Photometric determination of the mass accretion rates of pre-main sequence stars. I. Method and application to the SN1987A field

TL;DR

This paper introduces a new photometric method to identify and measure mass accretion rates in pre-main sequence stars without spectroscopy, and applies it to a field around SN1987A, revealing insights into star formation and disc erosion.

Contribution

The paper presents a self-consistent, spectroscopic-free method to determine mass accretion rates in PMS stars using broad-band and narrow-band photometry, validated on SN1987A data.

Findings

Identified 133 PMS stars with Halpha excess in SN1987A field.

Median mass accretion rate of 2.6x10^-8 M_sun/yr consistent with previous studies.

Observed UV radiation from massive stars affects circumstellar disc longevity.

Abstract

We have developed and successfully tested a new self-consistent method to reliably identify pre-main sequence (PMS) objects actively undergoing mass accretion in a resolved stellar population, regardless of their age. The method does not require spectroscopy and combines broad-band V and I photometry with narrow-band Halpha imaging to: (1) identify all stars with excess Halpha emission; (2) derive their Halpha luminosity L(Halpha); (3) estimate the Halpha emission equivalent width; (4) derive the accretion luminosity L_acc from L(Halpha); and finally (5) obtain the mass accretion rate M_acc from L_acc and the stellar parameters (mass and radius). By selecting stars with photometric accuracy in Halpha better than 15%, the statistical uncertainty on the derived M_acc is typically <17% and is dictated by the precision of the Halpha photometry. Systematic uncertainties, of up to a factor of 3 on the value of M_acc, are caused by our incomplete understanding of the physics of the accretion process and affect all determinations of the mass accretion rate, including those based on a spectroscopic Halpha line analysis. As an application of our method, we study a field of 9.16 arcmin2 around SN1987A, using existing HST photometry. We identify as bona-fide PMS stars a total of 133 objects with a Halpha excess above the 4 sigma level and a median age of 13.5 Myr. Their median mass accretion rate of 2.6x10-8 Msolar/yr is in excellent agreement with previous determinations based on the U-band excess of the stars in this field, and with the value measured for G-type PMS stars in the Milky Way. Their L_acc shows a strong dependence on their distance from a group of hot massive stars in the field and suggests that the UV radiation of the latter is rapidly eroding the circumstellar discs around PMS stars.