HII region variability and pre-main-sequence evolution

TL;DR

This paper investigates how the evolution of massive protostars influences the variability of surrounding HII regions, showing that these regions can rapidly collapse or shrink during stellar evolution phases, on timescales of a few thousand years.

Contribution

It introduces numerical simulations of HII region variability driven by pre-main-sequence evolution of massive protostars, a novel approach to understanding observed rapid changes.

Findings

HII regions can shrink or collapse from 80-300 AU to near absence.

Variability occurs on timescales as short as ~3000 years.

Different protostellar models show consistent collapse behavior.

Abstract

Recent observations and simulations have suggested that HII regions around massive stars may vary in their size and emitted flux on timescales short enough to be observed. This variability can have a number of causes, ranging from environmental causes to variability of the ionizing source itself. We explore the latter possibility by considering the pre-main-sequence evolution of massive protostars and conducting numerical simulations with ionizing radiation feedback using the FLASH AMR hydrodynamics code. We investigate three different models: a simple ZAMS model, a self-consistent one-zone model by Offner et al. (2009), and a model fit to the tracks computed by Hosokawa & Omukai (2009). The protostellar models show that hypercompact HII regions around massive, isolated protostars collapse or shrink from diameters of 80 or 300 AU, depending on the model choice, down to near absence during the swelling of stellar radius that accompanies the protostar's transition from a convective to a radiative internal structure. This occurs on timescales as short as ~3000 years.