Time Variability in Simulated Ultracompact and Hypercompact HII Regions

TL;DR

This study uses 3D radiation-hydrodynamic simulations to analyze the time variability of ultracompact and hypercompact HII regions, showing flickering behavior consistent with observations and predicting detectable flux variations in a subset of these regions.

Contribution

The paper provides the first detailed simulation-based analysis of HII region flickering, revealing negative flux changes and estimating the fraction of observable variable regions.

Findings

Simulated HII regions exhibit flickering consistent with observations.

Approximately 10% of observed HII regions should show flux variations over 10% within 10 years.

Negative flux changes can occur, contrary to previous expectations.

Abstract

Ultracompact and hypercompact HII regions appear when a star with a mass larger than about 15 solar masses starts to ionize its own environment. Recent observations of time variability in these objects are one of the pieces of evidence that suggest that at least some of them harbor stars that are still accreting from an infalling neutral accretion flow that becomes ionized in its innermost part. We present an analysis of the properties of the HII regions formed in the 3D radiation-hydrodynamic simulations presented by Peters et al. as a function of time. Flickering of the HII regions is a natural outcome of this model. The radio-continuum fluxes of the simulated HII regions, as well as their flux and size variations are in agreement with the available observations. From the simulations, we estimate that a small but non-negligible fraction (~ 10 %) of observed HII regions should have detectable flux variations (larger than 10 %) on timescales of ~ 10 years, with positive variations being more likely to happen than negative variations. A novel result of these simulations is that negative flux changes do happen, in contrast to the simple expectation of ever growing HII regions. We also explore the temporal correlations between properties that are directly observed (flux and size) and other quantities like density and ionization rates.