The star formation histories, star formation efficiencies and ionizing sources of ATLASGAL clumps with HII regions

TL;DR

This study investigates the star formation processes in ATLASGAL clumps with HII regions, analyzing their luminosities, efficiencies, and histories to understand how massive stars and clusters form within these dense molecular clumps.

Contribution

It provides new estimates of stellar luminosities by subtracting accretion contributions, confirms the $m_{max}-M_{ecl}$ relation in embedded clusters, and reveals that star formation efficiency decreases with increasing clump mass.

Findings

Stellar luminosities are overestimated in low-luminosity clumps without correction.

The $m_{max}-M_{ecl}$ relation holds for the embedded clusters in HII regions.

Star formation efficiency decreases with increasing clump mass, median ~0.3.

Abstract

1226 ATLASGAL clumps with HII regions were matched with radio sources in the CORNISH-North/South surveys, and 392 of them have corresponding radio sources. We determined the stellar luminosity according to the Lyman continuum flux. When the bolometric luminosity of HII-clumps is less than $\approx$ 10$^{3.7}$ L$_{\odot}$, corresponding to a clump mass $\approx$ 10$^{2.55}$ M$_{\odot}$, the stellar luminosities derived from the Lyman continuum flux overestimate the actual stellar luminosities, because the accretion onto the protostars contributes significantly to the radio emission. After subtracting the accretion luminosity, we obtained reasonable estimates of the stellar luminosity. Using the 0.5 Myr isochrone, we calculated the stellar masses according to the stellar luminosities, and found that they roughly follow the $m_{\rm max}-M_{\rm ecl}$ relation of embedded clusters, consistent with the ionizing sources representing the most massive stars in the embedded clusters of HII-clumps. We also studied the contribution of the possible flaring activity to the observed stellar luminosity and found that they can be neglected. We further studied the change of SFE with the clump mass. According to the derived mass of the most massive star in each HII-clump, using the theoretical $m_{\rm max}-M_{\rm ecl}$ relation, we calculated the mass of the corresponding embedded cluster and then the SFE of the clump. The SFE decreases with increasing clump mass, with a median value of $\approx$0.3. We also independently derived the SFE for each HII-clump based on the model developed in our previous work. The SFEs of HII-clumps derived from the observation and the model are in good agreement. Concerning the star formation histories of the ATLASGAL clumps, low-mass clumps may reach the peak of star formation earlier than high-mass clumps, consistent with the shorter free-fall time of low-mass clumps.