Evolution of Massive Protostars with High Accretion Rates

TL;DR

This paper investigates the evolution of massive protostars under high accretion rates, revealing how such rates influence stellar growth, structure, and the upper mass limits of stars before reaching the main sequence.

Contribution

It provides a detailed model of massive protostar evolution at high accretion rates, highlighting the effects on stellar size, formation timescales, and mass limits, which differ from low-rate scenarios.

Findings

Protostellar radius exceeds 100 R_sun at high accretion rates.

Hydrogen burning begins at around 40 M_sun for high accretion protostars.

Upper mass limit for pre-main-sequence stars is approximately 60 M_sun.

Abstract

Formation of massive stars by accretion requires a high accretion rate of > 10^-4 M_sun/yr to overcome the radiation pressure barrier of the forming stars. Here, we study evolution of protostars accreting at such high rates, by solving the structure of the central star and the inner accreting envelope simultaneously. The protostellar evolution is followed starting from small initial cores until their arrival at the stage of the Zero-Age Main Sequence (ZAMS) stars. An emphasis is put on evolutionary features different from those with a low accretion rate of 10^-5 M_sun/yr, which is presumed in the standard scenario for low-mass star formation. With the high accretion rate of 10^-3 M_sun/yr, the protostellar radius becomes very large and exceeds 100 R_sun. It is not until the stellar mass reaches 40 M_sun that hydrogen burning begins and the protostar reaches the ZAMS phase, and this ZAMS arrival mass increases with the accretion rate. At a very high accretion rate of > 3 x 10^-3 M_sun/yr, the total luminosity of the protostar becomes so high that the resultant radiation pressure inhibits the growth of the protostars under steady accretion before reaching the ZAMS stage. Therefore, the evolution under the critical accretion rate 3 x 10^-3 M_sun/yr gives the upper mass limit of possible pre-main-sequence stars at 60 M_sun. The upper mass limit of MS stars is also set by the radiation pressure onto the dusty envelope under the same accretion rate at 250 M_sun. We also propose that the central source enshrouded in the Orion KL/BN nebula has effective temperature and luminosity consistent with our model, and is a possible candidate for such protostars growing under the high accretion rate. (abridged)