Photometric Determination of the Mass Accretion Rates of Pre-main-sequence Stars. I. Method and Application to the SN 1987A Field

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 broadband V and I photometry with narrowband Hα imaging to (1) identify all stars with excess Hα emission, (2) convert the excess Hα magnitude into Hα luminosity L(Hα), (3) estimate the Hα emission equivalent width, (4) derive the accretion luminosity L _acc from L(Hα), and finally (5) obtain the mass accretion rate \dot{M}_acc from L _acc and the stellar parameters (mass and radius). By selecting stars with an accuracy of 15% or better in the Hα photometry, the statistical uncertainty on the derived \dot{M}_acc is typically lsim17% and is dictated by the precision of the Hα photometry. Systematic uncertainties, of up to a factor of 3 on the value of \dot{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 Hα line analysis. As an application of our method, we study the accretion process in a field of 9.16 arcmin² around SN 1987A, using existing Hubble Space Telescope photometry. We identify as bona fide PMS stars a total of 133 objects with a Hα excess above the 4σ level and a median age of 13.5 Myr. Their median mass accretion rate of 2.6 × 10^-8 M _sun yr^-1 is in excellent agreement with previous determinations based on the U-band excess of the stars in the same field, as well as with the value measured for G-type PMS stars in the Milky Way. The accretion luminosity of these PMS objects shows a strong dependence on their distance from a group of hot massive stars in the field and suggests that the ultraviolet radiation of the latter is rapidly eroding the circumstellar disks around PMS stars.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555.