(s) towards the peripheries of the HII region. We found that in general weak-line T-Tauri stars (WTTSs) are somewhat older than classical T-Tauri stars (CTTSs). Also the fraction of CTTSs among the T-Tauri stars (TTSs) associated with the BRCs is found to decrease with age. These facts are in accordance with the recent conclusion by Bertout et al. (2007) that CTTSs evolve into WTTSs. It seems that in general the EW of H$\alpha$ emission in TTSs associated with the BRCs decreases with age. The mass function (MF) of the aggregates associated with the BRCs of the morphological type "A" seems to follow that found in young open clusters, whereas "B/C" type BRCs show significantly steeper MF.