Photometric determination of the mass accretion rates of pre-main sequence stars. II. NGC346 in the Small Magellanic Cloud

TL;DR

This study introduces a novel photometric method to identify and analyze pre-main sequence stars in NGC346, revealing their accretion rates, ages, and masses, and highlighting the significance of accretion in stellar evolution at low metallicity.

Contribution

It presents a new self-consistent photometric technique to determine mass accretion rates of PMS stars without spectroscopy, applied to the NGC346 field in the Small Magellanic Cloud.

Findings

Identified 680 PMS stars with masses 0.4-4 M_sun and ages 1-30 Myr.

Discovered an older PMS population with median age ~20 Myr.

Found that accretion rates decrease with stellar age and are higher at lower metallicity.

Abstract

[Abridged] We have studied the properties of the stellar populations in the field of the NGC346 cluster in the Small Magellanic Cloud, using a novel self-consistent method that allows us to reliably identify pre-main sequence (PMS) objects actively undergoing mass accretion, regardless of their age. The method does not require spectroscopy and combines broad-band V and I photometry with narrow-band Halpha imaging to identify all stars with excess Halpha emission and derive the accretion luminosity Lacc and mass accretion rate Macc for all of them. The application of this method to existing HST/ACS photometry of the NGC346 field has allowed us to identify and study 680 bona-fide PMS stars with masses from ~0.4 to ~4 Msolar and ages in the range from ~1 to ~30 Myr. This is the first study to reveal that, besides a young population of PMS stars (~ 1 Myr old), in this field there is also an older population of PMS objects with a median age of ~20 Myr. We provide for all of them accurate physical parameters. We study the evolution of the mass accretion rate as a function of stellar parameters and find that logMacc ~ -0.6 Log t + Log m + c, where t is the age of the star, m its mass and c a quantity that is higher at lower metallicity. The high mass accretion rates that we find suggest that a considerable fraction of the stellar mass is accreted during the PMS phase and that PMS evolutionary models that do not account for this effect will systematically underestimate the true age when compared with the observations.