Flux Density Variations at 3.6 cm in the Massive Star-Forming Region W49A

TL;DR

This study compares radio flux densities of ultracompact H II regions in W49A over 21 years, finding a significant decrease in one source, supporting theories of accretion flow instability in massive star formation.

Contribution

First long-term radio flux comparison in W49A, revealing flux variability in a specific ultracompact H II region, and providing observational evidence for accretion flow instability.

Findings

Most sources show no significant flux change over 21 years.

One source, W49A/G2, decreased by 20% in peak intensity.

W49A/G2's flux decrease supports theories of accretion instability.

Abstract

A number of ultracompact H II regions in Galactic star forming environments have been observed to vary significantly in radio flux density on timescales of 10-20 years. Theory predicted that such variations should occur when the accretion flow that feeds a young massive star becomes unstable and clumpy. We have targeted the massive star-forming region W49A with the Karl G. Jansky Very Large Array (VLA) for observations at 3.6 cm with the B-configuration at 0.8'' resolution, to compare to nearly identical observations taken almost 21 years earlier (February 2015 and August 1994). Most of the sources in the crowded field of ultracompact and hypercompact H II regions exhibit no significant changes over this time period. However, one source, W49A/G2, decreased by 20% in peak intensity (from 71+/-4 mJy/beam to 57+/-3 mJy/beam), and 40% in integrated flux (from 0.109+/-0.011 Jy to 0.067+/-0.007 Jy), where we cite 5 sigma errors in peak intensity, and 10% errors in integrated flux. We present the radio images of the W49A region at the two epochs, the difference image that indicates the location of the flux density decrease, and discuss explanations for the flux density decrease near the position of W49A/G2.